Overview of the Temperature Sensor

Like all sensors, temperature sensors convert changes in a physical medium into a readable measurement indicating a change. In a temperature sensor, such as the mercury thermometer for example, changes in external heat cause the liquid mercury within the glass to expand or contract, whereupon it rises or falls inside a fine tube marked with a temperature scale in linear proportion to changes in ambient heat energy. The bulb containing mercury is the thermometer’s temperature sensor, while the scale along the length of the glass tube is the readable measurement.

Temperature sensors play vital roles in an enormous range of domestic, commercial and industrial products. In household appliances, they ensure the correct functioning of ovens, refrigerators, and central heating thermostats, where they maintain temperatures within a specified range, activating refrigeration or heating components whenever the set range is transcended to return the ambient temperature to the specified homeostatic level.

In industrial applications, such as chemical engineering for example, they are required to be sensitive enough to detect minute shifts in temperature so that chemical reactions can be properly controlled.

All temperature sensors respond to thermodynamic changes: as heat energy increases, molecules become more agitated and mobile, and systems or media expand and rise in temperature.

History

It would be a mistake to imagine that the temperature sensor was born at a precise historical moment with a single invention. In the third century BCE, the engineer Philo of Byzantine was already aware that air expanded and contracted in relation to heat and constructed an instrument for demonstrating this property (a tube filled with air with one end in a vessel holding water). Fast-forward to the 16th and 17th centuries, and pioneers of science such as Galileo Galilee were adapting these primitive instruments to create the precursor to the thermometer, the “thermoscope”, which demonstrated reliably that changes in heat produced corresponding changes in volume within the device.

A major advance in temperature sensor technology occurred in 1665, when the Dutch mathematician and physicist Christiaan Huygens fashioned the first sealed thermometer containing a quantity of alcohol (earlier thermoscopes had also been open to air pressure which contaminated their ability to accurately reflect changes in temperature only). Huygen’s device exploited alcohol’s volatility, which meant that it would expand and contract markedly in response to alterations in ambient heat. But it wasn’t until 1724 that the first standard scale for measuring changes in temperature became widely adopted in the manufacture of thermometers. Bearing the name of its deviser, Daniel Gilbert Fahrenheit, it is still in use today. Fahrenheit’s thermometer used mercury rather than alcohol, because it expanded and contracted on a more consistently linear basis than alcohol in response to changes in temperature.

Today, many temperature sensors are electronic devices with digital displays.

Technical aspects

There are two broad categories of temperature sensor: those that make direct contact with the medium whose heat they are measuring and those that don’t (contact- and non-contact sensors respectively). Non-contact temperature sensors (or pyrometers) measure radiated heat rather than convected or conducted heat as contact sensors do.

Each category includes thermometers, thermocouples and resistance temperature detectors (RTDs), which measure either the expansion or contraction of a physical substance or changes in electoral resistance and conductivity in response to temperature fluctuations.

RTDs generally provide a highly accurate, electrically mediated measure of temperature by harnessing changes in electrical resistance in the metallic thermal sensor. Alterations in resistance mirror alterations in temperature on a reliably linear basis until the upper end of the device’s scale is exceeded: at temperatures above 700°C, the metal element tends to degrade and measurements become exceptionally inaccurate.

Thermocouples also yield electrically mediated measures of temperature change, although their technical operation is different to RTDs. Typically, two fine wires made from different metals are sheathed within a thin cylindrical casing or thermowell, which protects the delicate thermosensitive elements from chemical or mechanical damage. The wires are joined at one end of the thermocouple and at the other are terminated at a device for measuring voltages. The device depends on the different electrical conductivities of the two metals, a difference which becomes more pronounced at higher temperatures producing increasing voltage differences between them. As a consequence, they are often used in applications which reach exceptionally high temperatures but the scale of temperatures they can encompass is often exceptionally broad, ranging from -200°C to 2100°C.

Where the temperature sensor is used in manufacturing

Thermocouples are widely used in the steel industry to monitor chemical reactions and temperature during the manufacture of steel. Several thermocouples can be linked to computer programs to monitor temperatures at different stages of processing in chemical refineries and production plants.

A range of thermometers, including alcohol, mercury-in-glass and infrared, are used by climatologists to measure temperatures at different locations and depths of the world’s oceans; they’re also widely used to measure external environmental temperatures so that, for example, local authorities can determine whether or not to grit roads if icy conditions are likely to develop.

More recently, the newly emerging field of nanothermometry is measuring the temperature of particles less than one micrometre is size, a feat that had hitherto proved impossible.

How the temperature sensor differs from other sensors

The defining feature common to all temperature sensors is the conversion of a property in one medium (for example the volume of a specific quantity of mercury in a sealed thermometer or the conductivity of a metallic element) into a readable scale as it responds to changes in convected, conducted or radiated heat.

Unlike other sensors, they are not designed to measure changes in movement, create images using infrared light, or to read electronically encoded data inside chips or tags, as for instance RFID sensors do.