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Re: [Phys-l] glassware needed



On Jan 1, 2009, at 8:01 PM, Brian Whatcott wrote:

I have a lamp controller hooked to the halogen lamp standard by my desk.
I turned the light well down to a round current reading, and read the
following parameters from the inexpensive "Kill-a-watt 4400"
meter (Cheesy name! <$20)

Volts 122.4V
Amps 2.00 A
watts: 158W
va: 245 voltamps
Fx: 59.9 Hz
p.f. 0.64

Turned high, I read:
Volts: 122.2V
Amps: 2.48A
watts: 293W
va: 303 va
Fx: 59.9 Hz
p.f. 0.96

These look plausible for a microprocessor with an A/D that samples
reasonably fast.
Hmmm....how would I check, without a calibrated wattmeter to hand?
Oh, I know: warm a fluid sample at high pf and another at low pf and check
if the delta t is proportional to indicated power input? :-)

1) One way to calibrate a questionable wattmeter, no matter how irregular the heating current is, is to measure thermal energy Q=c*m*dT. I am thinking about using electric energy E=P*t to heat a fluid in a colorimeter. The mean P, measured by a good wattmeter, should be equal to Q/t, where t is the duration of heating and T is the temperature.

2) For fluctuating values of P, E=SIGMA[P(t)*dt]. How high should the sampling rate be? For a randomly fluctuating I(t) the sampling rate does not have to be very high. What is important is to collect enough samples. to get the mean P. The same is true for periodically fluctuating current, provided the sampling intervals fluctuate randomly. In my opinion, sampling intervals should always fluctuate randomly.

3) An electric arc (for example between welding electrodes in salty water) will produce a rapidly fluctuating current. This can be dangerous. Do not try it without a protection!

Ludwik Kowalski, a retired physics teacher
5 Horizon Road, Apt. 2702, Fort Lee, NJ, 07024, USA
Also an amateur journalist at http://csam.montclair.edu/~kowalski/cf/