Instructions

UM-0085-B09 DT80 Range User Manual Page 328
RG
CS1 – 5 Wire Carlson Meter
The 5-wire configuration allows the DT80 to perform a 4-wire measurement on each resistance. It can therefore fully
compensate for cable resistance, including unequal cable resistances. It does, however, require two analog input
channels per sensor.
Sometimes a sixth wire is included – an additional sense wire connected to the junction between the resistors. This can
either be ignored or it can be connected to the + input of channel 2 in place of the link shown.
Figure 151: Wiring for 5-wire Carlson Sensor
To measure
Use the command
R1
1R(4W,II)
R2
2R(4W,II)
CS2 – 4 Wire Carlson Sensor
With a 4-wire configuration, the DT80 is able to compensate for cable resistance, provided that the resistance is equal for
all four wires. This is achieved by taking three separate resistance measurements (M1, M2 and M3) which include R1,
R2 and the cable resistance (Rc) in various combinations.
Figure 152: Wiring for 4-wire Carlson Sensor
To measure
Use the command
M1 (R1 + R2 + 2Rc)
1*R(II)
M2 (R1)
1R(4W,II)
M3 (R1 + R2 + Rc)
1+R(II)
Using these three measurements and a little algebra we can eliminate Rc and calculate the actual resistance values:


= 



= 2





For example, the following DT80 program will calculate the required resistances:
BEGIN
RA1M
1*R("M1",II,W)
1R("M2",4W,II,W)
1+R("M3",II,W)
CALC("R1~Ohm")=&M2
CALC("R2~Ohm")=2*&M3-&M1-&M2
END
CS3 – 3 Wire Carlson Sensor
With this configuration the DT80 is not able to take three independent measurements, so automatic lead resistance
compensation is not possible.
This configuration will therefore require manual correction for cable resistance (Rc). This can be applied as the channel
factor (which, for resistance, is an offset adjustment in ohms). The Rc value may be supplied by the manufacturer, or it
can be measured manually (at a typical working temperature).