Best Blade Design for Wind Turbine

Courtesy: NORDEX

The efficiency of wind turbines is significantly influenced by the design of their blades. Finding the best wind turbine blade is a primary focus for engineers and researchers in the field of renewable energy. Advancements in technology and material science have led to significant improvements in blade design over the years. This article delves into the aspects of the best blade design for wind turbines, explores common materials used in blade construction, and discusses the typical lifespan of these critical components.

Best Blade Design for Wind Turbine

The “best” blade design for wind turbines is determined by several key factors: aerodynamic efficiency, cost-effectiveness, durability, and minimal environmental impact. Optimal designs ensure that wind turbines convert the maximum amount of wind energy into electrical power.

Factors Influencing Blade Design

Aerodynamic Efficiency

The shape and size of the blades are paramount in determining a turbine’s aerodynamic efficiency. The aerodynamics of wind turbine blades encompasses the study of airflow and its interaction with the blade surface. A well-designed blade minimizes resistance while maximizing lift, thereby extracting maximum energy from the wind. The design process involves sophisticated modeling techniques to simulate wind flow and predict performance under various conditions. This design principle enables the blades to rotate the turbine’s rotor more effectively, even in low wind speed conditions.

Environmental Considerations

Blades must withstand the specific environmental conditions, including variations in wind speed, density, and direction. This adaptability ensures consistent performance, power generation, and longevity.

Material Durability

The choice of materials directly affects the blade’s durability and performance. Optimal blade designs also consider material flexibility and durability to withstand the physical stresses exerted by wind forces. The ability of a blade to bend slightly without breaking reduces the risk of damage during high winds, thereby extending the turbine’s operational life. Innovations in material science continue to enhance the strength, flexibility, and lifespan of turbine blades, driving advancements in wind energy technology, including:

Use of Carbon Fiber Composites: One significant innovation in the field of wind turbine blade materials is the increasing use of carbon fiber composites. Unlike traditional materials like fiberglass, carbon fiber offers a superior strength-to-weight ratio, which allows for the construction of longer, more efficient blades. This material innovation enhances the aerodynamic performance of wind turbines and contributes to their overall durability and lifespan. For instance, the introduction of carbon fiber has enabled the production of blades that can exceed 100 meters in length, significantly increasing the energy output of wind turbines.
Bio-based Epoxy Resins: Another cutting-edge development is the use of bio-based epoxy resins in the manufacturing of turbine blades. Traditional epoxy resins, derived from petrochemicals, provide a matrix in composite materials but pose environmental and health concerns. In contrast, bio-based epoxies, made from natural sources such as plant oils, offer a more sustainable and less toxic alternative. These materials reduce the carbon footprint associated with blade production while maintaining the mechanical properties required for high-performance turbine blades.

Design Features

The most efficient blade designs incorporate specific features tailored to aerodynamic efficiency, cost-effectiveness, durability, while minimizing environmental impact.. The shape of the blade, often inspired by airfoil profiles, minimizes drag and maximizes lift, enhancing the turbine’s ability to capture wind energy. The length and pitch (angle) of the blades optimize specific wind conditions and operational requirements.

Design Types

The horizontal-axis wind turbines (HAWTs) are the most common, characterized by their upward-standing, propeller-like design. This configuration provides superior efficiency in converting wind energy at high wind speeds.

Vertical-axis wind turbines (VAWTs), while less common, offer advantages in lower wind conditions and are more versatile in urban or space-constrained environments:

Omnidirectional Wind Capture: VAWTs can capture wind from any direction without the need for yaw mechanisms or orientation towards the wind. This feature is particularly advantageous in urban or complex terrains where wind directions can be highly variable.
Lower Wind Speed Operation: VAWTs are capable of operating in lower wind conditions. Their design allows them to start generating power at lower wind speeds, making them effective in areas where high wind speeds are not consistently available.
Reduced Noise and Vibration: VAWTs tend to generate less noise and vibration compared to HAWTs. This characteristic makes them more suitable for residential areas or urban settings, where noise and vibration can be a concern.
Ease of Maintenance: The main components of VAWTs often reside at ground level, making maintenance easier and less costly than HAWTs, where components typically mount higher on the tower.
Space Efficiency: VAWTs have a smaller footprint relative to their height, which can make them more suitable for space-constrained environments such as urban areas. They can be installed closer together in wind farms, allowing for a more efficient use of land.

Common Materials

The evolution of materials used in wind turbine blades has paralleled technological advancements. Initially, materials such as wood and steel were common. However, the industry has shifted towards more advanced materials like fiberglass, carbon fiber, and composites. These materials offer an unmatched balance of strength, durability, and lightness, essential for efficient blade design.

Traditional Materials

Historically, wind turbine blades have been manufactured using materials such as fiberglass, known for its strength and durability. However, the weight of fiberglass limits the size and efficiency of the blades.

Advanced Materials

The introduction of advanced materials like carbon fiber composites has revolutionized blade design. These materials offer a higher strength-to-weight ratio, allowing for the construction of longer, more efficient blades. For instance, blades made from carbon fiber composites can be up to 20% lighter yet stronger than those made from traditional fiberglass, enabling an increase in blade length by up to 10-15% without a corresponding increase in weight. This extended blade length can significantly enhance the energy capture of wind turbines. Each meter of blade length produces up to 6-8% more energy, depending on specific wind conditions and turbine design.

Despite their higher cost, the benefits of using carbon fiber composites in turbine performance are substantial. The investment in these materials pays off through the increased efficiency and power output of the turbines. For example, a wind turbine utilizing blades made from advanced composites might produce 15-20% more electricity over its lifespan compared to a turbine with traditional materials, justifying the initial higher cost through improved energy generation and, potentially, reduced maintenance costs due to the material’s enhanced durability.

Sustainability

The sustainability of materials used in blade manufacturing is becoming increasingly important. The industry faces challenges in recycling old blades, prompting research into more sustainable materials and designs that are easier to recycle or repurpose at the end of their lifecycle.

Typical Lifespan

The lifespan of wind turbine blades can significantly impact the overall sustainability and cost-effectiveness of wind energy projects.

Average Lifespan

Wind turbine blades are designed to last between 20 to 25 years. The lifespan can be affected by the quality of the materials used, the precision of the blade design, environmental conditions, and maintenance practices employed.

Lifespan Influences

Factors such as rough weather conditions, operational stresses, and the quality of maintenance can all affect the longevity of a blade. Regular inspections and maintenance are crucial for maximizing blade life.

