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Temperature Sensors Notes
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- Mangosuthu University Of Technology (MUT)
Thermocouple (Temperature Sensors) Notes
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Two Ways to Measure Temperature Why Use a Thermocouple?Advantages
Using Thermocouples Feature • Temperature range: Most practical temperature ranges,
from cryogenics to jet-engine exhaust, can be served using
Simplicity, Accuracy, and Flexibility thermocouples. Depending on the metal wires used, a
thermocouple is capable of measuring temperature in the range
By Matthew Duff and Joseph Towey –200°C to +2500°C.
Introduction • Robust: Thermocouples are rugged devices that are immune
The thermocouple is a simple, widely used component for to shock and vibration and are suitable for use in hazardous
measuring temperature. This article provides a basic overview environments.
of thermocouples, describes common challenges encountered • Rapid response: Because they are small and have low thermal
when designing with them, and suggests two signal conditioning capacity, thermocouples respond rapidly to temperature
solutions. The first solution combines both reference-junction changes, especially if the sensing junction is exposed. They
compensation and signal conditioning in a single analog IC for can respond to rapidly changing temperatures within a few
convenience and ease of use; the second solution separates the hundred milliseconds.
reference-junction compensation from the signal conditioning to • No self heating: Because t her mocouples require no
provide digital-output temperature sensing with greater flexibility excitation power, they are not prone to self heating and are
and accuracy. intrinsically safe.
Thermocouple Theory Disadvantages
A thermocouple, shown in Figure 1, consists of two wires of • Complex signal conditioning: Substantial signal conditioning
dissimilar metals joined together at one end, called the measurement is necessary to convert the thermocouple voltage into a usable
(“hot”) junction. The other end, where the wires are not joined, temperature reading. Traditionally, signal conditioning has
is connected to the signal conditioning circuitry traces, typically required a large investment in design time to avoid introducing
made of copper. This junction between the thermocouple metals errors that degrade accuracy.
and the copper traces is called the reference (“cold”) junction.* • Accuracy: In addition to the inherent inaccuracies in
thermocouples due to their metallurgical properties, a
THERMOCOUPLE thermocouple measurement is only as accurate as the reference
METAL A junction temperature can be measured, traditionally within
WIRING TO SIGNAL
1°C to 2°C.
CONDITIONING
CIRCUITRY
• Susceptibility to corrosion: Because thermocouples consist of
two dissimilar metals, in some environments corrosion over
METAL B time may result in deteriorating accuracy. Hence, they may
need protection; and care and maintenance are essential.
MEASUREMENT REFERENCE
JUNCTION JUNCTION • Susceptibility to noise: When measuring microvolt-level
signal changes, noise from stray electrical and magnetic
Figure 1. Thermocouple.
fields can be a problem. Twisting the thermocouple wire pair
The voltage produced at the reference junction depends on the can greatly reduce magnetic field pickup. Using a shielded
temperatures at both the measurement junction and the reference cable or running wires in metal conduit and guarding can
junction. Since the thermocouple is a differential device rather reduce electric field pickup. The measuring device should
than an absolute temperature measurement device, the reference provide signal filtering, either in hardware or by software,
junction temperature must be known to get an accurate absolute with strong rejection of the line frequency (50 Hz/60 Hz)
temperature reading. This process is known as reference junction and its harmonics.
compensation (cold junction compensation.)
Difficulties Measuring with Thermocouples
Thermocouples have become the industry-standard method for It is not easy to transform the voltage generated by a thermocouple
cost-effective measurement of a wide range of temperatures with into an accurate temperature reading for many reasons: the
reasonable accuracy. They are used in a variety of applications voltage signal is small, the temperature-voltage relationship is
up to approximately +2500°C in boilers, water heaters, ovens, nonlinear, reference junction compensation is required, and
and aircraft engines—to name just a few. The most popular thermocouples may pose grounding problems. Let’s consider
thermocouple is the type K, consisting of Chromel® and Alumel® these issues one by one.
(trademarked nickel alloys containing chromium, and aluminum,
Voltage signal is small: The most common thermocouple
manganese, and silicon, respectively), with a measurement range
types are J, K, and T. At room temperature, their voltage
of –200°C to +1250°C.
varies at 52 μV/°C, 41 μV/°C, and 41 μV/°C, respectively. Other
*We use the terms “measurement junction” and “reference junction” less-common types have an even smaller voltage change with
rather than the more traditional “hot junction” and “cold junction.” The temperature. This small signal requires a high gain stage before
traditional naming system can be confusing because in many applications the analog-to-digital conversion. Table 1 compares sensitivities of
measurement junction can be colder than the reference junction. various thermocouple types.
