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4.2 Bimetallic Thermometers

T. J. CLAGGETT, R. W. WORRALL(1969, 1982) B. G. LIPTK (1995)

S. EDVI, J. E. JAMISON(2003)

Temperature Ranges : Minimum standard span is 150 F (50 C); maximum standard span is 800 F (450 C).Standard ranges in approximately 15 steps are available from 100 to 125 F ( 70 to50 C) up to 200 to 1000 F (100 to 550 C). According to DIN 16203, the heating,ventilation, and air conditioning (HVAC) sector has graduated, indicating ranges from 22 to 392 F ( 30 to 200 C).

Materials of Construction : External parts are made of copper alloys, stainless steel, or CrNi steel with acrylicor industrial sheet glass view windows. The thermowells can be brass, steel, stainlesssteel, or other special-order materials.

Dimensions : Dial diameters vary from 1 to 6.5 in. (25 to 165 mm); stem lengths vary from 2.5 to36 in. (63 to 914 mm).

Costs : A 3 in. diameter, back-connected unit with a 2.5 in. brass thermowell lists at $60; a5 in. diameter, multiangle unit with a 6 in. stainless steel thermowell is $65. Ther-mometers are usually discounted from 10 to 45 % when large quantities are purchased.

Partial List of Suppliers : ABB Kent-Taylor (www.abb.com )Abbeon Cal Inc. (Strap-on) ( www.abbeon.com )AMETEK Inc. U.S. Gauge Div. ( www.ametekusg.com )Anderson Instrument Co. ( www.andersoninstrumentco.com )Ashcroft-Heise Div. of Dresser Industries ( www.dresserinstruments.com )Bacharach Inc. ( www.bacharach-inc.com )Bristol Babcock Co. Helicoid Div. ( www.bristolbabcock.com )Brooklyn Thermometer Co. Inc. ( www.brooklynthermometer.com )Cooper Instrument Corp. ( www.cooperinstrument.com )Crosby Inc.Dresser Industries, Instrument Div. ( www.dresser.com/instruments )Duro United Instrument Corp. (www.uidproducts.com )Fenwal Controls Inc. ( www.fenwalcontrols.com )FW Murphy ( www.fwmurphy.com )Grifth Industrial Products ( www.versagauge.com )Jumo Process Control Inc. ( www.jumoprocesscontrol.com )Marshall Instruments Inc. ( www.marshbellofram.com )Marshalltown Instruments Inc. ( www.marshbellofram.com )Marsh Instrument Co. ( www.marshbellofram.com )Mercoid Div. of Dwyer Instruments Ltd. ( www.dwyerinstruments.com )Moeller Instrument Co. ( www.moellerinstrument.com )Noshok Inc. ( www.noshok.com )Omega Engineering Inc. ( www.omega.com )Pacic Transducer Corp. (Surface) ( www.ptc1.com )Palmer Instruments Inc. ( www.palmerinstruments.com )Princo Instruments Inc. ( www.princoinstruments.com )PSG Industries Inc. ( www.psgcontrols.com )Qualitrol Corp. ( www.qualitrolcorp.com )Reotemp Instrument Corp. ( www.reotemp.com )Stortz, Div. of PSG Industries Inc.

Taylor Thermometer Corp. of America ( www.metroscales.com )

TI

Flow Sheet Symbol

2003 by Bla Liptk

http://www.abb.com/http://www.abb.com/http://www.abbeon.com/http://www.ametekusg.com/http://www.andersoninstrumentco.com/http://www.dresserinstruments.com/http://www.bacharach-inc.com/http://www.bristolbabcock.com/http://www.brooklynthermometer.com/http://www.cooperinstrument.com/http://www.dresser.com/http://www.fluidproducts.com/http://www.fluidproducts.com/http://www.fenwalcontrols.com/http://www.fwmurphy.com/http://www.versagauge.com/http://www.jumoprocesscontrol.com/http://www.marshbellofram.com/http://www.marshbellofram.com/http://www.marshbellofram.com/http://www.dwyerinstruments.com/http://www.moellerinstrument.com/http://www.noshok.com/http://www.omega.com/http://www.ptc1.com/http://www.palmerinstruments.com/http://www.princoinstruments.com/http://www.psgcontrols.com/http://www.qualitrolcorp.com/http://www.reotemp.com/http://www.metroscales.com/http://www.metroscales.com/http://www.reotemp.com/http://www.qualitrolcorp.com/http://www.psgcontrols.com/http://www.princoinstruments.com/http://www.palmerinstruments.com/http://www.ptc1.com/http://www.omega.com/http://www.noshok.com/http://www.moellerinstrument.com/http://www.dwyerinstruments.com/http://www.marshbellofram.com/http://www.marshbellofram.com/http://www.marshbellofram.com/http://www.jumoprocesscontrol.com/http://www.versagauge.com/http://www.fwmurphy.com/http://www.fenwalcontrols.com/http://www.fluidproducts.com/http://www.dresser.com/http://www.cooperinstrument.com/http://www.brooklynthermometer.com/http://www.bristolbabcock.com/http://www.bacharach-inc.com/http://www.dresserinstruments.com/http://www.andersoninstrumentco.com/http://www.ametekusg.com/http://www.abbeon.com/http://www.abb.com/