End-of-Life Options

The end-of-life disposal of wind turbine blades presents a challenge due to the durability and size of the materials. Efforts are underway to develop recycling methods that can handle the composite materials prevalent in blade construction, aiming to enhance the sustainability of wind energy further.

The efficiency of wind turbines is intricately linked to the design of their blades. By optimizing blade design for aerodynamic efficiency, employing advanced materials, and adhering to maintenance protocols, the energy output of wind turbines can be maximized. As the wind energy sector continues to evolve, ongoing research and innovation in blade design and materials science will play a critical role in the sustainable expansion of this vital renewable energy source.

Ductile Metals

Ductile metals are metals that can undergo significant plastic deformation under tensile stress before failure. Ductility is an important consideration in engineering and manufacturing, as it describes a material’s suitability for certain operations and its ability to absorb mechanical overload. In this article, we review the most ductile metals, the least ductile metals, and how to measure ductility of materials.

The Most Ductile Metals

When it comes to the mechanical properties of metals, ductility takes a central role. The use of ductile metals is important in various industries, ranging from construction to electronics. This section reviews some of these metals found in commercial applications.

Gold

Gold has been long considered as the most ductile metal on earth as a single ounce can be drawn to a length of 50 miles without breaking. Also, it has a high malleability as it can be hammered into sheets as thin as a few millionths of an inch. These attributes are due to its face-centered microstructure that makes it a lot more flexible than most metals. In addition, it is one of the densest metals at 19.3 g/cm³. With typical ductility values ranging from 15 to 50%, gold plays a prominent role in manufacturing jewelry, electronics, and reliable connectors in various devices. Moreover, it serves in creating delicate crowns in dentistry, and it is aesthetics makes it a popular option for ornaments.

Platinum

Platinum is quite similar to gold in that it is highly dense, at 21.5 g/cm3, with a face-centered microstructure, making it one of the most ductile of metals. It is highly resistant to corrosion and is one of the most stable elements in nature. This stability, in addition to its high boiling point of about 3,220℉, its as well as its resistance to nitric and hydrochloric acids makes it a common choice as electrodes in laboratories. Platinum also serves in improving efficiency of pacemakers, wires, electrical contacts, and catalytic converters in automobiles, where resilience is crucial. It has a percentage elongation ranging from 25 to 40%.

Silver

Similar to gold and platinum, silver boasts high ductility and malleability that allows it to be flattened into sheets and drawn into extended wires. With a percentage elongation ranging from 20 to 45%, the ductility of silver is exploited in the production of mirrors, photography, and electrical conductors. Also, its high resistance to corrosion makes it a popular choice for jewelry and soldering alloys. Although silver dissolves readily in nitric acid and solutions of sodium or potassium cyanide, it does not dissolve in solutions of sodium or potassium hydroxide or fused salts. In addition, it resists acids such as hydrochloric, sulfuric, acetic, citric, lactic, phosphoric, oxalic, and benzoic.

Copper

Copper has remarkable ductility and conductivity, such that it can be drawn into thin wires without its conductivity diminishing. Typically, it has a percentage elongation ranging from 30 to 70%, thus, making it the number one choice for commercial electrical wiring. Copper is also used in plumbing, electronic components, and in the creating of intricate sculptures in architecture.

Aluminum

Aluminum’s low melting point (1220℉) and density (2.7 g/cm³) contribute to giving it excellent ductility and malleability. In addition, aluminum boasts of a high strength-to-weight ratio, remarkable corrosion resistance, as well as good electrical and thermal conductivity. Its lightweight helps in reducing manufacturing costs, and also minimizing deadweight and energy consumption in vehicles. Aluminum’s anti-corrosive properties makes it a good fit for making parts that are exposed to the environment and for food packaging. Good electrical and thermal conductivity with its percentage elongation ranging from 10 to 30% means it can serve in electrical transmission lines.

The Least Ductile Metals

Metals with low level of ductility can barely withstand plastic deformation before failure. Despite this, they are common in a variety of applications. This section reviews a few of them.

Cast Iron

Cast iron consists of 2 to 3.75% of carbon, alongside small amounts of sulfur, silicon, phosphorus, and manganese. It has exceptional compressive strength but very low tensile strength due to its percentage elongation being less than 1%. Because of its compressive strength it has good wear resistance and excellent vibration damping capacity. This makes it a popular choice for vehicle engine blocks, pipes, and cookware.

Bismuth

Bismuth is a brittle metal with percentage elongation less than 2%. It has several desirable properties making it common in certain commercial applications.

Bismuth has low toxicity so constitutes certain alloys for pharmaceuticals and cosmetics.
It acts as a catalyst for making acrylic fibers.
Although it is brittle, bismuth can be extruded into wires that are used in thermocouple applications, as they have the highest negativity known.
Alloying bismuth with tin and cadmium forms low-melting materials that serve in fire detection and extinguishing systems.

Bismuth has low ductility but is used in several applications. — *Courtesy: BelmontMetals*

Lead

Lead is one of the least ductile metals with a percentage elongation usually less than 5%. However, it is malleable with high compressive strength. Because it is poisonous, its application is limited to minimize human contact such as in:

Lead-acid batteries.
Sound insulation
Ballasts and counterweights.
Coloring element in ceramic glazes.
Protective glass of computer and TV screens to shield viewers from radiation.

Although lead has high compressive strength, it is one of the least ductile metals. — *Courtesy: ThoughtCo*

Zinc

Although zinc is brittle by nature, it exhibits better ductility at higher temperatures with a percentage elongation usually less than 10%. Its anti-corrosive properties make it valuable in galvanizing steel. While its toughness at high temperatures means that zinc oxide is an important additive to rubber to strengthen tires and prevent failure when hot.

Importance of Measuring Ductility

Assessing the ductility of materials has several benefits including:

Material Selection: Because ductility is a crucial parameter in certain manufacturing applications, having foreknowledge of it is key to ensuring successful implementation of a project.
Quality Control: For material manufacturers, ductility measurements ensures that batches remain within specifications.
Engineering Design: Ductility measurements allow more accurate finite element modelling of designs. This results in the shortening of the total design and test time, as well as parts and systems with better durability at lower costs.

How to Measure Ductile Metals

Generally, assessing ductile metals follows two primary methods:

Percentage Elongation: This method involves measuring the increase in length of a tensile specimen before fracture, expressed as a percentage of the original length. It provides insight into a material’s ability to deform plastically under tensile stress.

