Automatic thermocouple reference junction compensator

Abstract

A system for automatic cold-junction referencing of thermocouples in temperature controllers wherein a differential amplifier is used to amplify the thermocouple signal prior to establishing the cold-junction compensation and measuring range. This permits the use of command signals from any floating or single-ended source, making the controller compatible with existing traditional command generators and with computer systems with digital-to-analog converters. The amplifier isolates the thermocouple from the remainder of the circuitry such that it can be grounded on the heated part.

We claim as our invention: 
     
       1. Temperature measuring apparatus comprising a measuring thermocouple adapted to be placed in contact with an object whose temperature is to be measured, thermocouple reference junction means connected in series-opposition with said measuring thermocouple, .[.a.]. temperature compensation .[.resistor.]. .Iadd.means .Iaddend.whose .[.resistivity.]. .Iadd.output signal .Iaddend.varies as a function of ambient temperature, first and second differential amplifiers each having a pair of input terminals, ungrounded conductors connecting said measuring thermocouple and said reference junction means across the input terminals of said first differential amplifier, a source of bias voltage, and means .[.connecting said source of.]. .Iadd.for applying said .Iaddend.bias voltage, the output .Iadd.signal .Iaddend.of said first differential amplifier, said .Iadd.output signal of said .Iaddend.compensation .[.resistor.]. .Iadd.means .Iaddend.and a temperature command signal to one of the input terminals of said second differential amplifier, the other input terminal of said second differential amplifier.[., said source of bias voltage and said compensation resistor.]. being connected to ground. 
     
     
       2. The measuring apparatus of claim 1 wherein said compensation .[.resistor.]. .Iadd.means .Iaddend.is connected in series with a .[.second.]. resistor across a source of direct current voltage, the junction of said .[.resistors.]. .Iadd.resistor and said compensation means .Iaddend.being connected to the input of said second differential amplifier. 
     
     
       3. The measuring apparatus of claim 1 wherein said thermocouple reference junction means and said temperature compensation .[.resistor.]. .Iadd.means .Iaddend.are in close proximity to each other such that their temperatures are the same. 
     
     
       4. The measuring apparatus of claim 3 including impedance means connecting the output of said second differential amplifier to said one input terminal of the second differential amplifier.