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.

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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.

Repeatability with this device is excellent and if you are careful with your initial calibration, you should have trustworthy measurements.

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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