User manual

Chapter 5 Transducer Conversion Functions
NI-DAQ User Manual for PC Compatibles 5-4 ni.com
Some RTD documentation contains values for α and ∂, from which you can
calculate A and B using the following equations:
where α is the temperature coefficient at T = 0 °C.
C = –α ß/1,000,000
where ß is a characteristic of your RTD similar to the α and ∂ equation
coefficients.
Strain_Convert and Strain_Buf_Convert
These functions convert a voltage or voltage buffer that NI-DAQ read from
a strain gauge to units of strain.
Parameter Discussion
The bridgeConfig integer indicates in what type of bridge configuration
the strain gauge is mounted. Figure 5-1 shows all the different bridge
configurations and the corresponding values that you should pass in
bridgeConfig.
Vex is the excitation voltage (in volts) that you used. If the value of Vex
is 0, a default excitation voltage of 3.333 V is assumed. The SCXI-1121
module provides excitation voltages of 10 V and 3.333 V. The SCXI-1122
module provides an excitation voltage of 3.333 V.
GF is the gauge factor of the strain gauge.
v is Poisson’s Ratio (needed only in certain bridge configurations).
Rg is the strain gauge nominal value in ohms.
RL is the lead resistance in ohms. In many cases, the lead resistance is
negligible and you can pass a value of 0 for RL to the routine. Otherwise,
you can measure RL to be more accurate.
Vinit is the unstrained voltage of the strain gauge in volts after it is mounted
in its bridge configuration. Read this voltage at the beginning of your
application and save it to pass to the strain gauge conversion routines.
A α 1 ∂ 100⁄+()=
B α∂ 10 000 100
2
,,()⁄–=