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 how products and equipment are designed; finding their way into even the most trivial items.

In this series, we explore how engineers accomplished design goals in a time long before the semiconductor revolution by spotlighting ideas that combined brilliant engineering with innovative uses of material properties.

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.

THERMOSTATS

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 mode of its operation. But when a cold engine is first started, this cooling capacity becomes a hindrance, as it can overwhelm an engines ability to rapidly warm up to operating temperature. Thermostats are used to regulate this temperature.

BIMETALLIC SWITCHES

Mechanical control by thermal expansion is simple and very reliable but what if we need to perform a nonmechanical form 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. Bimetallic string bend when heated and can be configured into electrical thermal switches.

FLASHERS

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 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 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. This cycle of opening and closes forms a thermal flasher.

COIL THERMOSTAT

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.

THERMOCOUPLE

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 manner. 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.

Other industrial configurations of control by heat exist, though these methods are more integrated into systematic designs, that are impractical for direct electronic control, they employ thermodynamic properties of working fluids such as air, combustion gases, steam or molten salt and as are generally used for power generation or transmission.

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