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Your opinion on a power meter I'd like to order.

Discussion in 'General Electronics Chat' started by alphacat, Jan 31, 2010.

  1. alphacat

    alphacat New Member

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    Hey,
    I've found the following power meter (specification) which is for rent.

    There is something I dont understand about its active power accuracy:
    [​IMG]

    What does it mean -
    Accuracy: ±(0.5% + 10digits) ?

    Thank you.
     

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    Last edited: Jan 31, 2010
  2. alphacat

    alphacat New Member

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    Anyone please?
    I just dont get what does it mean ±10 digits
     
    Last edited: Jan 31, 2010
  3. RODALCO

    RODALCO Well-Known Member

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    Doesn't makes much sense to me, normally it is + or - 2 or 3 digits.

    Whatever the reading is will have a ± 10 digits error in it, on top of the 0.5%

    e.g. 800W range , resolution is 100 mW , is 0.1 Watt,
    ± 10 digits makes the error ± 1 Watt.
     
    Last edited: Jan 31, 2010
  4. dave

    Dave New Member

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  5. alphacat

    alphacat New Member

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    Thank you very much RODALCO.

    Please let me see if I got it straight.

    for 800W @ 100mW resolution:
    ±0.5% = ±4W (= ±0.5% * 800W)
    ±10 digits = ± 1W (= ±10 * 100mW)

    Am I correct so far?

    If so, does it mean that the max error at 800W range is ±5W?
     
  6. RODALCO

    RODALCO Well-Known Member

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    Correct !

    Bear in mind that it is the max. error.

    Overall error will be less in most cases.
     
  7. canadaelk

    canadaelk Active Member

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    What are you meassuring? AC, DC, LF or HF. What accuracy do you need?

    If it is for simple 110VAC power consumption try the P4400, available through Amazon for 20 bucks.

    Cheers, E
     
  8. alphacat

    alphacat New Member

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    RODALCO
    Got it.
    Thank you very much.

    canadaelk
    The P4400 could be excellent, but since i'm from Europe, the mains voltage here is 230Vac.
    Are you familiar with a similar power meter that works at 230Vac?
     
    Last edited: Jan 31, 2010
  9. canadaelk

    canadaelk Active Member

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    R:

    My original info was from GB. it was a 230VAC/60Hz instrument. You should be able to find it. Maybe conrad.de

    If you can not get there let me know and I will try and retrace my search.

    E
     
  10. The Electrician

    The Electrician Active Member

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    In another thread you explained that the reason you want to measure power accurately is:

    "I have a Power Sensor IC which its theoretical accuracy (according to datasheet) is 0.1% (typical).
    I want to test its accuracy and to calibrate it if needed, and for that i need a very accurate power meter."

    One of the things you seem most interested in is the measurement of standby power of appliances. The P4400 is not going to give you the claimed .2% accuracy at low powers. It doesn't have enough resolution at low power levels.

    Let me give you a method to check the calibration of your Power Sensor IC without using an accurate wattmeter. If you start with a load which is a pure resistance, the power factor will be 100%, and you can measure the power consumed by the load by simply measuring the voltage and current separately. Separate DVMs (they must be true RMS instruments) can be used to make these two measurements, and even moderate cost DVMs should be able to make those measurements with .1% accuracy. Especially, you can do this at low power levels, similar to the standby power of an appliance.

    Be aware that the grid voltage is not constant to a .1% level. Connect a DVM to the grid and measure the voltage. Watch it for a minute or so, and you will see that it varies every few seconds. That's why it might be a good idea to have two meters to measure voltage and current, and be prepared to write down the readings simultaneously. Some DVMs have a "hold" function which might be helpful.

    So, get a resistor that will consume about 1 to 10 watts, connect it to the grid (I assume you are being safety conscious in all this), and measure the applied voltage and the current taken. Multiply the two numbers, and you have the power. Compare with what your Power Sensor IC says, and you have a calibration check.

    And, of course, you can use different resistors for higher power levels. It's important that the resistive loads used are pure resistance. Heaters would not be pure resistance if they have a fan in them. You can use incandescent light bulbs for moderate power levels.

    Now the only remaining thing to do is to verify that when the power factor is less than 100%, your Power Sensor IC still measures the true power correctly.

    Measure a resistor accurately, with a value suitable for whatever power level you would like to calibrate, making sure that it's a good quality resistor whose value doesn't change when it gets hot. For example, suppose you want to check the accuracy of your Power Sensor IC at a 2 watt level; use a resistor of about 25k ohms. Apply the grid voltage (230 VAC where you are) to the resistor alone and measure the voltage (E) and current (I) carefully (with true RMS meters). The power consumed will be E*I watts. If you have measured the resistance, R, accurately, the power consumed should also be given by E²/R.

    Now connect a .1µF film capacitor, C, of suitable voltage rating in parallel with the resistor. Measure the applied voltage and use the formula E²/R to calculate the power consumed. The consumed power is not changed by the capacitor, only the power factor. You can calculate the power factor knowing the value of R and C. Using different values of capacitor you can create various power factors. The capacitors must be good quality with low dissipation factor. For higher values, motor run capacitors would be good.

    So, your Power Sensor IC can be checked to be sure that it accurately measures the real power consumed at various power factors less than 100%, and by using different values of resistive load, you can check the accuracy at different power levels.
     

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