Why Are Some NI Switches Not Recommended for 2-Wire Resistance Measurements?

Issue Details

I read in the specification sheets of some NI Switch cards , that they are not recommended for 2-wire resistance measurements. What is the reason for that?

 

Solution

The NI Switch cards, which are not recommended for 2-wire resistance measurements, are all using electro-mechanical relays.

Electro-mechanical relays are not hermetically sealed with a noble gas, and therefore require a minimum switching current to ensure the mechanical contact surfaces remain clean.
For example, the NI 2575 is specified with a minimum switch load of 20 mV / 1 mA.

NI DMMs use a test current between 1 µA and 1 mA, depending on the range setting.
This current is too small to guarantee to clean the contact surface. This leads to a increased resistance of the relays.

In combination with the fact, that 2-wire-resistance measurements include the path resistance, NI does not recommand switch card with electro-mechanical relays for 2-wire-resistance measurements.
Instead you should use 4-wire-resistance measurements, or use a switch card with a different type of relays, which do not need a minimum switch load.