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| Which DMMs can reasonably measure RMS? |
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| bdunham7:
--- Quote from: W6EL on April 27, 2023, 04:33:05 am ---In order to sort of establish how well things were accurate, relative to each other, I ran a 60 Hz sine wave first. That ought to be something any reasonable meter can measure, right? And indeed, all instruments were under 2% of error, which is pretty good for my bunch. I'm using the HP scope as my reference since it actually arrived calibrated (it's since lapsed, but not that long ago). --- End quote --- A couple of things that jumped out at me and I'm curious about them. First, does the HP 54645D actually give you an explicit VRMS reading or are you calculating it somehow? If it gives you a reading, what is its specified accuracy? Scopes are typically not competitive with good DMMs in basic accuracy and I think the basic DC-gain tolerance is 1.5% or so for this model. Second, you give readings for the HP 400EL to four significant digits which seems impossible on an analog meter--that last digit corresponds roughly with the thickness of the paint on the needle! Are you just squinting extra hard or do you have some other method? I suspect your hand-tuned Radio Shack meter may be even more accurate than you think. |
| PartialDischarge:
UniT 181A gives 2.8995 for 100Hz, 2.89 for 1KHz and 2.895 for 200KHz. 4.1Vpp square wave with 2.05V offset... |
| W6EL:
--- Quote from: bdunham7 on April 27, 2023, 02:18:15 pm --- --- Quote from: W6EL on April 27, 2023, 04:33:05 am ---In order to sort of establish how well things were accurate, relative to each other, I ran a 60 Hz sine wave first. That ought to be something any reasonable meter can measure, right? And indeed, all instruments were under 2% of error, which is pretty good for my bunch. I'm using the HP scope as my reference since it actually arrived calibrated (it's since lapsed, but not that long ago). --- End quote --- A couple of things that jumped out at me and I'm curious about them. First, does the HP 54645D actually give you an explicit VRMS reading or are you calculating it somehow? If it gives you a reading, what is its specified accuracy? Scopes are typically not competitive with good DMMs in basic accuracy and I think the basic DC-gain tolerance is 1.5% or so for this model. Second, you give readings for the HP 400EL to four significant digits which seems impossible on an analog meter--that last digit corresponds roughly with the thickness of the paint on the needle! Are you just squinting extra hard or do you have some other method? I suspect your hand-tuned Radio Shack meter may be even more accurate than you think. --- End quote --- The scope has an RMS "measurement" you can bring up (among other things). I turned on 8x averaging as well. Yes, scopes are not exactly known for their vertical accuracy, but it's the best I can do. That it agrees (sine wave) with the HP-400EL does say something. The HP 400EL has more sig figs than you might expect, provided you are in the correct range. I used the 3 volt scale. Each of the most minor tic marks near the 2 volt mark is 0.020 volts, and I can discern values in between to a degree. Years of working in laboratories has refined this skill, although with most equipment being digital I don't get to use it as much as I wish. I'll grant you that maybe the 0.005 is generous though. I looked inside the radioshack meter this evening. Nothing fancy really. A single quad-pack chip seems to handle most of it. I did not see a dedicated RMS chip, although I did not look under the LCD for fear of breaking it. I do note that the six batteries are tapped such that a negative rail is available inside. The PWB reads "Tandy 1992" :) Also, the Owon B35T does indeed have an RMS chip (AD8439JCPZ, see here for the insides: https://lygte-info.dk/review/DMMOwon%20B35T%20UK.html). |
| W6EL:
--- Quote from: PartialDischarge on April 27, 2023, 02:55:52 pm ---UniT 181A gives 2.8995 for 100Hz, 2.89 for 1KHz and 2.895 for 200KHz. 4.1Vpp square wave with 2.05V offset... --- End quote --- I like the readout showing AC, DC, and the combined result. Fantastic. Tells a lot more of the story! |
| shakalnokturn:
--- Quote from: W6EL on April 27, 2023, 02:57:48 pm ---The scope has an RMS "measurement" you can bring up (among other things). I turned on 8x averaging as well. Yes, scopes are not exactly known for their vertical accuracy, but it's the best I can do. --- End quote --- Not having the vertical scale set as near as possible to ADC's full scale would add inaccuracy, I'm not sure what the optimal horizontal scaling would be for RMS measurement on a DSO although I'd suspect one full cycle and that it may vary from one scope to another. |
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