I have an old Fluke 79-III and I'm trying to check the resistance of some resistors which should be 0.22 Ohms. Unfortunately, they are all reading 0.6 Ohms and so I'm wondering how accurate I should be expecting the main resistance function of my meter to be.
| Function | Range | Resolution | Accuracy | Burden Voltage (Typical) | | Ohm | 400.0 Ohm | 0.1 Ohm | ±(0.4%+2) | Not applicable | | | 4.000 kOhm | 0.001 kOhm | ±(0.4%+1) | | | 40 Ohm | 40 Ohm* | 0.01 Ohm | 5% Typical*** | Not applicable | | * In 40 Ohm and 40mV ranges, thermals may introduce additional errors. To | | maximize accuracy, keep both probe tips at similar temperatures. | | *** Accuracy applies after lead resistance compensation. |
I believe that I understand the resolution to mean that while I shouldn't expect 0.22 ohm, I should at least expect the meter to read 0.2 or 0.3 ohms.
I don't understand the accuracy rating though. Does the +2 mean that I should expect it to read high by up to two in the least significant digit? Is the 0.4% accuracy 0.4% of the 400 ohm range (i.e. 1.6 ohms), or 0.4% of the current reading?
Looking closer, the spec from the Fluke 79-II user manual confirms that:
Accuracy specifications are given as: ± ([% of reading] + [number of least significant digits])
So B Pete's answer of ±0.2 ohms looks good.
Edit 2) Measuring the resistance of the leads by shorting them gives 0.3 ohms, so the 0.6 ohm I originally measured is well within the accuracy envelope of 0.22±0.2 ohm + 0.3±0.2 ohm.
Edit 3) Also, I did originally try the meters Lead resistance compensation (40 ohm) mode (instruction sheet p6) and measured 0.15 ohm, so I discounted it as also being inaccurate.
With only 5% accuracy, this should be correct to ±0.011 ohms.
Checking again now however, I see that when I do a Lead resistance compensation, the longer I keep the
range button pressed it, the more stable it becomes, converging on a reading of 0.05 ohms. Measuring the resistor, it now shows 0.17, sum these and (ta-da) we get 0.22 ohms.
Not bad for a 10+ year old meter that hasn't been calibrated in at least 6 years. *8')
Ultimately, should I expect my multimeter to get close to being able to measure 0.22 Ohm, or should I not expect much accuracy below a few ohms?