Analog Dialogue 44-10, October (2010) www.analog.com/analogdialogue 1
, Table 1. Voltage Change vs. Temperature Rise (resistance temperature-detector). The thermocouple voltage
(Seebeck Coefficient) for Various Thermocouple Types at 25°C. reading is then compensated to reflect the reference junction
temperature. It is important that the reference junction be read
Thermocouple Seebeck Coefficient as accurately as possible—with an accurate temperature sensor
Type (𝛍V/°C) kept at the same temperature as the reference junction. Any
E 61 error in reading the reference junction temperature will show
J 52 up directly in the final thermocouple reading.
K 41 A variety of sensors are available for measuring the reference
N 27 temperature:
R 9 1. Thermistors: They have fast response and a small package;
S 6 but they require linearization and have limited accuracy,
T 41 especially over a wide temperature range. They also require
Because the voltage signal is small, the signal-conditioning current for excitation, which can produce self-heating, leading
circuitry typically requires gains of about 100 or so—fairly to drift. Overall system accuracy, when combined with signal
straightforward signal conditioning. What can be more difficult conditioning, can be poor.
is distinguishing the actual signal from the noise picked up on the 2. Resistance temperature-detectors (RTDs): RTDs are
thermocouple leads. Thermocouple leads are long and often run accurate, stable, and reasonably linear, however, package size
through electrically noisy environments. The noise picked up on and cost restrict their use to process-control applications.
the leads can easily overwhelm the tiny thermocouple signal. 3. Remote thermal diodes: A diode is used to sense the
Two approaches are commonly combined to extract the signal from temperature near the thermocouple connector. A conditioning
the noise. The first is to use a differential-input amplifier, such as chip converts the diode voltage, which is proportional to
an instrumentation amplifier, to amplify the signal. Because much temperature, to an analog or digital output. Its accuracy is
of the noise appears on both wires (common-mode), measuring limited to about ±1°C.
differentially eliminates it. The second is low-pass filtering, which 4. Integrated temperature sensor: An integrated temperature
removes out-of-band noise. The low-pass filter should remove sensor, a standalone IC that senses the temperature locally,
both radio-frequency interference (above 1 MHz) that may cause should be carefully mounted close to the reference junction,
rectification in the amplifier and 50 Hz/60 Hz (power-supply) hum. and can combine reference junction compensation and signal
It is important to place the filter for radio frequency interference conditioning. Accuracies to within small fractions of 1°C can
ahead of the amplifier (or use an amplifier with filtered inputs). be achieved.
The location of the 50-Hz/60-Hz filter is often not critical—it can Voltage signal is nonlinear: The slope of a thermocouple
be combined with the RFI filter, placed between the amplifier and response curve changes over temperature. For example, at 0°C a
ADC, incorporated as part of a sigma-delta ADC, or it can be T-type thermocouple output changes at 39 μV/°C, but at 100°C,
programmed in software as an averaging filter. the slope increases to 47 μV/°C.
Reference junction compensation: The temperature of the There are three common ways to compensate for the nonlinearity
thermocouple’s reference junction must be known to get an of the thermocouple.
accurate absolute-temperature reading. When thermocouples were
Choose a portion of the curve that is relatively f lat and
first used, this was done by keeping the reference junction in an
approximate the slope as linear in this region—an approach
ice bath. Figure 2 depicts a thermocouple circuit with one end at
that works especially well for measurements over a limited
an unknown temperature and the other end in an ice bath (0°C).
temperature range. No complicated computations are needed.
This method was used to exhaustively characterize the various
One of the reasons the K- and J-type thermocouples are popular
thermocouple types, thus almost all thermocouple tables use 0°C
is that they both have large stretches of temperature for which the
as the reference temperature.
incremental slope of the sensitivity (Seebeck coefficient) remains
COPPER WIRE fairly constant (see Figure 3).
+
TEMPERATURE
CONSTANTAN
+ 70
UNKNOWN
J-TYPE
THERMOCOUPLE V 60
–
SEEBECK COEFFICIENT (¿V/îC)
IRON –
50
K-TYPE
T-TYPE
40
30
ICE BATH
REFERENCE REFERENCE
JUNCTIONS 20
10
Figure 2. Basic iron-constantan thermocouple circuit.
But keeping the reference junction of the thermocouple in an ice 0
–200 0 200 400 600 800 1000
bath is not practical for most measurement systems. Instead most
TEMPERATURE (îC)
systems use a technique called reference-junction compensation,
(also known as cold-junction compensation). The reference Figure 3. Variation of thermocouple sensitivity with
junction temperature is measured with another temperature- temperature. Note that K-type’s Seebeck coefficient is
sensitive device—typically an IC, thermistor, diode, or RTD roughly constant at about 41 μV/°C from 0°C to 1000°C.
2 Analog Dialogue 44-10, October (2010)
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