In the field, mechanics routinely use cable tensiometers to check tensions on aircraft control cables. The process is simple: Clamp the tensiometer on the cable, take a reading, compare it to the calibration chart with the tool (referencing cable size), and you get the resulting tension. All of this is done while the cable is installed in the aircraft and under tension.
When the Essco technician calibrated my tensiometer, he did something completely different. He mounted the tensiometer on a loose cable on the calibration machine, then he tightened the cable until the calibrated machine read the proper tension. He did this dozens of times for various tensions and cable sizes until he could properly fill out the new calibration chart for my tensiometer.
If you didn’t catch on yet to why my mind was blown while witnessing this process, let me explain. As mentioned earlier, when we use a cable tensiometer in the field, we clamp it onto a cable already under tension and read off the resulting number. This means that the number we measure accounts for the strain that the measuring tool itself places on the cable (it deflects the cable in order to measure the tension). However, when the tool is calibrated, Essco technicians clamp the tool onto the cable first, then pull the cable until the cable is under a known load (with the cable tensiometer attached). This is a completely different process that results in a completely different tension.
To prove the point, I asked the technicians to run a test. We used Essco’s calibration machine to place a cable in a fixed position at a fixed tension (15 pounds), then we attached my tensiometer. The calibration machine now read 20 pounds. Next, we did the process in reverse (the way that they calibrate). We attached the tensiometer, then brought the tension up to 15 pounds on the calibration machine. Then we held the cable in place and removed the tool. The tension measured dropped to 10 pounds. That’s a 30-percent error in measurement!
Essco didn’t do anything wrong. The technicians were following the industry standard for calibration practices, but it took me weeks of research to get to the bottom of this mystery. Here is what I found:
As an industry standard, cable tensiometers are calibrated to measure tensions of a “free cable.” Imagine attaching a cable to the ceiling and suspending a
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