User manual

Chapter 5 Transducer Conversion Functions
© National Instruments Corporation 5-3 NI-DAQ User Manual for PC Compatibles
The
RTD_Convert
routine has two remaining parameters—RTDVolts is
the voltage that NI-DAQ read from the RTD, and RTDTemp is the return
temperature value.
The
RTD_Buf_Convert
routine has three remaining parameters—numPts
is the number of voltage points to convert, RTDVoltBuf is the array that
contains the voltages that NI-DAQ read from the RTD, and RTDTempBuf
is the return array that contains the temperatures.
Using This Function
The conversion routines first find the RTD resistance by dividing
RTDVolts (or each element of RTDVoltBuf)byIex. The function
converts that resistance to a temperature using a solution to the Callendar
Van-Düsen equation for RTDs:
R
t
=R
0
[1 + At + Bt
2
+C(t–100)t
3
]
For temperatures above 0 °C, the C coefficient is 0 and the equation reduces
to a quadratic equation for which we have found the appropriate root. Thus,
these conversion routines are accurate only for temperatures above 0 °C.
Your RTD documentation should give you R
0
and the A and B coefficients
for the Callendar Van-Düsen equation. The most common RTDs are 100 Ω
platinum RTDs that either follow the European temperature curve (also
known as the DIN 43760 standard) or the American curve. The values for
A and B are as follows:
• European Curve (DIN 43760):
A = 3.90802 × 10
–3
B = –5.80195 × 10
–7
(α =3.85× 10
–3
; ∂ = 1.492)
• American Curve:
A = 3.9784 × 10
–3
B = –5.8408 × 10
–7
(α =3.92× 10
-3
; ∂ =1.492)