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Thread: Brand Power Meter Accuracy

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    Default Brand Power Meter Accuracy

    So recently we had some issues with our Extech power meter in the lab. A couple of units failed during testing while connected to it, including one that gave a dead short, and the unit hasn't worked right since, even after RMA. So I proposed we use a Brand 20-1850 instead, at least until we could get a higher end meter.

    We picked up the Brand meter and I was initially quite pleased. However, when I tested the first unit with it, I found that I was getting efficiency results about 5% above expected values! The unit was reading 5% low.

    Well I looked up the calibration guide, then constructed a precision resistive load using 1% low temp co 50W 500 ohm resistors. Using that and my Fluke meter to measure the AC voltage, I could get accuracy to 1% in the 50-120W range. However, outside of that range, again, the reading is off by increasing amounts as I go up in wattage. Overall, we were 1-3% out compared to the Extech and the results from our EU and Taipei labs.



    I called up Brand Electronics and spoke to Ethan, who seems to be the engineer and sole proprietor. Apparently the Brand meters are only 2-3% accurate in practice. He offers calibration service to get them to 1% accurate at a specified load, and then provide a cal curve for higher and lower loads, but the unit itself cannot be calibrated tighter than that.

    However, he does offer a higher-precision version, which has a more robust analog frontend and uses a 24-bit ADC, instead of the uC's native ADC, which he can cal to 0.5%, or can be caled by a NIST lab to 0.1%, with low drift. This is a "semi-custom" part according to Ethan, which is why it isn't listed on the website, but he offers it for $350 on request.




    I thought I'd just share this info for anyone reviewing units using a Brand power meter. Keep in mind that these have a distinct error curve outside of their calibrated wattage. The "semi-custom" version may be a better solution.

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    Default

    This is good info, but I want to put it in the correct sub-category.

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    Default

    No problem, sorry for the wrong section!


    Based on my discussion with Ethan, the sources of inaccuracy in the 20-1850 are these:
    - Accuracy and resolution of the main uC ADC (16-bits I believe)
    - Anti-aliasing to compensate for low resolution
    - Current noise below ~200mA in sensing circuit
    - Component accuracy rating, temperature co-efficient, and drift
    - Power supply noise (he's using a regular 5V LDO and bypass caps)

    The "semi-custom" version addresses the first three issues and improves the fourth, thereby achieving much greater accuracy.

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