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Re: [Phys-l] glassware needed

Bernard Cleyet wrote:
As it happens, I have a moving disk watt-hour meter as well as a cheapy sampling wattmeter.
No reason why I cannot compare both of them, though I am not up to generating 100 amp 10 microsecond pulses. Pity!
I'll let you know...
bc, can't wait!, and suffers from stream of consciousness writing.

Hmmm..I promised a another check, so here it is, as disappointing as it is.

I weighed several foam cups and filled them with water and weighed them with an old Ohaus
which meets calibration with lab masses.
I hooked two 100 ohm 10 watt 5% ceramic resistors in series, to a power lead,
courtesy RadioShack.
I heated a sample to 35.3 degC and transferred the heater and thermocouple to a cool sample,
checked the rise in temp, to gain a value for equivalent water mass of the heating hardware.

Then I heated another sample, taking temperatures every thirty seconds, and fitted
a straight line to the readings, getting 0.12340 DegC/second as the slope, with a
respectable max deviation of 0.5 deg and average deviations of 0.2 degrees.

Sadly, unless the properties of water have changed recently, I calculated the
input power to produce this result as 91 watts into 176.3 g. water.
The wattmeter was indicating 84 watts for this. Checking the heaters, I read
a resistance of 195 ohms, and the input voltage 121 V.
This should have provided a heating rate of 75 watts.
This complete lack of agreement put a stop to the next phase, which would have been
arranging to heat a sample with a small phase angle of input into a single 100 ohm resistor
at the same indicasted power input as before.

To complete the disarray, I measured the time for a rotation of the wattmeter disk
while metering a 300 watt nominal load - a halogen lamp.
A rotation time of 100 seconds in a meter marked "7.2Kh" should indicate this rate:
3600 X 7.2 / (rotation time) = watts
Timing a nominal 300 watt halogen load gave a power estimate well below 300 W.
Instrumental error gave me a beating, in a manner of speaking! :-)

So much for the joys of H.S. experimental procedure.....