Explains the principles underlying all devices used to sense temperature, and how the sensor signals are processed to convert them to indications of temperature. Describes the fundamental concepts of temperature, the Thermodynamic Temperature Scale, and the International Practical Temperature Scale. Covers sensor choice, response characteristics, heat transfer conditions, installation errors, instrumentation compromises, and mechanisms of sensor deterioration.

1. Principle of temperature
measurements

2. Different scales and their relations:
How Temperature scales are Designed
When pressure approaches zero, all real gas will behave like ideal gas,
that is, pV of a mole of gas relying only on temperature. Therefore we
can design a scale with pV as its argument. Of course any bi jective
function will do, but for convenience's sake linear function is the best.
Therefore we define it as "Thermometers and the Ideal Gas Temperature
Scale“
The ideal gas scale is in some sense a "mixed" scale. It relies on the
universal properties of gas, a big advance from just a particular
substance.

3. Practical Applications
Celsius scale: It is used for all scientific work everywhere, and for
all ordinary temperature work everywhere but in the United
States.
Fahrenheit scale: Most scientific and industrial processes now use
Celsius instead of the traditional Fahrenheit scale, and Fahrenheit
is only used primarily in three areas :
1)Atmospheric temperatures as publicly announced. (Celsius
equivalents when in written forms)
2)Baking and cooking temperatures (to avoid confusion with
older publications and users)
3)Body temperatures for medical uses (again, to conform to older
information sources

4. Practical Applications
Kelvin scale: Kelvin scale starts at absolute zero, where all motion
ceases and entropy is zero. All 'action' takes place above this
state of inactivity. Using degrees Celsius won't work in the ideal
gas law. If we give a negative temperature in Celsius. Using PV =
nRT, we would get a negative value for either P, V, or n. we can't
have a negative volume, pressure or number of moles.
Rankine scale: A few engineering fields and scientist measure
thermodynamic temperature using the Rankine scale

5. Different types of Thermometer:
Mercury thermometer
1. The physical quantity that is used to determine the
temperature of a body by means of a mercury thermometer
is the length of the thread mercury, or to be more exact, the
volume of mercury.
2. When the temperature increases, the volume of the
mercury increases too.
3. The sensitivity of a mercury thermometer can be increased
by
a. reducing the diameter of the capillary tube.
b. increasing the size of the bulb.

6. Resistance thermometer
1. Thermometers which use liquids inside the glass are
not suitable to be used for measuring a wide range of
temperature. e.g temperature ranging from -250 degree
celcius to about 700 degree celsius.
2. A suitable thermometer which is used for the above
range of temperatures is a resistance thermometer.
3. Therefore, tis thermometer is very accurate.

7. Thermocouple thermometer
1. An electromotive force (e.m.f) will be produced in
a thermocouple when there is a temperature
difference between the hot junction and the cold
junction. Once this happens, a current will flow.
2. This thermometer is very sensitive and responds
towards slight change in temperature.
3. A thermocouple thermometer is a very sensitive
thermometer which is suitable for measuring
temperatures ranging from -250 degree celsius to
1600 degree celsius.

8. Platinum Resistance Thermometer:
Working Principle
They are slowly replacing the use of thermocouples in many
industrial applications below 600 °C, due to higher
accuracy and repeatability. Common RTD sensing elements
constructed of platinum copper or nickel have a unique,
and repeatable and predictable resistance versus
temperature relationship ( R vs T) and operating
temperature range. The R vs T relationship is defined as the
amount of resistance change of the sensor per degree of
temperature change.

9. Construction
These elements nearly always require insulated leads attached. At
temperatures below about 250°C PVC, silicon rubber or PTFE
insulators are used. Above this, glass fiber or ceramic are used. The
measuring point and usually most of the leads, require a housing
or protective sleeve, often made of a metal alloy which is
chemically inert to the process being monitored. Selecting and
designing protection sheaths can require more care than the
actual sensor, as the sheath must withstand chemical or physical
attack and provide convenient attachment points.

10. Applications
Sensor assemblies can be categorized into two groups by how they are
installed or interface with the process: immersion or surface mounted.
#Immersion sensors take the form of an SS tube and some type of
process connection fitting. They are installed into the process with
sufficient immersion length to ensure good contact with the process
medium and reduce external influences. A variation of this style includes
a separate thermo well that provides additional protection for the
sensor.
# Surface mounted sensors are used when immersion into a process fluid
is not possible due to configuration of the piping or tank, or the fluid
properties may not allow an immersion style sensor.

11. Advantages and Disadvantages:
Advantages:
1. With this thermometer we can conveniently measure the
temperature over a wide range of -200c to 1200c, as in the
case of gas thermometer.
2. It is extremely accurate over the whole of this range. With
certain modifications, it can read temperature correctly up
to 0.002c within the range -190c to600c.
3. It has a large size, because of which it can be conveniently
used for measuring the average temperature of a large
volume.

12. Disadvantages:
1. On account of the low thermal conductivity of the sheath
enclosing the thermometer and also of the large thermal
capacity of the thermometers wire itself, it takes a long
time to acquire the temperature of the bath in which it is
placed.
2. Because of its large size, it is not suitable for measuring
temperature at a point.
3. It is rather a costly instrument because highly pure
platinum is necessary for its construction.