Our Ultra Precision Scales are driven by surface acoustic wave (SAW) technology. This new design is 10 to 20 times more accurate than standard scales—those that rely on strain gage technology—and rival the accuracy offered by MFR (magnetic force restoration) scales at about a third of the cost.
Yet before you can truly appreciate the simplicity and effectiveness of this technology, it’s important to understand the limitations and vulnerabilities found in the two most common weighing technologies.
What is a Load Cell?
A load cell is a type of transducer that converts force into an electrical signal and is the driving force behind the majority of weighing devices. It’s the device responsible for converting the weight of an object into the numbers you see on the scale’s display.
What is a Strain Gage?
A strain gage is a sensor whose resistance varies with applied force. When applied to a load cell, the strain gage can convert the weight of an object into an electrical resistance that can be measured.
When an external force (the weight of the object you want to weigh) is applied to a stationary object (your scale), it causes stress (an internal resistant force) and strain (displacement and deformation) in the stationary object.
How Does It All Work?
Inside the load cell is an object called a spring element. In its most basic form, the spring element is a specialized metal piece that’s attached to the scale floor at one end and the underside of the scale platform at the other.
When weight is applied to the platform, the spring element experiences stress and it therefore bends. How far the spring element bends is directly proportional to the weight applied to the scale’s platform. (You can compare it to a diving board at the pool: the board bends more when an adult stands on it than it does when a child stands on it.)
During the manufacturing process, four thin sections called flexures are incorporated into the design of the spring element. Strain gages are then bonded to these flexures and wired together to form a Wheatstone’s bridge—an electrical circuit that is used to measure an unknown electrical resistance.
So ultimately when you place an item on the scale, this is what happens:
• The spring element bends and causes the flexures to bend
• The bending of the flexures changes the resistance of the strain gages
• The changing resistance of the strain gages alters the voltage output of the Wheatstone’s bridge
• The voltage produced by the bridge is passed through an ADC (analog-to-digital signal converter)
• The digital signal is sent to the scale’s display
• You read the weight of the object you placed on the platform
Precise Enough for Most Weighing Tasks
Strain gages are the most common choice in load cell design for a reason—they perform well enough to serve the vast majority of weighing needs. When the load cell is made from a high quality material (like stainless steel) its durable, rugged and reliable, which means they’re accurate and sturdy enough for industrial environments.
Yet there is a smaller niche of industries that is not well served by a standard scale. This niche includes research facilities, laboratories, paint mixers, semiconductor manufacturers and any other operation that requires precise high resolution measurements because fractions of a pound—like 0.0172 lb. of material—can affect the quality, consistency, usability and/or performance of the final product.
The Limitations of Strain Gage Load Cells
How accurate a scale is depends on how close its measurements are to the true value of an object’s weight. Now all scales will deviate somewhat from this true value, but the degree to which this happens depends on the weighing technology you choose and the standards of the manufacturer you buy from.
Several conditions—including the ADC signal conversion and the adhesives used to bond the spring element and strain gages—affect the accuracy.
Strain gage scales are generally accurate to 1/10 of 1%. This means weight values can be ± 0.1 lb. of the true weight. And while 0.1 lb. of material won’t make a difference in the taste or texture of a processed food, that much pigment can have a drastic effect on the consistency of paint mixer’s palette and the exact color they’re producing.
There is also a relationship between maximum capacity and resolution—the number of digits to which a scale can accurately weigh—that can also be limiting. Scales with higher maximum capacities offer lower resolutions and vice versa.
For example, a bench scale with a 10 lb. maximum capacity has a resolution of 0.002 lb. while a platform scale with a 500 lb. maximum capacity has a resolution of 0.1 lb. Yet the accuracy of these measurements is still ± 0.1 lb. regardless of the resolution.
Magnetic Force Restoration Scales
The first solution to this issue with strain gage scales was the introduction of MFR scales. The design completely eliminated the load cell, relying instead on a lever system and an electromagnet to oppose the weight of the object on a platform.
Yet there are a few drawbacks to this technology including:
• Very low maximum capacities
• Expensive purchase prices
• Sensitivity characteristics that limit accuracy
These scales generally won’t weigh loads higher than 50 pounds, which makes them impractical for large-scale operations.
They cost thousands of dollars, which can make them hard to budget. Repair and replacement costs drive costs even higher.
They’re vulnerable to drifts in accuracy caused by changes in temperature, air currents and vibration caused by other machines, which means they’re also not practical for use in industrial environments where these conditions are almost certain.
So while they provide the high resolution measurements required by these niche industries, they don’t do so in a meaningful way.
The Most Precise Weighing Technology
To address the design vulnerabilities found in strain gage and MFR scales, we took the same basic theory of load cell design found in strain gage scales for its ruggedness, durability and strength and applied our new weighing technology—surface acoustic wave. This innovation allowed us to develop an extremely precise, high resolution scale that’s far more affordable than MFR scales and better equipped to handle tough industrial conditions.
How SAW Technology Works
Two surface acoustic wave (SAW) transducers are placed at opposite ends of the spring element. One acts as a transmitter and the other as a receiver. When you place an object on the platform, a bulk wave is created in the substrate of the spring element. The transmitter detects this wave and passes it to the receiver where it is amplified and sent back to the transmitter, creating a feedback loop between them.
The frequency at which oscillates between the transducers is relative to the distance between them, which is determined by the bending of the spring element. And this, as we already know, is proportional to the weight of the object on the platform.
The frequency is measured digitally and sent to the indicator where you see the weight value.
Advanced Design Creates a Superior Product
We fabricate our SAW transducers using semiconductor-grade crystalline substrates. Electronics are deposited thereon using the same photolithography process used in the manufacture of advanced semiconductor devices.
This design creates a fully digital output stream, eliminating the need for an ADC signal conversion, and produces weight values that are inherently more accurate than those offered by strain gage devices.
It also creates a weighing device that is not vulnerable to drifts in accuracy caused by changes in temperature, air currents or vibrations, which means they can be used in any industrial environment.
The Numbers Speak for Themselves
Readability is a term scale manufacturers use to describe the performance of their products. It’s calculated by dividing the maximum capacity by the resolution and is expressed as parts of a whole.
For example, a 10 lb. capacity strain gage scale has a resolution of 0.002 lb. You would calculate (10 / 0.002 = 5,000) and say that the scale has a readability of 1 part in 5,000. It means that the scale can display 5,000 unique weight values between 0 and 10 pounds with an accuracy of 1/10 of 1%.
1 part in 5,000 is the typical readability for strain gage models.
Yet this number pales in comparison with our Ultra Precision Scales. At a 10 lb. capacity, they offer a resolution of 0.0001 lb. So if you calculate (10 / 0.0001 = 100,000), you would say that these scales have a readability of 1 part in 100,000—20% better than strain gage scales! And with an accuracy of 1/100 of 1%, they’re also ten times more accurate!
Unlike the low capacity levels of MFR scales, our Ultra Precision Scales offer maximum capacities of 500 pounds and higher!
Yet a 500 lb. capacity strain gage scale has a resolution of 0.1 lb. while SAW scale at the same capacity offers an unrivaled precision of 0.005 lb.!
Our SAW scales are inherently more accurate because they digitally measure displacement instead of stress, which means they require much less bending from the spring element and no ADC conversion. With this new design, we were able to improve the performance of our scales by 200%!
If you’d like to learn more about our ultra precision technology or are interested in purchasing one of our Ultra Precision Scales, please contact us today!