$%EL=\frac{final\, gauge\, length-initial\, gauge\, length}{initial\, gauge\, length}=\frac{l_{f}-l_{i}}{l_{i}}\times 100$

Reduction in Area: This method quantifies the decrease in the cross-sectional area of the specimen at the point of fracture. Thus, it provides additional information on the material’s ductile behavior under stress.

$%RA=\frac{change\, in\, area}{original\, area}=\frac{A_{O}-A_{f}}{A_{O}}\times 100$

Tipping Angle

The tipping angle of a load is an important consideration when dealing with the rotational equilibrium of machines or structures. In this article, we will learn more about the applicability of tipping angle to loads, calculation of tipping angle, and tipping hazards.

Applicability of Tipping Angle to Loads

Discussing the application of tipping angle to loads always involves the concepts of torque and center of gravity. To understand what all these are, we conceptualize an arbitrary object laying on a flat surface as the figure below shows.

Object laying on flat surface, with its weight acting through its center of gravity. — **Figure 1: Object Laying on Flat Surface**

From Figure 1, the object is experiencing two forces: its weight acting through its center of gravity (CoG), and the normal force as a reaction from the surface. These two forces act in direct opposition to each other and keep the object in equilibrium.

Also, the CoG is located at a height, h, from the supporting surface, and at a lateral distance, x, from the pivot or rotation point. As the surface tilts, a torque develops due to the normal force, tending to rotate the object in a clockwise direction. The weight of the object tends to rotate the object in the anti-clockwise direction about the pivot in Figure 2.

Object remains in equilibrium because it is yet to reach tipping angle. — **Figure 2: Object tilting but not yet at tipping angle**

At this point, the object remains in equilibrium because the CoG is still above the support base of the object. As tilting progresses, the CoG arrives at the pivot, which is the end of the support base. This point is the object’s tipping point and the angle (θ) is the tipping angle, as Figure 3 shows.

Object rotation has reached tipping angle. — **Figure 3: Object tilting at tipping angle**

Tilting the surface beyond this angle causes the object to lose equilibrium and overturn under the influence of its weight. Thus, designers must know the tipping angle for a structure or machine as well as provide resistance against any force that wants to initiate its occurrence.

Calculation of Tipping Angle

The tipping angle of a structure is not dependent on any external force that it is experiencing, although these forces can initiate it. Rather, it is due to the interaction between its center of gravity and its geometry. Thus, the first step of calculating the tipping angle of a structure is finding its center of gravity. This is straightforward for structures with uniform density because the center of gravity coincides with the center of area. However, for structures not having uniform density, the process is more tedious as it requires integration of the mass across the structure’s axes to determine the mass distribution. After determining the CoG location, the calculation of the tipping angle (θ) is as follows:

$\theta =\tan^{-1}\left ( \frac{x}{h} \right )$

In the equation above, x represents the perpendicular distance between the CoG and the tilting edge of the support base. While h is the height of the CoG from the support base.

Tipping Hazards

The concept of tipping angle is important in industry, as the general approach is to avoid it, thereby preventing the initiation of tipping loads.

Preventing Tipping of Vehicles

Designing against tipping is a key aspect for vehicle manufacturers, especially as cars traverse roads of varying slopes. Generally, they achieve this by making the CoG as low as possible. Particularly in race cars, which move at high speeds and experience large overturning torques. For trucks, this is not easy to achieve because of their height and loads that they carry. This is even more complex when it comes to articulated trucks that tip sand, gravel, etc. As they offload their contents, the CoG constantly changes, thus the need for the following safety measures:

Ensure the ground is stable by tipping only when the vehicle is on a level and firm surface. Because instability of the ground can quickly change the CoG and the vehicle could reach its tipping angle.

Tipping on stable ground to avoid tipping angle vs tipping on unstable or tilted ground. — *Courtesy: Worksafe*

Set up exclusion zones around the truck to ensure the safety of personnel and other equipment during tipping. Generally, this is about 15 feet behind the truck, and 1.5 times its length on each side.
Utilize trained spotters to guide drivers. Especially if the vehicle is long or large, due to limited visibility of the driver.
The vehicle should be in good condition so that it performs according to the manufacturer’s specifications.

Tipping Hazards in Lifting

Lifting operations is another activity where equipment is prone to attaining their tipping angle, if not properly managed. To avoid tipping, cranes typically use outriggers and other measures to counterbalance the load weight.

Also, the lifting point should be directly above the CoG of the load. This ensures that the forces in each sling balance themselves out. Another parameter to control when ensuring stability is the load radius, which is the distance between the crane and the load. The further away the load is, the larger its overturning torque on the crane. As a result, it is best practice to lift heavier loads as close as practicable to the crane’s center.

Flexible Metals

Generally, the term ‘flexible’ is not technically accurate for describing the characteristics of metals. Rather, a flexible metal is one that combines two key metallic attributes, which are malleability and ductility. In this article, we discuss the most flexible metals in commercial use, as well as why some metals are more flexible than others.

The Most Flexible Metals

Ductility refers to the ability of a metal to undergo plastic deformation under tensile stress without rupture/failure. A metal exemplifies its ductility by being drawn into a wire without losing strength or breaking. On the other hand, malleability is the ability of a metal to undergo plastic deformation under compressive stress without rupture. Moreover, malleability quantifies a metal’s ability to be beaten into sheets. Although both characteristics appear similar, ductility in any metal does not necessarily equate to malleability, and vice versa. For example, lead can undergo significant compressive stress, but fails easily under tension. So, for metals to be flexible, they need to have these two features in significant measure.

Gold

Gold offers a perfect combination of ductility and malleability, which is why it ranks highest in the list of flexible metals. A single ounce can be drawn to a length of 50 miles. Similarly, an ounce of gold can be beaten to form a 187 ft² gold leaf. These attributes are due to its microstructure and it being one of the densest metals at 19.3 g/cm³. As a result, gold serves effectively as premium wires and in other electrical applications, because it is a good conductor of heat and electricity. Its malleability enables its formation into various shapes and sizes for jewelry, electronics, medical implants, etc.

