3-28 Basic DMM Operation Model 2750 Multimeter/Switch System User’s Manual
4-wire RTDs
For 4-wire RTDs, the temperature measurement range is -200°C to 630°C (0.01°C resolu-
tion). RTD types that are supported include D100, F100, PT385, and PT3916. A USER
type is available to modify RTD parameters, such as the resistance at 0°C. The USER type
can be enabled from the front panel, but the settings can only be changed using remote
programming.
The RTD has a metal construction (typically platinum). The resistance of the RTD
changes with change in temperature. The Model 2750 measures the resistance and calcu-
lates the temperature reading. When using default RTD parameters, the resistance of the
RTD will be 100Ω at 0°C.
Of all the temperature transducers, the RTD exhibits the most stability and linearity. The
Model 2750 performs the 4-wire measurement using offset-compensated ohms. This pro-
vides the most accurate way to measure the low resistance of the RTD.
NOTE The equation to calculate temperature vs. resistance readings listed is in the
RTD reference tables provided in Appendix F.
NOTE Only one USER RTD per scan list.
Connections
NOTE When using the front panel inputs, the INPUTS switch must be in the “F” (out)
position. For switching modules, it must be in the “R” (in) position.
Thermocouple connections
Connections for thermocouples are shown in Figure 3-8. Thermocouples are color coded
to identify the positive (+) and negative (-) leads (Table 3-3). Note that the negative (-)
lead for U.S. type T/Cs is red.
For front panel inputs, you need to use a simulated reference junction for thermocouple
temperature measurements. An ice bath, as shown in Figure 3-8A, serves as an excellent
cold junction since it is relatively easy to hold the temperature to 0°C. Notice that copper
wires are used to connect the thermocouple to the Model 2750 input.
NOTE The positive lead of the type T thermocouple is made of copper. Therefore, that
lead can be connected directly to the input of the Model 7700. It does not have to
be maintained at the simulated reference temperature (i.e., immersed in ice
bath).
For the Model 7700 switching module, you can also use a simulated reference junction as
shown in Figure 3-8B, or you can connect the thermocouple wires directly to the screw
terminals (internal reference junction) as shown in Figure 3-8C. Using a simulated refer-
2750-900-01.book Page 28 Wednesday, August 3, 2011 7:56 AM
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