Q: I am looking for a differential temperature transmitter. We have used the SPT in the past and know that it will work, but I don’t know if you can use two inputs of two different 4-wire RTDs with the SPT.
A: The SPT has only four input terminals; therefore the number of RTD wires is limited. For differential temperature, one RTD can be a 3-wire and the other RTD can only be a 2-wire. The lead length compensation wire of the 3-wire connection compensates for the lead length of both RTDs. This is only useful if you make sure that all five wires have identical resistance.
To answer your question, you may use two 4-wire RTDs, but you cannot connect all of the wires unless you do it as described above. Wiring details are shown in Figure 17 of the installation manual.
A better option is to use our DIN rail mounted THZ3 HART temperature transmitter, a new model which can accept two three-wire RTDs and provide a differential measurement output. The configuration of the THZ3 is done by utilizing DTMs with PACTware on your PC or by using a HART compatible handheld device or HART host.
Details about the input connections of the THZ3 are in Figure 2.2 of the installation manual.
Model Example: THZ3/PRG/4-20MA/12-42DC [DIN]
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