Measuring Temperature with an RTD or Thermistor
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This tutorial is part of the National Instruments Measurement Fundamentals Series.
Each tutorial in this series, will teach you a specific topic of common
measurement applications, by explaining the theory and giving practical
examples. This tutorial introduces and explains the concepts and
techniques of measuring temperature with an RTD or Thermistor.
Table of Contents:
You can also view an on demand web event on strain gauge measurements.
For more information, return to the Measurement Fundamentals Main Page.
the temperature of an object determines the sensation of warmth or
coldness felt by touching it. More specifically, temperature is a
measure of the average kinetic energy of the particles in a sample of
matter, expressed in units of degrees on a standard scale.
Resistance temperature detectors (RTDs) operate on the principle of
changes in electrical resistance of pure metals and are characterized
by a linear positive change in resistance with temperature. Typical
elements used for RTDs include nickel (Ni) and copper (Cu), but
platinum (Pt) is by far the most common because of its wide temperature
range, accuracy, and stability.
RTDs are constructed by one of two different manufacturing
configurations. Wire-wound RTDs are constructed by winding a thin wire
into a coil. A more common configuration is the thin-film element,
which consists of a very thin layer of metal laid out on a plastic or
ceramic substrate. Thin-film elements are cheaper and more widely
available because they can achieve higher nominal resistances with less
platinum. To protect the RTD, a metal sheath encloses the RTD element
and the lead wires connected to it.
RTDs are popular because of their excellent stability, and exhibit the
most linear signal with respect to temperature of any electronic
temperature sensor. They are generally more expensive than
alternatives, however, because of the careful construction and use of
platinum. RTDs are also characterized by a slow response time and low
sensitivity; and because they require current excitation, they can be
prone to self-heating.
RTDs are commonly categorized by their nominal resistance at 0 °C.
Typical nominal resistance values for platinum thin-film RTDs include
100 Ω and 1000 Ω. The relationship between resistance and temperature
is very nearly linear and follows the equation
For <0 °C RT = R0 [ 1 + aT + bT2 +cT3 (T - 100) ] (Equation 1)
For >0 °C RT = R0 [ 1 + aT + bT2 ]
Where RT = resistance at temperature T
R0 = nominal resistance
a, b, and c are constants used to scale the RTD
The resistance/temperature curve for a 100 W platinum RTD, commonly referred to as Pt100, is shown below:
Figure 1. Resistance-Temperature Curve for a 100 Ω Platinum RTD, a = 0.00385
The most common RTD is the platinum thin-film with an a of 0.385%/°C and is specified per DIN EN 60751. The a value depends on the grade of platinum used, and also commonly include 0.3911%/°C and 0.3926%/°C. The a
value defines the sensitivity of the metallic element, but is normally
used to distinguish between resistance/temperature curves of various
Table 1. Callendar-Van Dusen Coefficients Corresponding to Common RTDs
* For temperatures below 0 °C only; C = 0.0 for temperatures above 0 °C.
Thermistors (thermally sensitive resistors) are similar to RTDs in that
they are electrical resistors whose resistance changes with
temperature. Thermistors are manufactured from metal oxide
semiconductor material which is encapsulated in a glass or epoxy bead.
Thermistors have a very high sensitivity, making them extremely responsive to changes in temperature. For example, a 2252 W thermistor has a sensitivity of -100 W/°C at room temperature. In comparison, a 100 W RTD has a sensitivity of 0.4 W/°C. Thermistors also have a low thermal mass that results in fast response times, but are limited by a small temperature range.
Thermistors have either a negative temperature coefficient (NTC) or a
positive temperature coefficient (PTC). The first has a resistance
which decreases with increasing temperature and the latter exhibits
increased resistance with increasing temperature. Figure 2 shows a
typical thermistor temperature curve compared to a typical 100 W RTD temperature curve:
Figure 2. Resistance versus Temperature for a Typical Thermistor and RTD
RTDs and thermistors are resistive devices, you must supply them with
an excitation current and then read the voltage across their terminals.
If extra heat cannot be dissipated, I2R heating caused by
the excitation current can raise the temperature of the sensing element
above that of the ambient temperature. Self-heating will actually
change the resistance of the RTD or thermistor, causing error in the
measurement. The effects of self-heating can be minimized by supplying
lower excitation current.
The easiest way to connect an RTD or thermistor to a measurement device is with a 2-wire connection.
