Industrial thermometers are used for temperature measurement in many industrial applications. They are generally more durable than laboratory thermometers.  The way an industrial thermometer works depends upon the type of temperature sensor it uses. Sensor options include bimetal, liquid or gas-filled, electronic, or infrared (IR).

Liquid or gas-filled thermometers use a gas or liquid as the temperature-sensing element. The liquid expands as the temperature rises in the tube and indicates the temperature. The basic design for this type of thermometer is a small-bore glass tube with a thin-walled glass bulb at the lower end. Typically, the liquid that fills is mercury or alcohol. As heat is transferred into the substance, it expands and pushes the column of liquid or gas higher up the capillary, which indicates the temperature.

When choosing a liquid or gas-filled thermometer, there are several parameters to consider, including:

Material in tube: gas or liquid — Mercury is commonly used to fill thermometers due to its thermal expansion characteristics.  Red alcohol spirit fill is a fill choice that is safer than mercury.

Material of tube: glass or metal — Glass is more common, but metal tubes are best for more rugged applications.  Stainless steel is often used to prevent corrosion and to allow for thermometer mounting.

Level of liquid or gas in the tube — The appropriate volume of liquid or gas in the tube depends on the expansion coefficient of the substance used, the area of the capillary and the range of temperature required. The governing formula is:

h = Volume / Area = b * V_o (T - T_o) / A

Bimetal or bi-metallic thermometers use a bimetal spring as the temperature-sensing element. This coil spring is made of two different types of metals that are welded or fastened together. Choices include copper, steel and brass - as long as one metal has low heat sensitivity and the other metal has high heat sensitivity. Whenever the welded strip is heated, the two metals change length based on their individual rates of thermal expansion. Since the two metals expand to different lengths, the bimetallic strip is forced to bend or curl towards the side with a lower coefficient of thermal expansion. The movement of the strip is used to deflect a pointer over a calibrated scale, which then indicates temperature to the user.

Advantages of bimetal thermomenters include their low cost, durability, ease of installation and use, and accuracy over a wide range of temperatures.  Disadvantages include that only indicating types are available. There's also the possibility of calibration change because of use or environment. Bi-metallic thermometers are not as accurate as glass stem thermometers.

Electronic devices such as thermistors measure changes in electrical resistance and convert them to changes in temperature. For more information, please see Digital Thermometers.

Infrared thermometers are non-contact devices that convert infrared (IR) energy to an electrical signal that can be displayed in units of temperature. For more information, please see Noncontact Infrared Temperature Sensors.

Specifications

Important specifications for industrial thermometers include display options, dimensions, stem material, stem adjustments and operating environment.

Display Characteristics

Thermometers display options include:

• Fahrenheit display, display range, and scale divisions
• Celsius or Centigrade display, display range, and scale divisions
• Both Fahrenheit and Celsius options
• Max/min readings
• Glass or plastic viewing windows

Display range is the minimum and maximum values of temperature that can be displayed. Scale division is the smallest division of degrees that can be displayed. It is sometimes referred to as resolution in digital instruments.

Thermometer Dimensions

Thermometer buyers may need to specify the following dimensions and mounting options:

• Stem extension — The stem is a highly thermal conductor that effectively extends the thermometer bulb into the environment.
• Scale length is the length of the thermometer's readable face. It is the overall length of the measuring portion of the thermometer
• Insertion portion is included in the total stem length. The insertion portion is the part of the stem that gets immersed into the environment.
• Mounting options can be varied and include duct mounts, tanks or process lines mounts and handheld.

Stem Material

Stem materials commonly used are:

• Brass
• Aluminum
• Stainless steel

Mechanical Features

Industrial thermometers may have a stem with an adjustable or permanent angle.

Adjustable angle stems allow for the face and/or stem of the thermometer to be angled for easier use.

Permanently angled stems have a face and stem which cannot be adjusted. This option is best used for applications in which wall mountings are needed.

Operating Environment

Operating environment parameters include:

Operating temperature is the temperature range in which the device will be used. Thermometers are offered in a variety scales which can be used in a wide range of temperatures.

Maximum shock and vibration the device will be exposed to while in use. High levels of shock and vibration can affect the temperature sensing mechanism making the device less accurate.

Weather exposure could affect the life span of the thermometer. Thermometers used for meteoroidal or in outdoor settings should be sealed against water and humidity as well as study enough to face high winds, snow and direct sunlight.

Watertight thermometers are rated for washdown or wet environment applications.

Sanitization is an important specification if the thermometer is going to be used in a clinical setting or in the food and beverage industry.

Applications

Industrial thermometers are used in manufacturing, automotive, HVAC, food and beverage, construction, and other applications.

References

OMEGA's Temperature Measurement and Control Glossary

Singh, S. K. Industrial Instrumentation and Control. 3rd ed. New Delhi: Tata McGraw-Hill, 2009. Print.

Volume Expansion Thermometer

Image credits:

WATTS | Baumer Ltd. | Tel-Tru