Clever Uses Of Thermal Expansion

On any given day we depend on dozens of hidden computers seamlessly integrated into our lives to function The low-cost flexibility and ease of rapid product development of embedded microprocessors have fundamentally changed the way products and equipment are designed and used. break through even the most trivial of elements in this series we explore how engineers achieved design goals long before the semiconductor revolution by highlighting ideas that combine brilliant engineering with innovative use of material properties, possibly One of the most prevalent requirements in the industrial world is temperature sensing and control From weather sensing and control to automotive and industrial needs we rely on the ability to regulate temperature in myriad ways in modern temperature sensing. electrical temperature characteristics ature in a semiconductor component such as a diode, thermistor, or thermocouple are used as sensors in or simpler designs, these signals feed analog electronic circuitry 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 means of its microprocessor so that the control is carried out in the software.

Some

thermal

sensors perform this sampling and quantification in a stand-alone system. package or within an integrated circuit that communicates digitally with the microprocessor, even in low-cost products. This is a common mechanism for temperature detection and control. Valve switches and other control mechanisms are designed around a simple but powerful property of most materials: they expand when heated. Thermal

expansion

is one of the most common physical phenomena that we experience on a daily basis. Most materials expand when heated. When a material is heated, the kinetic energy of that material increases as its atoms and molecules move ab 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 Fahrenheit thermometer refined by German physicist Danielle Gabriele in the early 1700s. The thermometer evolved into a numerical scale measurement that quantified the thermal

expansion

and contraction of alcohol, and eventually mercury, which complemented the Fahrenheit temperature scale, was the Celsius scale. invented by a swedish astronomer under Celsius around the same time period aligned his units with the freezing and boiling point of water most vehicle engines work best around the boiling point of water keeping the heat generated by combustion in check Thermal is a liquid cooling system that coolant flows in a circuit between the engine and a radiator, usually the capacity of the cooling system is large enough to cool the engine in all operating modes, but when turned on With a cold engine for the first time, this cooling ability becomes a hindrance, as it can overwhelm the engine's ability to quickly warm up to operating temperature.

Ambient temperatures and air flowing through the radiator may cool the running engine below its ideal operating temperature range to maintain optimum coolant operating temperature. A temperature regulating device known as a thermostat is used. the mechanical properties of thermal expansion and contraction to regulate the flow of cold between the engine and the radiator by regulating valves the key to how it converts heat into mechanical motion is wax when wax is heated it typically expands by about 5 and 20 percent by volume as it evaporates melts if the volume of wax is enclosed, a wax motor known as a linear actuator is created around 18 0 to 195 degrees Fahrenheit or 82 to 91 degrees Celsius, the Wax begins to melt, expands, and opens a valve that allows coolant to flow through the radiator.

If the engine temperature starts to drop, the wax solidifies and shrinks, causing the valve to close and reopen. Allowing the thermostat to lock out the mechanical control of coolant flow 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 similar way to wax? Metals expand when heated, although different metals expand at different rates this difference in expansion rates allows for some interesting applications if we take two different metal strips, say steel and brass, and join them along their length, we create a bimetallic strip because brass expands more than steel when a bimetallic strip is heated and bent toward the steel side it can also bend in the opposite direction if cooled below its initial temperature b Because metals are excellent electrical conductors, bimetallic strips can be used to control electricity.

If we configure a bimetallic strip in a way that allows thermal movement at a specific temperature to break or complete the contact points of an electrical switch, we now have a temperature controlled switch. this forms a simple but reliable electric thermostat. These types of switches are often used as thermal breakers in motors and other high current electrical equipment. If the temperature exceeds a risk limit, the switch opens and the current is cut off as the equipment cools. The switch closes and restores power. We can further extend the functionality of bimetal switches by mounting an electrically resistive heating element on the bimetal strip as current flows through the heating element electrical resistance ca

uses

heat dissipation raising the temperature of the bimetal strip as it heats thermal movement ca

uses

the bimetallic element to ignite the fl The flow of electrical current is then diverted from the heating element, cooling it.

