Temperature measurement

TEMPERATURE
MEASUREMENT
BY
ASHVANI SHUKLA
MANAGER(C&I)
RELIANCE

INDEX
 Introduction
 Liquid and gas thermometer
 Bimetallic strip thermometer
 Resistance thermometers, which include RTD and
Thermistor
 Thermocouple
 Junction semiconductor sensor
 Radiation Pyrometer

Introduction
 In previous days the thermometer was only used for
measuring human body temperature and atmospheric
temperature, but now days due to drastic industrial
developments, it is essential to measure temperature
of various situations and conditions much accurately
and precisely. To achieve required accuracy and
precision of temperature measurement, different types
of temperature sensor have been developed.
Depending upon the working principle of temperature
sensors, they have broadly classified in various
types.it will be explaines in next slide.

Liquid and gas
thermometer
 This is the oldest process of temperature
measurement. The method is very simple. In gas
thermometer, one glass made bulb is filled with some
specific gas and it is observed the different volumes of
the gas inside the bulb at different temperatures. Here,
Charles Law of Gas is applied, this law states that
volume of gas is directly proportional to kelvin
temperature of the gas when pressure is kept
constant. In gas thermometer, the temperature is
calibrated against volume of the gas inside the bulb,
considering the pressure of the gas is constant as
shown in the basic figure above.

Liquid and gas
thermometer
 The reading of this type of temperature sensors is not quite
reliable as it is highly effected by atmospheric pressure. Another
little bit improved version of gas thermometer was also
introduced, where volume of the gas inside the bulb is kept
constant at all temperatures by adjusting the level of capillary
tube attached to the bulb and observed what is the change of
head of the liquid in side the capillary tube. Here, also Charles
Law of Gas is applied. As we know this law also states that
pressure of an enclosed gas is directly proportional to its kelvin
temperature when its volume is constant. Here, changed
pressure of the gas is calibrated to its temperature. As both of
these gas thermometers sense temperature, they can be
considered as oldest and simplest version of temperature
sensor.

Bimetallic Strip
Thermometer
 This is another simple form of thermometer. Here two
strips of different metals are joint together to form a
bimetallic strip. The coefficient of expansion these two
metal must be different enough. Due to these different
coefficient of expansion, the metal strips will be
expanded or contracted differently for the same
temperature change in surrounding. Consequently,
whole bimetallic strip assembly will bend or be
deformed in shape. This mechanical deformation of
the strip, is then caught by a pointer in an indication
dial by means of lever - gear arrangement.

Bimetallic Strip Thermometer
 Every metal and alloy has its won coefficient of expansion. That means they
are expanded in their size differently for same temperature increase. If we
couple two strips made of two different metals or alloys, then due to
dissimilarity in coefficient of expansion they will be expanded or contracted
differently during temperature change and consequently the whole bimetallic
strip assembly will bend or be deformed. As this deformation of shape of a
specific bimetallic strip is due to temperature rise or fall, this deformation can
also be measured in the scale of temperature. From this principle, the
concept of Bimetallic Strip Thermometer came. Bimetallic strip mainly used
in industries in temperature control devices. It is assembled with temperature
controller system. When temperature reaches to a preset value, the
bimetallic strip is so bent it closes an NO contact which initiates the cooling
system to decrease the temperature of the system. Bimetallic strip
thermometer is also widely used in industries because of their simplicity and
robustness. There are mainly two types of bimetallic strip thermometer are
available in the market. Both are same in working but differ in construction.

 Spiral Strip Bimetallic Thermometer
 Here the bimetallic strip used is in spiral shaped. When temperature rises,
due to bimetallic property, the spring twists more. Due to this mechanical
deformation of the spring, a pointer attached to the dial moves and indicates
the temperature, as the dial of this bimetallic strip thermometer is calibrated
in temperature scale.
 Cantilever Strip Bimetallic Thermometer
 Here a straight bimetallic strip is attached as a cantilever. When temperature
rises or falls, the strip bends either sides and the movement of the front end
of the strip is transferred to a pointer dial system via gear - lever system, to
take reading of temperature.
 Advantages of Bimetallic strip Thermometer
 There are mainly three major advantages of this instrument. one they are
robust two they are simple and three they are fully mechanical devices no
need of power source.


