Energy meters domestic plug in wall type 1

[asimpson]

There are plenty of domestic plug-in electric energy meters for monitoring consumption about the home or office.

These will refresh display every 10-20 seconds.

I wish to monitor a rapidly switching device and I am concerned the energy meter may not be getting a true value of consumption if the sampling and switching are out of step.

Does anybody know if a typical energy meter for about $15 has the smarts to overcome the worry I have?

http://www.ustudy.in/node/6449

Thanks

 

[VEBill]

If you can't get a clear answer, then you might have to run some tests.

It would make sense that the meter would sample faster than the display update, and average the results. Or perhaps it would integrate under the curve using analog circuitry. But maybe not... ...perhaps it simply samples.

In the long run, even samples should average out to the correct value.

 

[davidbeach]

In the long run, even samples should average out to the correct value.

Unless, of course, the sampling rate is such that it always sees either the on or off state of the device being switched...

 

[dpc]

If your device is a non-linear load, you will also need to be concerned about the accuracy of the power measurement in the presence of harmonic voltages and currents. This is an issue even with (supposed) revenue-grade energy meters.

 

[VEBill]

Those KillWatt-style energy meters are intended to be plugged in and left in place for a day or more to gather the average power consumption over a variety of conditions (e.g. a refrigerator, a TV, etc.).

Over any reasonably long period of time, unlucky synchronization is unlikely. The meter should catch the On Power pulses in proportion to the duty cycle, thus there's a good chance that the averages will work out. Unless something else is going on, which might be the OP's fundamental concern.

If the OP can determine the On Power, the Off Power, and the duty cycle, and assuming they are stable - then it's obviously trivial to calculate the precise average power; for comparison with what the $20 meter says.

If the OP needs dynamic measurements, then use a different method.

 

[asimpson]

Thanks all.

The application is a 3 kW heater I wish to test in the field. I cant predict normal cycling of heater. It is a prety rough test so I cant expect too much accuracy but I just wish to limit unknowns.

Perhaps I am being too cautious and the issues will average themselves out.

In lab tests we use much more sophisticated instruments so I should test the accuracy myself against these and see what sort of correlation I get. However this is often out of my control.

Could anybody recommend a good low cost consumer energy meter (<$50) for running over week or are they all prety much the same?

Thanks again

 

[Compositepro]

A heater is on or off, so to know the energy used you just need to know how long it is on during a certain period. Just hook-up a timer to the heater power. It could be as simple as a synchronous motor clock. How far its time is behind the true time is how long the heater has been off since the start of measurement. Or how far it moved from noon when it was set is how long the heater has been on.

 

[IRstuff]

Can't quite imagine that a 3kW heater will be cycling so fast that a typical power meter would have trouble keeping up. I would think that a typical operational cycle would be on the order of minutes.

Watt's Up Pro is higher than your bogey:

http://www.ebay.com/sch/i.html?_from=R40|R40&_sacat=0&_nkw=watts+up+pro+meter&_sop=15&rt=nc,

but it's able to datalog to a PC at 1-second intervals:

https://www.wattsupmeters.com/secure/downloads/manual_rev_9_corded0812.pdf

TTFN
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[waross]

A few years ago, Skoggs had an issue with some revenue grade electronic meters and a mismatch between the sampling rate and the switching rate. As I remember the meters were reading unacceptably high.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[itsmoked]

Couldn't possibly expect revenue meters to ever read low could you, Bill?

asimpson; The WattsUp unit I have has worked very well for me.

This from their highlights:

The Standard model measures voltage and current thousands of times a second so it has an incredibly fast response time which enables you to "see the surge" of power when appliances are first turned on. The peak value display captures this surge so it is displayed even if it happens too fast to see live. The minimum RMS voltage display shows the line quality and voltage dips. Minimum RMS current and minimum watts can be used to monitor variances in appliances that run continuously.

[URL unfurl="true"]https://www.wattsupmeters.com/secure/products.php?pn=0[/url]

Keith Cress
kcress -

http://www.flaminsystems.com

 

[zappedagain]

3 KW? Not from a standard 120VAC plug in the USA. If you are elsewhere with 208/240 VAC you might find something available, otherwise you will probably have to go with dedicated test equipment for a higher price. You can probably rent the equipment that you need. Besides, if you are already going out into the field, don't you want to make sure you have the right equipment, not the least expensive equipment? How much will it cost if you have to repeat the trip a second time with the right equipment?

Z

 

[mikekilroy]

Keith, thanks for the watts up link....

We had (have) a need for a customer to accurately measure rms current on load that is cyclic at rate of 100msec to 3000msec. during each cycle, the load changes from .5 to 18 amps. we found no wattmeter on the market for less than thousands of dollars that did not have > 1 sec sample rate, so none could make our measurements... I ended up providing a true rms averaging digital current meter and CT transformer that would rms 4000 samples/sec over adjustable 1-60 second period. It agreed with our rms scope readings. Our customer had to then multiply this true rms current by the 230v 1ph to get true rms watts.

I say this because even your link says it samples no faster than 1 sec. The only model that says it samples 1000's of times per sec is the cheap 'watts up' model, and it appears to just give instantaneous peak and min values and does not seem to collect samples over time like the more expensive ones do? the more expensive ones are all limited 1 sec or longer sample rates also. Am I reading the data wrong on these? I hope so, as I would love to be able to provide a $100 true rms wattmeter that samples faster than 1/sec!

http://www.KilroyWasHere<dot>com

 

[itsmoked]

I didn't see the OP's second post's 3kW aspect as zapped did, so I doubt the utility of the product for him.

As for the sample period the low-end one I have is, I believe, the same as the the one we're seeing on the site. While I never knew it sampled above 1kHz it has always appeared to be very fast to me. The numbers seem to change instantly as compared to occasional updating like most DMMs.

It does cough-up rational peak currents of values you'd expect from inrush. The WattsUp came out just before all the power calc chips hit the market so I believe they did a nice hardcore read the instantaneous current and voltage and multiply together rapidly, type design. As for logging it really doesn't "log" it totalizes. It will tell you the duty cycle, kwhrs, average amps, etc. You can program your energy cost and it will give you a running energy cost. It's very nice for assaying stuff around your house. Plug each item into it and leave for 24 hours and log it to a notebook. Wander around the house and do everything and in two weeks you know who the energy hogs are. Refer, cable box, too-large flat screen, etc.


I think the ad-speak is a bit confused in that I believe the newer versions work the same but only log the data second by second.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

It may be that the Watts Up unit samples at a high rate but only logs the data every second. The energy consumptionmay be accumulated correctly for a fast duty cycle even though it is not recorded.
That is the total consumption may be correct for your duty cycle even though you can not see the actual duty cycle with the 1 second update.
I looked up the manual for the Watts Up unit and was not reasured.
Do you need total consumption or do you also need the load profile?

Bill
--------------------
"Why not the best?"
Jimmy Carter