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Thread: Power Meter with accuracy above 700W

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    Someone with a load tester should do a roundup of sub- $150 power meters. There are quite a few:

    Seasonic Power Angel
    Watts UP
    UPM
    Kill a Watt

    I wonder which is the most accurate.

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    Quote Originally Posted by SKYMTL View Post
    Someone with a load tester should do a roundup of sub- $150 power meters. There are quite a few:

    Seasonic Power Angel
    Watts UP
    UPM
    Kill a Watt

    I wonder which is the most accurate.
    Well the Seasonic and the Kill-A-Watt are based on the same underlying hardware AFAIK (UPM is which one again? Becuase i think there is a europena model also based on teh Kill-A-Watts hardware)....so I figure they will be the same. The Watts-UP is the one i would be interested in knowing about.

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    UPM is this one:

    LINK

    Specs say it is good to 1800W and it doesn't look like it is based on the Kill-a-Watt hardware.

    I wonder if Seasonic will release a new load accessory for their Power Angel. It would be nice to have a 100W-500W variable load accessory for the Power Angel.

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    Quote Originally Posted by SKYMTL View Post

    As for the power meter, do you mean somthing like this:
    http://www.extech.com/instrument/pro...99/380801.html

    For some reason, I can't find it for sale on that same site.....
    That's the one I'm talking about.

    Quote Originally Posted by SKYMTL View Post
    Someone with a load tester should do a roundup of sub- $150 power meters. There are quite a few:

    Seasonic Power Angel
    Watts UP
    UPM
    Kill a Watt

    I wonder which is the most accurate.
    I'd love to do that, but I'd have to buy the meters. $$$

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    How about you buy them, test them and I offer to buy the best performing one from you?

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    Quote Originally Posted by jonnyGURU View Post
    I'd love to do that, but I'd have to buy the meters. $$$
    You can try to contact the companies (or the distributors who sell these things) and ask for a free "evaluation unit" for an "upcoming review", or something like this. You probably already know how to do that properly

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    I wish I could send you the Watts up Pro that I have sitting in my office right now... BUT... Inventory Item. I have a Uei clamp on also. It has served its purpose of the last little bit. Don't know about accuracy because I have never compared it to anything else.
    "...this Silencer is a lot less expensive and it has the PCP&C name on it so it must be 133t r0x0r raaaaaarrrr1!!!" -JonnyGuru
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    Quote Originally Posted by SKYMTL View Post
    So the time has come to upgrade my test system to something that can can stress 700W+ PSUs.

    The problem is that I am a bit worried about the accuracy of my UPM power meter above 500W after reading Jonny's post about how the Kill-A-Watt became inaccurate at higher loads.
    I have considerable amount of experience with AC power measurement and currently own two power meters. Used to own three, but one was bought back by the manufacturer due to unresolved issues.

    The Kill-A-Watt is without doubt, amazing value for the cost. To get something with a comparable functionality from an instrument manufacturer you'll be paying a mid three to low four figures for their low end stuff. Four to five figures for the top of the line stuff.

    What is it about Kill-A-Watt you find not good enough and most importantly, what is the worst accuracy that meets your purpose? Accuracy cost a lot of money in general in the metrology world.

    You won't find one that meets the published specs for a bargain. High accuracy instruments are individually calibrated by hand by skilled metrologists and the factory specs are only guaranteed to be valid for usually a year, then it has to be sent back for a recertification and a calibration.

    Even if the calibration is expired by a few years you'll get a good ballpark figure but looks like that's not what you are after, so doing away without a current calibration certificate is apparently not an option for you. The usability depends on the amount of error that is acceptable to you. You can pull "I reckon it's accurate within 5%" out of your butt. You won't be able to say "I'm 95% confident that my measurements fall within xx% of the measured value"

    You might find one at a significant discount online second hand, but expect the calibration to be outdated. If you want it to be at rated specs, you'll have to send it out to a calibration lab and shell out a minimum of $100.

    I am looking for something that is rated a bit higher and is a bit more of a "Professional" tool. So far I have found a few options:

    Brultech2020

    WattsUp Pro with PF logger
    Brultech2020. No idea. It does not give you the frequency bandwidth nor does it give you the accuracy specifications to my satisfaction.

    Watts Up: Definitely avoid. Bad experience. After a replacement and unresolved issue, the manufacturer had to buy it back for an issue at engineering level.

    Fluke Clamp meter which I could use with my Fluke 177. The only problem with this approach is (if I read right) the Fluke 177 can only read up to 10A accurately if I am reading this correctly:
    Fluke 187 Specs

    Or a true clamp meter
    Useless. You won't be able to do the basic measurement even the Kill-A-Watt can do.

    Volt x Amp = Watts, if and only if you are operating a pure resistive load.

