Chapter 2
Operating the NI 435x Device
2-18
ni.com
Figure 2-7.
3-Wire RTD Measurement and Lead Wire Resistance Compensation
Refer to Figure 2-10 for an example of how you can use different
transducers connected to analog channels in the same measurement setup.
Note
For best results, use the 1 mA current source when using the NI 4351 with RTDs
with resistances below 1 k
Ω
. Refer to the
readme.htm
file found on the NI 435
x
CD for
software issues regarding the 1 mA source. For resistances above 1 k
Ω
or with the NI 4350,
use the 25
µ
A current source to avoid self-heating. Refer to the
section for
further details.
Introduction to Thermistors
A thermistor is a piece of semiconductor made from metal oxides, pressed
into a small bead, disk, wafer, or other shape, which is sintered at high
temperatures and finally coated with epoxy or glass. The resulting device
exhibits an electrical resistance that varies with temperature.
There are two types of thermistors—negative temperature coefficient
(NTC) thermistors and positive temperature coefficient (PTC) thermistors.
An NTC thermistor is one whose resistance decreases with increasing
temperature. A PTC thermistor is one whose resistance increases with
increasing temperature. NTC thermistors are much more commonly used
than PTC thermistors, especially for temperature measurement
applications.
R
RTD
= R
CH
n
– R
CH
(n +1)
if R
L1
= R
L2
= R
L3
I
EX+
, I
EX0+
, or I
EX1+
RTD
R
L1
R
L2
R
L3
CH
n+
CH
n–
I
EX–
, I
EX0–
, or I
EX1–
CH
n +1
CH
n +1