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Watt meter recommendation

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Posted by Tom E on June 30, 2008, 12:16 pm
 
I would like to know how much my various appliances are consuming.
Where can I get a device (for the UK) that will measure the energy
consumed by individual devices?

I have looked at things like "Brennstuhl Wattage and current meter
PM 230" and the "Kill-A-Watt", but they look rather cheap, and I'm
not convinced they will accurately measure loads with unusual
waveforms (my computer for one thing).

Any better suggestions?

Tom


Posted by Johnny B Good on July 1, 2008, 1:51 am
 


 You're quite right to be suspicious of such meters (that's not to say
such digital technology can't provide accurate fractional wattage
readiings with extremely distorted waveforms, I'm just saying such
accuracy can't be done on the cheap).

 Quite a few years back, I spotted a very rare piece of test kit at a
radioham rally going for the princely sum of 35 quid. Normally, I
wouldn't have paid more than a tenner for the more mundane multimeter
type of tester usually on offer at this type of event, but I made an
exception in this instance since it was a genuine analogue wattmeter for
use at mains voltages ranging from 100 to 500 volts in three scales of
100, 200 and 500 volts fsd and 3 current ranges of 1, 5 and 25 amps fsd
(both with a times 2 over range) for both single and 3 phase supplies.

 It is a german made unit with the name "Metrawatt". At the time I
thought I'd very likely only make occaisional use of it to satisfy "Idle
Curiousity", but I've had it in line on the repair bench to monitor
power draw of the many PCs I've had in for repair over the past 7 or 8
years and it's proved to be a very useful diagnostic aid on many
occasions. Quite frankly, I'd feel rather lost without it now.

 The hundred watt scale is marked in 2 watt divisions allowing
interpolation by eye alone to a half watt accuracy (leaving the +/- 3%
accuracy issue aside). For sub watt readings, it's possible to
interpolate down to less than 200mW with the aid of a jeweler's loupe
and the mirror backed anti-paralax scale where the +/- 3% error (of
deflection) is lost amongst the inevitable interpolation errors involved
in taking such readings.

 Trying to get a reading of a 5 watt load with one of those cheap
digital meters can quite easily range over the zero to 15 watt mark
which is worse than useless since such readings are simply totally
misleading.

 I rather suspect a digital wattmeter capable of measuring such low
wattages with any degree of confidence (let's say within 20% of reality)
is likely to cost hundreds of pounds. On the other hand, I rather fancy
you could get hold of an old analogue meter similar to mine for less
than a hundred pounds if you searched hard enough (but they were a very
uncommon piece of test kit even back then, so it might take a bit of
searching).

 Actually, after doing quite a bit of googling just now, it seems the
chances of finding such an instrument would appear to be almost zero. It
makes me feel rather priviledged that I should possess such a venerable
(and valuable) piece of test kit. Also, it does turn out that a digital
equivilent _will_ cost hundreds of pounds. :-(

 I'm sorry I can't be more helpful.

--
Regards, John.

 Please remove the "ohggcyht" before replying.
The address has been munged to reject Spam-bots.


Posted by Tom E on July 1, 2008, 9:35 am
 Johnny B Good wrote:

That's a shame, but thanks for the info!  Out of interest, do you
have a picture online of your device?

Thanks,

Tom


Posted by Johnny B Good on July 1, 2008, 12:38 pm
 

 Sadly,no, although in my googling results last night (wee small hours
:-), I did see a picture of something very like (a mavowatt, I think)
with 4 ranges of both amps and volts but I think it used electronics for
its 'multimeter functions'. Unfortunately, after spending nearly another
hour googling, I haven't been able to find it again, otherwise I'd have
provided a link.

 If you have a favourite 'photobucket' site that's simplicity to post
photos to that you'd care to recommend, I'd gladly upload a photo or
two.

--
Regards, John.

 Please remove the "ohggcyht" before replying.
The address has been munged to reject Spam-bots.


Posted by Neon John on July 1, 2008, 12:25 pm
 On Tue, 1 Jul 2008 02:51:50 +0100, Johnny B Good


Actually, such accuracy CAN be done on the cheap and the Kill-A-Watt does it
quite nicely.

I own several (originally 10, minus whatever have been lost here and there).
At one time had 10 of 'em hooked in a series daisychain along with a GE analog
"1/4% accuracy class" lab wattmeter and a NIST traceable GE watt-hour transfer
standard meter.

All 10 instrument read within a digit of one another and they all agreed with
the traceable instruments to within my ability to read them.  The first meter
in the chain did read 1 watt higher which accounted for the burden of the
other 9 plus the losses in the short cords connecting them together.

It is pretty evident from looking at the internal construction that each
device is flashed with its own calibration factors during manufacture.  That
means that the current shunt and voltage divider resistors don't have to be
accurate, just stable.  That they are.

OTOH, the very much higher priced Watts Up instrument is garbage once the
power factor drops much below unity.  Price is no indication of quality with
such instruments.

No experience with that Brennstuhl Wattage instrument so I'll not comment.

John
--
John De Armond
See my website for my current email address
http://www.neon-john.com
http://www.johndearmond.com  <-- best little blog on the net!
Tellico Plains, Occupied TN
So you're a feminist... Isn't that cute!


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