i am building a display for a project to demonstrate the electricity usage for a LED bulb vs an Incandescent and a CFL. I believe that i need a wattmeter, but cant find one online for a reasonable (cheap) price. i have found an ammeter for a good price, but im only trying to demonstrate the difference between a 100 watt, 32 watt, and a 7 watt bulb, and would like to be able to demonstrate the usage, rather than just referring to the package. any ideas where i could find one of these or any ideas?
Gone2011-04-28T17:27:09Z
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Power in an AC circuit is VOLTS X AMPS X POWER FACTOR!!!
Power factor is 1 for an incandescent lamp, so it doesn't matter, but for a CFL, power factor is around 0.5. If you use an ammeter, the power will appear to be double the actual value. Look carefully at your CFL bulb. You will see something like "23W 120VAC 60HZ 380mA" 380 milliamps is 0.38 amps. 120 volts X 0.38 amps is 45.6 voltamperes. The power factor is 23 watts / 45.6 voltamperes = 0.5.
Your best bet is to buy a Kill A Watt wattmeter for $20 or so.
EDIT1 Not looking for accurate measurement? If the CFL package says 32 watts and your ammeter with watts marked on the dial says 64 watts, you are going to have a lot of explaining to do! I wouldn't give a very good grade for that kind of demonstration. I know Bill specializes in quantity not quality in his answers, but this is pathetic.
An ammeter will do. Amps x voltage = power in watts. Voltage is fixed, so you can put a new scale on the ammeter that measures watts.
In other words, if this is an analog ammeter, you can past a new scale on it that reads in watts. I don't know what ammeter you got, but it should be able to read 100/120 = 0.8 amps near full scale.
If it's a 1 amp FS meter, which would be ideal, then that is equivalent to 120 watts full scale. (assuming you have a 120 volt line, in the US)
EDIT, power factor doesn't matter in a demonstration such as this. We are not looking for an accurate measurement.