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Originally Posted by MaxxxRacer
wheere woudl the variation be? I will measure it with the DMM and clampeter at each setting before i do anything so i doubt it will be an issue.
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Actually, it's a big issue.
The variations will come from many different places. The voltage will most certainly fluctuate, as Bill said, from minute to minute, from hour to hour, as well as from day to day. Everytime one of your neighbour runs a load of laundry or bakes something in the oven, your voltage drops.
Then you've got line noise, aka voltage spikes which may not seem relevant, but may have an impact on the power output (more below).
After that, you have frequency shifts; it's usually 60 Hz (in North America), but the electric company doesn't guarantee that. Also, the AC is supposed to be a perfect sine wave, but that's no guarantee either.
All of the above become an issue for two reasons:
1-you really don't know the actual power that you're putting in, and it could be different each time
2-it's really difficult to measure the electrical power, under those circumstances.
So...
For the sake of repeatability, which is important in testing because you really need to make sure that you're being fair, AC is a very poor choice.
Now...
Using DC is infinitely easier to measure; slap a good calibrated DMM on it, and use a shunt as Bill described, to measure the current. Ideally, the DC power supply has very little fluctuation in its output, and I recomend that you at least borrow an oscilloscope to take a look at it. What you'll find, more than likely, is a very small AC signal (aka voltage ripple) at a frequency that's pretty high, which will have almost no impact on the power output of the heater (the voltage sine wave has a low and a high, and they essentially cancel themselves out, usually so fast that the heater doesn't have time to react).
I know that it's not easy nor cheap to get a powerfull DC power supply, so I'd concentrate on finding a source for a used one.
There are other issues that impact repeatability, that fall outside of the whole heater issue. Bill's already hinted that you're going to have an issue with heat within the room. Specifically, you're going to have an issue maintaining a steady air inlet temperature. Your first heat source is the heater (primary and secondary losses) and the second is that most power supplies operate with an efficiency as low as 67%: generating 350 Watts in DC may require over 500 Watts of AC power, and the difference is going to heat up your room.
How are we doing so far?
