Chapter 4
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 OM-USB-TEMP-AI hardware level-shifts the thermocouple’s output voltage into the A/D’s common mode
input range by ap2.5 V to the thermocouple’s low side at the C#L input. Always connect thermocouple
sensors to the OM-USB-TEMP-AI 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 OM-
USB-TEMP-AI terminal blocks produce two additional thermocouple junctions. 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 OM-USB-TEMP-AI subtracts the cold
junction voltage from the thermocouple voltage.
The OM-USB-TEMP-AI has one high-resolution temperature sensor integrated into the design. The CJC sensor
measures the average temperature at the terminal block 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 OM-USB-TEMP-AI is equipped with 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.
RTD and thermistor measurements
RTDs and thermistors are resistive devices that require an excitation current to produce a voltage drop that can
be measured differentially across the sensor. The OM-USB-TEMP-AI measures the sensor resistance by forcing
a known excitation current through the sensor and then measuring (differentially) the voltage across the sensor
to determine its resistance.
After the voltage measurement is made, the resistance of the RTD is calculated using Ohms law – the sensor
resistance is calculated by dividing the measured voltage by the current excitation level (±
Ix
) source. The value
of the ±
Ix
source is stored in local memory.