    Clean sinusoidal current with phase shift, such as an ideal induction motor
    volt x amp x cosφ

    Reality:
    Combination of phase shift and non-sinusoidal current.
    AC power measurement in real life is not a trivial matter. The Kill-A-Watt accomplishes this task using an ASIC DSP.

    The sum of instantaneous volt x amp divided by the number of samples during the integration period. The number of samples needs to be high enough to account for every part of the current waveform. So, if the display is to refresh twice a second, the chip would have to take the reading of both the voltage x amperage 500 times every 1/2 second, then divide the sum of the products over 500 to give a true average power(over a half second period). This isn't a hard process for custom made DSP.

    PF = the wattage you get above/V*A average
    Don't get it?

    Draw two waves, 90 degrees out of phase on a sheet of paper. Call them y1 and y2 If the volt =2 and amp = 2 and you simply multiply the two together, you get 4. This is called 4VA.

    Now, multiply y1 and y2 along the same x value. No matter how many times you do it, you'll get a 0 everytime. So the result is you have 0W, 4VA, power factor = 0.00.

    They do it just like this with a DSP based meter. On an analog process meter, this is done using an analog integrator and the analog output is converted to digital with a A/D converter.


    Theory of AC power measurement:
    http://www.yokogawa.com/tm/tr/tm-tr0605_01.htm

    The best you can obtain with a voltmeter and an ammeter is the VA. If your ammeter isn't a true RMS, you can't even do this. Power factor is predictable and it is around 0.65 on standard power supplies and 0.98 on active PFC power supplies, so you can guess a wattage this way, but this isn't a measurement. This is a rough estimate.

    So, what do you guys think? What should I do?
    None of the above. Sorry. Buy a REAL instrument.

    Brands to look for:
    HP-Agilent, Hioki, Yokogawa, Voltech, Dranetz, Fluke and any reputable manufacturer who can actually produce a detailed specifications and make NIST traceable certificate available.

    Upper range, but not quite lab grade:
    http://www.epd.com/power_meters.html
    Last edited by Kvar; 07-04-2007 at 01:43 PM.

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    Quote Originally Posted by eightballrj View Post
    I wish I could send you the Watts up Pro that I have sitting in my office right now... BUT... Inventory Item. I have a Uei clamp on also. It has served its purpose of the last little bit. Don't know about accuracy because I have never compared it to anything else.
    Ok, now I'm going to talk about the Watts Up Pro. I bought one, and I no longer own one.

    After I bought mine, I noticed my unit showed an error that is well beyond the 2% or whatever accuracy they claimed when it was used to measure non-unity (which means power factor is not 100%) loads. I don't have the figures off top of my head, but it was a GROSS error.

    The Watts Up series uses a current transformer while the Kill-A-Watt use a shunt. Current transformer is more vulnerable to phase error and would not have a linear response to harmonic current which can extend into thousands of heartz. I believe the core was made of iron, so I would expect it to truncate much of the higher frequency harmonics present in a computer power supply type load and add to inaccuracy.

    It measured voltage and current reasonably well, however the problem was that it showed a significant amount of power factor measurement error. This was confirmed using a lab grade power meter daisy chained with the Watts Up Pro. The lab power meter is not powered directly from the power source under test, so connecting it downstream of the Watts Up does not create an error.

    The two meters agreed well with a resistive load. With an induction motor load drawing around 150W, the two meters showed a significant discrepancy. Amp and volt readings were close together, which means the Watts Up had a phase error. I contacted the manufacturer with my concerns.

    I provided them with data reported from both instruments, as well as the data from my Kill-A-Watt. I heard back from the company president Mr. Brad Volin and he asked me to return the meter. I got a replacement meter. Same exact problem, however the absurd skew was in the opposite direction. The meter went back to Doubleed as soon as I got the return label from them. I requested that they repair my unit or provide me with yet another replacement that meets published specs.

    Mr. Volin reported back to me that they were able to duplicate the problem and that their engineering team was looking into it. I was told there was an issue beyond the specific units I have handled. I didn't need the thing right away, so I gave them some time, but after a few months, they could not provide me with a unit that meets the specs and offered to buy back my unit.

    This was in late 2005 or 2006. So unless they've come up with a fix since then, I would avoid it. The actual data got buried somewhere, but the amount of error was obscene by my standards. If you contact Mr. Volin, he maybe able to provide you with the actual data so you can see if the error is acceptable or not yourself.

    To their credit, the computer software features are nice, but with a serious flaw in the fundamental operations, it was not practical for me.
    Last edited by Kvar; 07-04-2007 at 02:32 PM.

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    The funny thing is with the Kill-A-Watt it occasionally gives readings of over unity with certain power supplies.

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