What is Temperature?
Temperature is defined as the degree of hotness or coldness
measured on a definite scale. Hotness and coldness are the result
of molecular activity. As the molecules of a substance move faster,
the temperature of that substance increases.
What is Heat?
Heat is a form of energy and is measured in calories or BTU's
(British Thermal Units).
Why temperature is measured as a process variable?
There are changes in the physical or chemical state of most
substances when they are heated or cooled. The
measurement of temperature is also important for protection
of the equipment, as uncontrolled high or low temperatures
can cause structural deterioration of pipelines and vessels.
The flow of heat is transferred in three ways: convection, conduction,
a) C d ti
When heat is applied to one part of a substance, it is transferred to all
parts of the substance. The movement is from molecule to molecule.
Gases and liquids are poor conductors. The flow of heat by conduction
takes place most effectively in solids.
Heat transferred by the actual movement of portions of a gas or liquid from one place
to another is called convection. This movement is caused by changes in density due
to rising temperat re For example, in a forced air heating s stem the warm air
entering the room through the supply duct is less dense, and therefore, lighter than
the cooler air already in the room. As the warm air-cools, it drops and moves through
the cool air return and back through the heating system
Heat energy is transferred in the form of rays sent out by the heated
substance as its molecules undergo internal change. Only energy is
transferred. The direction of the flow of heat is from the radiating
source. The radiant energy is then absorbed by a colder substance or
object. Radiation takes place in any medium (gas, liquid, or solid), or in
Absolute zero is the temperature at which the movement of molecules completely stops
Temperature value on given scale can be converted to express on other Scales:
C = ( F-32) x 5/9
F = ( C x 9/5) + 32
K = ( C + 273.2)
Temperature Measuring Sensors
common temperature measuring sensors
3.Resistance Temperature Detectors (RTDs)
These are thermometers filled with either a liquid such as mercury or an
evaporating fluid such as used in refrigerators. In both cases the inside
of the sensor head and the connecting tube are completely full.
Any rise in temperature produces expansion or evaporation of the liquid
so the sensor becomes pressurised. The pressure is related to the
temperature and it may be indicated on a simple pressure gauge.
The problems with glass thermometers are that
1.Mercury solidifies at -40oC.
2.Alcohol boils at around 120oC.
3 Accurate manufacture is needed and this makes
accurate ones expensive.
4.It is easy for people to make mistakes reading
b) Bimetallic Strips
A bimetallic strip is constructed by
two metals with different
coefficients of thermal expansion.
If heat is applied to one end of the
strip, the metal with the higher
coefficient of expansion will expand
more readily than the lower one.
As a result, the whole metallic strip
will bend in the direction of the metal
with the lower coefficient.
Bimetallic strip applications:
it can be used as a fast acting
thermostat to control air
They are usually used to measure
process parameters for local readout.
A thermocouple consists of two pieces of dissimilar metals with
their ends joined together (by twisting soldering or welding).
When heat is applied to the junction, a voltage, in the range of
millivolts (mV), is generated. A thermocouple is therefore said
to be self-powered. Shown is a completed thermocouple circuit.
In order to use a thermocouple to measure process temperature, one
end of the thermocouple has to be kept in contact with the process
d f h h
while the other end has to be kept at a constant temperature. The end
that is in contact with the process is called the hot or measurement
junction. Circuit emf = Measurement emf - Reference emf
If circuit emf and reference emf are known, measurement emf can be
calculated and the relative temperature determined.
To convert the emf generated by a thermocouple to the standard 4-20
mA signal, a transmitter is needed.
Types of thermocouples
Thermocouples exist in many different types, each with its own color codes for the
dissimilar-metal wires. Here is a table showing the more common thermocouple
types and their standardized colors, along with some distinguishing
characteristics of the metal types to aid in polarity identification when the wire
colors are not clearly visible:
An output of 40 millivolts at 1000ºF can be compared to 30 mv for type J and 22 mV
for type K. Type E has more tendencies to change characteristics with time than type
J, K and T.
Three Basic Types Of Thermocouple Assembly
Some thermocouple assemblies are manufactured as follw:
o The exposed junction is often used for the measurement of
static or flowing non-corrosive gas temperatures where the
time must b minimal.
t be i i
o The ungrounded junction often is used for the measurement of
static or flowing corrosive gas and liquid temperatures in critical
o The grounded junction often is used for the measurement of
static or flowing corrosive gas and liquid temperatures and f
d li id
T/C Operation and Installation Aspects
In every thermocouple circuit there must be both a measurement
junction and a reference junction: this is an inevitable consequence of
forming a complete circuit (loop) using dissimilar-metal wires.