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4.2 Bimetallic Thermometers 591

Tel-Tru Mfg. Co. ( www.teltru.com )Trend Instruments Inc. ( www.trendinst.com )H.O. Trerice ( www.hotrerice.com )Universal Enterprises ( www.ueitest.com )Weiss Instruments Inc. ( www.weissinstruments.com )Weksler Instruments Corp. ( www.dresserinstruments.com )Wika Instrument Corp. ( www.wika.com )

BIMETALLIC SPRINGS

Bimetallic thermometers make use of two fundamentalprinciples: (1) metals change volume with temperature, and(2) this coefcient of change is not the same for all metals(see Figure 4.2a). If two different straight metal strips arebonded together and heated, the resultant strip will bendtoward the side of the metal with the lower expansion rate.Deection is proportional to the square of the length and thetemperature change and inversely proportional to the thickness.A bimetallic spring (Figure 4.2b) can be calibrated to producea predictable deection at a preset temperature. This is the basisof operation of the many bimetallic temperature switches inhousehold appliances.

The motion produced by a bimetallic spring is small; toamplify it in a reasonably sized space the bimetal strip may bewound in the form of a spiral or a helix. A classic example of an ambient air temperature thermometer is shown inFigure 4.2c. The outside edge of the spiral is pinned to the frame

and a pointer is connected to the center. As the temperatureincreases, the spiral winds up deecting the pointer clockwise.

Knowing the coefcients of expansion of the two metals,their thickness, and the desired scale length and range, thetotal length of the spiral can be computed. A favorite com-bination of metals is low-expanding Invar (64% Fe, 36% Ni)against high-expanding nickel-iron alloy with chromium ormanganese added.

Thermometers

Most industrial or residential bimetal thermometers use ahelical coil which can be designed to t into a stem more easilythan the spiral. The element is surrounded by a protectingtube or thermowell (see Section 4.14 ). The device can bemounted to measure the temperature of the gas or liquid insidea duct. The design is frequently used on domestic furnacesand over the years has replaced most of the glass stem ther-mometers used earlier. Figure 4.2d illustrates a typical bimetal-lic thermometer using a helical coil. Also shown is a protectivethermowell, which allows the removal or replacement of thethermometer without opening up the process tank or piping.

When only periodic checks are needed on a particular tem-perature point, a thermowell only is installed and is protectedfrom dirt accumulation by a cap.

A single helix moves axially as it winds or unwinds withheat and cold. This requires clearance for a vertical movementof the pointer. The difculty can be overcome, if desired, byusing a multiple element, wound coaxially so as to form coilswithin coils. This construction is more costly but has an advan-tage in requiring less immersion depth ( Figure 4.2e ).

FIG. 4.2a

Thermal expansion of materials for stem-type expansion thermom-eters. (Courtesy of WIKA Instrument Corporation.)

FIG. 4.2b

The operation of a bimetallic spring.

Temperature

E x p a n s i o n m m

/ m

0(32)

200(392)

400(752)

600(1100)

800(1500)

1000 C(1800 F)

5

10

15

A l u

m i n

u m S t a i

n l e s

s

N i c k

e l

B r a

s s

I n v a

r

P o r c e l a i n

Quartz

Cold PositionHigh

ExpansionMetal

Hot Position

LowExpansion

Metal

FIG. 4.2c

Bimetallic ambient air thermometer.

LowExpanding MetalCase

Bimetal SpiralFastened to Case

and Pointer

High Expanding Metal

2003 by Bla Liptk

http://www.teltru.com/http://www.trendinst.com/http://www.hotrerice.com/http://www.ueitest.com/http://www.weissinstruments.com/http://www.dresserinstruments.com/http://www.wika.com/http://1083ch4_14.pdf/http://www.wika.com/http://www.dresserinstruments.com/http://www.weissinstruments.com/http://www.ueitest.com/http://www.hotrerice.com/http://www.trendinst.com/http://www.teltru.com/http://1083ch4_14.pdf/

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4.2 Bimetallic Thermometers 593

Carlson, D.R., Temperature Measurement in Process Control, InTech ,October 1990.

Hashemian, H.M. and Peterson, K.M., Assuring Accurate TemperatureMeasurement, InTech , October 1989.

Hormuth, G.A., Ways to Measure Temperature, Control Engineering ,Reprint No. 948, 1971.

Magison, E., Temperature Measurement, InTech , November 2001.

Plumb, H.H., Temperature: Its Measure and Control in Science and Industry, Vol. 4, 5th Symposium on Temperature, National Bureau of Standards,American Institute of Physics, Instrumentation, Systems, and Auto-mation Society, Pittsburgh, PA, 1972.

Schupp, A.B., A Report on Progress in the Field of Bimetallic ActuatedThermometers, Measurement and Control News , October 1990.

2003 by Bla Liptk