Bimetal Thermometer   Capillary ThermometerVapor Actuated Thermometer 

Bimetal Thermometer.                 Capillary Thermometer                     Vapor Actuated Thermometer

Image Credit: Dywer                      Image Credit: Kobold                   Image Credit: WIKA

 

What are Dial Thermometers?

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.

How do Dial Thermometers work?

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.

 

Selection Criteria for Dial Thermometers

The GlobalSpec SpecSearch database allows industrial buyers to select dial thermometers by type, temperature sensor, display options, size, features, configurations, operating environment, and applications.

 

Type

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.

o   Advantages include: Low cost, tough, easy to use and install, accurate over a wide range of temperatures

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

Bimetal Coil

Bimetal coil reacts to light. Image Credit: Hustvedt

  • 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.

o   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

Filled

Temperature range (°C)

Scale linearity

Speed of response

Superior features

Less desirable features

Liquid

-87 to 371

Linear except at low temperatures

6-7 sec.

Smaller bulb sizes

Narrower spans

Lower cost

Shortest capillary

Compensation made difficult

Vapor

Above ambient to 343, and below ambient to -184

Scale divisions increase with temperature increases

4-5 sec. except when passing through ambient temperatures

Long capillary length available

Ambient temperature compensation not required

Fast speed

Nonlinear scale

Difficult to provide through ambient temperatures

No overrange capacity

Gas

-268 to 760

Linear except at very low temperatures

4-7  sec.

No head effect

Greatest overrange

Greatest range of temperature

Largest bulb size

Widest spans requires

Least power for control

Mercury

-40 to 649

Linear

4-5 sec.

Most linear scale

Least difficult to compensate

Most power for control element

Objection to mercury on accidental breakage

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 subclasses of vapor systems..

o   Class IIA operates with the measured temperature above the temperature of the rest of the system.

o   Class IIB operates with the measured temperature below the temperature of the rest of the system.

o   Class IIC measures temperatures above and below the temperature of the system.

o   Class IID can overcome the cross-ambient limitation by using a second nonvolatile liquid.


Display Options

GlobalSpec provides information about thermometers with several display options. These 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.

Bi-Metal Thermometer

Bi-Metal Thermometer Display Face. Image Credit: Winters Instruments

 

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

Hot or not? Temperature and thermometers

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?