Operator`s manual

Figure 4-6. Extended Ohms range.
4.3.7 Effects of Thermo-Voltaic Offset
Resistance measurements are sensitive to Thermo-Voltaic (Thermal EMF) errors, which can
leads, relay contacts and other elements in the measurement path. These errors affect all mea
be caused by poor test
surement methods,
l
ected to any load, wait for a short time (about 2 minutes), than
easure the voltage across the shorted relay contacts. Make sure to short the DMM leads and set ‘relative’ to clear
count all relay contacts, which are in series
als in 4-Wire mode). To calculate the
V EMF
Error due to
100 µV EMF
Error due to
1mV EMF
including 2-Wire, 4-Wire, 6-Wire and 3-Wire (guarded 2-Wire ohms). To quantify this error, consider a system in
which signals are routed to the DMM via a relay multiplexing system. Many vendors of switching products do not
provide Thermal EMF specification. It is common to find relays that have more than 50 µV EMF. With severa
relay contacts in the path, the error is very significant. It is possible to measure it using the SM2040 330mV DC
range. To do this, close a single relay that is not conn
m
the DMM offset prior to the measurement. To calculate worst-case error,
with the measurement (V,Ω+, V,Ω- terminals in 2-Wire, and I+, I- termin
worst-case error, multiply this count by the Thermal EMF voltage. Use Ohms law to convert this voltage to
resistance error as in the following table.
Resistance Measurement Errors due to Thermo-Voltaic offsets.
SM2042 Ohms DMM Error due to
Range Current Resolution
10 µ
33 Ω
10 mA
10 µΩ 1 mΩ 10 mΩ 100 mΩ
330 Ω
1 mA
100 µΩ 10 mΩ 100 mΩ 1 Ω
3.3 kΩ
1 mA
1 mΩ 10 mΩ 100 mΩ 1 Ω
33 kΩ
100 uA
10 mΩ 100 mΩ 1 Ω 10 Ω
330 kΩ
10 uA
100 mΩ 1 Ω 10 Ω 100 Ω
3.3 MΩ
1 uA
1 Ω 10 Ω 100 Ω 10 Ω
33 MΩ
100 nA
100 Ω 100 Ω 1 kΩ 100 Ω
330 MΩ
10 nA
10 kΩ 1 kΩ 10 kΩ 100 kΩ
31 Signametrics
4.3.8 Guarding High Value Resistance Measurements (SM2044)
Measuring high value resistors using the 2-Wire function require special attention. Due to the high impedances
involved during such measurements, noise pickup and leakage could be very significant. To improve this type of