Chapter 3
NI 4350 Operation
©
National Instruments Corporation
3-25
Self-Heating
The current source on the NI 4350 instrument is designed such that any
error resulting from self-heating is negligible in most cases. This
section explains how that occurs.
When current is passed through an RTD or a thermistor (both are
resistive devices), 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,
which is the power required to heat the thermistor by 1° C from ambient
temperature and is usually has 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 will dissipate:
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 will self-heat by only 0.003° C. Thus, the small value of
the current source helps you prevent any appreciable error due to
self-heating.
RTDs are inherently immune to this problem of self-heating because
their resistance is relatively small—100
Ω
at 0° C, for example. Here,
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
Table 3-5.
Guidelines for Resistance Measurement
Resistance Being
Measured (
Ω
)
Measurement
Technique
R
≤
1 k
Ω
Four-wire
1 k
Ω
< R
≤
10 k
Ω
Four-wire or three-wire
R > 10 k
Ω
Four-wire, three-wire, or two-wire
