Model DMM7510 7½ Digit Graphical Sampling Multimeter Reference Manual
Section 4: Measure considerations
DMM7510-901-01 Rev. B / May 2015
4-9
•
Keep the two materials forming the junction at the same temperature.
•
Keep the two junctions close together.
•
Allow test equipment to warm up and reach thermal equilibrium in a constant ambient
temperature.
•
Keep all junctions away from air currents; in some cases, it may be necessary to thermally
insulate sensitive junctions to minimize temperature variations.
•
When making a copper-to-copper connection, apply sufficient pressure to ensure the connection
is gas tight to prevent future oxidation.
•
In some cases, you may need to connect the two thermal junctions together with good thermal
contact to a common heat sink. Unfortunately, most good electrical insulators are poor heat
conductors. In cases where low thermal conductivity may be a problem, you can use special
insulators that combine high electrical insulating properties with high thermal conductivity. Some
examples of these materials include hard anodized aluminum, sapphire, and diamond.
Using relative offset to minimize thermoelectric EMFs
Some systems may still have residual thermoelectric offsets after following the guidelines in
Minimizing thermoelectric EMFs
(on page 4-8). If the offsets are relatively constant, you can use the
relative offset feature in the Model DMM7510 to cancel them. Refer to
(on page 3-4)
for information.
Magnetic fields
When a conductor loop cuts through magnetic lines of force, a very small current is generated. This
phenomenon can cause unwanted signals to occur in the test leads of a test system. If the conductor
has sufficient length or cross-sectional area, even weak magnetic fields can create signals that affect
low-level measurements.
To reduce these effects:
•
Reduce the lengths of the connecting cables.
•
Minimize the exposed circuit area.
•
Change the orientation of the leads or cables.
•
Minimize cable loop area or introduce cable twisting
•
In extreme cases, you may require magnetic shielding. Special metal with high permeability at low
flux densities (such as mu metal) is effective at reducing these effects.
Even when the conductor is stationary, you may have problems with magnetically-induced signals.
Fields can be produced by sources such as the AC power line voltage. Large inductors, such as
power transformers, can generate substantial magnetic fields. Keep the Model DMM7510 voltage
source and connecting cables away from these potential noise sources.
