Page 20 – Power Measurement - Scales and Tacho
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!
