a

Clever Uses Of Thermal Expansion

on any given day we rely on dozens of

hidden computers seamlessly integrated

into our lives to function the low-cost

flexibility and ease of rapid product

development of embedded micro processors

have fundamentally changed how products

and equipment are designed finding their

way into even the most trivial of items

in this series we explore how engineers

accomplished design goals in a time long

before the semiconductor revolution by

spotlighting ideas that combine

brilliant engineering with innovative

use of material properties arguably one

of the most prevalent requirements in

the industrial world is the sensing and

control of temperature from climate

sensing and control to automotive and

industrial needs we rely on the ability

to regulate temperature in countless

ways in modern temperature sensing the

electrical characteristics of

temperature on a semiconductor

components such as a diode a thermistor

or a thermocouple are used as sensors on

older or simpler designs these signals

feed analog electronic circuits for

feedback control but the abundance and

low cost of modern microcontrollers make

it more feasible and common to sample

and quantify these signals by your

microprocessor for control to be

performed in software some thermal

sensors perform this sampling and

quantifying on a self-contained package

or within an integrated circuit that

communicates digitally with the

microprocessor even on low-cost products

this is a common mechanism for

temperature sensing and control however

in the pre electronic era and even today

where mechanical simplicity and

reliability is required we use heat in

clever ways valves switches and other

control mechanisms are engineered around

a simple but powerful property of most

materials they expand when heated

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thermal expansion is one of the more

