[Encyclios]

Methods and instruments of measuring temperature

Measurement of temperature cannot be accomplished by direct comparison with basic standards such as length and mass. A standardized calibrated device or system is necessary to determine temperature. In order to measure temperature, various primary effects that cause changes in temperature can be used.

The temperature may change due to changes in physical or chemical states, electrical property, radiation ability, or physical dimensions. The response of the temperature-sensing device is influenced by any of the following factors:

• Thermal conductivity and heat capacity of an element.
• Surface area per unit mass of the element.
• Film coefficient of heat transfer.
• Mass velocity of a fluid surrounding the element.
• Thermal conductivity and heat capacity of the fluid surrounding the element.

Temperature can be sensed using many devices, which can broadly be classified into two categories: contact- and non-contact-type sensors. In case of contact-type sensors, the object whose temperature is to be measured remains in contact with the sensor. The inference is then drawn on the assessment of temperature either by knowing or by assuming that the object and the sensor are in thermal equilibrium. Contact-type sensors are classified as follows:

• Electric thermometers
• Liquid-in-glass thermometers
• Pressure thermometers
• Bimetallic strip thermometers
• Chemical thermometers
• Radiation thermometers

In the case of non-contact-type sensors, the radiant power of the infrared or optical radiation received by the object or system is measured. Temperature is determined using instruments such as radiation or optical pyrometers. Non-contact-type sensors are categorized as follows:

• Radiation pyrometers
• Optical pyrometers
• Fiber-optic thermometers