# Changes in Multimeter reading on applying increasing contact probe pressure.

#### mojizs

Joined Aug 23, 2023
15
I have read different ways or reasons that could affect the measurement of multimeter readings. I am measuring resistance across a circuit at two points using a kelvin probe (Keithly 5808). I get different readings every time I take measurements of the same circuit in intervals of time. I tried to eliminate other possibilities, and I am left with either of the two.
1. The probes itself are not the right one.
2. Applying varying pressure when making contact can change the value.
I want to know if there is a way to determine if the varying pressure can change the value and by how much. A relation to describe it or any references that supports it? Or is there anything else that I am missing?
Thank You

#### MrChips

Joined Oct 2, 2009
29,850
Welcome to AAC!

The pressure applied when making contact is not likely to affect a measurement unless there is a flaw on the board itself and it is sensitive to board flexing.

The make and model of the meter itself is just as important as the probe. You need to state the meter model, the impedance of the meter at the selected range setting, and the type of circuit under test and its impedance where the probe is applied. In other words, we need to know more about the two points where the measurements are made.

#### WBahn

Joined Mar 31, 2012
29,512
The measurement with probes like that include the resistance of the probe tips AND the contact resistance with the circuit. So, yes, if the contact point with the circuit is not good and clean such that varying the pressure will vary the contact resistance, that will upset the measurement.

#### crutschow

Joined Mar 14, 2008
33,358
I am measuring resistance across a circuit at two points using a kelvin probe (Keithly 5808).
What is the meter model?

#### WBahn

Joined Mar 31, 2012
29,512
What is the expected value of the resistance you are trying to measure?

Is the resistance you are trying to measure isolated from the rest of the circuit?

Is power removed from the circuit?

#### Ya’akov

Joined Jan 27, 2019
8,539
Welcome to AAC.

What is the total resistance, and what is the magnitude of the variation?

#### mojizs

Joined Aug 23, 2023
15
The make and model of the meter itself is just as important as the probe. You need to state the meter model, the impedance of the meter at the selected range setting, and the type of circuit under test and its impedance where the probe is applied. In other words, we need to know more about the two points where the measurements are made.
Thank You
Here are the details:
Meter: HP 34401A Multimeter
Input Impedance: 1 MΩ ± 2% in parallel with 100 pF

Circuit: Not powered/active. It's a PCB with the traces having a gold pin attached to each end of it. The measurement is taken across those two pins for a single trace on the board.
I have no knowledge of the probe impedance at contact. The probe used is of copper gold plated material.

#### mojizs

Joined Aug 23, 2023
15

#### mojizs

Joined Aug 23, 2023
15
What is the expected value of the resistance you are trying to measure?

Is the resistance you are trying to measure isolated from the rest of the circuit?

Is power removed from the circuit?
I have no expected value of resistance, but we are trying to collect some data from the circuit.
The circuit is a PCB with traces that have a gold pin connected on each end. It is not powered/active. The resistance is measured for one trace only.

#### mojizs

Joined Aug 23, 2023
15
Welcome to AAC.

What is the total resistance, and what is the magnitude of the variation?
Variation is in between: 0.01 -0.05 ohms. We are not sure of the expected total resistance.

#### ericgibbs

Joined Jan 29, 2010
18,234
hi m,
Are these the probes you are using.?
E

#### Attachments

• 178.5 KB Views: 27

#### mojizs

Joined Aug 23, 2023
15
hi m,
Are these the probes you are using.?
E
Hi,
Yes that is correct.

#### crutschow

Joined Mar 14, 2008
33,358
Variation is in between: 0.01 -0.05 ohms. We are not sure of the expected total resistance.
What is the value of the total resistance you measure?

#### mojizs

Joined Aug 23, 2023
15
What is the value of the total resistance you measure?
Measured total resistance: 0.1148 -0.0972 ohms

#### Ya’akov

Joined Jan 27, 2019
8,539
Variation is in between: 0.01 -0.05 ohms. We are not sure of the expected total resistance.
.05Ω is very small.

What metal is the conductor you are trying to measure? Does it have solder mask or other coatings? Did you clean it with something like 99% IPA?

#### crutschow

Joined Mar 14, 2008
33,358
Variation is in between: 0.01 -0.05 ohms. We are not sure of the expected total resistance.
Measured total resistance: 0.1148 -0.0972 ohms
That's a pretty small difference which can be just from where you placed the probe on the specimun.
What, exactly, is the thing you are measuri9ng?

#### DickCappels

Joined Aug 21, 2008
10,104
clean your probes and the contact surface on the resistance you are measuring. You might even have to sharpen the probes to obtain somewhat consistent readings, and as crutschow suggests, make sure you have the probes in the same place (and with similar pressure) for each measurement.

Measurements like what you are attempting are difficult to make repeatability. One technique that helped with a similarly impossible measurement problem is to take many measurements and then average them. This would then be useful in comparing with the average of several future readings.

#### WBahn

Joined Mar 31, 2012
29,512
For measuring resistances that small, you probably should be using a different probe, such as the Keithley 5805, which makes a true four-point measurement and is specifically designed for measuring circuit board traces.

#### mojizs

Joined Aug 23, 2023
15
.05Ω is very small.

What metal is the conductor you are trying to measure? Does it have solder mask or other coatings? Did you clean it with something like 99% IPA?
The metal is a copper trace, gold plated and has a solder mask (0.0004 inches). I did clean with IPA and redid the test. I believe like @DickCappels said that repeatability for such small measurements will differ inevitably.