Aluminum

Aluminum is the most abundant metal on earth, often existing as oxides and salts due to its high chemical reactivity level. Like gold, aluminum has good ductility and malleability. Also, it has high strength-to-weight ratio and excellent resistance to corrosion, which makes it predominant in several applications. The low density and low melting point of aluminum contribute to its high ductility and malleability for a wide range of temperatures. Its features make it predominant in electrical applications for making terminals and contactors, but not common in wires, due to some pitfalls. They are excellent as sheet metals for vehicle bodies, roofing sheets, and cans for food storage.

Copper

Although copper is a heavy and dense metal, its face-centered cubic atomic structure allows for relative motion, without breaking bonds. This accounts for its good ductility and malleability, showing total elongation of up to 60% before fracture. In combination with its excellent electrical conductivity and relatively low cost, it is the number one choice for electrical wires globally. Its malleability also makes it a good choice for jewelry, cookware, pipes, and cladding.

Platinum

Platinum is a silver-colored, highly dense (21.5 g/cc) metal in its pure form. Also, it is rare and has several desirable physical properties, so quite expensive. Platinum is more ductile than gold, and arguably the most ductile of metals. It is highly resistant to corrosion with a boiling point around 3,220℉, and is one of the most stable elements in nature. Because platinum is immune to nitric and hydrochloric acids it commonly serves in laboratories as electrodes. It also serves in improving the efficiency of pacemakers, wires, and optical fibers. However, its most common application is as a catalytic converter in cars where it converts CO and other residual pollutants into CO₂ and water vapor. As one of the most inert flexible metals, it is suitable for making exclusive jewelry.

Silver

Like platinum, silver is one of the lustrous flexible metals as it exhibits good ductility and malleability. Its decorative aesthetic makes it a popular material for jewelries, but it is soft so often alloyed with materials such as nickel and palladium. Because it has high thermal and electrical conductivity, alloying silver with copper delivers premium electrical wiring and conductors. This alloy has high ductility, being able to achieve up to 40% elongation in a tensile test.

Why Are Some Metals Flexible?

Generally, ductility and malleability are a common feature with metals due to their microstructure. Within each metal, atoms form a metallic bond, which allows electrons from atoms to move freely between each other. As a result, atoms are able to slide past each other to an extent, while maintaining this bond, hence allowing metals to stretch. So, why are some metals more flexible than others? One of the reasons is the number of valence electrons. It is the valence electrons that are able to move freely between atoms. Thus, the higher the number of valence electrons, the more flexibility a metal possesses.

Common Crystal Lattices that determine if metals are flexible

Another factor that makes some metals more flexible than others is the type of crystal lattice they have. Typically, metals have either face-centered, body-centered, or hexagonal closed-packed lattice structure in their solid state. When deformation of a metal occurs, it happens along the slip planes of its lattice structure. So, the more slip planes a lattice structure has, the more possibilities for deformation, leading to more ductility and malleability. Out of the common lattice structures, face-centered and body-centered lattices have the most slip planes. However, metals with face-centered lattice structures are the most flexible because their slip planes have a closer packing in comparison to body-centered lattice. Thus, requiring significantly lower energy expenditure during deformation.

Selector Switch Diagram

A selector switch is a mechanical control device that enables switching between a minimum of two electrical circuits. It is common in applications where there is more than one control option, but only one is to be active per time. In this article, we will review how a selector switch works, and the selector switch diagram for 2, 3, and 4-position switches.

How a Selector Switch Works

The basic function of a selector switch is to provide flexible controls with minimal space. It usually achieves this by operating a knob, which rotates to specific positions to connect or disconnect various circuits. In some applications, the switching is automatic or remote, but the principle of operation remains the same. Basic components that make up a selector switch include:

Operator/Actuator: This part of the switch moves to either open or close a circuit. Manual selector switches usually come in the form of a knob. However, some recent versions utilize buttons, which are more popular in automatic and remote switches.
Contact Block: This component encapsulates the contactors of each circuit in the system. Generally, the gap between the actuator and contactor is 0.5mm when a circuit is not active. Whereas for high accuracy and high durability applications, the gap is much smaller around 0.25mm. However, such a small gap will make the system prone to arcing if there is significant vibration or shock. To make the system resistant to these, the contact gap is typically 1 and 1.8mm in systems that experience vibration.

Exploded View of Selector Switch Components — *Courtesy: Elprocus*

LED Block: The LED block has indicator lighting that shows the position of the actuator.
Casing: Houses all internal components of the switch, so it is durable and comes in a variety of shapes. Predominant materials are plastic and metals. Also come in a variety of IP or NEC ratings depending on its operating environment.

Categorization of selector switches depends on the number of circuits it is connecting. Moreover, the most common options in the industry are 2, 3, and 4-position switches.

2-Position Selector Switch

This type of selector serves in switching between two circuits. It also goes by the names 2-way switch and single-pole double throw switch. Typically, they serve in on-off applications such as the following:

Lighting: 2-position selector switches are popular in simple lighting applications for switching on or off a lighting point.
Motor Control: This can serve in switching on and off a motor, as well as in changing the direction of motor rotation in reversible motors.
Temperature Control: 2-way switching is useful in controlling the temperature in a room by changing between two different cooling or heating sources.
Audio/Video Routing: When there are two different audio/video sources and one output, a 2-position selector is effective in switching between them.

3-Position Selector Switch

A 3-position selector goes a step further by enabling switching between three different circuits. So, this type of switch has three position states, which often go by the name ‘Hand – Off – Auto’ (HOA). The HOA name is largely because of its frequent use in motors that have the auto start function. When utilized on a motor starter, the ‘hand’ position indicates a manual operation of the motor. The ‘Auto’ position allows the automatic starting of the motor using a means such as a PLC or sensor. Finally, the ‘Off’ position is for shutting down the system. This selector is similar to the 2-position switch but enables input from two different circuits where there is a requirement to have both circuits normally open.

Three-position selector switch — *Courtesy: SCADAware*

Although the HOA arrangement is quite common for 3-position selector switches, there is a downside to it, especially when on large motors. If the selector is on ‘Hand’ with the motor running and needs to switch to ‘Auto’, the starter will have to drop off before reenergizing as it goes through the ‘Off’ position. This is why some motors utilize the ‘Off – Auto – Hand’ switch instead. However, having the ‘Off’ position at the center is desirable in certain applications for safety reasons, as it enables a quick shutdown.

3-Position Selector Switch Diagram

This section highlights how a 3-position selector switch works in a lighting application, using a diagram. In this example, the selector switch serves to control two different lighting points where only one light is to be on per time. As the figure below shows, the switch has 1-0-2 positions which are akin to the ‘Hand-Off-Auto’ positions. Also, the switch is in the ‘Off’ position, so neither of the lights are on.

