Temperature Sensors

History of Temperature

“Temperature” indicates the magnitude of kinetic energy of molecules. Historically, temperature measurement has been a mechanical process that utilizes the thermal expansion of gases and liquids. With the changing times, however, the industrial temperature range has been expanding from very low to very high beyond the measurement range of mechanical thermometers, and the demand for measurement accuracy has become more stringent, leading to the development of electronic sensors instead of mechanical sensors. On the other hand, due to increasing demands for measurement accuracy, absolute value discrepancies between sensors could no longer be ignored and the “International Practical Temperature Scale” was established to specify a temperature scale to match the thermodynamic temperature in a practical manner. Since its first adoption as an international scale in 1927, it has gone through multiple revisions due to the expansion of the temperature range and improvements in accuracy and has become the international standard as the “1990 International Temperature Scale (ITS-90).” Below are the units of temperature in use.

a. Fahrenheit [°F]

Derived from the name of a German physicist (Gabriel Daniel Fahrenheit). He was a thermometer researcher who studied the boiling points of various liquids and discovered that different substances had different boiling points and that these boiling points varied with atmospheric pressure. After inventing the alcohol thermometer, he made the first mercury thermometer in 1714 and introduced the Fahrenheit scale. It is currently used in the United States and the United Kingdom.

b. Celsius [°C]

Derived from the name of a Swedish astronomer and experimental physicist (Anders Celsius). In 1742, he created a temperature measurement scale by setting 0 ℃ as the boiling point and 100 ℃ as the freezing point of water, which was reversed after his passing. The Celsius is a temperature unit widely adopted in the world.

c. Absolute Temperature [K]

An Irish physicist, Lord Kelvin of Largs, theoretically defined a temperature scale in 1848 that was independent of the material properties of the thermometer. Later he was elected first President of the International Electrotechnical Commission. Also known as ‘Kelvin temperature’, it is based on the laws of thermodynamics. Absolute temperature is very useful for expressing physical phenomena in equations.

Type of Sensors

Temperature sensors are the oldest among all sensors. They are used not only for temperature control of home appliances and temperature measurement in chemical plants, but also for measurement and control of water level, humidity, flow velocity, and pressure. Temperature sensors can be generally divided into two types: contact and non-contact, as shown below. The contact type directly touches an object for measurement and is widely used because of its simple sensor configuration. Typical examples are platinum resistance thermometers, thermistors and thermocouples. Non-contact type detects the infrared radiation emitted from an object and measures the temperature of the object from the amount of infrared radiation. The sensor configuration is complex. Non-contact type sensors include thermopiles.

a. Platinum Resistance Thermometers

Sensors that measure temperature by measuring electric resistance of metals are called resistance thermometers. Among these, resistance temperature sensors made of platinum, which are chemically stable and easy to obtain in high purity, are used as standard thermometers and IEC 60751 specifies requirements for industrial platinum resistance thermometer sensors. Most of these sensors are made of thin platinum wire wound in a coil shape.
The disadvantage of these sensors is that they are larger in size than other sensors, but smaller versions have been commercialized by vapor deposition or other methods.

Platinum resistance thermometers with protecting tube

Thermal characteristics of a platinum resistance thermometer

Thermal properties of resistance of temperature sensors

b. Thermistors

Thermistors are temperature sensors that utilize the characteristics of semiconductors. Although they have poor linearity in the change of electrical resistance with temperature change [or, in temperature coefficient of resistance] and are less accurate in measurement, they are currently the most widely used temperature sensors because they are compact and about 10 times more sensitive than platinum resistance thermometers.

There are mainly two different types of thermistors: NTC and PTC. PTC thermistors cannot cover a wide temperature range, but they are useful for low temperature because the temperature coefficient of PTC thermistors is nearly an order of magnitude bigger than that of NTC thermistors.

There is also another type called CTR, which has the characteristic of a sudden decrease in internal resistance after exceeding a certain temperature.

Type	Characteristics	Range	Characteristic Curve	Usage
NTC	Resistance decreases as temperature increases Negative Temperature Coefficient	-50 to +400°C		Temperature measurement
PTC	After a certain temperature, internal resistance suddenly increases. Positive Temperature Coefficient (Switching characteristic)	-50 to +150°C		Temperature switch
CTR	After a certain temperature, internal resistance suddenly decreases. Switching characteristic	-50 to +150°C		Temperature alarm

Applications of thermistors:

• Digital thermometers
• Refrigerators, freezers
• Air conditioners

Apart from temperature sensing, it can also be used for other applications such as wind velocity sensors, micro-flow velocity sensors, vacuum sensors, and gas sensors.

c. Thermocouple Probes

A thermocouple is a temperature sensor that utilizes the Seebeck effect, the phenomenon in which a thermoelectromotive force is generated and a current is produced in a circuit containing two different metals when the junctions between the metals are maintained at different temperatures.
In principle, thermocouples measure the electromotive force between the measuring junction and the reference junction. In order to know the temperature of the measuring junction, the temperature of the reference junction must be constant. So, the electromotive force is generally defined with the reference junction at 0°C. (In laboratories, the measurement is often made by placing the reference junction in sherbet-like ice water.)

Characteristics of thermocouples

• Relatively inexpensive and easy to obtain.
• Simple measurement method, high accuracy, and relatively small measurement time delay.
• Enables temperature measurement of a wilder range than thermistors, etc.
• The type and strand diameter can be selected according to sensitivity, life span, and other conditions.
• Enables temperature measurement of small objects and in confined spaces.
• The distance between the measuring object and the instrument can be large, and even if a local temperature change occurs somewhere along the circuit, it has almost no effect on the measured value.

Thermocouples and their range of temperature　(JIS C 1602)

Characteristics of electromotive force of thermocouples

We provide a wide variety of thermocouple thermometers. Thermocouple probes of various materials and forms are available including customized probes. Please feel free to contact us for further information.

Thermocouple probes

Here is the catalog for our thermocouple probe products. Please refer to <FAQ about thermocouples> as well.

d. Thermopiles

Thermopiles, which are simple infrared sensors, consist of thermocouples arranged in series in a very small area.

Reference

Useful information for selecting appropriate sensors and for your measurement.

Temperature range for each sensor