Ohms = E squared / Power

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cadpoint said:
Rolling along with 19 and 20 threads here, note, one, two and five here .
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That's good.
I've found that, as I got older (and I'm old) you just seem to "know" things without thinking too hard about them or how they are derived.
 
Well I just performed an actual test on a 2000W 230V 1PH heater that is labeled 8.7A, this heater is probably over 40 years old. Its elements are in parallel and I was able to touch the test leads on the actual terminals of each element individually. The rated resistance should be 52.9 ohms (R = 230 squared / 1000W)

Actual Resistance:
R = 50.0 cold, 95.4% of hot
R = 52.4 hot, 5% more than cold

Actual Wattage (233V & 8.8A):
P = 8.8 x 233
P = 2,050.4

Original question:
Is it most accurate/effective to test ohms resistance to qualify a heater element as good?

Findings:
  • It is most accurate & effective to test amperes rather than ohms.
  • Testing amperes involves less process.
  • Testing amperes is more accurate because this reveals actual use.
  • Resistance should remain constant as manufactured, therefore a likely different voltage should affect wattage rather than resistance usually causing an increase in wattage rather than ohms resistance.

I probably need to find a hobby :-?, but anyway just glad to share.

Harmonica would be a good hobby
 
tryinghard said:
Findings:
  • It is most accurate & effective to test amperes rather than ohms.
  • Testing amperes involves less process.
  • Testing amperes is more accurate because this reveals actual use.
  • Resistance should remain constant as manufactured, therefore a likely different voltage should affect wattage rather than resistance usually causing an increase in wattage rather than ohms resistance.
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That's what I said, ain't it? :smile: http://forums.mikeholt.com/showpost.php?p=929463&postcount=8
 
tryinghard said:
True and this is how I've always tested but I was curious about using ohms resistance to test.
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When you test for DC resistance, that's all you can really get. You'd have to know many things about the load to calculate running impedance from a DC resistance measurement.
 
tryinghard said:
[*] Testing amperes involves less process.
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Surely that's true only if you can get to the individual conductors without dismantling anything?
You can check total resistance by measuring it from the appliance plug terminals with no dismantling.
Anyway, if I measured 50 ohms against a calculated 52.4, I would have concluded that the heater element is sound.
But, as Larry says, you need to be cautious about interpreting DC resistance measurements. On a filament lamp all you could reasonably conclude is whether or not you get continuity.
 
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