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 [Music] you 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 you

Channel: New Mind

Views: 197,900

Rating: 4.8265371 out of 5

Keywords: bimetallic strip, before computers, thermal sensors, thermal circuit breaker, industrial controls, new mind, thermostat, thermocouple, thermoelectric effect, mercury thermometer, alcohol thermometer, thermometer, climate control, how a radiator works, celsius vs fahrenheit, thermoscope, flasher relay, temperature controller, explainer video, old technology, interesting engineering, engineering videos, heat control, thermal expansion, the use of thermal expansion, industrial

Id: EiR1gl3Lix8

Length: 13min 50sec (830 seconds)

Published: Mon Feb 04 2019

Reddit Comments

[u/read-a-lot]

Sauce?

[u/Holeysox]

Didn't someone just post this a month ago?

[u/TheBizzleHimself]

Most clever use of thermal expansion I’ve seen is in the Sunbeam Radiant Control toaster

[u/Sanjispride]

Well are they clever or common??