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Homemade Tensiometer thoughts
I have been experimenting with tensiometer designs and want to share my findings. My current version is based off of a design by Darrin Towers which is based off of a design in Kitplanes magazine.
The theory behind this is simple. The cable has a slight offset forced on it by the spring. The force required to create this offset is the two times the sin of the offset angle.
In this case, we have a 4 degree angle so 2 * sin(4) * cable tension is the measured force. In the case of 20#, it would be~2.8 pounds. This low force is why I can make this device on a 3D printer. There is a problem with this design. The more tension on the cable, the more the center point is deflected -which results in a lower angle so the increase in deflection is non-linear. Higher tensions move the needle less than lower tensions (the movement between 40 to 50# is less than from 20 to 30#. To get around this, I tried substituting the dial indicator/spring with an electronic strain gauge. The results were linear and worked well. However, that device had two issues: you need a battery, and you need to zero the circuit before taking a measurement. You can't attach it to your cable then power up. In the end, I decided to go for the simple dial indicator design.
Replies to This Discussion
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Permalink Reply by Douglas Binder on
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Calibration is simple: apply known tensions and note the reading. I plan to make a label that will cover the numbers on the dial indicator and mark the label every 5 pound increment. The way mine is set up, from 15 to 40 pounds is less than one full rotation so this label will work well. The dial indicator is in a slot on the plastic so can be adjusted up and down to control the initial deflection. Changes to this position will affect the total travel throughout the range.
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Repeatability with this device is excellent and if you are careful with your initial calibration, you should have trustworthy measurements.
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Permalink Reply by Dan Dempsey on
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I made one. I'll see if I can post some pictures. Not pretty, but it works. I use it every annual.
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