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Topic: How, specifically, does an electric meter measure wattage? (Read 3 times) previous topic - next topic

draythomp

Good one Lefty!  

I can think of a couple of other possibilities off hand.  If you only need a little power, just wrap some wires around one leg of the incoming before the meter and suck some power out inductively.  The other one is to use something that has a very low power factor and returns everything back to the power company as reflected power.  I used to do this with a fluorescent tube, just connect the ends without turning on the filaments then strike a spark with something else to get it to fire up and away you go, free light.  You can also heat the filaments with a battery and get it to fire and disconnect the battery; big battery though.

A long, long time ago with completely different meters than we have today, we would use a really powerful magnet and retard the aluminum disk by putting the magnet right up against the meter's glass and holding it on with duct tape.  This wouldn't stop the meter, but it would slow it down noticeably.  Doesn't work with any of the new meters though.

You could also slip a bribe to the guy that reads the meter.
Trying to keep my house under control http://www.desert-home.com/

strykeroz

"There is no problem so bad you can't make it worse"
- retired astronaut Chris Hadfield

mauried

About the only way I can think of doing this is to try and exploit the accuracy limitations that all
electronic electricity meters have.
Most modern electronic electricity meters are Class 1 meters which have a variety of accuracy specs
depending on how much energy they have measured.
A modern utility meter is Class 1 with a very tight requirement to be accurate to +/- 1% down to 5% of load.
If the Meter has a maximum current rating of 100 A, then the meter will be accurate within 1% down to
50 ma , so anything below this means that the meters accuracy is not defined.
If the supply voltage is 240 V , then a load of less than 12 watts will be below the meters minimum accuracy standard
and whilst the meter may measure this amount of power, you could most likley argue that the reading is not accurate.
Running a 10 mw led of such a meter would most likley not be recorded at all.


PeterH

#18
Jan 09, 2013, 02:18 pm Last Edit: Jan 09, 2013, 02:28 pm by PeterH Reason: 1
I've no practical experience of how electricity meters work, but I seem to remember when I was taught about power factors being told that these meters typically measure current, and that the mean square adjustment and integration over time was then used to calculate the transferred charge and hence energy. This was to explain why industrial consumers were concerned about power factor - because they are charged for current but get value from energy so they want the power factor to be as high as possible so they don't pay for energy they didn't use.

Given that these devices are specifically designed to measure current accurately over a wide range of conditions, I doubt that it will be easy to cheat them. You could demonstrate reducing the power factor so that the meter shows power that you didn't get, but there's no way (that I know of) to get a power factor greater than 1.

What scope do you have to interfere with the power supply coming in to the meter? Is it entirely under your control, are you allowed to use inductive coupling etc, or is it completely inaccessible to you? If you only need to demonstrate power going through the meter without being measured and the whole circuit is under your control, then you could try either using DC (I have no idea whether meters would detect that) of a very high frequency AC well outside the range the meter is designed to pick up.
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David82

....the whole circuit is under your control, then you could try either using DC (I have no idea whether meters would detect that) of a very high frequency AC well outside the range the meter is designed to pick up.

what do you mean by this?

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