Thermocouple transmitter is resistance transducer; Thermocouple transmitter is voltage transducer. Thermocouple output of uV signal transmitter needs preamplifier signal amplified first, then the voltage current transform circuit 4 ~ 20 ma output signal. Basic principle of thermocouple output is the resistance of change, is to give the thermocouple electricity, measuring the current and voltage, calculate according to the ohm theorem is resistance. For thermistor sensitivity high, usually directly using the resistance method of partial pressure for change of voltage, the voltage current transform again 4 ~ 20 ma output circuit. For the low sensitivity thermocouple ( Such as PT100, Cu50, etc. ) , often using bridge principle, extracted and amplified variation, then the voltage current transform circuit 4 ~ 20 ma output signal. Note: the structure of thermocouple thermocouple is reliable, steady work, to ensure that the requirements for its structure is as follows: 1, the composition of the thermocouple two hot electrode welding must be strong; 2, two hot electrode should be well insulated between each other, in case of short circuit; 3, compensation conductor should be connected to the free cod of a thermocouple to be convenient and reliable; 4, protect casing thermal electrodes should be able to make sufficient isolation and harmful medium.