Bend me, shape me, anyway you want me
Long as you love me, it's all right
Bend me, shape me, anyway you want me
You got the power to turn on the light

("Bend Me, Shape Me," by S. English & L. Weiss, 1966)

BIMETALLIC STRIP THERMOMETER TECHNOLOGY

PTOA Readers and Students who are reading the PTOA Segments in the intended sequential order know that the measurement response lag associated with fluid-filled systems limits their usefulness for measuring and indicating local process stream temperatures.

Because of the difference in thermal expansion rates between metals, the bending motion of Bimetallic Thermometers makes it possible to measure and indicate local process temperatures faster.

TIs that use bimetallic technology are widely used to indicate local process stream temperatures.

 

At their homes, PTOA Readers and Students observe bimetallic technology in use when noticing a temperature displayed on a common ambient thermometer ...

 

and also when selecting how much heat is needed to operate an electric iron ...

 

or when adjusting the temperature on some types of thermostats.

The use of bimetallic strip thermometers for ironing and thermostats significantly extends the usefulness of the technology from simple temperature indication into the realm of simple, mechanical On-Off Control (aka simple mechanical switch). 

 

HOW BIMETALLIC THERMOMETERS WORK

Coefficients of Thermal Expansion

Bimetallic Thermometers/Temperature Indicators/Switches work because two different metals exposed to heat/cold will expand/contract at different rates.

Many thanks to EngineeringToolbox.com for providing a table of thermal expansion coefficients for metals and other materials which can be accessed at the link below.

Table of Thermal Expansion Coefficients

The higher the number listed on the table, the faster the metal will expand upon being heated and contract upon being cooled.

The list shows that the coefficient of thermal expansion for Brass is 18.7.

The coefficient of thermal expansion for Invar is 1.5.

Conclusion: Brass will expand/contract over 12 times as fast as Invar because:

18.7 / 1.5  = 12.3

(In this case the units do not matter to us. Taking a ratio of one expansion rate to the other means the units cancel out).

 

TEMPERATURE MEASUREMENT VIA  BIMETALLIC EXPANSION

The term "bimetallic technology" hints that the device will involve two different metals working together.

The construction of a Bimetallic Strip is  explained below and impacted by:

• The choice of the two metals.
• The length of the two metal strips.
• The thickness of the two metal strips.

 

The Bimetallic Strip

A bimetallic strip is made out of two different metals, like the above-mentioned  Brass and Invar.

The metal strips are the same length and thickness.

The strips are welded together and fixed at one end.

When sensing heat, the metal with the higher rate of thermal expansion (Brass) will force the bimetallic strip to bend toward the side that has lower rate of thermal expansion (Invar).

Upon getting colder, the metal with the higher rate of thermal expansion (Brass) will rapidly contract, and this movement forces the bimetallic strip to bend toward the opposite side.

In either case, the bending causes the welded strip to deflect from its originally straight status.

Notice that the deflection of the welded strip changes the movement of heated metal molecules into a mechanical movement.

When the mechanical movement is linked to a dial, the temperature indication is expressed in terms that a human being can understand.

 

EVERYDAY USES OF BIMETALLIC EXPANSION TECHNOLOGY

The magnitude of deflection caused by two strips of horizontal or vertical metal welded together is not sufficient to measure temperature.

Making the bimetallic strip longer and then winding it into a spiral increases the mechanical movement of the strip per each degree of sensed change in temperature.

Now its possible to measure temperature with a bimetallic strip!

The graphic above illustrates how the spiral-wound bimetallic strip works in the ambient temperature thermometer shown to the right.

The photo on the below left is a spiral-wound bimetallic strip that has been removed from a thermostat ... just like the one shown to the below right.

 

 

 

 

PROCESS INDUSTRY USES OF TEMPERATURE MEASUREMENT WITH BIMETALLIC STRIPS

Winding the bimetallic strip into a helical structure increases the sensitivity of the instrument sufficiently to use the technology for local process stream temperature measurement.

The above graphic shows a helically-wound Copper-Invar strip at cold ambient conditions on the left hand side.

When heated, the difference in thermal expansion between the metals (copper and invar in this case) causes a twisting action of the shaft.

The shaft is linked to a (green) pointer that is expertly matched to work with the scale on the dial face.

Alert PTOA Readers and Students are probably wondering how the instrument can be inserted into the process piping and not be eroded by the processing gas or liquid stream?

Thermowells (ISA symbol=TW) are inserted into the piping to protect the TI.

 

Alert PTOA Readers and Students will next recognize that the thermowell adds a layer of measurement lag due to the need to conduct the process temperature through the thermowell wall.

Correctamundo! Thermowells are made out of highly conductive materials that are also compatible with the characteristics of the process stream that is having its temperature measured.

Silicone may also be used to fill bimetallic thermometers because that helps dampen some of the vibration from the process lines.

 

BIMETALLIC ON-OFF MECHANICAL SWITCHES

The simple bimetallic strip technology can also be used as a mechanical switch for on-off control.

In the graphic of a bimetallic switch shown below, the yellow metal has a greater rate of expansion compared to the light blue metal.

The picture on the left hand side shows the switch closed at a temperature of 25 °C.

The temperature being measured is increasing to 30 °C.

While the temperature is increasing, the yellow metal is expanding at a greater rate relative to the blue metal.

 

 

When the temperature reaches 30 °C, the yellow metal has expanded to the point that the force of deflection opens the switch.

The contact has been interrupted; we can assume the process has reached a 30 °C setpoint and will  not switch "on" again until the yellow metal cools down, contracts, and once again closes the switch.

Bimetallic strips can be used to make simple on-off control within a fairly wide range of temperatures

Household oil stoves use this technology to control temperatures.

TAKE HOME MESSAGES: Bimetallic Strips are another type of temperature-measuring technology based on thermal expansion. In this case it is not the volume of a liquid that expands with temperature but rather the increase/decrease in length (aka "linear expansion/contraction") that occurs when a metal is exposed to changes in temperature.

Bimetallic technology requires attaching two dissimilar metals together lengthwise.

An increase in temperature causes the metal with the faster thermal expansion rate to lengthen and bend toward the metal with the slower thermal expansion rate. 

Upon cooling, the bimetallic strip will bend in the opposite direction.

The deflection of the bimetallic strip changes the molecular movement of excited, heated metal molecules into a mechanical movement.

The mechanical movement experienced by the bimetallic strip can be linked via a shaft to a pointer that indicates the temperature on a scale that human beings called Outside Process Operators can understand.

Spiral-wound bimetallic strips have many non-industrial uses.

Helically-wound bimetallic strips have the sensitivity of movement needed to be useful measuring process stream temperatures.

TIs with bimetallic strips do not directly come in contact with process streams; they are protected by thermowells.

Bimetallic strip technology can also be used in simple mechanical on-off control switches.

©2016 PTOA Segment 00104
PTOA Process Variable Temperature Focus Study Area
PTOA Process Industry Automation Focus Study Area

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