Figure 3. Making a 2-Wire RTD/Thermistor Measurement
With this method, the two wires that provide the RTD or thermistor with
its excitation current are also used to measure the voltage across the
sensor. Because of the low nominal resistance of RTDs, measurement
accuracy can be drastically affected by lead wire resistance. For
example, lead wires with a resistance of 1 W connected to a 100 W platinum RTD cause a 1% measurement error.
A 3-wire or 4-wire connection method can eliminate the effects
of lead wire resistance. The connection places leads on a high
impedance path through the measurement device, effectively eliminating
error caused by lead wire resistance. It is not necessary to use a 3 or
4-wire connection method for thermistors because they typically have
much higher nominal resistance values than RTDs. A diagram of a 4-wire
connection is shown below.
Figure 4. Making a 4-Wire RTD Measurement
RTD and thermistor output signals are typically in the millivolt range,
making them susceptible to noise. Lowpass filters are commonly used in
RTD and thermistor data acquisition systems to effectively eliminate
high frequency noise in RTD and thermistor measurements. For instance,
lowpass filters are useful for removing the 60 Hz power line noise that
is prevalent in most laboratory and plant settings.
Using SCXI with RTDs and Thermistors
National Instruments SCXI is a signal conditioning system for PC-based
data acquisition systems. An SCXI system consists of a shielded chassis
that houses a combination of signal conditioning input and output
modules, which perform a variety of signal conditioning functions. You
can connect many different types of sensors, including RTDs and
thermistors, directly to SCXI modules. The SCXI system can operate as a
front-end signal conditioning system for PC plug-in data acquisition
(DAQ) devices (PCI and PCMCIA) or PXI DAQ modules.
Figure 5. SCXI Signal Conditioning System
SCXI offers a variety of analog and digital signal conditioning
modules for various types of signals, including RTDs and thermistors.
Table 1 includes the features of SCXI modules that can be used for RTD
and thermistor measurements.
Table 1. SCXI Signal Conditioning Modules for RTDs and Thermistors
SCXI-1102 w/ SCXI 1581
|Number of inputs||4||16 (devices in series)|
8 (4-wire scanning mode)
|Amplifier gains||1 to 2000 – jumper selectable||1 to 2000 – jumper selectable||1 or 100 – software selectable per channel|
|Filtering options||4 Hz or 10 kHz||4 Hz or 4 kHz – software programmable||2 Hz|
|Isolation||250 Vrms||480 Vrms||N/A|
|Excitation Values||3.33 V, 10 V|
0.15 mA, 0.45 mA
| 100 µA|
|Recommended terminal block for RTDs/Thermistors||SCXI-1320 or SCXI-1322||SCXI-1322||SCXI-1300 or SCXI-1303|
Recommended Starter Kit for RTD or Thermistor SCXI DAQ System:
1. PCI-6052 DAQ board
2. SCXI-1000 chassis
3. SCXI-1349 cable assembly
4. SCXI modules and terminal blocks (See Table 1 above)
5. Refer to ni.com/sensors for recommended sensor vendors
Using SCC with RTDs and Thermistors
National Instruments SCC provides portable, modular signal conditioning
for DAQ systems. SCC modules can condition a variety of analog I/O and
digital I/O signals. SCC DAQ systems include an SC-2345 Series shielded
carrier, SCC modules, a cable, and a DAQ device. Figure 4 below
illustrates an SC-2345 carrier with SCC modules.
Figure 4. SC-2345 with SCC Modules
The SCC-RTD01 RTD module accepts up to two RTD input signals from 2, 3, or 4-wire RTDs of the following types:
- Pt100 (-100 to +850 °C)
- Ni120 (-80 to +320 °C)
- Cu10 (0 to +260 °C)
The RTDs are excited by a 1 mA precision current source provided on the SCC-RTD01.
The RTD inputs are filtered and passed into a differential amplifier
with a gain of 25. The output of the amplifier passes through a 3-pole
30 Hz filter and is buffered to allow maximum scan rates. Because of
the fixed gain of 25, the maximum input voltage is 400 mV.
Figure 5. Schematic of SCC-RTD01 Used in 4-Wire Mode
Recommended Starter Kit for RTD or Thermistor SCC DAQ System:
1. PCI-6052 DAQ board
2. SC-2345 module carrier
3. SCC-RTD01 (1 per 2 RTDs/thermistors)
4. Refer to ni.com/sensors for recommended RTD and thermistor vendors
Return to Sensor Fundamentals or the Measurement Fundamentals Main Page.
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