For the DT8X Family of Data Loggers
To measure the resistance of a particular device, the bestselling dataTaker DT8x family of data loggers uses the common technique of sourcing a highly accurate current through the device, measuring the resulting voltage, and then calculating the resistance as V/I. There are 2 current sources available at 2.5 mA and 200 uA, and to avoid a reduction in accuracy, the voltage measurement is performed without the high voltage attenuators so that the maximum voltage that can be measured is 3.0 volts, effectively limiting the maximum resistance that can be measured to approximately 10,000 ohms.
Sometimes it may be necessary to measure a larger resistance, for example in the case of a thermistor or potentiometer. The measurement range can be extended by wiring a known resistor in parallel with the resistance being measured. This will, however, reduce the resolution of low-resistance measurements.
In DeTransfer software, the command to make the measurement is as follows:
Then the actual resistance is calculated using the following expression:
As shown above, users first read the combined resistance and store it in a channel variable (1CV), then calculate the value of R where Rp represents the value of the parallel resistor in ohms. As well as the 4-wire configuration shown here, a parallel resistor can also be used with a 3-wire or 2- wire resistance measurement. In all cases, the parallel resistor (Rp) should be located near the sensor (R), as shown above, so that the lead resistances can be correctly compensated for.
If it is not practical to locate the resistor near the sensor, then it can be located at the logger end of the cable. In this configuration the best accuracy will be obtained by connecting the sense inputs (+ and -) across Rp (if its resistance is significantly less than R). If Rp is greater than R, then the sense inputs should instead be connected across R, although in this case the effect of cable resistance is likely to be negligible, given that both R and Rp are high resistances.
Calculating Parallel Resistor Value
The required value of the parallel resistor Rp is given by the formula
where Rmax is the maximum resistance required to be measured. For example, to measure up to 100 kO, a parallel resistor of about 10kO would be suitable.