Page 20 – Power Measurement - Scales and Tacho


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With our Stirling engines and their low levels of power, we can use kitchen scales
and a bicycle speedometer to take our measurements.

You can see the setup in the photo opposite.

Some bicycle speedometers have to be told the wheel diameter to get correct readings.
Most have a setting for what they call “cadence” and you and I would call rpm.

Both these devices give digital read out. I find this far better than reading a dial.
When trying to read a needle over a dial, there is always flicker.
The observer is required to make “an interpretation”.

With a digital display there is certainty as to what the digits are.
Also, the value can be read more quickly than from a flickering pointer over a scale.


The two readings, grams and rpm, must be taken rapidly. Ideally, simultaneously.

Grams and rpm! Yes, a complete muddle of units. Imperial and SI. That’s the world we live in!

Having got a pair of readings, grams load and rpm, this is the formula you need

…………………….
Watts = WEIGHT Kg
……………………………..….x DIAMETER cms
……………………………..…….x RPM
……………………………….……./ 195

The diameter is twice the length of the prony brake arm, or radius.

See the Wattage calculation in the StirlingEngineBoats.com "Engine Room" for the derivation.

Time for a computer spread sheet – then you can draw a graph.

A picture is worth a thousand numbers!