Measurement Computing WLS-TEMP - Manual (Page 20)

WLS-TEMP Specifications
20
error voltage = resistance of the thermocouple x 105 nA
To reduce the error, reduce the length of the thermocouple to lower its resistance, or lower the AWG of the wire
by using a wire with a larger diameter. With OTD disabled, a maximum of 30 nA of input leakage current is
injected into the thermocouple.
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 WLS-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 voltage, resistance or temperature
format.
Data linearization
An onboard microcontroller automatically performs linearization on RTD and thermistor measurements.
 RTD measurements are linearized using a Callendar-Van Dusen coefficients algorithm (you select DIN,
SAMA, or ITS-90).
 Thermistor measurements are linearized using a Steinhart-Hart linearization algorithm (you supply the
coefficients from the sensor manufacturer's data sheet).
AC power supply
The external power supply is an AC-to-USB 2.5 W supply that is used to power the WLS-TEMP during remote
wireless operations (MCC p/n USB Power Adapter.)
External components
The WLS-TEMP has the following external components, as shown in Figure 11.
 Screw terminals
 USB connector
 Status LEDs (Command, Wireless Power, Transmit, Receive, Received Signal Strength indicators)
 LED Test button