Bimetal temp gauge
Bimetal temp gauge
Basic Principle:
Basic Principle:
These thermometers use the following two
These thermometers use the following two principles:
principles:
1. All metals change in dimension, that is expand or contr
1. All metals change in dimension, that is expand or contr act when there is a
act when there is a change in temperature.
change in temperature.
2. The rate at which this expansion or contraction takes place depend on the temperature co-efficient of
2. The rate at which this expansion or contraction takes place depend on the temperature co-efficient of
expansion of the metal and this
expansion of the metal and this temperature coefficient of expansion is different for
temperature coefficient of expansion is different for different metals.Hence the
different metals.Hence the
difference in thermal expansion rates is used
difference in thermal expansion rates is used to produce deflections which is proportional to
to produce deflections which is proportional to temperature
temperature
changes.
changes.
The bimetallic thermometer consists of a bimetallic strip. A bimetallic strip is made of two thin s
The bimetallic thermometer consists of a bimetallic strip. A bimetallic strip is made of two thin s trips of metals
trips of metals
which hae different coeffcients of expansion. The two metal
which hae different coeffcients of expansion. The two metal strips are !oined together b
strips are !oined together b " bra#ing, welding or
" bra#ing, welding or
reeting so that the relatie motion between them is arrested.
reeting so that the relatie motion between them is arrested.
The bimetallic strip is in the for
The bimetallic strip is in the for m of a cantileer beam. An increase in temperature will result
m of a cantileer beam. An increase in temperature will result in the deflection of
in the deflection of
the free end of the strip as shown i diagram. This deflection is linear and can be related to temperature
the free end of the strip as shown i diagram. This deflection is linear and can be related to temperature
changes.
changes.
The radius of the cur
The radius of the cur ature of the bimetallic strip which was
ature of the bimetallic strip which was initiall" flat is determined using the following
initiall" flat is determined using the following
relationship.
relationship.
$% t&'(1)m*+ ) (1)mn*m+)1mn/0(h-l*(T2-T1*(1)m* +
$% t&'(1)m*+ ) (1)mn*m+)1mn/0(h-l*(T2-T1*(1)m* +
where,
where,
$% radius of the curature at the temperature T2.
$% radius of the curature at the temperature T2.
T % total thickness of the bimetallic strip % (t1)t2*
T % total thickness of the bimetallic strip % (t1)t2*
m%t1t2 % Thickness of lower
m%t1t2 % Thickness of lower expansion metalthickness of higher expansion metal.
expansion metalthickness of higher expansion metal.
l% coefficient of expansion of lower expansion metal.
l% coefficient of expansion of lower expansion metal.
h% coefficient of expansion of higher expansion metal.
h% coefficient of expansion of higher expansion metal.
T1 % 3nitial temperature.
T1 % 3nitial temperature.
T2 % temperature.
T2 % temperature.
The following are the important proporties
The following are the important proporties a material should hae to be
a material should hae to be selected for bimetallic thermometers.
selected for bimetallic thermometers.
1
1..
4
4o
oe
effffiic
ciie
en
nt
t o
of
f e
ex
xp
pa
an
ns
siio
on
n..
2
2..
5
5o
od
du
ullu
us
s o
of
f e
ella
as
sttiic
ciitt"
"..
'
7.
6lectrical conductiit".
8.
9uctilit".
0.
5etallurgical abilit".
Different common forms of bimetallic sensors are listed.
1.
Helix t"pe.
2.
piral t"pe.
'.
4antileer t"pe.
7.
;lat
t"pe.
5etals used in bimetallic strips are:
High expansion
1.
<rass
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.
'.
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.
'.
The" can with stand 8@? oer range in temperaures.
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. <oth these metals are !oined together at one end b" either welding or rieting. 3t is bonded so strong that there will not be an" relatie motion between the two. The image of a bimetallic strip is shown below.
"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. <imetallic strips are aailable in different forms like helix t"pe, cantileer, spiral, and also flat t"pe.
