Temperature measurements are fundamentally important to many industrial processes. Many measurements are made with a probe in or on a pipe or vessel. However, if an object cannot be contacted, measuring temperature may be difficult. Contact sensors such as thermometers, thermocouples, and RTD’s are accurate and cost effective but not always practical. For many industrial applications, optical and infrared temperature sensors can carry out measurements virtually impossible for contact sensors.

Contact devices

There are several popular methods for measuring temperature by contact. Readings are produced by the expansion of a liquid or metal, the generation of a current in the presence of heat, or the generation of resistance to current flow in the presence of heat. The first two are called active devices; they generate a signal. The third is called a passive device; it creates a blockage. All these devices react proportionally to the temperature they contact.

Liquid-in-glass thermometers

Liquid-in-glass thermometers consist of a glass envelope, a responsive liquid, and an indicating scale. They were invented about 300 yr ago and are still the most commonly used temperature-indicating devices.

Some advantages of these thermometers are:

• Low cost

• Simple to use

• Ease of checking for damage

• No power requirement.

• Bimetal thermometers

  Bimetal thermometers use a composite material made up of strips of two or more metals joined together that have different expansion rates. The strip tends to change its curvature when subjected to a change in temperature.

  These devices will retain their accuracy indefinitely. Accuracy is within 1% of scale range at any point on the scale. They can be used beyond their range by 100% below 250 F, 50% up to 500 F, and 10% up to 750 F without damage.

  Thermocouples

  When dissimilar metal wires are joined and the joint is heated, a voltage is produced that is proportional to the temperature. Various combinations of metal wires respond differently to temperatures.

  The selection of thermocouple wire depends on a variety of factors including:

  • Temperature to be measured

  • Compatibility with the ambient atmosphere

  • Cost

  • Millivolt output

  • Linearity.

  • RTDs

    Resistance temperature detectors (RTDs) are based on the change of electrical conductivity with temperature. They consist of an encapsulated coil of wire (the sensing element), internal lead wires, supporting and insulating material, and a protective case. Platinum wire has the optimum characteristics for service over a wide temperature range.

    Noncontact Devices

    When temperatures are extremely high or the object to be measured is remotely located, noncontact devices must be used. These devices use the radiant energy from the hot source to determine its temperature.

    Optical pyrometers

    Optical pyrometers measure the intensity of the radiant flux emitted in a narrow wavelength interval in the visible spectrum. The temperature of the target is determined from its spectral radiant intensity. Temperatures from 1400 to 5200 F can be measured. With special absorbing screens, this can be extended to 10,000 F.

    Infrared sensors

    Infrared sensors collect radiation from a target. The infrared energy is isolated and measured using photosensitive detectors. The detectors convert the infrared energy to an electrical signal, which is then converted into a temperature value. For accurate temperature readings, the target should be larger than the instrument’s field of view. This device can measure temperatures from -32 to 5000 F.

    Temperature sensors

    Contact	Noncontact
    Liquid-in-glass	Optical
    Bimetallic	Infrared
    Thermocouple
    RTD

    Liquid-in-glass Thermometers

    Liquid	Temperature range, F
    Mercury	-38 to +1110
    Mercury alloys	-76 to +250
    Organic alloys	-328 to +450

    Thermocouple comparison

    ISA Type	Positive wire	Negative wire	Recommended temperature, F	Acceptable ambient conditions
    T	Copper	Constantan	-450 to +750	Oxidizing reducing
    J	Iron	Constantan	0 to +1650	Reducing
    K	Chromel	Alumel	0 to +2300	Oxidizing or neutral
    E	Chromel	Constantan	-300 to +1600	Oxidizing
    R,S	Platinum-rhodium	Platinum	0 to +2800	Oxidizing
    B	Platinum 70-Rh30	Platinum 94-Rh6	0 to +3200	Inert or slow oxidizing

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