DMM vs LCR: a confused beginner on resistance measuring

[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]

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.

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]

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?

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.

[ArthurDent]

macboy is correct. Also if you were to measure the D.C. resistance of an inductor with a DMM then you would just be measuring the resistance of the length of the coil of wire used to make that inductor but that DMM reading wouldn't tell you anything about the A.C. characteristics of the inductor. That is why some wire-wound resistors that might be used on A.C. are wound non-inductively to try to cancel out the inductance you'd get by winding the entire coil of wire in one direction. In this drawing the two bottom resistors are wound non-inductively and ones like this would probably agree much closer on both your instruments.

Try a few different resistors and see if you can find some non-inductive ones by the readings you get.

[ejeffrey]

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

Yes, for DC resistance, 4 wire measurement is only needed when the lead resistance isn't negligible compared to the DUT.  For LCR meters it is more important it is quite common for lead (or fixture) capacitance or inductance to be large compared to the DUT.

[Moriambar]

thanks for the detailed explanation folks

[tggzzz]

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?

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.

Precisely.

A company I worked for made a test set with an accuracy of 0.1dB but a resolution of 0.001dB. The customer didn't care about the 0.1dB, since they had other ways of measuring absolute values. OTOH they did care about the stability and repeatability of the 0.001dB, since they were looking for slow changes in tests that took a week.

[David Hess]

At low values of resistance, the DE-5000 is specified to be more accurate and have higher resolution when making an AC measurement.  This may reflect the fact that an AC measurement ignores thermocouple errors which become significant at lower resistances where the sensed voltage is lower for a given excitation current.

So AC measurements go down to 20 ohms with 0.001 ohm resolution while DC measurements are limited to 200 ohms with 0.01 ohm resolution.  AC measurements on the 200 ohm range are specified to be more than twice as accurate as on the DC measurements.