The connection wire from the Remote RTD Module to the RTD will typically have all
three conductors the same size and length. This means that the resistance of each
conductor will be the same. Call the resistance of each conductor R
lead
.
From the above, the following equations can be seen;
V
V
V
V
= 20mA R
+10mA R
+10mA
RH
R lead
RTD
H lead
lead
RTD
=
+
+
´
´
´
R
lead
V
V
V
= 10mA R
+ 20mA R
CR
C lead
R lead
lead
lead
=
+
´
´
Subtracting these two equations and subsituting R
lead
for the different lead resistance
provides the following;
(
) (
)
V
V
30mA R
10mA R
30mA R
RH
CR
lead
RTD
lead
-
=
´
+
´
-
´
V
V
10mA R
RH
CR
RTD
-
=
´
This calculation gives the voltage across the RTD independant of the connection wire
resistance. By performing this calculation, the Remote RTD Module compensates for
the conductor resistance and gets the true value for the RTD resistance.
The Remote RTD Module then converts the measured voltage across the RTD to the
temperature that the RTD is measuring. This is possible because the RTD conforms to
a standard resistance vs temperature curve.
5
1. INTRODUCTION
