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| DMM vs LCR: a confused beginner on resistance measuring |
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| Moriambar:
Hi folks. I own both the eevblog brymen and the DER EE de-5000 LCR meter, and albeit using the latter only for capacity and esr, I wondered how it could perform on resistance measuring. Since I'm still a total beginner, I will write down what I think and what it does not sound right in my head, hopefully someone will be albe to make me understand the thing. First of all I refer to these manuals about the specs: Brymen, page 25 DER EE page 39-40 First of all I understand that there are two main differences in the measurement: * The DER EE uses 4 wire connection * when using normal R mode, the LCR calculates phase angle and a bunch of stuff to determine what the DUT is and then calculates the resistance. Anyhow, let's focus on measuring a 1kOhm resistor. Let's also assume that its value at current temperature and humidity is 1.0000000000000000000000000 kOhms. The range specs of the Brymen indicate an accuracy of 0.3%+2d, so what I expect is that the display has 3 decimal places (and that it will read between 0.995 and 1.005 k. Indeed the brymen shows three decimal places. Then I go to the LCR meter. First of all I notice it has two different accuracies: one for AC resistance and one for DC resistance (DCR). I'm quite puzzled here because I do not understand the difference (perhaps it is related on how the R is measured?). But I will carry on. At 1kHz the AC resistance accuracy in the 2k range is the same 0.3%+2d (the DCR is 0.1% better though). I start wondering about where is the increased precision of a 4 wire connection. Regardless I expect the same as before. Nevertheless the LCR meter shows an extra meaningless digit, and I do not understand why! I mean if the display is 0.9979 or 0.9970 is basically the same since the last digit is not at all accurate ie meaningless. Why is that so? Am I missing something? I know: I should buy proper equipment to do precise measurements, but I can't afford them (and it's not as easy to score on ebay here in Italy). Anyway thanks for reading! Cheers |
| PTR_1275:
It’s a slippery slope you’re getting into (expensive equipment ahead) The thing to keep in mind is there is a difference between resolution and accuracy. An extra digit is useless for accuracy as you mentioned, but it can help you see slight changes that would take longer to see on a meter with one digit less. As for 4 wire resistance measurements, I forget the point that the benefit becomes irrelevant, but it is used mostly on lower resistances to remove the test lead resistance. If you have test leads with a resistance of 0.5 ohm when measuring a 1M resistor, the lead resistance is insignificant. |
| Moriambar:
--- Quote from: PTR_1275 on December 10, 2018, 10:40:24 am ---It’s a slippery slope you’re getting into (expensive equipment ahead) The thing to keep in mind is there is a difference between resolution and accuracy. An extra digit is useless for accuracy as you mentioned, but it can help you see slight changes that would take longer to see on a meter with one digit less. As for 4 wire resistance measurements, I forget the point that the benefit becomes irrelevant, but it is used mostly on lower resistances to remove the test lead resistance. If you have test leads with a resistance of 0.5 ohm when measuring a 1M resistor, the lead resistance is insignificant. --- End quote --- Great. Believe me: I do not have the money to go on the slippery slope, which I think for once is a good thing. Anyway thanks. Perfectly clear. So 4wire is good for low resistances, otherwise the inaccuracies will be greater than the change brought by the measurement method. Good Cheers |
| mvs:
--- Quote from: Moriambar on December 10, 2018, 10:10:33 am ---Nevertheless the LCR meter shows an extra meaningless digit, and I do not understand why! I mean if the display is 0.9979 or 0.9970 is basically the same since the last digit is not at all accurate ie meaningless. Why is that so? Am I missing something? --- End quote --- Resolution and absolute accuracy are different things, so its fully ok that your LCR displays a digit more. Higher resolution is helpful for relative measurements, where only short term stability and linearity matters. |
| macboy:
A DMM resistance measurement puts a fixed DC current through the device and measures the voltage, giving a resulting resistance reading (from Ohms law, R=V/I). An LCR goes further. It can make a DC measurements, but it also can make an AC measurement, putting an AC signal of a certain frequency through the device, and measuring both the amplitude and phase of the resulting current. This can be used to determine the equivalent series inductance of the resistor, and/or its equivalent parallel capacitance. These are "parasitics" which result from the physical construction of the device. Since most resistors are made of a coil of wire (or a metal film on a cylindrical form, cut into a spiral), they have some inductance. The LCR bridge can tell you about it, but the DMM is ignorant of it. |
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