THERMOCOUPLE TEMPERATURE MEASUREMENT (C, F)
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Measurement Accuracy Specifications

Hardware and Software Setup

Temperature measurement using a thermocouple involves connecting the two thermocouple leads to a pair of instruNet Vin+ and Vin- input terminals.  It is recommended that you install an additional wire from the instruNet Vin- screw terminal to the instruNet GND screw terminal, at the instruNet hardware, as illustrated below, and as noted here.  instruNet calculates the value of the temperature in degrees Celsius using a polynomial linearizing equation.

Fig 3.15 - Temperature Measurement (Thermocouple)

To do temperature measurement using a thermocouple you must:

1.   Set the Sensor field in the Hardware settings area to the appropriate Thermocouple type (e.g. J, K, T, E, R, S).

2.   Set the Wiring field in the Hardware settings area to Vin+ - Vin- (i.e. differential analog voltage measurement).

3.   Set the Range field in the Hardware settings dialog depending on the temperature range being measured, as noted here.  For example, with the i100 box, one would set the voltage measurement range to +-10mV if working with J <170C, K <230C, T <180C, R <900C, or S <900C; and set the voltage range to +-80mV if working with higher temperatures.  Typicaly measurement resolution is approximately 1.2µV in the +/- 10mV range and approximately 10µV in the +/- 80mV range.

4.   Wire your voltage source per figure 3.15, click here if you need more guidance setting up the software, and click here if the measured value is not correct. 5, 10   If your thermocouple leads are backwards, then the measured temperature will be shown as varying in the opposite direction from ambient (e.g. if the instruNet terminals are at 25°C, the thermocouple is at 35°C, and the leads are backwards, then instruNet will report 15°C.).

Troubleshooting

If your thermocouple is not working properly, please see Troubleshooting Thermocouples.  Attaching Vin- to Gnd with a small wire solves many RFI problems.  Integrating for .001 to .001666 seconds solves many noise problems.

Theory of Operation

Thermocouple Temperature Vs. Voltage
The voltage across a thermocouple is a non-linear function of temperature at the thermocouple tip, as summarized below.

Thermocouple Measurement Errors
The maximum thermocouple measurement error is the sum of the following component errors:

1. Errors within the thermocouple itself, which are often due to impurities in the thermocouple materials. For more information, please consult your thermocouple supplier.
2. Cold Junction Compensation measurement and linearization error, which is typically < +/- .25C.
3. Maximum thermocouple linearization software error: J: +/- .05C; K: +/- .08; T: +/- .05C; E: +/- .04C; R: +/- .03C; S: +/- .03C; B: +/- .04C; N: +/- .05C; C: +- .5C; D: +-.5C; G: +-.67C for 0 to 300C, +-.26C for 300 to 2315C;
4. Thermocouple voltage measurement error (e.g. +/- 10uV maximum voltage measurement error with the i100 box on the +/-10mV range with 0.001 seconds of integration).