Presentation on Temp Measurement

1. CLASSIFICATION OF FILLED SYSTEMS
1. Class I-filled with liquids other than mercury.
2. Class II-filled with vapour.
3. Class III-filled with gas.
4. Class V -filled with mercury

2. Class I-Liquid Filled Systems.
This class of filled systems use liquids other than mercury and are sub-divided into three
groups as under :
Class I-uncompensated.
Class I A - fully compensated.
Class I B -case compensated only.

3. Class II-Liquid Vapour Systems
There are four subdivisions of this class of systems.
Class II A. These are designed for bulb placement in a process whose
temperature always exceeds the case ambient temperature.
Class II B. These are designed for bulb placement in a process whose
temperature is always below the case ambient temperature.
Class II C. These are designed for bulb placement in a process whose
temperature can exist periodically both above and below the case ambient, but
whose temperature when near case ambient is unimportant.
Class II D. These are designed for bulb placement in a process whose
temperature measurement is important when it is near about. case ambient
and fluctuates about it periodically

4. Class II A
Class II B
Class II D
Class II C

5. Class lIl Gas Filled Systems.
Class III systems are of two types
(i) class III A
(ii) class III B .
Class III A systems are fully compensated. They are compensated with a bulb
less capillary and its Bourdon tube and with a short capillary section or a
bimetallic strip.
Class III B are case compensated systems which use a bimetallic strip as a
compensator.

6. Class V -Mercury Filled Systems
There are two types of mercury filled systems
(i) Class V A
(ii) Class VB.
Both these systems are completely filled with mercury or mercury-thallium
eutetic amalgam.
Class V A is completely compensated system provided with standard case and
capillary compensation techniques.
Class V B are case compensated systems

7. Salient Features of Filled Systems
(i) They are one of the most economical, versatile, and widely used
devices for industrial temperature measurement applications.
(ii) They are rugged in construction and there is very little possibility of any
damage during transportation, installation and usage.
(iii) Remote indication at a distance of about 60 m is quite satisfactory.
(iv) The transient response is primarily dependent on the bulb size and the
thermal properties of the enclosed, fluid. The highest response can be
achieved by using a small bulb connected to some type of electric pressure
transducer through a short capillary.
(v) They are usually low in cost, stable in operation and accurate within ±
1°C.
(vi) compensation is required in case of changes in ambient temperature
and also in case long capillary tubes are used.

9. BIMETALIC THERMOMETERS
Bimetallic thermometers are extensively used in process
industries for local temperature measurements.
1. These thermometers use two fundamental principles
2. all metals expand or contract with change in temperature
and the temperature co-efficient of expansion is not the
same for all metals and therefore their rates of expansion
or contraction are different. The difference in thermal
expansion rates is used to produce deflections
proportional to temperature changes.

10. Two Strips of metals with different thermal expansion coefficients are bonded
together at the same temperature such that they cannot move relative to each
other
Since all metals try to change their physical dimensions at different rates when
subjected to same change in temperature, these two metallic strips change their
lengths at different rates.
The differential change of expansion of two metals results in bending of the
bimetallic strip with change in temperature

11. In most practical applications. the metals and their dimensions are so chosen that their
moduli of elasticity and thicknesses are equal i.e EB = EA or n = I and tA = tB or m = 1.
Therefore. from above Eqn. we have.

12. Let us consider the case of a bimetallic strip in the form of a cantilever of length L as
shown in Fig. .
The strip is assumed to bend through a circular arc when sl1bjected to a change in
temperature.
The thickness of each metal forming the strip is t/2. Therefore.
In case one of the metals has a very low thermal
expansion co-efficient say B, we have as αB= O