Three-position selector switch diagram showing selector in the off position — *Off Position*

In this ‘off’ position, none of the contactors in either light position is connected. To put on the light in position 1, an operator turns the knob accordingly, thus resulting in the connection of the contactors on line 1.

Three-position selector switch diagram showing selector in the hand position — *Switch in Position 1*

Similarly, the light along line 2 can become active by turning the selector to the corresponding position, as the figure below highlights.

Three-position selector switch diagram showing selector in the auto position — *Switch in Position 2*

4-Position Selector Switch

The 4-position selector switch is quite similar to its 3-position counterpart. In that, it enables switching between multiple circuits – in this case, three – as well as an ‘off’ position. Thus, the three circuits can meet the requirements of being normally open at the same time. This type of switch is common in both residential and industrial applications, where it serves as fan regulators, blender speed regulators, switching between motors, etc.

Product Spotlight: Bio-Check Valve

The biocheck valve solves an expensive problem that exists with lagoons and biogas. Colin Burton of Punchlist Zero sat down with Brian Crook to talk about this valve and potential future applications.

PL0: Tell me about this Bio-Check valve. Where did you get the idea?

Brian: The Bio-Check Valve started with an old friend of mine in the valve industry. He’s been supplying piping, flanges, and all sorts of stuff for the biogas industry. In the United States, the biogas industry is in its infancy. As opposed to Europe, they’ve been doing it for roughly 20 years, they have a good system for it and a good supply chain.

In the United States, the predominant way to store biogas is via a lagoon. The lagoon is a giant pond, maybe the size of four to eight football fields. It’s lined with a rubber membrane that rests on top. The biogas generation process places manure, meat scraps, and biomass material into a pile and allows it to decompose.

The decomposition produces methane gas, which flows into ponds. A significant issue occurs because the ponds lose pressure and rests on the water. The pressure then drops and the digesters membrane hits the water surface. What results is the system sucks up water, resulting in shut down of the gas processing equipment.

My friend Kenny Despot came to me and told me about this problem that biogas companies were having. The more he talked, the more I realized we could solve this problem. We took the concept to a biogas trade show in May 2023. People were blown away and said, Hey, this solves a problem that we have.”

PL0: How is this valve different from what’s currently in the market?

Brian: What’s currently on the market, you have ball checks and swing checks. You have different things like vacuum breakers, all of those things serve a specific purpose. So if you look at most of your check valves, they control flow in one direction. What we weren’t trying to do with this was control flow.

We were trying to keep the liquids from leaving the digester. We came up with a way to actually shut off the orifice when the water level rises. We designed a mechanical vacuum breaker to break the vacuum when the water level drops. It is a repeatable device, it’s self-operating, and it doesn’t require any input from any control system. It can serve digesters with multiple hatches and entry points.

PL0: How has your experience in the industry helped with the creation of this new valve?

Brian: I believe that my career has been made because I’ve always focused on solving people’s problems. The Bio-Check solves a problem and that’s a reflection of who I am and what my career has been about.

And, it’s kind of like a crowning moment to say, Hey, I did something that maybe changed the industry in a positive way. So I think that that’s what drove the creation of it, as wanting to solve someone’s problem and then having the knowledge to do it and then executing it. So it hasn’t been easy. It’s been a lot of work, but it’s been worth it. Rewarding.

PL0: Who needs to know about this valve?

Brian: Anybody that is in the biogas digester business. We’re dealing right now with the largest biogas digester manufacturer in the United States. They’re kind of what got this process started. There are a few other ones that are in the market as well. It is a growing business. This is a new problem because it’s a new industry and that’s why our product is unique because it’s a new problem, a new industry, and it’s a new product that solves that problem that they’ve discovered as they’ve started building these. Currently, there are probably about 200 dairy digesters in service in the United States.

It makes sense to take cow manure that normally goes in a pile and instead put it in a pond and capture that gas. It’s responsible, caring for the environment and it’s just logical to use stuff that just sits there.

For those working with dairy farmers, my advice to them is to look into a digester because it’s affordable and it produces additional revenue for them. I think that the Bio-Check is about to become a standard on these digester systems because it solves a very expensive problem that exists with lagoons.

So we’re looking forward to that and looking forward to providing solutions to it. We’ve had people ask us if there are other products that we’d be willing to look at designing and whatnot and the answer to that is yes. Lee Bares and I are both partners of SVC and are flexible in solving our customer’s problems.

PL0: Where do you see the Bio-Check valve taking you in the future?

Brian: We’re working on a lot of other products. The culture of our company is built around hard work, being responsive, producing a great product, an also solving problems, which is reflected in all of our employees. Perhaps the Bio-Check valve is the beginning of something new for the industry or the beginning of maybe a brand of products that are used for the biogas digesters.

I’m open to looking at other applications and developing other products that can really change people’s lives. We can produce a product and employ a good staff of people who provide for their families. That’s really my goal at the end of the day.

Email Brian Crook for further information on the Bio-Check valve at brian@sovalvecontrol.com.

C900 Pipe

C900 is an American Water Works Association (AWWA) standard PVC pressure pipe, for water transmission and distribution applications. It is equivalent to cast iron pipe for the same applications in terms of outside diameter. In this article, you will learn more about C900 pipe, review some specifications, and compare it to Schedule 40, as well as C905 pipe.

More on the C900 Pipe

C900 pipe offers a safe, long-lasting, and stable solution, so it is common in modern infrastructure. Although the pipe standard was initially developed for only potable water service, recent revisions include wastewater and reclaimed water systems. The benefits of deploying this pipe in an application include:

It is immune to almost all types of internal and external corrosion because it is not a conductor. Thus, it offers a cheaper alternative to metal pipes as it does not require coating or cathodic protection. In addition, it is not susceptible to acid or alkaline soil conditions, which makes it effective in underground applications.
The C900 maintains its smooth interior over several years of service, so there is hardly any frictional loss. Moreover, it possesses superior flow characteristics with a Hazen-Williams flow coefficient of 150. This puts less demand on pumping capacity, thus, offering savings in pumping costs and on pipe size requirements.
Because the hydrostatic proof test of the C900 is up to four times its pressure class, it is able to offer long-term durability. The standard also ensures that the bell-gasketed joint meets the same strength requirement as the pipe. Combining this strength with the flexibility of PVC pipes, the joint and pipe assembly can bend without breaking to compensate for minor earth movements.
It is easy to install and carry out repairs, as it can be field-cut using a power saw or handsaw and beveled, without the aid of expensive machinery.

