Sources of Error in Resistance Measurements
External Voltages
Any voltages present in the system cabling or connections will affect a resistance measurement. The
effects of some of these voltages can be overcome by using offset compensation (as described on the
above section).
Settling Time Effects
The internal DMM has the ability to insert automatic measurement settling delays. These delays are
adequate for resistance measurements with less than 200 pF of combined cable and device capacitance.
This is particularly important if you are measuring resistances above 100 kΩ. Settling due to RC time con-
stant effects can be quite long. Some precision resistors and multifunction calibrators use large parallel
capacitances (1000 pF to 0.1 µF) with high resistance values to filter out noise currents injected by their
internal circuitry. Non-ideal capacitances due to dielectric absorption (soak) effects in cables and other
devices may have much longer settling times than expected just by RC time constants. Errors will be meas-
ured when settling after the initial connection, after a range change, or when using offset compensation.
You may need to increase the channel delay time before a measurement in these situations.
High-Resistance Measurement Errors
When you are measuring large resistances, significant errors can occur due to insulation resistance and sur-
face cleanliness. You should take the necessary precautions to maintain a “clean” high-resistance system.
Test leads and fixtures are susceptible to leakage due to moisture absorption in insulating materials and
“dirty” surface films. Nylon and PVC are relatively poor insulators (10
9
ohms) when compared to PTFE insu-
lators (10
13
ohms). Leakage from nylon or PVC insulators can easily contribute a 0.1% error when meas-
uring a 1 MΩ resistance in humid conditions. The table below shows several common insulating materials
and their typical resistances.
Insulating Material Resistance Range Moisture Absorbing
PTFE 1 TΩ to 1 PΩ N
Nylon 1 GΩ to 10 TΩ Y
PVC 10 GΩ to 10 TΩ Y
Polystyrene 100 GΩ to 1 PΩ N
Ceramic 1 GΩ to 1 PΩ N
Glass Epoxy (FR-4, G-10) 1 GΩ to 10 TΩ Y
Phenolic, Paper 10 MΩ to 10 GΩ Y
N = No; Y = Yes
Strain Gage Measurements
You can measure a strain gage using a 4-wire resistance measurement with scaling.
When a force is applied to a body, the body deforms. The deformation per unit length is called strain (ε).
Strain may be either tensile (+) or compressive (-). Practical strain values are usually quite small (typically less
than 0.005 inch/inch for most metals) and are often expressed in micro-strains (µε). There are three
4Measurement Tutorials
206 Keysight DAQ970A User's Guide
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