# Evaluating the "Usefulness" of force calibration equipment

#### March 4, 2020

Usefulness may be the one word we often neglect to consider when purchasing force-measuring equipment, or any test equipment. Let's think about that for a bit and let it sink in. Most people I have run across seem to be concerned with price, a manufacturer's specification sheet, and maybe the physical size of the force-measuring device. However, when we discuss "usefulness," we start to think about what matters most. Is the equipment able to accomplish what my organization or I need it to do?

Figure 1 S-beam or S-type load cell

### Price

Figure 2 Acceptance limits of a 0.5 % tolerance with a device known to within 0.25 %

Think about it this way, I have a device that is known to within 0.25 % and I want to make a measurement that is within 0.5 % if I take measurement uncertainty into account, which I definitely should do if I'm using a decision rule that requires accounting for measurement uncertainty, my window for passing a device with a tolerance of 0.5 % is low. Figure 2 above shows the acceptance limits of between 9975.679 lbf through 10024.321 when taking the measurement uncertainty into account. It means I may have to adjust what I am testing as my device does not give me enough of a window to "pass" without adjustment. Then the customer has a potential failure that is caused by my equipment that was deemed just good enough. I would not call this equipment very useful if it costs more of my or the companies time to frequently make adjustments. Furthermore, we have simplified the equation as a device that is accurate to 0.25 % at the time of calibration often carries a much higher uncertainty when stability, repeatability, reproducibility, environmental conditions, resolution, and other CMC uncertainty parameters are not considered in these examples.

Figure 3 Acceptance limits of a 0.5 % tolerance with a device known to within 0.1 %

What if we change the reference equipment to a better load cell and meter combination that could achieve a realistic 0.1 % tolerance? Figure 3 shows that acceptance limits will increase if the reference standard uncertainty decreases. In this scenario, the end-user can "pass" more instruments as the acceptance limits with less than 2 % risk increases to 9960.286 to 10,039.714

### Manufacturer's Specification

Figure 6 Morehouse Precision Shear Web Cell

### Conclusion

In this industry, I have not seen something comparable to Consumer Reports, or websites that truly rank load cells, and other force measuring equipment. I would love to see reviews for a S-beam type load cell that says if you load it through top and bottom threads and get the centering perfect, the output will be very symmetrical and repeatable, however, if you are out of alignment by 0.1 cm, or have a side load of 0.1 degrees, that the error will be close to 1 %. Does that 1 % potential error make the device useful when the price is good, size is great, and the spec sheet looks okay? It all depends on what the end goal is. If the expectation is to calibrate tensile machines to ASTM E4 or ISO 7500, then the 1 % error is not going to make the load cell very useful, will it? If the end goal is to verify a press has a stamping force known to within 5 %, then maybe it could be quite useful. Figure 6 above is an example of a load cell that will tolerate some misalignment and still produce confident measurements as slight misalignment will increase the error by less than 0.003 %. It would be the recommended choice for ASTM E74 & E4, and ISO 376 & 7500 applications. In the end, it comes down to forming a relationship with a company you can trust. A company that listens to your problems and finds solutions to your measurement needs. A company that provides the right equipment and the right service so that you can have force-measuring equipment that is indeed "useful."

I take great pride in our knowledgeable team at Morehouse, who will work with you to find the right solution. We have now been in business for over a century and have a focus to be the most recognized name in the force business. That vision comes from educating our customers on what matters most and having the right discussions. Morehouse will not commit to providing a system if we cannot meet your expectations.

Everything we do, we believe in changing how people think about force and torque calibration. We challenge the "just calibrate it" mentality by educating our customers on what matters, what causes significant errors, and focus on reducing them. Morehouse makes simple to use calibration products. We build awesome force equipment that is plumb, level, square, rigid, as well as provide unparalleled calibration service with less than two week lead times.

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