Sadly, the Australian market still does not have a low cost consumer power meter like that one.
*sigh* One was released recently, but the supplier removed it from sale because it had accuracy issues on the low end of the scale.Dave.
I checked my 20 inch Sony and it was drawing 2 watts when turned off, but 5 VA (volt-amperes, the product of separately measured volts and amps; the same thing you did).
The power was measured with a Yokogawa electromechanical wattmeter specifically designed to measure power when the power factor is low.
These two measurements mean that the power factor was about .4.
What you should know is that the measurement you made is an upper bound for the true power. For example, if the power factor (which you don't know) is .33333, then the true power consumption is 1/3 of what you measured and you are paying less than 15 cents a month for that TV's idle power; more like 5 cents. Do you really need to know more than that? If so, then get a Kill-a-watt, but be aware that the Kill-a-watt will have inadequate resolution for really accurate measurements at such low powers.
"The Phantom" hn
** Like hell the trolling, scumbag cretin did.You need a " true rms" amp meter to measure VA.
....... Phil
How about as a work around for the poor low end accuracy; Put three 100 watt light bulbs on as a load, measure the power then add the low power load he wants to measure and note the difference. Would the power meter have the resolution to do this? Mike Mike
If the gadget has a transformer first thing, yes. But if it's a switcher, with a rectifier and filter cap first, and he uses a not-true-RMS meter, the error could go in the opposite direction.
"E I cos theta" is sort of meaningless for radical waveforms.
John
messagenews: snipped-for-privacy@b40g2000prf.googlegroups.com...
Your average punter is not going to want to do this, and even I couldn't be bothered. Also, that is not a solution for long term consumption monitoring which is what these things are good at also. I've got a silicon chip power meter kit (which I got ridiculously cheap), and that is pretty good, but I wouldn't mind a one or two more cheap commercial ones. They are dime-a-dozen in the US, but not here.
Dave.
For those playing along at home, this is the new power meter in question available on the Australian market:
The ATA who was selling the unit (not via eBay like the link above) initially had this to say in the advert:
"Note: Some of the readouts on the display on this meter may be hard to read if your eyesight isn't so good, although the main readout uses around 9mm high digits. Also, meter accuracy measuring small values, below 10 watts, is not great, so it may not display accurate reading for small loads. "
But then removed it from sale for the following reason:
"Low cost digital power meter possible accuracy issues: Our new low cost power meter has been a very popular item, however some customers have reported accuracy issues with the meter. Our initial testing showed it to work quite well, but it seems the accuracy is variable, with some meters working well, and others being inaccurate.
Because of this problem, we have decided to no longer stock the meter. We are instead looking for alternatives and expect to have a replacement meter available in a couple of months at most.
If you have recently purchased a low-cost energy meter from ATA and have doubts about its accuracy, or have any other problems with the meter, you are welcome to return it for a refund."
Seems like eBay sellers are still happy to supply it, and probably haven't heard about any problems with it.
Dave.
Cool, thanks, Bob, I'll check that out.
--HC
thing
Tim Wescott in the first response to the OP's question mentioned the need for a correct RMS measurement. I expected that the issue was settled then and my comments assumed as much.
Just to be pedantic, the procedure of using an average-reading AC meter for current, then for voltage, ignores all polarity. So, the reading can indicate that the power grid is powering the appliance, OR that the appliance is powering the grid, you need more info to determine which. This uncertainty as to power direction also applies instant-to-instant, so it is easy to see that the net power could be alternately positive and negative.
Power forward from grid to appliance yields 'power factor' =3D 1. Power backward from appliance to grid yields 'power factor' =3D -1 Power alternating can get a 'power factor' anywhere in the range of (+1, -1)
But it IS the formula of interest when you have a pure sinewave for 'E', which is nearly correct for mains power. The correct way to measure I is not just RMS, but filtered for exactly the frequency and phase of E. So two RMS meters won't tell you the power factor any more than two average-meters would. You need a power meter like the one the power company bolted to your house, OR its electronic equivalent, that actually measures E times I first, then integrates up all the timeslices (the multiply/accumulate function here is KEY for digital signal processors, because it shows up in LOTS of real problems).
The 'cos theta' is a number in the range (+1, -1) which is also called the power factor, and it is the missing piece in any of the schemes that measure E and I separately.
No, just ignore the trolls, of which Phalluson is one of the worst.
This is sci.electronics.basics, where no question is unwelcome - rudeness like his (and a few others) is severely frowned upon.
Cheers! Rich
"Rich Grise"
** Especially Rich Grise.A monumental f****it, a smug turd, an asshole and a net cop jerk off.
** The bone-headed moron thinks he OWNS the newsgroup and his bizarre opinions are unchallenagble.He could not be more wrong.
About everything.
Piss the fool off, or killfile him.
........ Phil
"The Phantom"
** What a spectacularly STUPID assumption to make.But only one of a great many.
........ Phil
Phil loiters around the "basics" newsgroup looking for - in his opinion - people who know less than he does, people who ask "basic" questions, so he can abuse them and feel smart. He probably picks fist-fights with six year old girls, too, so he can win.
John
The usual definition of power factor doesn't admit of negative values.
See:
But it is possible to have a pure sinewave for 'E' and a highly distorted waveform for the current.
If the current waveform is highly distorted, how do you define the relative phase between E and I?
I don't think anybody said that two RMS meters (alone) would tell you the power factor, did they?
Actually, the meter bolted to your house is an energy meter, not a power meter.
Incorrect. Amps times Volts gives VA (volt-amperes) which is 'apparent power'.
Finding true watts (real power) for a complex load is well .. complicated. There may be phase angle issues between the volts and the amps (which make the watts lower than the VA) and non-sinusoidal waveforms (which make the watts HIGHER than the VA).
Fun eh ?
Graham
As measured with true RMS meters, right?
If watts are higher than VA, the power factor is greater than 1, since power factor is defined as watts/VA.
According to
Can you give a specific example of non-sinusoidal waveforms where the power factor would be greater than 1?
He gave up fighting six year old girls. They kept beating the crap out of him.
-- Service to my country? Been there, Done that, and I\'ve got my DD214 to prove it. Member of DAV #85. Michael A. Terrell Central Florida
Just to be sure I understand, are you saying that if a person measures the voltage and current with two RMS meters, those two numbers, plus a wattmeter measurement, can't be used to give power factor?
"Eeysore"
** Plus defies the law of conservation of energy.All hell breaks loose then ......
...... Phil
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