Products > Test Equipment
Low R measurement on a DMM
pope:
Forgive me if this not the correct section for the following question but I'm not sure where else I should post it. Mods, please feel free to move it if needed.
Considering a DMM, which reads a value of 0.1Ω when the probes are sorted. If we do a continuity test on a device and we measure 0.3Ω, can we assume that the actual resistance between the two points is 0.2Ω? i.e 0.3Ω minus the resistance of the probes (0.1Ω)?
And if so, what's the benefit of a 4W multimeter?
grumpydoc:
1) yes
2) to avoid the problem encountered in 1)
Kleinstein:
It is OK to subtract the reading one gets from just shortening the probes. That is about the best one can do with 2 wire ohm mode, though the resitance to subtract may not be that constant - it can vary with the quality of the contact, though one may not see this with 0.1 ohm resolution.
The 4 wire ohm mode avoids the contact and cable resistance problem, with no need to the contact resistance to be stable.
Fungus:
--- Quote from: pope on March 27, 2023, 12:08:11 pm ---Considering a DMM, which reads a value of 0.1Ω when the probes are sorted. If we do a continuity test on a device and we measure 0.3Ω, can we assume that the actual resistance between the two points is 0.2Ω? i.e 0.3Ω minus the resistance of the probes (0.1Ω)?
--- End quote ---
No, because there will be two extra resistances between the points of the probes and the circuit.
--- Quote from: pope on March 27, 2023, 12:08:11 pm ---And if so, what's the benefit of a 4W multimeter?
--- End quote ---
It takes the resistance of the probes completely out of the equation.
nb. You can also do it with two multimeters and a current source, you don't need a special 4W meter.
grumpydoc:
--- Quote from: Fungus on March 27, 2023, 12:35:38 pm ---No, because there will be two extra resistances between the points of the probes and the circuit.
--- End quote ---
Well, one extra point of contact anyway.
You're right of course, there's an extra unknown (or unknowns) but in practice who would not just subtract the 0.1Ω either mentally or using the meter's "rel" function (if it has one, though all but very low end DMMs do).
Navigation
[0] Message Index
[#] Next page
Go to full version