Functional Details; Thermocouple Measurements; Cold Junction Compensation (Cjc); Data Linearization - Measurement Computing USB-TEMP User Manual

Functional Details

Thermocouple measurements

A thermocouple consists of two dissimilar metals that are joined together at one end. When the junction
of the metals is heated or cooled, a voltage is produced that correlates to temperature.
The USB-TEMP hardware level-shifts the thermocouple's output voltage into the A/D's common mode
input range by applying +2.5 V to the thermocouple's low side at the C#L input. Always connect
thermocouple sensors to the USB-TEMP in a floating fashion. Do not attempt to connect the
thermocouple low side C#L to GND or to a ground referencing resistor.

Cold junction compensation (CJC)

When you connect the thermocouple sensor leads to the sensor input channel, the dissimilar metals at the
USB-TEMP terminal blocks produce an additional thermocouple junction. This junction creates a small
voltage error term which must be removed from the overall sensor measurement using a cold junction
compensation technique. The measured voltage includes both the thermocouple voltage and the cold
junction voltage. To compensate for the additional cold junction voltage, the USB-TEMP subtracts the
cold junction voltage from the thermocouple voltage.
The USB-TEMP has two high-resolution temperature sensors that are integrated into the design of the
USB-TEMP. One sensor is located on the right side of the package, and one sensor is located at the left
side. The CJC sensors measure the average temperature at the terminal blocks so that the cold junction
voltage can be calculated. A software algorithm automatically corrects for the additional thermocouples
created at the terminal blocks by subtracting the calculated cold junction voltage from the analog input's
thermocouple voltage measurement.
Increasing the thermocouple length
If you need to increase the length of your thermocouple, use the same type of thermocouple wires to
minimize the error introduced by thermal EMFs.

Data linearization

After the CJC correction is performed on the measurement data, an on-board microcontroller
automatically linearizes the thermocouple measurement data using National Institute of Standards and
Technology (NIST) linearization coefficients for the selected thermocouple type.
The measurement data is then output as a 32-bit floating point value in the configured format (voltage or
temperature).

Open-thermocouple detection (OTD)

The USB-TEMP is equipped with an open-thermocouple detection for each analog input channel. With
OTD, any open-circuit or short-circuit condition at the thermocouple sensor is detected by the software.
An open channel is detected by driving the input voltage to a negative value outside the range of any
thermocouple output. The software recognizes this as an invalid reading and flags the appropriate
channel. The software continues to sample all channels when OTD is detected.
Input leakage current
With open-thermocouple detection enabled, 105 nA (max.) of input leakage current is injected into the
thermocouple. This current can cause an error voltage to develop across the lead resistance of the
thermocouple that is indistinguishable from the thermocouple voltage you are measuring. You can
estimate this error voltage with the following formula:
error voltage = resistance of the thermocouple x 105 nA
4-1
Chapter 4