Evaluating the "Usefulness" of force calibration equipment.

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

Figure 4 Morehouse 600K lightweight load cell

Physical Size

Figure 5 Morehouse Precision Load Cell Specification Sheet

Manufacturer's Specification