National Instruments NI 435 Series, Руководство пользователя, Страница: 44 / 77

Chapter 2 Operating the NI 435x Device
NI 435x User Manual 2-26 ni.com
Self-Heating
The current source on the NI 435 x is designed so that any error resulting
from self-heating is negligible in most cases.
When current is passed through an RTD or a thermistor (both are resistive
devices), the power dissipated is equal to
I
2
R , which heats the resistive
devices. This phenomena is called self-heating and is typically specified by
manufacturers in the form of the dissipation constant. The dissipation
constant is the power required to heat the thermistor by 1 °C from ambient
temperature, and it is usually represented in units of mW/°C. The
dissipation constant depends significantly on how easily heat is transferred
away from the thermistor, so the dissipation constant may be specified for
different media—in still air, water, or oil bath.
Thermistors, with their small size and high resistance, are particularly
prone to these self-heating errors. Typical dissipation constants range
anywhere from less than 0.5 mW/°C for still air to 10 mW/°C or higher for
a thermistor immersed in water. A 5,000
Ω
thermistor powered by a 25
µ
A
excitation current dissipates as follows:
I
2
R = (25
µ
A)
2
×
5,000
Ω
= 3.1
µ
W
If this thermistor has a dissipation constant of 10 mW/°C, the thermistor
self-heats by only 0.003 °C. Thus, the small value of the current source
helps prevent any appreciable error due to self-heating.
RTDs are relatively immune to the problem of self-heating because their
resistance is relatively small, such as 100
Ω
at 0 °C. Also, the amount of
self-heating depends significantly on the medium in which the RTD is
immersed. An RTD can self-heat up to 100 times higher in still air than in
moving water. The self-heating in RTDs due to the built-in 25
µ
A is
negligible. When using 1 mA excitation current, a 100
Ω
RTD dissipates as
follows:
I
2
R = (1 mA)
2
×
100
Ω
= 0.1 mW
If this RTD has a dissipation constant of 5 mW/°C, the RTD self-heats by
0.02 °C.
AC Noise Effects
The NI 435 x rejects AC noise as specified in NMR in Appendix A,
Specifications . However, if the amplitudes of the AC noise are large
compared to the DC signal, or if the peak value (AC plus DC) of the