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Watt meter recommendation

Discussion in 'Home Power and Microgeneration' started by Tom E, Jun 30, 2008.

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  1. Tom E

    Tom E Guest

    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
     
  2. The message <g4airr$q6n$>
    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.
     
  3. Tom E

    Tom E Guest

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

    Thanks,

    Tom
     
  4. The message <g4ctp6$cog$>
    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.
     
  5. The message <>
    That's interesting. Did any of your test loads include the 1 to 15 watt
    range? What sort of test loads did you test with (less than unity PF or
    narrow conduction angle loadings and so on)?
    That's true (FSVO).
    Same here, but I have to confess that the 20 dollar Kill-A-Watt meter
    sounds like the yank equivilent to the £9.99 plug in digital watt meters
    available over here in the UK (and Europe since the stores involved are
    Aldi, Lidl and Netto), so I rather assumed they'd suffer the same
    shortcomings with sub 40 watt loads noted by most owners of such
    devices.

    The one such meter I have is a DEM1379 and it can be totally misleading
    with 11 watt loads and less. Since it can't display fractions of a watt,
    it's totally unsuitable for checking the standby power of most wall
    warts, particularly the smpsu based designs where it can be as low as
    200mW.
     
  6. daestrom

    daestrom Guest

    Someone over in alt.engineering.electrical did a pretty intensive test of a
    couple of 'Kill-A-Watt' units. Compared them with some pretty high-end
    instruments and found the 'Kill-A-Watt' was pretty darn good (<2% I think it
    was).

    The bang-for-the-buck is pretty good for these. Of course, I don't know if
    they would work in the UK, but surely their web site can tell you more.

    daestrom
     
  7. M Q

    M Q Guest

    Neon John wrote:

    ....
    ....
    I have used a US K-A-W to measure 240 volt loads here, but its kind of kludgy:
    The load has to be balanced (no neutral current).
    You have to wire it up screwy:
    The K-A-W measures the current on the neutral, so you have to put one
    leg of your hot through the K-A-W neutral, and then connect the K-A-W hot
    to your neutral. Multiply the resulting power reading by 2.

    If you didn't follow what I was doing, I would recommend not trying it.
     
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