The bimetallic strip is then shrunk back to its original state. This opens the switch and restores power to the heating element. The self-opening and closing cycle of a switch from the movement created by heating. and cooling is called thermal flashing by adjusting the resistance of the heating element and the temperature trigger point of the bimetal strip we can change the rate of flashing thermal flashes are quite common and are still in use today although they are being phased out with Built-in electronics and LED lighting in flickering incandescent lights, such as Christmas tree lights, special flashing thermal bulbs are used in-line to produce flickering lights in older cars. created by cycling the turn signal thermal switch by balancing the current draw between the heat of the ignition element in the turn signal light and the draw from the bulbs being turned on in the circuit the ability to detect the operation of a bulb is makes it possible when a bulb fails the amount of current flowing through the heating element changes causing the change in rate of heating this results in a faster flash rate this is why our turn signals or hazard signals flash faster when a signal bulb fails bi-metal strips are durable are easily formed and can be used in various configurations if we wind a bi-metal strip thermal movement causes the coil to tighten or unwind creating rotation if we calibrate the movement at the temperature of the bimetallic coil we create a rotational movement relative to the temperature we add grad uations and an indicator needle and now we have a dial thermometer this simple purely mechanical mechanism allows not only temperature measurement but also the ability to control in an adjustable manner this is how non-electronic residential adjustable thermostats operate the key to their function is a glass bulb containing mercury that works as an electric liquid tilt switch if we connect it to a bimetallic coil the combination of gravity and a rotating coil now works as an adjustable thermostat this works by allowing the temperature to toggle the mercury when it drifts From the desired temperature set by the position of the bimetallic coil as the coil contracts and rotates, it tilts the mercury switch activating it and signaling heat as the ambient temperature rises and the coil rotates in the other direction, the switch Tilts back to off position When activating heater ment, the signals are reversed for cooling, although the fundamental operation remains the same.

A variant of the adjustable coil thermostat is the linear adjustable thermostat used on older gas furnaces. This adjustable thermostat uses a bi-metal rod that activates a gas valve as the rod heats up. expands by pushing the valve closed while cooling its contraction opens the gas valve the amount of dist The way this thermal movement has to be overcome to actuate the gas valve is controlled by an adjustable screw that connects directly to the throttle dial. temperature setting. By changing the actuation distance, we control the operating temperature of the thermostat by combining different metals in order to detect the temperature. it also occurs in other forms when a bond is formed between two dissimilar metals, such as with chromel and alumel alloys.

A thermoelectric effect is produced. An electric potential difference develops across the junction with the change of voltage in a matter dependent on temperature. This is known as a thermocouple. thermocouples. they are simple, robust, inexpensive, and interchangeable, although they are not precise. They are used as temperature sensors for simple and digital control systems. They perform well at extreme temperatures that are impractical for other solutions, such as with indirect flames, due to their simplicity and ability to generate small amounts of electricity. All current thermocouples are still used today in flame failure devices or monitoring devices. of flame in some gas combustion equipment, such as dryers, ovens and water heaters. is persistent it is critical that this pilot flame is maintained to avoid filling the surrounding space with flammable gases by placing a thermocouple directly into the flame and using Carnot generators to hold open a pilot gas electromagnetic valve we create a fail-safe valve that will shut off the gas supply if the heat of the flame is not felt.

Variations of this mechanism are used even with modern electronic ignition combustion equipment, as it offers a reliable, independent safety mechanism to prevent fire hazards. Other industrial configurations of heat control. exists, although these methods are more integrated into systemic designs that are impractical for direct electronic control. they employ the thermodynamic properties of the working fluid, such as air, flue gases, steam, or molten salt, and are generally used for power generation or transmission with the proliferation of low-cost embedded microcontrollers and microprocessors embedded sensors and software function design flexibility heat activated control use is declining even trivial enhancement devices such as thermostats climate control flashers and valves are slowly being replaced by software synthesis control pumps actuated electrical devices, relays, solenoids, and valves that consolidate functionality within the program code.