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Temperature Measurement

In these slides, many figures have been taken from the Internet during my search in Google. Due to the lack of

space and diversity of references, most of the references have not been cited here. I would like to sincerely

thank all of these useful references for their valuable help.

Mechanical/Chemical Temperature Sensors

Liquid Thermometer (usually Mercury, ALCOHOL)

The mercury-in-glass or mercury thermometerwas invented by physicist Daniel GabrielFahrenheit in Amsterdam (1714). It consists ofa bulb containing mercury attached to a glasstube of narrow diameter; the volume ofmercury in the tube is much less than thevolume of the bulb. The volume of mercurychanges slightly with temperature; the smallchange in volume drives the narrow mercurycolumn a relatively long way up the tube. Thespace above the mercury may be filled withnitrogen or it may be at less than atmosphericpressure, a partial vacuum.

Note: Temperature sensors are also called Thermometer.

Thermostat with bimetal coil at (2)

Bimetallic Strip both sensor and actuator

A bimetallic strip is used to convert a temperature change into mechanical displacement.The strip consists of two strips of different metals which expand at different rates as theyare heated, usually steel and copper,

Pressure Thermometer

Applying thermal expansion of liquid or gas.

Change-of-state Sensors

Change-of-state temperature sensors measure just that - a change in the state of a material broughtabout by a change in temperature, as in a change from ice to water and then to steam. Commerciallyavailable devices of this type are in the form of labels, pellets, crayons, or lacquers.

Reversible Liquid Crystal Temperature Label

Electrical Temperature Sensors

A thermocouple is an electrical deviceconsisting of two dissimilar electricalconductors forming electrical junctions atdiffering temperatures. A thermocoupleproduces a temperature-dependent voltage asa result of the thermoelectric effect, and thisvoltage can be interpreted to measuretemperature.

Thermocouple

Seebeck effect

Resistance Thermometer (RTD)

Resistance thermometers, also called resistance temperature detectors (RTDs), are sensors used tomeasure temperature. Many RTD elements consist of a length of fine wire wrapped around a ceramic orglass core but other constructions are also used. The RTD wire is a pure material, typically platinum,nickel, or copper. The material has an accurate resistance/temperature relationship which is used toprovide an indication of temperature. As RTD elements are fragile, they are often housed in protectiveprobes.

RTDs, which have higher accuracy and repeatability, are slowly replacing thermocouples in industrialapplications below 600 °C.

Use electrical resistance and require a power source to operate.

The three main categories of RTD sensors are thin-film, wire-wound, and coiled elements.

Common RTD sensing elements constructed of platinum, copper or nickel.

The resistance ideally varies nearly linearly with temperature.

PT100 sensors (Platinum Resistance Thermometers or RTD sensors)

Two-wire configuration Three-wire configuration Four-wire configuration

The four-wire resistance configuration increases the accuracy of measurement of resistance. Four-terminal sensingeliminates voltage drop in the measuring leads as a contribution to error. To increase accuracy further, any residualthermoelectric voltages generated by different wire types or screwed connections are eliminated by reversal of thedirection of the 1 mA current and the leads to the DVM (digital voltmeter). The thermoelectric voltages will beproduced in one direction only. By averaging the reversed measurements, the thermoelectric error voltages arecancelled out

Thermistor

A thermistor is a type of resistor whose resistance isdependent on temperature, more so than in standardresistors.

Thermistors are of two opposite fundamental types:

With NTC thermistors, resistance decreases astemperature rises.

With PTC thermistors, resistance increases astemperature rises.

Thermistors differ from resistance temperaturedetectors (RTDs) in that the material used in athermistor is generally a ceramic or polymer, whileRTDs use pure metals.

The temperature response is also different; RTDs are usefulover larger temperature ranges, while thermistors typicallyachieve a greater precision within a limited temperaturerange, typically −90 °C to 130 °C. Compared to thermistors,platinum RTDs are less sensitive to small temperaturechanges and have a slower response time. However,thermistors have a smaller temperature range and stability.

Infrared Thermometer / Pyrometer

They are sometimes called laser thermometers as a laser is used tohelp aim the thermometer, or non-contact thermometers ortemperature guns, to describe the device's ability to measuretemperature from a distance.

(Non-contact Sensor)

Infrared thermography (IRT), thermal imaging, and thermal video are examples of infraredimaging science (Infrared Camera). Thermographic cameras usually detect radiation in thelong-infrared range of the electromagnetic spectrum (roughly 9,000–14,000 nanometers or9–14 µm) and produce images of that radiation, called thermograms.

Integrated Circuit (IC) Temperature Sensors

DHT12

Analog Output

I2C Output

1-Wire Output

1-Wire Output

I2C Output

Samples of IC Temperature Sensors

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