Page 30 – The Strain Gauge Setup

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Strain Gauges measure force electrically. But what you buy is called a load cell.

This is a strain gauge and an amplifier together.

The strain gauge needs to be mounted between a post and a clamp on the generator.

A multimeter can then be used to measure the output voltage form the amplifier

Prior to installation on the generator, the output voltage needs to be calibrated with known weights – use metric weights. This will results in a scale factor – such as 1v = 0.85 newtons.

A newton is about the weight of an apple – about 100 grams weight
For accuracy, calibrate the load cell for the force (weight) of 1 Kg = 9.81 Newtons.

When installed between the post and the generator, the distance between the fixing point
of the strain gauge and the axis of the generator must be measured.
Use meters. These are the SI units. Torque is measured in Newton meters (Nm).

To get the wattage (Power = Torque x Rotational Speed), the engineering unit of revolutions
per minute (RPM) must be converted to the SI unit of Radians per second

A radian is about 60 degrees.
To convert from a tacho RPM value to get to radians per second multiply by 0.105
Each radian per second is about 10 RPM.

When I am carrying out a measurement run I record the load cell volts on my multimeter
and the RPM from my bicycle speedometer meter (Cyclists call RPM “cadence”)

I key Volts and RPM to a spreadsheet and use column maths to convert to Nm and Rads/sec.

Then I chart Torque vs Rotational Speed (Nm vs Rads/sec) and calculate peak power.