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
<imetallic thermometers are generall" aailable with 2, ', 7, 8 or 0-inch concentric dials, preferabl" of the non-parallax t"pe (i.e. not isuall" misaligned or displaced*, with external #ero ad!ustment, and 12-inch mounting thread. The stem should be of tainless teel '10*, haing a 17-inch diameter, and of a customi#ed length to suit process reDuirements. The dial orientation ma" be bottom or back, known as EstraightF or EangleF, respectiel". Howeer, an all angle ad!ustable swiel connection is preferred in order to enable the dial to be read from the most conenient location.
<imetallic thermometers are not recommended for continued use aboe 72@ degree 4elsius. The thermal stabilit" of the bimetallic thermometer is an inherent characteristic of the metals used and continued operation cannot be assured aboe 7G14.
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
c
Ther
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
ed
Sy
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.
Listed below is some temperature measurement applications for which the use of a $T9 is normall" preferred oer a thermocouple:
• !emperature o tur$ine inlet steam %hose design temperature is close to the maximum allo%a$le
temperature or piping and e&uipment
• !emperature o permanent tur$ine test points • Average temperature o nuclear reactor coolant • Average com$ustion tur$ine inlet air temperature
• 'ondenser cooling %ater inlet to outlet temperature gain • 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.
<imetallic Thermometers
3ntroduction
Bimetallic thermometers are made up of bimetallic strips formed b" !oining two different metals haing different thermal expansion coefficients. <asicall", bimetallic strip is a mechanical element which can sense temperature and transform it into a mechanical displacement. This mechanical action from the bimetallic strip can be used to actiate a switching mechanism for getting electronic output. Also it can be attached to the pointer of a measuring instrument or a position indicator. Marious techniDues such as rieting, bolting, fastening can be used to bond two la"ers of dierse metals in a bimetallic strip. Howeer the most commonl" used method is welding. ince two metals are emplo"ed to construct a bimetallic strip, hence the" are named so.
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.F1
5ain ;eatures
These t"pes of thermometers work best at higher temperatures, since their accurac" and sensitiit" tends to reduce at low
temperatures.
<imetallic strip thermometers are manufactured in arious designs. Jne of the most popular design i.e. flat spiral is shown
in the figure below. The" can also be wound into a single helix or multiple helix form.
<imetallic thermometers can be customi#ed to work as recording thermometers too b" affixing a pen to the pointer. The
pen is located in such a wa" that it can make recordings on a circling chart.
<imetallic strips often come in er" long si#es. Hence, the" are usuall" coiled into spirals which make them compact and
small in si#e. This also improes the sensitiit" of bimetallic strips towards little temperature ariations.
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
<imetallic strips are one of the o ldest techniDues to measure temperature. The" can be designed to work at Duite high temperatures i.e. upto 8@@; or 20@4. 5a!or application areas of a bimetallic strip thermometer include:
;or arious household appliances such as oens etc.
Thermostat switches
Ball thermometers
Crills
Bimetallic Strip
Thermometers-Working 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
<imetallic strip is a mechanical element that senses temperature and conert it into mechanical
displacement. This mechanical action can be used to actiate a switching mechanism in order to get
electronic output. o bimetallic strip is a deice that measure temperature and gie output in for of
mechanical displacement of electrical output .
Bimetallic thermometers are the thermometers made of bimetallic strips which is formed b" !oining two
different metals that hae different thermal expansion coefficients. There are number of techniDues that
can be used to bind 2 la"ers of different metals .These techniDues include rieting, bolting and
fastening .Jther most commonl" used method is welding.
<imetallic strip 3mages
Borking Krinciple of bimetallic strip
<imetallic strip work on 2 principles
•
Bhen change in temperature takes place , either expansion or contraction occurs in metals .
•
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.
<rass and steel are common metals that are bonded together in a bimetallic strip.
!ore : List of all t"pes of thermometer
<imetallic thermometer features
•
These thermometers work best at higher temperatures. This is due to the fact that their sensitiit"
and accurac" is less at low temperatures.
•
There are arious designs of <imetallic strip thermometers aailable .5ost common and popular
design flat spiral .Jther designs are single helix or multiple helix form.
•
4ertain customi#ation can be done in <imetallic thermometers such as fixing a pen to the pointer.
Bith this pen one can make recordings on a circling chart.
•
