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National Instruments Corporation
7
SCC-RTD01 Resistance Temperature Detector User Guide
Using the SCC-RTD01
Note
If you have an RTD that is not supported in Measurement & Automation
Explorer (MAX), Read Voltage Data and scale it in your ADE.
Converting Voltage Measurements to Temperature Measurements
If you use MAX to configure the SCC-RTD01, the reading you get from
the E/M Series DAQ device is properly scaled. Otherwise, you must scale
the readings and convert the voltage measurement to temperature by
performing the following steps to use the standard equations for platinum
RTDs:
1.
Measure the RTD voltage.
a.
Read the RTD channel on the E/M Series DAQ device and call the
value
V
ESERIES
[CH(
X
)].
b.
Calculate the RTD voltage using the following formula:
where
V
RTD
is the SCC-RTD01 input voltage.
V
ESERIES
is the E/M Series DAQ device voltage.
2.
Use polynomial expressions or a conversion table provided by the RTD
vendor to convert the RTD voltage to temperature.
Note
NI programming environments include RTD conversion utilities that implement the
voltage-to-temperature conversions. Refer to the ADE documentation for more
information on these utilities.
Although the RTD resistance-versus-temperature curve is relatively
linear, accurately converting resistance to temperature requires curve
fitting. The Callendar-Van Dusen equation is commonly used to
approximate the RTD curve:
R
t
=
R
0
[1 + A
t
+ B
t
2
+ C(
t
– 100)
3
]
where
R
t
is the resistance of the RTD at temperature
t.
R
0
is the resistance of the RTD at 0
°
C.
A, B, and C are the Callendar-Van Dusen coefficients shown in
Table 1.
t
is the temperature in
°
C.
V
RTD
V
ESERIES
25
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