Specifications

Since the initial development of the AWWA C900 in 1975, there have been several revisions of the standards with changes to aspects of the pipe.

Dimensions, Pressure Classes, and Applications

Although standard lay lengths remain at 20 ft +/- 1-inch, the pipe size range has expanded from 4 to 12 inches, to 4 to 60 inches. Thus, it encapsulates the C905 standard, which previously served for pipes greater than 12 inches. By extension, more dimension ratios (ratio of pipe outside diameter to minimum wall thickness) have been included as the table below shows.

Pipe Size (in)	Dimension Ratio	Pressure Class (psi)
4, 6, 8, and 12	27.5 25 21 18 17 14	150 165 200 235 250 305
14 and 16	41 32.5 27.5 25 21 18 17 14	100 125 150 165 200 235 250 305
18, 20, and 24	51 41 32.5 27.5 25 21 18 17 14	80 100 125 150 165 200 235 250 305
30	51 41 32.5 27.5 25 21 18 17	80 100 125 150 165 200 235 250
36	51 41 32.5 27.5 25 21 18	80 100 125 150 165 200 235
42	51 41 32.5 27.5 25 21	80 100 125 150 165 200
48, 54, and 60	51 41 32.5 27.5 25	80 100 125 150 165

C900 Nominal Sizes, Dimension Ratios, and Pressure Classes

In addition, the scope of the C900 now includes non-potable water applications in recent revisions.

C900 Pipe Testing

A major cause of C900 pipe failure stems from manufacturing defects. These defects include incomplete fusion of extrusion knit lines, incomplete resin fusion, and inhomogeneity of the filler content. Thus, testing is crucial to verify the quality of products. Some of these tests include:

Sustained Pressure Test: This is a 1000-hour test for the qualification of the compound and extrusion process.
Burst Pressure Test: Carried out at specific frequencies, this destructive test determines the absolute maximum pressure at which the pipe fails catastrophically, or begins to plastically deform when pressurized.
Ring Tensile Test: This safer alternative to the burst pressure test involves cutting a piece of piece out and pulling it to test its ductility.
Hydrostatic Pressure Test: This internal pressure test is a requirement for each piece of standard length.
Fusion Joint Test: The method for qualifying fusion joints has become a popular field joint method in modern installations.

Pressure testing of the C900 depends on the dimension ratio of the pipe as the table below highlights.

Dimension Ratio	Pressure Class (psi)	Sustained Pressure Test (psi)	Burst Pressure Test (psi)	Hydrostatic Pressure Test (psi)
14	305	650	985	610
17	250	530	800	500
18	235	500	755	470
21	200	420	630	400
25	165	350	535	330
26	160	340	500	320
27.5	150	320	475	300
32.5	125	270	400	250
41	100	210	315	200
51	80	170	260	160

Pressure Testing for C900

Safety Factor and Temperature Coefficients

Because of variations in manufacturing, materials, installation, and unforeseen factors, the C900 standard uses a safety factor of 2.0 to calculate pressure capacities. In addition, these capacities are adjusted by temperature coefficients, as all PVC pipes experience a decrease in strength with rising temperature.

Temperature		Pressure Reducing Coefficient
℉	°C	Pressure Reducing Coefficient
80	27	0.88
90	32	0.75
100	38	0.62
110	43	0.50
120	49	0.40
130	54	0.30
140	60	0.22

Temperature Coefficients

Schedule 40 vs C900 Pipe Standards

Both Schedule 40 and C900 are common options for water pipe systems, but they have basic differences as the following table highlights.

Schedule 40	C900
Materials for making Schedule 40 include galvanized mild or low carbon steel, as well as PVC.	This standard only serves for PVC pipes.
Generally, are thin-walled pipes.	C900 has relatively thicker walls vs Schedule 40.
Ideal for low to medium-pressure installations where pipe sizes are 4 inches or less such as residential buildings. Due to its typically thinner walls and high avaialability, it is generally cheaper than C900.	Ideal for low to medium-pressure installations, but generally serves a much broader size range than PVC.
It is a glue joint pipe, so it provides a restrained system. However, becomes susceptible to leakages at joints over time.	It is a gasketed pipe, thus requires thrust restraints at bends, tees, etc. As a result, provides more robust connections than PVC, minimizing leakages.

Schedule 40 vs C900

C905 vs C900 Pipe Standards

The C905 and C900 are both pipe standards from the AWWA, with the table below highlighting their differences.

C905	C900
The first edition was published in 1988 to cover potable water pipes with nominal sizes between 12 and 48 inches.	The first C900 edition was published in 1975 covering potable water pipe sizes from 4 to 12 inches.
Earlier revisions have smaller safety factors because their design is for larger transmission lines with minimal surge pressure spikes. Also, hydrostatic pressure testing for individual pipes in earlier revisions was at a pressure two times its pressure class.	Generally, they have larger safety factors due to smaller pipes being more susceptible to surges in pressure. While individual pipes were hydrostatically tested at a pressure that is four times its pressure class.
The C905 is no longer published since the 2016 revision, as it has been absorbed into the C900-16.	Earlier revisions have smaller safety factors because their design is for larger transmission lines with minimal surge pressure spikes. Also, hydrostatic pressure testing for individual pipes in earlier revisions was at a pressure two times it pressure class.

C905 vs C900

Three Phase Voltage

Three-phase voltage refers to power supply using three separate conductors carrying the same voltage, but in different phases. Moreover, any equipment that is connected to all three phases would experience peak electrical power from each phase at different times. As a result, steadier power output results than in single phase ac supply. In this article, you will learn more about three-phase voltage, review its formula, and the three-phase voltage stabilizer.

More on Three Phase Voltage

Generally, a three-phase voltage configuration consists of three conductors carrying the same power. However, each phase is at 120° to the other, meaning that each power line achieves peak voltage/current/power at different times. This configuration is an improvement of the single-phase systems as it offers the following advantages:

Ideal for running industrial loads and does not need additional starters for large motors, unlike single phase voltage.
The 120° phase difference between lines helps in delivering a smooth power output.
Because the power is delivered by three separate lines, it allows for the use of smaller size conductors. Thus, it is a cost-effective solution for powering large loads.

