Measurement Computing WEB-TEMP - Manual (Page 18)

WEB-TEMP User's Guide Signal Connections
18
 IC+ is the current excitation source for channel 4 and channel 5
 ID+ is the current excitation source for channel 6 and channel 7
The WEB-TEMP makes two, three, and four-wire measurements of RTDs (100 Ω platinum type) and
thermistors.
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 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 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 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, you can 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. Note : There are two high/low
channel pairs on each terminal. The # indicates the channel number. Do not make connections to pin marked
"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 pin marked "NC".