 Spiral Strip Bimetallic Thermometer
 Here the bimetallic strip used is in spiral shaped. When
temperature rises, due to bimetallic property, the spring twists
more. Due to this mechanical deformation of the spring, a
pointer attached to the dial moves and indicates the
temperature, as the dial of this bimetallic strip thermometer is
calibrated in temperature scale.
 Cantilever Strip Bimetallic Thermometer
 Here a straight bimetallic strip is attached as a cantilever. When
temperature rises or falls, the strip bends either sides and the
movement of the front end of the strip is transferred to a pointer
dial system via gear - lever system, to take reading of
temperature.
 Advantages of Bimetallic strip Thermometer
 There are mainly three major advantages of this instrument.
one they are robust two they are simple and three they are fully
mechanical devices no need of power source.

 Disadvantages of Bimetallic strip
Thermometer
 Main disadvantages are they are not
very accurate and they are not
suitable for measuring lower
temperature as the metals and
metallic alloys show nearly same
expansion or contraction in lower
range of temperature.

Resistance Temperature Detector
 Construction and Working
Principle
 A Resistance Thermometer or Resistance Temperature Detector is a
device which used to determine the temperature by measuring the
resistance of pure electrical wire. This wire is referred to as a
temperature sensor. If we want to measure temperature with high
accuracy, RTD is the only one solution in industries. It has good linear
characteristics over a wide range of temperature. The variation of
resistance of the metal with the variation of the temperature is given as,
 Where, Rt and R0 are the resistance values at t°C and t0°C
temperatures. α and β are the constants depends on the metals.

Resistance Temperature
Detector

Detector
 This expression is for huge range of temperature. For small range of
temperature, the expression can be.
 Rt=Ro(1+At)
 In RTD devices; Copper, Nickel and Platinum are widely used metals. These
three metals are having different resistance variations with respective to the
temperature variations. That is called resistance-temperature characteristics.
Platinum has the temperature range of 650°C, and then the Copper and
Nickel have 120°C and 300°C respectively. The figure-1 shows the
resistance-temperature characteristics curve of the three different metals.
For Platinum, its resistance changes by approximately 0.4 ohms per degree
Celsius of temperature. The purity of the platinum is checked by measuring
R100 / R0. Because, whatever the materials actually we are using for
making the RTD that should be pure. If it will not pure, it will deviate from the
conventional resistance-temperature graph. So, α and β values will change
depending upon the metals.

Detector
 Construction of Resistance
Temperature Detector or RTD
 The construction is typically such
that the wire is wound on a form
(in a coil) on notched mica cross
frame to achieve small size,
improving the thermal
conductivity to decrease the

Resistance Temperature Detector
 Signal Conditioning of RTD
 We can get this RTD in market. But we must know the
procedure how to use it and how to make the signal
conditioning circuitry. So that, the lead wire errors and other
calibration errors can be minimized.
 In this RTD, the change in resistance value is very small with
respect to the temperature. So, the RTD value is measured by
using a bridge circuit. By supplying the constant electric current
to the bridge circuit and measuring the resulting voltage drop
across the resistor, the RTD resistance can be calculated.
Thereby, the temperature can be also determined. This
temperature is determined by converting the RTD resistance
value using a calibration expression. The different modules of
RTD are shown in below figures.

Detector
 In two wires RTD Bridge, the dummy wire is absent. The output
taken from the remaining two ends as shown in fig.3. But the
extension wire resistances are very important to be considered,
because the impedance of the extension wires may affect the
temperature reading. This effect is minimizing in three wires
RTD bridge circuit by connecting a dummy wire C. If wires A
and B are matched properly in terms of length and cross
section area, then their impedance effects will cancel because
each wire is in opposite position. So that, the dummy wire C
acts as a sense lead to measure the voltage drop across the
RTD resistance and it carries no current. In these circuits, the
output voltage is directly proportional to the temperature. So,
we need one calibration equation to find the temperature.

Detector
 Expressions for a Three Wires
RTD Circuit

Detector
 if we know the values of VS and
VO, we can find Rg and then we
can find the temperature value
using calibration equation. Now,
assume R1 = R2:

Detector
 If R3 = Rg; then VO = 0 and the
bridge is balanced. This can be
done manually, but if we don’t
want to do a manual calculation,
we can just solve the equation 3
to get the expression for Rg.