Common three-phase configurations are the Delta and the Wye or Star configurations.

Delta Configuration

The delta configuration is the ideal setup for AC power transmission with its name stemming from its system of interconnection. All three coils are in series to form a close mesh as the figure below shows.

This interconnection appears to create a short circuit, but this is not the case because of the phase difference between each source. Rather, a balanced system produces an algebraic sum of all voltages around the mesh as zero. Most delta transmissions utilize three wires, but it is possible to introduce a neutral line by a center tap on one of the coils.

Wye Configuration

This is the more popular option in AC power distribution. Forming this configuration involves connecting similar ends (starting or finishing) of each coil together to form a neutral point. So, this configuration usually utilizes a neutral line.

At transformers, where power transmission transits into distribution, it is possible to maintain the same three-phase voltage configuration or change it depending on the following scenarios.

A Wye-Wye configuration is used for small currents and high voltage.
Delta-Delta is ideal for large currents and low voltages.
Delta-Wye connection suits step-up transformers at power generation stations.
Wye-Delta connection is for step-down transformers where transmission ends.

How to Calculate Three-Phase Voltage

When calculating three-phase voltage, there are two important concepts to consider, which are line and phase voltage. Line voltage is the amount of voltage between any two lines in a balanced system. Whereas, phase voltage refers to the voltage across any one line or component in a balanced system. Determining any of these parameters depends on the configuration in place.

Calculating Voltage in Delta Configuration

For delta configurations, the line voltage (E_line) and phase voltage (E_phase) are always the same. This is because the connection between each pair of line conductors is directly across a single winding in the circuit.

$E_{line}=E_{phase}$

As a result, the delta configuration can only deliver a single voltage value.

Calculating Voltage in Wye Configuration

For a Wye configuration, the line voltage always exceeds the phase voltage. If the system is balanced, the relationship between the line voltage (E_line) and the (E_phase) phase voltage will be as follows:

$E_{line}=E_{phase}\sqrt{3}$

As a result, the Wye configuration can deliver different voltages to a load in a system. Thus, it is more flexible than the Delta configuration.

Three Phase Voltage Stabilizer

Because of the sensitivity and cost of certain industrial and commercial equipment, it is necessary to finetune incoming power. Three-phase voltage stabilizers serve as a gateway between the power source and the load to achieve this finetuning. These stabilizers continuously monitor the incoming mains supply and automatically control the output voltage to meet prerequisite levels.

As a result, the load enjoys protection from harmful surges, spikes, and transients. More sophisticated applications use voltage stabilizers equipped with power line conditioning features to provide enhanced protection and noise filtration. Historically, servo electronic-based stabilizers have dominated the market. But in recent times, static electronic stabilizers have become popular due to the speed of microprocessor control.

3-Phase Voltage Stabilizer, Courtesy: Direct Industry

There are several factors such as load type, output power, and power factor, which influence the selection of a voltage stabilizer. Moreover, key tips for selecting the capacity of voltage stabilizers, otherwise known as automatic voltage regulators, are as follows:

The three-phase voltage stabilizer should be between 1.5 to 2 times the power if protecting purely resistive loads. Examples of these are electric heaters, resistance wires, and incandescent lights.
If the loads are capacitive such as water pumps, air conditioners, and computers, then the three-phase stabilizer should be three times the load. Moreover, when the capacitive load is very large, there is greater demand on the starting current. In some occasions, this could be between 5 to 8 times more. The voltage stabilizer has to deliver over three times more power than the load.

Corner Grounded Delta

A corner grounded delta system is a three-phase delta service with one phase having the ground connection. This configuration goes by several names including grounded leg/phase service, end-grounded delta system, and grounded B phase system. In this article, we review the application of corner grounded delta, review the dangers of this system, and compare it vs a high leg.

Applications of Corner Grounded Delta

The corner grounded delta (CGD) is one of the earliest forms of grounded systems in electrical installations. Because of the disadvantages of not grounding systems, electrical designers had to integrate a ground reference to existing configurations. Doing this is technically challenging and expensive, however, the corner-grounded system offers a low-cost method of grounding a delta configuration. Generally, the grounding could be on any of the three phases from the transformer secondary, but the choice is usually the second or B phase for easy tracking. A CGD system offers the following benefits:

Reduces the generation of transient overvoltage that occurs in systems without grounding.
Establishes a ground reference for each current-carrying conductor.
Offers a low-cost option to introduce a neutral line that will be grounded.

Typical corner grounded delta industrial setup — *Courtesy: IAEI*

Despite these advantages, the use of corner grounded delta is limited to old installations. This is because recent grounding methods are more suitable and reliable. However, it is common to still encounter CGD systems today for the following reasons:

Almost all low-voltage systems in the past used transformers with delta-connected secondaries for power distribution. So, grounding the corner of one of the delta phases was the simplest way of making these systems safe.
It helps avoid the installation of equipment ground fault protection on solidly wye electrical services, which is an NEC requirement.
Also, CGD systems promote the use of less expensive equipment, as two-pole switches and the neutral could serve in three-pole applications.

Examples of CGD Devices

In modern applications of corner-grounded delta system, it is necessary to have equipment ground fault protection. This is due to the potential high fault currents on the first ground fault. As a result, equipment used on a CGD system requires testing and approval from industry regulators. Examples of approved devices that are UL-listed include:

Molded case circuit breakers.
Safety switches.
QMB and QMJ switches.
BP fusible switches.
Panelboards such as QMB panelboards, I-Line circuit breaker panelboards, as well as NQOD and NF circuit breaker panelboards.
QED switchboards.

Dangers and Disadvantages

There are several factors that make corner-grounded delta systems less popular in modern applications. Some of the dangers of using this system are:

The potential to the ground is significantly higher than in a neutral-grounded system. For example, in a 480V system, the potential is 480V to ground rather than 277V. This can lead to significant voltage delivery for any device on the grounded leg, making them prone to damage.
Also, this system experiences more severe fault switching for the clearing device, which could greatly reduce its ratings.

Other disadvantages of this system include:

Corner-grounded delta systems do not work with lower-cost slash-rated circuit breakers such as 480/277V and 240/120V.
Any equipment in this configuration requires significant testing and approval before use. Also, having equipment ground fault protection is necessary for safeguarding equipment. It is recommended to follow UL listings if connecting equipment on this configuration.
Because the grounding is on one of the phases, ground fault sensing for that phase is undefined. As a result, this configuration cannot be used in locations where ground faults are required such as healthcare facilities.
To avoid confusion, the corner grounded phase needs some form of identification throughout the network. Else, it could be mistaken for a faulty line as it would have a zero potential to ground, while the other two lines will have higher voltage to ground.

