Electronic – Voltmeter resolution increase by having several in series

resolutionvoltagevoltage measurementvoltmeter

The problem

I am trying to measure quite precisely a voltage around 7.2V on a line of a PCB with a voltmeter. In this voltage range, my voltmeter only displays 3 digits. So the reading is something like 7.26V. However, I would like to have 4 digits of resolution.

Idea to solve the problem

To increase the resolution, I connected two voltmeters in series to have the voltage they each measure drop at half the range and benefit from the 4 digit resolutions. When doing so, the first voltmeter is stabilizing at 3.626V. The second one starts its reading, stabilize shortly at 3.626V but soon after restart its reading and repeat this sequence indefinitely.

The question

Is it possible to consider the total voltage to be twice 3.626V, so 7.252V? Is there a risk that the voltmeters are interfering each other in their reading, and I cannot use this result?

Schematic

How I connected the voltmeter on my circuit.

simulate this circuit – Schematic created using CircuitLab

Figure 1. Two digital meters with nominal 10 M input impedance but second one is 10% below nominal.

Since your meters are in series the same current flows through both. The impedances of the meters will differ slightly (although yours are very close) and the voltage read by each will be proportional to their resistances. In my illustration I have assumed that the meters are the 199.9 type which have a full-scale reading of 199.9 mV.

Is it possible to consider the total voltage to be twice 3.626V, so 7.252V? Is there a risk that the voltmeters are interfering each other in their reading, and I cannot use this result?

You would need to understand the inner workings of the meters a bit more. They shouldn't interfere but if the second meter is switching its auto-ranging resistors in and out then it might. If you can turn off auto-range it might help.

Be aware of your meters' capabilities. Look up their specifications to work out the % error (of full scale - not your reading) and how many digits the reading can be out by.