What is a Bimetallic Thermometer : Construction and Its Working

Temperature is the most frequently measured factor in industrial and commercial settings. There are different kinds of industries that rely on the temperature process in a certain range like processing of food, manufacturing of paper, cold storage, pharmaceuticals, etc For that, there are various types of sensing devices are used to detect the temperature in the industries. So temperature detecting device like thermometer plays a key role in the industries due to the reasons like flexible, reliable and economical. The thermometer is one kind of mechanical device, used to measure the temperature rise in different applications using different principles. Measuring the temperature in industries is very important as they affect on many things which we rely on. This article discusses an overview of the bimetallic thermometer and its working.

What is Bimetallic Thermometer?

Definition: A thermometer which uses two different metal strips for converting the displacement of temperature into mechanical. The metals used in the thermometer are steel, copper & brass. These strips are connected and they will enlarge at different rates once they heated. This change will compare with the real temperature & moves a needle beside the scale. These thermometers are low-cost, simple, and strong. The bimetallic thermometer diagram is shown below.


Construction of Bimetallic Thermometer

The construction of this thermometer can be done using two bimetallic strips that are connected to form dissimilar thermal expansion coefficients. Generally, it is a mechanical device where the mechanical action of this device can be used for activating the switching mechanisms to acquire electronic output.


The two metallic strips can be connected by using a welding technique or different techniques like bolting, riveting and fastening, and there is no relative motion among these metals. The metallic strip can be designed with two metals like copper, steel, etc


Working Principle of Bimetallic Thermometer

The working principle of this thermometer mainly depends on two basic temperature properties of the metal like the following.

Once the temperature changes, then there will be a change in the physical dimension of the metals. Whenever the temperature rises, the metal strip turns in the direction of the less temperature coefficient metal. Similarly, when the temperature reduces, then the strip turns in the direction of a high-temperature coefficient metal.


Types of Bimetallic Thermometer

These thermometers are classified into two types like the spiral strip and helical type. These two thermometers are used to keep the thermometer size in a manageable limit.

Spiral Type Bimetallic Thermometer

The designing of this thermometer can be done by wrapping the bimetallic strip into this thermometer. The inside of the spiral can be connected in the direction of the housing, and a pointer can be connected to the outside of the spiral. The temperature which is measured can be read off a standardized scale.

This kind of design is expensive but also saves space. The drawback of this device is, we cannot detach the temperature sensor as well as the dial from each other. These thermometers are used to determine the room temperature or in refrigerators.

Helical Type Bimetallic Thermometer

In many situations, it is essential to detach the pointer from the bimetallic coil. The temperature sensor must be placed in the pipe and the temperature can be displayed at the outside of the pipe.

Bimetal thermometers are designed with a bimetal strip and it is wrapped into a helical coil. The helical bimetal is connected at one end in a measuring tube. A metal rod that can be turned in the direction of the helical coil, is connected to another end.

A pointer in this thermometer can be connected to the higher side of the metal rod. Whenever the measuring tube is heated up, then the helical bimetal winds up to turn the metal rod. The equivalent temperature will be read off over the calibrated scale.

In these thermometers, switch contacts can also be used to close an electrical circuit once the temperature is increased or decreased. Switch contacts are used to control the heating pump.

How to Select?

The following factors need to consider while selecting this thermometer, they are

  • The size of the case or the dial
  • Length of capillary or stem
  • The type of connection
  • Range of temperature

The inappropriate applications may be harmful to the thermometer, so it causes failure, property damage or personal injury. Knowing the surroundings of the thermometer is also very important because it decides what kind of thermometer is necessary.

The different types of errors that occur while measuring the temperature are convection, conduction, radiation, noise, response time, issues in grounding.

Advantages of Bimetallic Thermometer

The advantages are

  • Installation is easy
  • Simple maintenance
  • Accuracy is good
  • Less cost
  • Temperature range is wide
  • Linear response
  • Robust and simple

Disadvantages of Bimetallic Thermometer

The disadvantages are

  • If the measure in low-temperature then they will give a less accurate result.
  • If they handled roughly then calibration can be disturbed
  • These are not suggested for above 400’C temperature.
  • When these thermometers are frequently used, then the bimetallic of this device may permanently bend so errors will occur.


The applications of bimetallic thermometer are

  • Used in control devices
  • A spiral strip-type thermometer is utilized in AC thermostats.
  • The helix strip type is used in refineries, tire vulcanizes, oil burners
  • These thermometers are utilized in household devices which include AC (air conditioner), oven, and apparatus in industries like hot wires, refineries, tempering tanks, heater, etc.


1). Who invented the bimetallic thermometer?

Generally, this thermometer is invented by John Harrison

2). How do you restore the accuracy of a bimetallic thermometer?

The accuracy of thermometers can be lost once they are bumped, exposed to a temperature change, dropped and we can restore their accuracy by adjusting them which is known as calibration.

3). What is a bimetallic strip?

A mechanical element used to sense temperature & change it into a mechanical displacement is known as a bimetallic strip.

4). What is the most accurate method for calibrating bimetallic thermometers?

The most accurate method is the ice water method.

5). What is a bimetallic stemmed thermometer?

This thermometer measures the temperature with the help of a metal probe & a sensor to the end.

Thus, this is all about an overview of the bimetallic thermometer. The measurement of temperature is essential in the process of industries like manufacturing, monitoring, safety, health, etc. Here is a question for you, how to select a bimetallic strip thermometer?