EEVblog Electronics Community Forum
General => General Technical Chat => Topic started by: erupter on July 28, 2012, 09:20:54 am
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I bought a PowerBay SSM T8 mains power meter over ebay
video (https://www.youtube.com/watch?v=JNn6Swb7g8I#ws)
It came with an australian standard plug (https://en.wikipedia.org/wiki/AC_power_plugs_and_sockets#Australian_standard_AS.2FNZS_3112_.28Australasian_10.C2.A0A.2F240.C2.A0V.29).
So I thought: since I want to meter my computer or other devices, and the wall sockets are in a position where reading the meter is difficult, what about putting an extension to it?
My idea is to have a cable of 1.5m go from the meter to the plug/socket for the appliance.
I suppose that this gets into transmission lines theory (which I always hated) so I'm asking: what kind of distortion I'm gonna get from such a setup?
Ideally this thing would be calibrated for measuring at its own plug/socket, so installing an extension will surely modify what the thing reads. But by how much?
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It shouldn't make any difference where in line the power meter goes. I think you're over thinking this. :)
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Try it with an extension lead :D.
Obviously make sure the load is constant,
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So I thought: since I want to meter my computer or other devices, and the wall sockets are in a position where reading the meter is difficult, what about putting an extension to it?
My idea is to have a cable of 1.5m go from the meter to the plug/socket for the appliance.
I suppose that this gets into transmission lines theory (which I always hated) so I'm asking: what kind of distortion I'm gonna get from such a setup?
Ideally this thing would be calibrated for measuring at its own plug/socket, so installing an extension will surely modify what the thing reads. But by how much?
You might be shocked to discover, if you looked behind the wall socket, that there is a really long cable extension running through the wall back to the distribution panel and meter cabinet. And before that there is a way long cable running all the way back to the utility transformer. Who knows what kind of distortion is going to happen over all that cabling?
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Transmission lines? Purely as an exercise, its moot when lenght < 1/10th the wavelength. At 60Hz that's about 500 km; at 50 Hz, 600 km. At those lengths you've probably have far more IR losses much sooner than reflection losses.
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The watt meters sold for domestic use are so inaccurate that it would not make much difference if you wrapped the cable round the moon and back, if you use one and at the same time use a couple of DMM's to measure amps an voltage you will in all likely hood find a wide disparity.
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In the USA the killawatt has been tested by many independent state and local government bureaus. Its about 1% accurate overall, it claims 0.2%, and is even lent from government libraries for people to check out. I've tested it myself with a DMM on many types of loads. More discussion is in eevblog archives.
(http://www.p3international.com/products/images/main_p4400.jpg)
http://www.cityofboston.gov/environmentalandenergy/conservation/kill-a-watt.asp (http://www.cityofboston.gov/environmentalandenergy/conservation/kill-a-watt.asp)
The watt meters sold for domestic use are so inaccurate that it would not make much difference if you wrapped the cable round the moon and back, if you use one and at the same time use a couple of DMM's to measure amps an voltage you will in all likely hood find a wide disparity.