Chemical Engineering June 2015 - 48

Actual sensor
Linearization curve with
transmitter-sensing matching
Ω
138.5 Ω
138.2 Ω
100oC
Process temperature
100oC
Transmitter linearizes
138.2 Ω to 100oC
Figure 5. Transmitter-sensor matching produces a more accurate linearization curve than the ideal
curve. The green line on the right shows the transmitter output that was linearized using transmitter-
sensor matching
the vast majority of industrial RtDs
are Pt100 devices. the primary reason
for choosing a Pt500 or Pt1000
is for improved resolution due to
the larger slope of their resistance
change per degree celsius.
For a Pt100 RtD at a given resistance
value, the transmitter assumes
that the corresponding process temperature
matches the ideal curve for
a Pt100 RtD, as shown in Figure 3.
as previously explained, this will not
be the case.
For example, using the ideal curve
for a Pt100 RtD, the resistance at a
process temperature of 100°c would
be 138.5 Ω. this is also the information
that the associated transmitter
is expecting to see. However, this
particular RtD actually provides a resistance
value of 138.2 Ω when the
process temperature is 100°c.
a transmitter using ideal-curve
programming will provide the control
system with a temperature value
of 99.2°c based upon the 138.2
Ideal curve
Ideal curve
Ω resistance value it receives, versus
a temperature value of 100°c
for this resistance. this produces
a 0.8°c measured error, as seen
in Figure 4.
iec 60751 identifies a process for
platinum Pt100 RtDs where an equation
developed by Hugh longbourne
callendar and M.s. van Dusen can
be used to identify the unique performance
curve for an individual RtD.
this equation, called the cvD equation,
is given in equation (1). For a
more detailed discussion of the cvD
equation, please see the box in Part
1 of this Feature Report on p. 44.
Rt = R0[1 + AT + BT2 + C(T-100)T3]
(1)
in the cvD equation, Rt is resistance
at temperature T, R0 is the resistance
when T = 0°c, and A, B, and C are
constants, commonly referred to as
cvD coefficients. these values are
specifically derived from each RtD
sensor during calibration using labIINNOVATIONNNOVATION
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48
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Chemical Engineering June 2015

Table of Contents for the Digital Edition of Chemical Engineering June 2015