Instructions

UM-0085-B09 DT80 Range User Manual Page 311
RG
R6 – High Resistance Input with Parallel Resistor
Note: This configuration fits to Series 3 or older as the resistance measurements are limited to a maximum of about 10kΩ. Measuring
resistance above 1MΩ in Series4 has not been tested
Resistance measurements are limited to a maximum of about 10kΩ. This 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.
Figure 138: Wiring for 4-wire resistance input, using a parallel resistor
To measure
Use the command
R
1R(4W,W)
CALC("R~ohm")=(Rp*&1R)/(Rp-&1R)
As shown above, we first read the combined resistance, and then calculate the value of R using an expression that
references the combined resistance measurement (&1R). 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:


=
10000 



10000
where R
max
is the maximum resistance required to be measured. For example, to measure up to 100kΩ a parallel resistor
of about 10kΩ would be suitable.
Bridges
Because of its sensitivity, the Wheatstone bridge circuit is commonly used for the measurement of small changes in
electrical resistance
. Applications include load cells, pressure sensors and strain gauges.
Figure 139: Wheatstone bridge
Bridges are designed such that under quiescent conditions the ratios R1/R4 and R2/R3 are equal, resulting in a zero
output voltage, V
out
. A small change to one or the resistances will then cause a corresponding change to V
out
, which can
then be measured accurately using the DT80’s sensitive 30mV range.
When one of the four resistors in a bridge is active (that is, sensitive to the quantity being measured) the circuit is called
a quarter bridge, and the remaining three resistors are called bridge completion resistors. Similarly, half and full
bridges imply two and four active gauges. All completion resistors should be close-tolerance precision resistors.
The DT80 returns all bridge measurements in a ratio metric form with units of parts per million (ppm):


=




10

where:
• V
out
is the measured bridge output voltage
• V
ex
is the excitation voltage
For a bridge measurement to be accurate, both of these voltages must be known accurately, and any lead or connector
resistances must be compensated for.