Detector
 This expression assumes, when
the lead resistance RL = 0.
Suppose, if RL is present in a
situation, then the expression of
Rg becomes,

Thermocouple
What is a Thermocouple?
A thermocouple circuit is formed when two
dissimilar metals are joined at both ends and
there is a difference in temperature between the
two ends. This difference in temperature creates
a small current and is called the Seebeck effect
after Thomas Seebeck who discovered this
phenomenon in 1821.

When there is a difference in temperature
between the two ends of this circuit, a small
voltage is formed within the circuit. This voltage
or EMF (electro motive force) is usually
measured in the 1/1000th of a volt (milli volt).
Most people’s body produces more voltage than
that! The higher the difference in temperature,
the higher the voltage. If the right pairs of
materials are used, these thermocouple circuits
can be used to measure temperature.

The junction that is put into the process in
which temperature is being measured is
called the HOT JUNCTION. The other
junction which is at the last point of
thermocouple material and which is almost
always at some kind of measuring
instrument, is called the COLD
JUNCTION.

In the above example, one end of the thermocouple is @ 1000°
and the other end is @ 100° so the difference is 900°. If we
wanted to measure the temperature in a furnace, we could use a
thermocouple to do so. If the above example were used, the
temperature inside the furnace is 1000° and the temperature
outside is 100°, the thermocouple would indicate a difference in
temperature between the inside and outside of 900°. The only
problem with the example above is that we want to know the
temperature inside the furnace, not the difference between the
outside and the inside. To do this with a thermocouple, we need
to apply “Cold Junction Compensation”. To apply this cold
junction compensation, all we need to know is the temperature
of the cold junction.

The measuring instrument normally does
this cold junction compensation. The
instrument measures the temperature at
the point where the thermocouple attaches
and adds that temperature back in to the
equation as per the above example. The
instrument then displays the result of this
equation. It is important to maintain
thermocouple material throughout the
circuit as in the case of a sensor that is
located some distance from the measuring
instrument. Specially coded extension wire
is normally used.

Thermocouple Extras
According to ASTM color code guidelines, which apply to most North American sensor
manufacturers, the Red leg is always negative.
2 types of thermocouples (types J and K) have one leg, which is magnetic. With these 2
types, you can use a magnet to determine polarity.
The hot junction of a thermocouple can be made by any means possible as long as there
is good, constant contact between the two wires.
Special limits of error thermocouple sensors do not have to have special limits of error
extension wire.

Non-thermocouple materials can be used in
thermocouple circuits under the right conditions. Non-
thermocouple connectors, terminals and slices can be
used as long as there is no temperature gradient
present at the areas where these items are used.
Extension wire does not have to be a large gauge to
work in an application where the sensor is placed a
long way from the measuring instrument. Most modern
temperature monitoring instruments are current based
so lead wire resistance is not critical.
It is possible to get an average temperature reading
using multiple thermocouples as long as the sensors
are wired in parallel and the resistance of these
different sensors is the same.

Working principle
The working principle and construction of an Optical
Pyrometer are quite simple. We have drawn an
experimental model of this type of temperature
sensors. It is a measuring instrument that measures
temperature of a hot glowing object. The instrument
has an illuminated reference, with which the brightness
of that of the hot body is matched by controlling the
input electric current of the reference. When, the glow
of the reference matches with the hot object through an
eye piece, that electric current is measured to calibrate
the temperature of the hot body.

Construction of Optical Pyrometer
It is quite simple. Consider it as a cylinder,
which has a lens in one end and in the
other end there is an eye piece. In
between there is a lamp. In front of the eye
piece there is a coloured glass (usually
red), to make lights monochromatic. The
lamp is connected to a battery source
through an ammeter and a rheostat as
shown in the figure.

The optical pyrometer works in a certain simple
process. The process is, the brightness of the filament
of the lamp, that we are using through a battery source
can be controlled by the rheostat. Now by controlling
the incoming current, the brightness of the filament is
increased or decreased. Going through this process
there will be a certain point, when the filament of the
lamp will not be visible from the eye piece. That very
moment the brightness of the filament matches with
the brightness of the hot body as seen through the
monochromatic glass. From the reading of the
ammeter of that particular condition we can get the
temperature of the hot body, as the ammeter is
previously calibrated in temperature scale.

Temperature measurement

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