As we already know, the voltage received by the measuring instrument
from a thermocouple will be the difference between the voltages
produced by the measurement and reference junctions.
Since the purpose of most temperature instruments is to accurately
measure temperature at a specific location, the effects of the reference
junction’s voltage must be “compensated” for by some means, either a
special circuit designed to add an additional canceling voltage or by a
i l i it d i
d t dd
software algorithm to digitally cancel the reference junction’s effect.
Thermocouple reference tables
When using thermocouples, temperature reference
t bl can be used to convert the mV signal from the
thermocouple into a temperature reading.
All table values are referenced to a cold junction
temperature of 0° C. if the reference junction of a
thermocouple is not at 0°C, the tables can still be
used by applying an appropriate Correction to
compensate f th diff
t for the difference between the reference
junction and 0 ºC.
3. Resistance Temperature Detectors
The resistance of a conductor usually increase as the temperature
increase .if the properties of that conductor are known, the
temperature can be calculated from the measured resistance.
Any conductor can be used to construct an RTD, but a few have been
identified as having more described characterstics than others. The
characteristics which are desired include
1.Stability: in the temperature range to be measured. The material
must not melt, correde, embattle or change electrical characteristics
when subjected to the environment in which it will operate.
2.Linearity: The resistance change with temperature should be as liner
as possible over the rang of interst to simplify readout.
3.High resistively: Less material is needed to manufactor an RTD with
a specified resistance when the matrial has a high characteristic
i ti l
4.Workability: The material must be suitable for configuring for
insertion into the media.
has been accepted as the material, which best fit all
the criteria and has been generally accepted for industrial
measurement between –300 and 1200° F (-150 and 650 °C ).
are commercially available with resistances from 50 to
RTD known as Pt100, because it has 100 Ohm
resistance at 32°F ( 0°C) and increase resistance 0.385 ohms
f every °C of temperature rise.
the resistance of the RTD is found by measurement, the
temperature can b calculated:
be l l
°C = ( Ohms reading – 100 ) / 0.385
accuracy of this calculation is determined primarily by the
accuracy of the reading.
2 wire connection:
To detect the small variations of resistance of the RTD, a temperature
transmitter in the form of a Wheatstone bridge is generally used. The
circuit compares the RTD value with three known and highly accurate
A problem arises when the RTD is installed some distance away from the
transmitter. Since the connecting wires are long, resistance of the wires
changes as ambient temperature fluctuates. The variations in wire
resistance would introduce an error in the transmitter To eliminate this
problem, a three-wire RTD is used.
3-wire RTD connection:
The connecting wires (w1, w2, w3) are made the same length and
therefore the same resistance. The power supply is connected to one
end of the RTD and the top of the Wheatstone bridge.
It can be seen that the resistance of the right leg of the Wheatstone
bridge is R1 + R2 + RW2.
The resistance of the left leg of the bridge is R3 + RW3 + RTD.
Since RW1 = RW2, the result is that the resistances of the wires cancel
and therefore the effect of the connecting wires is eliminated
Thermistor is resistance temperature element made from a semiconductor
material and basically do the same job as an RTD. These elements
generally have a negative Temperature coefficient (NTC) but positive
temperature coefficients are also available over a limited range. They are
only used for a typical range of -20 to 120oC and are commonly used in
small hand held thermometers for every day use.
Th advantage of a Th
Thermistor i it i hi hl sensitive t t
i t is
changes making them useful in temperature trip alarms. Unfortunately they
posses highly non-linear resistive properties which restrict their useful
The mo ell are used to p ote t the dete to and so that the detector can
detector nd o th t
be changed without interrupting the process. One downside of using a
thermowell is the time delay it introduces into the measurement system
due to thermal lag.
Thermowlls should be installed where a good representative sample of the
process fluid temperature can be measured
The optimum immersion length of a thermowell depends on the
If the well is installed perpendicular to the line, the tip of the well should
be between one half and one third of the pipe diameter.
If the well is installed in an elbow, the tip should point towards the flow.
The speed of response of a sensor in a thermowell will be slower than that
of an unprotected buib. Keeping the clearance between bulb and pocket
down to an absolute minimum and filling the space with oil or glycol
(antifreeze) can reduce this effect.