Measurement Computing WEB-TEMP - Manual (Page 22)

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Chapter 4
Functional Details
Thermocouple measurements
The WEB-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 WEB-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 connecting the thermocouple sensor leads to the sensor input channel, the dissimilar metals at the WEB-
TEMP 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 WEB-TEMP subtracts the cold junction voltage from
the thermocouple voltage.
The WEB-TEMP 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)
The WEB-TEMP 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
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 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 a resistance or temperature format.