National Instruments NI 435 Series, Руководство пользователя, Страница: 32 / 77

Chapter 2 Operating the NI 435x Device
NI 435x User Manual 2-14 ni.com
voltage with the calibrated value of the precision current source stored
onboard.
Introduction to RTDs
An RTD is a temperature-sensing device whose resistance increases with
temperature. An RTD consists of a wire coil or deposited film of pure
metal. RTDs can be made of different metals and can have different
resistances, but the most popular RTD is made of platinum and has a
nominal resistance of 100
Ω
at 0 °C.
RTDs are known for their excellent accuracy over a wide temperature
range. Some RTDs have accuracy as high as 0.01
Ω
(0.026 °C) at 0 °C.
RTDs are also extremely stable devices. Common industrial RTDs drift less
than 0.1 °C/year, and some models are stable to within 0.0025 °C/year.
RTDs can be difficult to measure because they have relatively low
resistance (100
Ω
) that changes only slightly with temperature (less than
0.4
Ω
/°C). To accurately measure these small changes in resistance, you
may need to use special configurations that minimize errors from lead wire
resistance.
Relationship of Resistance and Temperature in RTDs
Compared to other temperature devices, the output of an RTD is relatively
linear with respect to temperature. The temperature coefficient, called
alpha (
α
), differs between RTD curves. Although various manufacturers
may specify
α
differently,
α
is most commonly defined as the change in
RTD resistance from 0 to 100 °C, divided by the resistance at 0
°
C, divided
by 100
°
C as follows:
α
(
Ω
/
Ω
/°C) = [(R
100
– R
0
)/R
0
]/100
°
C
where R
100
is the resistance of the RTD at 100
°
C, and R
0
is the resistance
of the RTD at 0
°
C.
For example, a 100
Ω
platinum RTD with
α
= 0.00385 measures 138.5
Ω
at 100
°
C. Figure 2-3 shows a typical resistance-temperature curve for a
100
Ω
platinum RTD.