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Chapter 4
Functional Details
Thermocouple measurements
OM-WEB Series 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 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 connecting the thermocouple sensor leads to the sensor input channel, the dissimilar metals at the device
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 device subtracts the
cold junction
voltage from the thermocouple
voltage.
OM-WEB Series hardware has four high-resolution CJC temperature sensors integrated into the design. The
CJC sensors are configured as one sensor for each channel pair. CJC sensors measure 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)
OM-WEB Series hardware is equipped with open-thermocouple detection for each analog input channel. OTD
is automatically enabled when the channel pair is configured for thermocouple sensors.
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
firmware continues to sample all channels when OTD is detected.
RTD and thermistor measurements (OM-WEB-TEMP only)
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-WEB-TEMP 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.
Once the resistance value is calculated, the value is linearized in order to convert it to a temperature value. The
measurement is returned by software as a 32-bit floating point value in either temperature or resistance.
