OM-WEB Series User's Guide
Signal I/O Connections
19
Thermocouple connections
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.
OM-WEB Series hardware make fully differential thermocouple measurements without requiring ground-
referencing resistors. A 32-bit floating point value in either a voltage or temperature format is returned by
software. Open thermocouple detection (OTD) is available for each thermocouple input. OTD automatically
detects an open or broken thermocouple.
Use the web browser or InstaCal to select the thermocouple type (J, K, R, S, T, N, E, and B) and one or more
input channels to connect the thermocouple.
Wiring configuration
Connect the thermocouple to the device using a differential configuration, as shown in Figure 3.
Note
: There
are two high/low channel pairs on each terminal. The # indicates the channel number. Do not make connections
to the pin labeled "NC".
Figure 3. Typical thermocouple connection
Connect thermocouples such that they are floating with respect to AGND (pins 9, 19, 28, 38). The AGND pins
are isolated from earth ground, so connecting thermocouple sensors to voltages referenced to earth ground is
permissible as long as the isolation between the AGND pins and earth ground is maintained.
When thermocouples are attached to conductive surfaces, the voltage differential between multiple
thermocouples must remain within ±1.4 V. For best results, use insulated or ungrounded thermocouples when
possible.
Maximum input voltage between analog input and ground
The absolute maximum input voltage between an analog input and the isolated AGND pins is ±25 VDC when
the device is powered on, and ±40 VDC when powered off.
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.
RTD and thermistor connections (OM-WEB-TEMP only)
A resistance temperature detector (RTD) measures temperature by correlating the resistance of the RTD
element with temperature. A thermistor is a thermally-sensitive resistor that is similar to an RTD in that its
resistance changes with temperature — thermistors show a large change in resistance that is proportional to a
small change in temperature. The main difference between RTD and thermistor measurements is the method
used to linearize the sensor data.
RTDs and thermistors are resistive devices that require an excitation current to produce a voltage drop that can
be measured differentially across the sensor. OM-WEB-TEMP, OM-WEB-TCs feature four current excitation
sources (±IA to ±ID) for measuring resistive type sensors. Each current excitation source is dedicated to one
channel pair.
The OM-WEB-TEMP makes two, three, and four-wire measurements of RTDs (100 Ω platinum type) and
thermistors.
