Bimetal temp gauge Basic Principle: These thermometers use the following two principles: 1. All metals change in dimension, that is expand or contr act when there is a change in temperature. 2. The rate at which this expansion or contraction takes place depend on the temperature co-efficient of expansion of the metal and this temperature coefficient of expansion is different for different metals.Hence the difference in thermal expansion rates is used to produce deflections which is proportional to temperature changes.
The bimetallic thermometer consists of a bimetallic strip. A bimetallic strip is made of two thin s trips of metals which hae different coeffcients of expansion. The two metal strips are !oined together b " bra#ing, welding or reeting so that the relatie motion between them is arrested. The bimetallic strip is in the for m of a cantileer beam. An increase in temperature will result in the deflection of the free end of the strip as shown i diagram. This deflection is linear and can be related to temperature changes. The radius of the cur ature of the bimetallic strip which was initiall" flat is determined using the following relationship. $% t&'(1)m*+ ) (1)mn*m+)1mn/0(h-l*(T2-T1*(1)m* +
where, $% radius of the curature at the temperature T2. T % total thickness of the bimetallic strip % (t1)t2* m%t1t2 % Thickness of lower expansion metalthickness of higher expansion metal. l% coefficient of expansion of lower expansion metal. h% coefficient of expansion of higher expansion metal. T1 % 3nitial temperature. T2 % temperature. The following are the important proporties a material should hae to be selected for bimetallic thermometers.
1.
4oefficient of expansion.
2.
5odulus of elasticit".
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6lastic limit after cold rolling.
7.
6lectrical conductiit".
8.
9uctilit".
0.
5etallurgical abilit".
Different common forms of bimetallic sensors are listed. 1.
Helix t"pe.
2.
piral t"pe.
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4antileer t"pe.
7.
;lat t"pe.
5etals used in bimetallic strips are:
High expansion 1.
2.
=ickel-iron allo"d with chromium > manganese.
Low Expansion 3nar (allo" of nickel > iron*.
Application of bimetallic strips and thermometers 1.
The bimetallic strip is used in control deices.
2.
The spiral strip is used in air conditioning thermostats.
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The helix strip is used for process application such as refineries, oil burners, t"re ulcanisers etc.,
Advantages of bimetallic thermometers 1.
The" are simple, robust and inexpensie.
2.
Their accurac" is between )or- 2? to 8? of the scale.
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The" can with stand 8@? oer range in temperaures.
7.
The" can be used where er a mecur" in-glass thermometer is used.
Limitations of bimetallic thermometer: 1.
The" are not recommended for temperature aboe 7@@4.
2.
Bhen regularl" used, the bimetallic ma" permanentl" deform, which inturn will introduce errors.
Bimetallic Thermometer 3n an industr", there is alwa"s a need to measure and monitor temperature of a particular spot, field or localit". The industrial names gien to such temperature sensors are Temperature 3ndicators (T3* or Temperature Cauges (TC*. All these temperature gauges belong to the class of instruments that are known as bimetallic sensors.
Two basic principles of operation is to be followed in the case of a bimetallic sensor. The" are
1.
A metal tends to undergo a volumetric dimensional change (expansion/contraction), according to the change in temperature.
2.
Diferent metals have diferent co-ecient o temperatures. !he rate o volumetric change depends on this co-ecient o temperature.
Working
The deice consists of a bimetallic strip of two different metals and the" are bonded together to form a spiral or a twisted helix.
"imetallic #trip
A change in temperature causes the free end of the strip to expand or contract due to the different co-efficients of expansion of the two metals. This moement is linear to the change in temperature and the deflection of the free end can be read out b" attaching a pointer to it. This reading will indicate the alue of temperature.
The figure below shows the working of a bimetallic sensor. Two metals, blue and red are rieted together. 3f it is used in an oen, the red metal would expand faster than the blue metal. 3f it is used in a refrigerator, a rise in temperature causes the blue metal to expand faster than the red one. As a result the strip will bend upward and short circuits with a metal wore so that current begins to flow. 3f the si#e of the gap between the strip and the wire is ad!usted, "ou can control the temperature.
"imetallic !hermometer
Selection of Temperature Measurement Devices
Isuall" a gien temperature measurement can be satisfactoril" made b" seeral different t"pes of temperature sensing deices. $anges and capabilities of the arious temperature measuring deices oerlap in man" instances. The following shall be used as guidelines for selection of Temperature 6lements.
