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What are the different methods of temperature measurements?

The different methods of temperature measurement are :

1. Mechanical  2. Electrical.

Mechanical methods:


1. Mercury in glass thermometers:
This consists of a glass tube of very fine bore joined to a reservoir at the bottom and sealed at the top. A measured quantity of mercury is the enclosed. When the thermometer is heated the mercury expands much more than the glass and is therefore forced to rise up in the tubing A scale is fixed at the side.

2. Bimetallic Thermometer:
Two metals whose coefficient of linear expansion is different are welded and rolled together to the desire thickness. The actual movement of a bimetal is its flexivity with one end fixed, a straight bimetal strip deflects in proportion to its temperature, to the square of its length and inversely with its thickens.

3.Pressure Spring Thermometers: There are four classes of pressure spring thermometers.

1. Liquid filled      =  class 1

2. Vapor pressure =  class 2

3. Gas filled          =  class 3

4. mercury filled    =  class 4

Liquid filled & Mercury filled :

Both type, operate on the principle of thermal expansion. Where the bulb is immersed in a heated substance. The liquid expands causing the pressure spring to unwind. The indicating, recording or controlling mechanisms are attached to pressure spring.

Compensated Thermometer System:
Compensations are provided in order to nullify the effect of changes in ambient temperature. The compensation in liquid filled expansions thermal system consists of the second tubing and helical element, both liquid filled. The two elements are so constructed that the measuring helical floats on a movable base the position of which is governed by the compensating helical. The two tubing and helicals are matched in volume so that variation in temperature at the instrument case and along the capillary tubing produce equal motion from both helicals. Such motion nullity each other so that only motion produced by varying the bulb temperature actuates the recorder pen.

Gas filled Thermometers:
This type depends upon the increase in pressure of a confirm gas (constant volume) due to temp. increase. The relate between temp. and pressure in this kind of system follow Charles law and may be expressed.

                                                        P1      T1
                                                      ----- = -----
                                                        P2      T2

The system is filled under high pressure. The increase pressure for each degree of temperature rise is therefore greater than if the filling pressure were low. Nitrogen the gas most after used for such systems, because it chemically insert and possesses a favorable coefficient thermal expansion.

Vapor - Pressure Thermometers:
Vapor pressure thermometers depend upon vapor pressure of liquid which only partially fills the system. At low temperatures the vapor pressure increase for each unit temperature charge is small, at higher temperature the vapor pressure change is much greater.

Electrical method of temperature measurement:
1. Thermocouples: It is a simple device consisting of a dissimilar metal wires joined at their ends. when an of each wire is connected to a measuring instrument thermocouples becomes an accurate and sensitive temperature measuring device.


Hot and Cold Junction
Thermocouples Types and Range:

Type T/C

          Positive wire & color

Negative wire & color

   Range °F

    ( J )

Iron & White

Constantan & Red

-300 to1400

   ( K )

Chromel & Yellow

Alumel & Red

-300 to2300

   ( E )

Chromel & Purple

Constantan & Red

-300 to1600

   ( T )

Copper & Blue

Constantan & Red

-300  to 650

   ( R ) 

Platinum and 10%Rhodium & Black

Platinum & Red

  32  to 2700

   ( S )

Platinum and 13%Rhodium & Black

Platinum & Red

  32  to 2700

 

Resistance – Temperature Detectors (RTD):
RTD's are generally used for precise temperature measurement. It consists of a five wire wrapped around an insulator and enclosed in a metal. The most sheath of a resistance thermometer resembles that of bimetallic thermometer bulb.
PRINCIPLE : "Resistance increases as temperature increase"
Rt. = Ro (1 + α t )
Rt. = Resistance of Temperature to measured.
Ro. = Resistance of zero temperature.
α    = Co. off  of thermal (expansion).
t     = Temperature to be measured.
These metals have a positive temperature co-efficient of expansion. Therefore resistance increases as the temperature  increases.

Types of material used: (1) Platinum  (2) Nickel
These metals have a positive temperature co-efficient of expansion. Therefore resistance increases as the temp. increases.

Calculation of  Resistance or Pt100.
Ro.      = 100
x for platinum  = 0.00385 /c.
To calculate Resistance at 100'c.
R100  = 100 [ 1+ ( 38.5  x 10  4  x 100 ) ]
           = 100 + (100  x  0.385)
R100  = 138.5
Resistance at 100'c = 138.5

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written by bob, September 17, 2015
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written by mmmmm, March 16, 2016
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written by mmmmm, March 16, 2016
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written by BOB, April 05, 2016
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    about temp. measuring instruments
    written by praveen singh, March 01, 2017
    It's a good but not a sufficient for more information

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