Operation
3-18
3.4
Measurement considerations
Low-level measurements made using the Model 1801 can
be adversely affected by various types of noise or other
unwanted signals that can make it very difficult to obtain
accurate readings. Some of the phenomena that can cause
unwanted noise include thermoelectric effects (thermo-
couple action), source resistance noise, magnetic fields,
and radio frequency interference. The following para-
graphs discuss the most important of these effects and
ways to minimize them.
3.4.1 Thermoelectric potentials
Thermoelectric potentials (thermal EMFs) are small elec-
tric potentials generated by differences in temperature at
the junction of dissimilar metals. The following para-
graphs discuss how such thermals are generated and ways
to minimize their effects.
Table 3-9
Differential temperature bus commands
Command
Description
[:SENSe[1]]
:TEMPerature
:DTCouple
:TYPE <type>
:TYPE?
:USLope <value>
:USLope?
:RTEMperature <value>
:RTEMperature?
:TRANsducer DTC
:TRANsducer?
Sense subsystem.
Temperature path.
Differential path.
Select thermocouple type (type = J|K|T|E|R|S|B| USER).
Return thermocouple type (J|K|T|R|R|S|B| USER).
Select thermocouple slope (value = slope in V/°C, 0 is invalid).
Return thermocouple slope (V/°C min = -0.099999; max =
0.099999; default = 20E-6).
Select reference thermocouple temperature (value = reference tem-
perature in degrees)
Return thermocouple reference temperature in degrees; min=-273°C;
max=2000°C; default=0°C).
Select differential thermocouple transducer.
Return transducer type (DTC).
Notes:
1. Angle brackets (<>) are used to indicate parameter type. Do not include brackets in programming message.
2. Upper-case letters indicate command short form.
3. Brackets ([]) indicate command is optional.
Thermoelectric coefficients
As shown in Table 3-10, the magnitude of thermal EMFs
generated depends on the particular materials involved.
Best results are obtained with clean copper-to-copper
connections as indicated in the table.
Table 3-10
Thermoelectric coefficients
Material
Thermoelectric potential
Copper-Copper
Copper-Silver
Copper-Gold
Copper-Lead/tin
Copper-Kovar
Copper-Silicon
Copper-Copper Oxide
≤
0.2µV/°C
0.3µV/°C
0.3µV/°C
1-3µV/°C
40µV/°C
400µV/°C
1000µV/°C