Corner Grounded Delta vs High Leg Delta

Another common configuration is the high leg delta. This configuration also goes by the names wild-leg, stinger leg, and dog leg delta. They are common in three-phase delta transformers that intend to also supply single-phase electricity. As a result, a neutral line is introduced by center-tapping one of the phases. The voltage between this neutral line and the line opposite it is higher than for the other two lines adjacent to the neutral. Thus, the name “high leg” delta.

Differences between the corner grounded and the high leg configurations are as follows:

Corner Grounded Delta	High Leg Delta
The ground connection is at the corner of one of the phases.	While the ground connection is at the center point of one of the phases.
Can provide a variety of voltages at a lesser cost.	It is a three-wire system and supplies only three-phase power.
Supplies only a single voltage.	Usually serves in residential power supply where the single phase delivers 120V for lighting and low-load devices. While the three-phase sends 240V to high-load equipment such as washing machines and HVAC. So, it is effective as a split phase supply.
Usually serves in residential power supply where the single phase delivers 120V for lighting and low-load devices. While the three-phase sends 240V to high-load equipment such as washing machines and HVAC. So, it is effective as a split-phase supply.	On the other hand, one of the legs has a higher voltage to the ground than the other two legs.
Two legs have a higher voltage to the ground, while the phase with the grounding has zero potential to the ground.	Even without markings, it is easy to identify lines on this system. Because the high leg and every third circuit afterward will be either a three-pole breaker or a blank.
Requires some means of identification for the corner grounded line throughout the system.	Can serve in industrial applications such as 240V and 480V power supplies.

Corner Grounded Delta vs High Leg Delta

Surface Finish Symbols

Surface finish symbols characterize the overall texture of a surface. Often, this characterization is in terms of only the surface roughness. But an in-depth description covers other parameters such as the lay and waviness of the surface. In this article, we discuss the use of surface finish symbols and relevant standards, discuss the Ra and Rz parameters, as well as compare them.

Surface finish parameters — *Courtesy: Get-it-made*

Why Use Surface Finish Symbols

Surface finish symbols are key features in engineering drawings when communicating geometrical dimensions of a product. These symbols indicate the quality of a surface by describing its appearance or feel. There are several reasons why these symbols are important to engineers, product designers, and other professionals involved in manufacturing.

Friction and Wear: The texture of a material’s surface determines the amount of friction it is subject to. For example, a smooth surface experiences less friction and wear, whereas a surface with high roughness experiences more friction. Either could be desirable when manufacturing a part, depending on its functionality.
Adhesion: The surface roughness of a part also influences its ability to adhere to paint, coatings, and other materials. Surfaces for painting are best manufactured with a bit of roughness to ensure adhesion. Smoother surfacess have less surface area, and subsequently less adhension.
Aesthetics: Having a visual appeal is important for certain products and surface finish symbols help communicate this.

Other ways that a surface finish can influence a product include:

It eliminates surface defects.
It improves conductivity and adds surface electrical conductions.
Surface finish impacts the corrosion and chemical resistance of a material.

Different parts of the world utilize unique surface finish symbols and standards. Out of these, the most recognized are the ASME Y14.36 in the US, while the rest of the world uses ISO 1302.

ISO Surface Finish Symbols

Basic Symbol: This symbol indicates that the manufacture of a surface could be by any method.

ISO surface finish symbol indicating the manufacture of surface via any method.

Material Removal by Machining: Adding a horizontal bar to the basic symbol indicates the requirement of material removal to produce a surface.

Symbol indicating material removal requirement.

Material Removal Prohibited: The introduction of a circle to the basic symbol indicates the prohibition of material removal. Rather, processes such as casting, cold or hot finishing, and powder metallurgy should be used in forming the surface.

Symbol indicates material removal prohibition.

In addition, there is a symbol ideal for surface finish callouts with room for several parameters as the figure below shows.

Surface texture symbol alongside parameters.

Manufacturing method – c.
First surface texture requirement – a.
Second surface texture requirement – b.
Surface lay as well as orientation – d.
Machining allowance – e.

ANSI/ASME Surface Finish Symbols

Similar to the ISO standard, the ASME Y14.36 has a surface texture symbol with several parameters.

The average roughness value (Ra) – a.
The production method, coating, or additional information – b.
Roughness sampling length in millimeters or inches – c.
The direction of the surface lay. – d.
Minimum material removal requirements in millimeters – e.
The symbol as well as the value of an alternate surface finish parameter – f.

ASME surface finish symbols also include those indicating the direction of lay as the figure below shows.

ASME surface finish symbols for the direction of lay.

Ra

Ra is a roughness parameter that indicates the average variation of the roughness profile from the mean line. It is the most common surface finish parameter and is also referred to as arithmetic average or common line average. Because of the averaging, the Ra value is usually lower than the actual height of the roughness variations. So, utilizing it alone does not completely characterize the roughness of a surface as the figure below highlights.

Surfaces with different profiles but the same Ra value. — *Courtesy: CNC Cookbook*

Clearly, the profiles of the five surfaces (a – e) in the figure above are different, however, they all have the same Ra value. Showing the insufficiency of using only Ra to define a surface roughness. Despite this shortcoming, Ra remains predominant because of its ease of measurement using either analog or digital devices.

Rz

Another common surface finish parameter is Rz. It measures the average values of the five largest differences between peaks and valleys within five consecutive sampling lengths of a surface. This makes it more sensitive to extremes in comparison to Ra. As such, it is the preferable parameter in Europe, especially Germany.

Ra vs Rz Surface Finish Units

Ra and Rz are important surface finish parameters. — *Courtesy: ResearchGate*

Characteristics	Difference Between Ra and Rz
Definition	Ra indicates the average surface roughness of a profile. Whereas Rz quantifies the average maximum height of a profile.
Sensitivity to Extremes	Ra is relatively insensitive to a few extremes, while Rz is sensitive to extremes that it measures.
Conversion	Because Ra and Rz are measuring different quantities, there is no relationship between them. However, statistics show that if Rz is known, then Ra is about seven times less than it. On the other hand, if Ra is the known quantity, Rz is about 20 times more.
Measuring	Ra is easier to measure than Rz.

Ra vs Rz