common physical phenomena we experience

daily most materials expand when heated

when a material is heated the kinetic

energy of that material increases as its

atoms and molecules move about more at

the atomic level the material will take

up more space due to its movement so it

expands Naturalist santorio santorio or

possibly his friend galileo galilei were

the first to make use of this property

of materials to observe the relative

difference in heat between objects known

as a thermal scope this instrument

eventually evolved into the thermometer

refined by German physicist Danielle

Gabriele fahrenheit in the early 1700s

the thermometer evolved into a

numerically scaled measurement that

quantified the thermal expansion and

contraction of alcohol and eventually

mercury complementing fahrenheits

temperature scale was the Celsius scale

invented by Swedish astronomer under

Celsius around the same time period it

aligned its units with the freezing and

boiling point of water most vehicle

engines operate best around the boiling

point of water keeping the heat

generated by combustion in thermal check

is a liquid cooling system that flows

coolant in a circuit between the engine

and a radiator typically the cooling

system capacity is large enough to cool

the engine at all modes of its operation

but when a cold engine is first started

this cooling capacity becomes a

hindrance as it can overwhelm the

engines ability to rapidly warm up to

operating temperature furthermore it's

possible for the ambient temperature and

air flowing through the radiator to cool

the operating engine below its ideal

operating temperature range in order to

maintain the optimum operating

temperature of the coolant a temperature

regulating device known as a thermostat

is used an automotive thermostat relies

purely on the mechanical properties of

thermal expansion and contraction to

regulate the flow of cool

between the engine and the radiator by

valve regulation the key to how it

converts heat to mechanical motion is

wax

when wax is heated it typically expands

5 to 20 percent in volume as it melts if

the wax volume is enclosed a wax motor

is created known as a linear actuator at

around 180 to 195 degrees Fahrenheit or

82 to 91 degrees Celsius the wax begins

to melt expanding and pushing open a

valve that allows coolant to flow

through the radiator if the engine

temperature begins to drop the wax

solidifies shrinking it causing the

valve to close and once again allowing

the thermostat to block coolant flow

mechanical control by thermal expansion

is simple and very reliable but what if

we need to perform non mechanical forms

of temperature based control such as

electrical switching in a manner similar

to wax metals expand when heated though

different metals expand at different

rates

this difference in expansion rates

allows for some interesting applications

if we take two strips of dissimilar

metals say steel and brass and bond them

together along their length we create a

bimetallic strip because brass expands

more than steel as a bimetallic strip

warms up it bends towards the steel side

it can also Bend in the opposite

direction

if cooled below its initial temperature

because metals are excellent electrical

conductors bimetallic strips can be used

to control electricity if we configure a

bimetallic strip in a manner that allows

the thermal motion at a specific

temperature to break or complete the

contact points of an electrical switch

we now have a temperature driven switch

this forms a simple yet reliable

electrical thermostat these types of

switches are often used as thermal

circuit breakers in motors and other

high current electrical equipment if the

temperature exceeds a Hazzard limit the

switch opens and current is cut as the

equipment cools the switch closes

restoring current we can expand on the

functionality of bimetallic switches

further by mounting an electrically

resistant heating element to the

bimetallic strip as current flows

through the heating element the

electrical resistance causes a

dissipation of heat raising the

temperature of the bimetallic strip as

it heats up the thermal motion causes

the bimetallic element to switch on the

flow of electricity current is then

shunted away from the heating element

cooling it the bimetallic strip then

contracts back to its original state

this opens the switch restoring current

back to the heating element the cycle of

self opening and closing a switch from

the motion created by heating and

cooling is called a thermal flasher by

tuning the resistance of the heating

element and the temperature trigger

point of the bimetallic strip we can

change the rate of flashing thermal

flashes are quite common and are still

to use today though they are slowly

being phased out with embedded

electronics and LED lighting on

incandescent blinking string lights such

as Christmas tree lights special thermal

flashing bulbs are used in line in order

to produce blinking lights on older cars

Thermal flashers are the mechanism by

which turn signals and hazard signals

flash that clicking sound is directly

created by the cycling of the flashers

thermal switch by balancing the current

draw between the heating element in the

flasher and the

draw off the lightbulbs being flashed in

the circuit the ability to detect a

bulbs operation becomes possible when a

light bulb fails the amount of current

flowing through the heating element

changes causing the change in the

heating rate this results in a faster

flashing rate this is why our turn

signals or hazard signals flash faster

when a signal bulb fails bimetallic

strips are durable easily formed and can

be used in various configurations if we

coil a bimetallic strip the thermal

motion causes the coil to tighten or

unwind creating rotation if we calibrate

the motion to the temperature of the

bimetallic coil we create rotational

motion relative to temperature add

graduations and an indicator needle and

we now have a dial thermometer this

simple purely mechanical mechanism not

only allows for measuring temperature

but also the ability to control it in an

adjustable manner this is how

residential non electronic adjustable

thermostats operate the key to its

function is a glass ampule containing

mercury which functions as an electrical

liquid tilt switch if we mouth is to a

bimetallic coil the combination of

gravity and a rotatable coil now

functions as an adjustable thermostat

this works by allowing the temperature

to toggle the mercury switch as it

deviates from the desired temperature

set by the bimetallic coil position as

the coil contracts and rotates it tilts

the mercury switch activating it and

signaling for heat as the ambient

temperature rises and the coil rotates

in the other direction the switch tilts

back to off the activating heating the

signals are inverted for cooling though

the fundamental operation remains the

same a variant of the adjustable coil

thermostat is the linear adjustable

thermostat used an older gas ovens this

adjustable thermostat uses a bimetallic

rod that actuates a gas valve

as the rod is heated it expands pushing

the valve closed as it cools its

contraction opens the gas valve the

amount of distance this thermal motion

has to overcome to actuate the gas valve

is controlled with an adjustable screw

which is directly attached to the

temperature setting dial by changing the

distance of actuation we control the

operating temperature of the thermostat

combining dissimilar metals for the

purpose of temperature sensing also

comes in other forms when a junction

between two different metals are formed

such as with the alloys chromel and

alumel the thermoelectric effect occurs

an electrical potential difference

across the junction develops with the

voltage changing in a temperature

dependent matter this is known as a

thermocouple

thermocouples are simple rugged

inexpensive and interchangeable though

they aren't precise they are used as

temperature sensors for both simple and

digital control systems they function

well at temperature extremes that are

impractical for other solutions such as

with indirect flames because of their

simplicity and ability to generate small

amounts of electrical current

thermocouples are used even today in

flame failure devices or flame

supervision devices in some gas

combustion equipment such as dryers

ovens furnaces and water heaters

a tiny flow of gas is used to maintain a

small flame for the purposes of ignition

because this flow of gas is persistent

it is critical that this pilot flame is

maintained in order to prevent filling

the surrounding space with flammable

gases by placing a thermocouple directly

in the flame and using the Carnot

generates to hold open an

electromagnetic pilot gas valve we

create a fail-safe valve that will cut

the gas supply if the heat of the flame

isn't sense variations of this mechanism

are used even with modern electronic

ignition combustion equipment as it

offers a self-contained reliable safety

mechanism for preventing fire hazards

other industrial configurations of

control by heat exists though these

methods are more integrated into

systemic designs

that are impractical for direct

electronic control they employ

thermodynamic properties of working

fluid such as air combustion gases steam

or molten salt and are generally used

for power generation or transmission

with the proliferation of inexpensive

embedded microcontrollers and micro

processors integrated sensors and the

design flexibility of function by

software the use of heat trigger control

is diminishing even trivial improving

devices such as thermostats climate

control flashers and valving are slowly

being replaced with software synthesize

control actuated electric pumps relays

solenoids and valves that consolidate

functionality within program code

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