1.Bi met al l i cTher momet er s 3f the temperature to be measured is not reDuired for automatic control, recording, or indication in the control room, a bimetallic thermometer should be used.
2.Fi l l edSy s t em Ther momet er s Jccasionall" a temperature transmitter, recorder andor controller has to be installed in a location or under circumstances where electrical power is not aailable or inconenient to use, while instrument air (at 2@ psig or aboe* is present. Inder these circumstances, a filled s"stem thermometer is an ideal choice because it can be combined with a commerciall" aailable pneumatic transmitter, recorder andor controller. 3f a local temperature measurement is in an inaccessible location so that a bimetallic thermometer cannot be easil" read, a filled s"stem thermometer is recommended because its capillar" tubing can be led to a n indicator that can be installedlocated in a conenient place.
A liDuid filled thermometer should be considered for corrosie areas or where ibration is a problem.
3.Ther mocoupl es Thermocouples are generall" used for high temperature applications. At high temperatures there is a clear preference for thermocouples oer $T9s. Kro!ect specific reDuirement should be checked before selection of Thermocouple or $T9 for specific application.
4.Res i s t anc eTher mome t er s
Listed below is some temperature measurement applications for which the use of a $T9 is normall" preferred oer a thermocouple:
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!emperature o tur$ine inlet steam %hose design temperature is close to the maximum allo%a$le temperature or piping and e&uipment
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!emperature o permanent tur$ine test points
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Average temperature o nuclear reactor coolant
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Average com$ustion tur$ine inlet air temperature
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'ondenser cooling %ater inlet to outlet temperature gain
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otor stator %inding and $earing temperatures
Thermocouples are not suitable for narrow range temperature measurements. This is because the change in the 65; deeloped b" a thermocouple oer a narrow temperature range is er" small and difficult to measure. Therefore, for narrow spans or small temperature differences a $T9 is recommended.
Borking The working of a bimetallic strip thermometer is based upon the fact that two dissimilar metals behae in a different manner when exposed to temperature ariations owing to their different thermal expansion rates. Jne la"er of metal expands or contracts more than the other la"er of metal in a bimetallic strip arrangement which results in bending or curature change of the strip. The working principle of a bimetallic thermometer is illustrated in figure below. EJne end of a straight bimetallic strip is fixed in place. As the strip is heated, the other end tends to cure awa" from the side that has the greater coefficient of linear expansion.F
1
5ain ;eatures
These t"pes of thermometers work best at higher temperatures, since their accurac" and sensitiit" tends to reduce at low temperatures.
EThe bimetallic strip can be scaled up or down. Jn a large scale, it can proide literall" tones of force for mechanical control or other purposes. Jn a smaller scale, it can proide the force a nd moement for micro machine integrated circuits (MMIs).F2
Applications
;or arious household appliances such as oens etc.
Thermostat switches
Ball thermometers
Crills
4ircuit breakers for electrical heating deices
Bimetallic Strip ThermometersWorking principle and features MARCH 2, 2016 BY SHUTUP LEAVE A COMMENT
3n this post we are going to make "ou familiar with some of the co ncepts related to bimetallic strip thermometer such as what are bimetallic thermometers and where these are used .Be will also discuss the working principle , construction and arious features of these t"pe of thermometers . Let us know what basicall" bimetallic thermometers are and how these are different from other t"pe of thermometers .
Definition construction of Bimetallic thermometer
Borking Krinciple of bimetallic strip
Bhen change in temperature takes place , either expansion or contraction occurs in metals .
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The rate of expansion or contraction depends on the temperature co-efficient of expansion of the metal. This temperature coefficient is different for different metals. Therefore difference in thermal expansion rates is used to produce deflections and these deflections are proportional to temperature changes.
3t means that two dissimilar metals behae in a different manner when exposed to temperature ariations due to owing to different thermal expansion rates. Jne la"er of metal expands contracts more than other metal la"er of metal due to which there occurs bending or curature change of strip. Simple explanation of Bimetallic strip thermometer ;ix one end of a straight bimetallic strip and then start heating the strip . As the strip is heated, other end( end of the strip that is not fixed * tends to cu re awa" from the side that has the greater coefficient of linear expansion.
Explanation of this temperature measuring device
!ore : List of all t"pes of thermometer
These thermometers work best at higher temperatures. This is due to the fact that their sensitiit" and accurac" is less at low temperatures.
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There are arious designs of
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4ertain customi#ation can be done in
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The bimetallic strip can be scaled up or down.
