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 microprocessors have fundamentally changed the way products and equipment are designed, finding their way into even the most trivial aspects. In articles in this series we explore how engineers achieved design goals in an era long before the semiconductor revolution, highlighting ideas that combine brilliant engineering with innovative use of materials properties. Arguably one of the most prevalent requirements in the industrial world is temperature sensing and control, from climate sensing and control to automotive and industrial needs, we rely on the ability to regulate temperature in countless ways in the Modern temperature, the detection of electrical characteristics of temperature in semiconductor components, such as a diode, a thermistor or a thermocouple, are used as sensors in older devices. 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 using your microprocessor so that control is performed in software.

Some

thermal

sensors perform this sampling and quantifying in a self-contained package or within an integrated circuit that digitally communicates with the microprocessor; even in low-cost products, this is a common mechanism for temperature detection and control; however, in the pre-electronic era and even today, where mechanical simplicity and reliability are required. We use heat intelligently. Valves, 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 we experience every day. Most materials expand when heated. When heated, the kinetic energy of that material increases as its atoms and molecules move more at the atomic level.

The material will occupy more space due to its movement, so it expands. The naturalist Santorio Santorio or possibly his friend Galileo Galilei were the first to do Using This property of materials to observe the relative heat difference between objects is known as

thermal

range. This instrument eventually evolved into the thermometer refined by German physicist Danielle Gabriele Fahrenheit in the early 18th century. The thermometer evolved into a numerical scale measurement that quantified the thermal

expansion

and contraction of alcohol and eventually mercury, complementing the Fahrenheits temperature scale, was the Celsius scale invented by a Swedish astronomer under Celsius around the same time period in which he aligned his units with the freezing and boiling points of water.

Most vehicle engines operate best around the boiling point of water, maintaining the The heat generated by combustion in the thermal check is a liquid cooling system that flows coolant in a circuit between the engine and a radiator. Typically, the capacity of the cooling system is large enough to cool the engine in all operating modes, but when starting a cold engine for the first time. This cooling capacity becomes a hindrance as it can overwhelm the engine's ability to quickly warm up to operating temperature; Additionally, the ambient temperature and air flowing through the radiator may cool the running engine below its ideal operating temperature range to maintain optimal level. coolant operating temperature a temperature regulating device known as a thermostat is used an automotive thermostat relies solely on 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 is converted The heat of mechanical movement is wax.

When wax is heated, it typically expands between 5 and 20 percent in volume as it melts. If the wax volume is closed, a wax motor known as a linear actuator is created at approximately 180 to 195 degrees Fahrenheit or 82 to 91 degrees. Celsius, the wax begins to melt by expanding and opening a valve that allows coolant to flow through the radiator. If the engine temperature begins to drop, the wax solidifies, shrinking, causing the valve to close and once again allowing the thermostat to block mechanical control of coolant flow. 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, although different metals expand at different rates, this difference in expansion rates allows for some interesting applications if we take two strips of different metals, say steel and brass, and join them together along their length. , we created a bimetallic strip because brass expands more than steel as a bimetallic strip is heated, it bends toward the steel side and can also bend in the opposite direction if cooled below its initial temperature because the 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 circuit. switch now we have a temperature controlled switch, this forms a simple but 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 danger limit, the switch opens and current is cut off as the equipment cools the switch closes restoring current We can further extend the functionality of bimetallic switches by mounting an electrically resistant heating element to the bimetallic strip as current flows through the heating element.

Electrical resistance ca

uses

heat dissipation by raising the temperature of the bimetallic strip as it heats up, thermal movement ca

uses

the bimetallic element to ignite the flow of electrical current, then deflects away from the heating element, cooling it, the bimetallic strip then contracts back to its original state, this opens the switch and restores power to the heating. element The automatic opening and closing cycle of a switch from the movement created by heating and cooling is called thermal intermittent by adjusting the resistance of the heating element and the temperature activation point of the bimetallic strip we can change the speed of the intermittent thermal flashes.

They are quite common and are still used today, although they are slowly being phased out with integrated electronics and LED lighting in strings of flickering incandescent lights, such as Christmas tree lights. Special in-line heat bulbs are used to produce flickering lights on older cars. Heated flashers are the mechanism by which turn signals and hazard signals flash. That clicking sound is created directly by cycling the flasher's thermal switch by balancing the current draw between the heating element in the flasher and the draw of the light bulbs in the circuit. To detect the operation of a bulb becomes possible when a 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, it is by That's why our turn signals or hazard signs flash faster when there is a signal the bulb fails bimetallic strips are durable, easily formed and can be used in various configurations if we wind a bimetallic strip the thermal movement causes the coil to tighten or unwind creating rotation if we calibrate the movement to the temperature of the bimetallic coil we create a relative rotation movement To the temperature we add graduations and an indicator needle and now we have a dial thermometer.

This simple, purely mechanical mechanism not only allows for temperature measurement but also the ability to control it in an adjustable manner. This is how non-electronic adjustable residential thermostats operate the key to their function is a glass bulb containing mercury that works like an electrical liquid tilt switch if we connect it with the mouth 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 change the mercury switch as it progresses. deviates from the desired temperature set by the position of the bimetallic coil as the coil contracts and rotates, tilts the mercury switch activating it and signaling heat as the ambient temperature increases and the coil rotates in the other direction, the switch tilts back to turn off heating activation.

The signals are reversed to cool, although the fundamental operation remains the same. A variant of the adjustable coil thermostat is the linear adjustable thermostat used in older gas furnaces. This adjustable thermostat uses a bimetallic rod that actuates a gas valve as the rod heats and expands by pushing. the valve closes as it cools its contraction opens the gas valve the amount of distance this thermal movement has to overcome to actuate the gas valve is controlled with an adjustable screw that is directly connected to the temperature adjustment dial by changing the distance of action that we control The operating temperature of the thermostat that combines different metals in order to detect the temperature also occurs in other forms when a bond is formed between two different metals, such as with chromel and alumel alloys.

The thermoelectric effect occurs. An electrical potential difference develops across the junction. the voltage change depends on temperature, this is known as a thermocouple. Thermocouples are simple, rugged, inexpensive, and interchangeable, although they are not precise. They are used as temperature sensors for simple and digital control systems. They work well in extreme temperatures that are not practical for other solutions, such as indirect flames. Due to their simplicity and ability to generate small amounts of electrical current, thermocouples are used even today in flame failure devices or monitoring devices. of flame in some gas-burning equipment, such as dryers, furnaces, furnaces, and water heaters.

Gas is used to maintain a small flame for ignition purposes because this gas flow is persistent, it is essential that this pilot light be maintained to avoid filling the surrounding space with flammable gases by placing a thermocouple directly in the flame and using the Carnot generated to keep open an electromagnetic gas pilot valve. We created a fail-safe valve that will cut off the gas supply if the heat from the flame is not perceived. Variations of this mechanism are used even with modern combustion equipment with electronic ignition, as it offers an autonomous and reliable safety mechanism to prevent fire risks.

There are other industrial heat control configurations, although these methods are more integrated into systemic designs that They are not practical for direct electronic control, they use thermodynamic properties of the working fluid, such as air, combustion gases, steam or melt. salt and are generally used for power generation or transmission With the proliferation of low-cost embedded microcontrollers and microprocessors, embedded sensors, and the flexibility of function design through software, the use of heat-triggered control is decreasing even devices Trivial upgrade items such as thermostats, turn signals, and climate control valves. are slowly being replaced by software that synthesizes control driven by electric pumps, relays, solenoids and valves that consolidate functionality within the program code that you