OM-WEB Series User's Guide
Signal I/O Connections
20
Use InstaCal or the web interface to select the sensor type and the wiring configuration. Once the resistance
value is calculated, the value is linearized in order to convert it to a temperature value. A 32-bit floating point
value in either temperature or resistance is returned by software.
RTD maximum resistance
Resistance values greater than 660 Ω cannot be measured by the OM-WEB-TEMP in the RTD mode. The
660 Ω resistance limit includes the total resistance across the current excitation (±Ix) pins, which is the sum of
the RTD resistance and the lead resistances.
Thermistor maximum resistance
Resistance values greater than 180 kΩ cannot be measured by the OM-WEB-TEMP in the thermistor mode.
The 180 kΩ resistance limit includes the total resistance across the current excitation (±Ix) pins, which is the
sum of the thermistor resistance and the lead resistance.
Two-wire configuration
The easiest way to connect an RTD sensor or thermistor to the OM-WEB-TEMP is with a two-wire
configuration since it requires the fewest connections to the sensor. With this method, the two wires that
provide the RTD sensor with its excitation current also measure the voltage across the sensor.
Since RTDs exhibit a low nominal resistance, measurement accuracy can be affected due to the lead wire
resistance. For example, connecting lead wires that have a resistance of 1 Ω (0.5 Ω each lead) to a 100 Ω
platinum RTD will result in a 1% measurement error.
With a two-wire configuration, connect either one sensor per channel pair or two sensors per channel pair.
Two-wire, single-sensor
A two-wire single-sensor measurement configuration is shown in Figure 4.
: 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 4. Two-wire, single RTD or thermistor sensor measurement configuration
When you select a two-wire single sensor configuration, connections to C#H and C#L are made internally.
Two-wire, two sensor
A two-wire, two-sensor measurement configuration is shown in Figure 5.
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 5. Two-wire, two RTD or thermistor sensors measurement configuration
