Dial thermometers use a metal pointer on a circular scale to indicate temperature measurements. They have two main components: the needle and a temperature sensor.  The way a dial thermometer works depends on the type of thermometer it is. Options include bimetal, liquid or gas-filled and vapor-tension, they are described below. 

Types of Dial Thermometers

Common dial thermometers include:

Bimetal or bi-metallic - A bimetal thermometer uses a bimetal spring as temperature sensing element. This technology uses a coil spring made of two different types of metals that are welded or fastened together. These metals could include copper, steel or brass as long as one has low heat sensitivity while 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 include: Low cost, tough, easy to use and install, accurate over a wide range of temperatures

Disadvantages include: only indicating types are available, possibility of calibration change due to use or environment, and not as accurate as glass stem thermometers. 

Liquid or gas-filled - A liquid or gas actuated thermometer uses a gas/vapor as temperature sensing element. The liquid expands as the temperature rises, rising in the tube and indicating 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. The liquid or gas that fills the tube is usually 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 things to consider, including:

• Material in tube - gas or liquid
• Material of tube - glass or metal
• Level of liquid or gas in the tube

Comparison Table for Various Filling-Fluid of Filled Thermometers.

Chart Credit: Industrial Instruments for Temperature Control

Vapor-tension-based. This type of thermometer is filled with a permanent gas made from a variety of liquids based upon the range of temperatures needed. It operates on the principle that when there is pressure in a vessel that contains only liquid, the liquid’s vapor increases with temperature and is independent of volume. With a vapor system, you measure temperature at the interface between the liquid and the vapor. For a vapor system to operate properly, the interface must remain in the bulb. It is a less expensive choice often used in HVAC applications.

There are four sub-classes of vapor systems:

• Class IIA operates with the measured temperature above the temperature of the rest of the system.
• Class IIB operates with the measured temperature below the temperature of the rest of the system.
• Class IIC measures temperatures above and below the temperature of the system.
• Class IID can overcome the cross-ambient limitation by using a second nonvolatile liquid. 

Display Options

Dial thermometers display options include:

• Fahrenheit or Celsius, or both on dual displays
• Min/max readings

Digital thermometers usually have an LCD or LED display. T Traditional dial thermometers have a round face with a metal needle, which identifies the measured temperature.

Size Characteristics

Important display or scale characteristics for dial thermometers include:

Display diameter. Common dial or display diameters range from one inch to five inches and up

Probe is a generic term used to describe many types of temperature sensors , which can be application specific. Probes can come in a variety of materials, and sensitivities.

Stem lengths can range from as short as two inches to greater than eighty inches.

Bulb diameter and length

Capillary length. Shorter capillaries provide greater accuracy.

Specialty Features

Specialty features often distinguish dial thermometers. These features can include

• Dial displays
• Digital displays
• Flexible capillary is a bulb or sensing element that is connected to the display with a flexible capillary tube. This feature is for liquid/gas/vapor types only.
• Zero adjustment thermometers can reset to a zero point.
• Max/min indication thermometers can record or indicate maximum and/or minimum values.
• Vibration dampening thermometers have a needle that is capable of absorbing vibrational or shock energy. This is usually done with a liquid filling the dial cavity. Vibration dampening is only available for mechanical needles.

Configurations

Common configurations and features for dial thermometers include

Probe style thermometers use a probe as a temperature sensor.

Surface measurement thermometers are configured for surface measurements only

Pocket style thermometers are small and made to carry in a pocket. They havea clip and/or sheath

National Pipe Thread (NPT) fittings are available on some thermometers

Flange fittings are available on some thermometers

Thermowell fittings are available for some thermometers. Thermowells are used to protect temperature sensors that monitor industrial processes while permitting accurate measurement. They are typically installed in piping systems and subject to both hydrostatic and aerodynamic forces

Operating Environment

Operating environment is an important criteria to consider when selecting a dial thermometer. Such criteria include:

• Operating temperature
• Maximum shock
• Maximum vibration
• Weather
• Humidity
• Sanitization

Applications

Common applications for dial thermometers include:

• Industrial
• Laboratory
• Food service
• Meteorological

References

OMEGA’s Temperature Measurement and Control Glossary

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

What is a Dial Thermometer?

Image credits:

Dwyer Instruments | Winters Instruments