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| Easy way to test the calibration of a DMM (Fluke 45)? |
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| CalibrationGuy:
You quote me but don't understand my words. I said unknown sources. Do you know what factory seconds are? How about counterfeit goods that pervade the sources you mentioned. Where in anything I said did you get the idea that I meant you cannot get an idea of approximate voltage using parts from a reputable manufacturer? In fact, I went out of my way to mention a device that is reasonably priced yet has remarkable performance. I mentioned my equipment and cost of calibration to emphasize that the result I posted was in fact true, NOT that the OP should waste his money for no reason. Go take your distortions elsewhere. TomG. |
| shapirus:
--- Quote from: shabaz on April 02, 2024, 10:48:21 pm ---"3) You cannot verify with any certainty, 20mV which meter is off using a reference from unknown sources without proof of calibration." That's demonstrably untrue. Modern silicon comes factory-trimmed to a greater precision. You could pick up a $5 voltage reference (half the cost of the AliExpress one!) and with little care (a couple of capacitors) end up with a result accurate to within single-digit millivolts. So yes, it is certainly possible to identify which of two multimeters (or both, or any number of them) are faulty, if their discrepancy is 20 mV. --- End quote --- If, and only if, the reference has been measured with a trusted voltmeter! Otherwise, you have three devices, DMM 1, DMM 2, and voltage reference, showing the same or different readings, and you have no way of telling which reading is true, because none of the devices have been verified against a known-good reference. That's exactly the problem with the Chinese references. They aren't that bad in terms of design and parts. But how do we know whether their output voltages match what's written on the sticker, and if it does, then with what exactly degree of uncertainty? Oh and regarding the modern silicon... What did you have in mind? If we take, for example, LM399, then its initial tolerance is specified at 2%. That's 0.139V at its nominal 6.95V output. Definitely not good for checking DMMs out of the box without measuring and adjusting. What's the highest precision single-chip reference? Is there anything that has at least a 0.05% tolerance? |
| J-R:
Recommending that someone buy the cheap/unknown references to check their equipment is an indefensible position. You will go down with that ship... It's been well established here on the forums that those cannot be trusted and that is essentially their only job. Someone who is new to this might try to calibrate their TE or apply attributes to the reference that it cannot support. So it's more dangerous to have one than not. Fine, if you're bored and want to play around with them go ahead, but don't suggest anything further. |
| shabaz:
Oh, so a $5 reference chip is totally fine? Now you agree! There's every chance that there could be a fault with a reference from a reputable manufacturer too. If it was drifty, or in fact had an error greater than 20 mV, you'd easily be able to detect it. You literally created an argument that wasn't there. You dreamt it up. No-one stated that a $10 board should be relied on (or a $5 one for that matter) for trusting your multimeter is performing to spec. That would be an idiotic thing to do. It was simply to determine if a multimeter had a significant discrepancy or not, so that you'd know which one to send for calibration. |
| CalibrationGuy:
--- Quote from: shapirus on April 02, 2024, 11:15:21 pm --- --- Quote from: shabaz on April 02, 2024, 10:48:21 pm ---"3) You cannot verify with any certainty, 20mV which meter is off using a reference from unknown sources without proof of calibration." That's demonstrably untrue. Modern silicon comes factory-trimmed to a greater precision. You could pick up a $5 voltage reference (half the cost of the AliExpress one!) and with little care (a couple of capacitors) end up with a result accurate to within single-digit millivolts. So yes, it is certainly possible to identify which of two multimeters (or both, or any number of them) are faulty, if their discrepancy is 20 mV. --- End quote --- If, and only if, the reference has been measured with a trusted voltmeter! Otherwise, you have three devices, DMM 1, DMM 2, and voltage reference, showing the same or different readings, and you have no way of telling which reading is true, because none of the devices have been verified against a known-good reference. That's exactly the problem with the Chinese references. They aren't that bad in terms of design and parts. But how do we know whether their output voltages match what's written on the sticker, and if it does, then with what exactly degree of uncertainty? --- End quote --- That's exactly my point. There were numerous discussions about certain suppliers who were writing random numbers on the included "calibration" sheets. You need to start with a known good reference. It doesn't have to be as accurate as ours, that wasn't the point, the point was that it has to be of known values which meets your needs, tolerance wise. TomG. |
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