Bimetallic Thermometer-FAQ

The bimetallic thermometer is a temperature measurement instrument that changes temperature into mechanical displacement using a metallic strip. You can measure the temperature via thermal expansion. The thermometer has a bi-metal spring that acts as a temperature sensing element.

The bimetallic thermometer is suitable for various applications. For getting in-depth knowledge about this instrument, this guide will help you understand the functioning and the usage of bimetallic thermometers in detail.

1. What is the bimetallic thermometer?

In any industrial or commercial setting, measuring temperature is a crucial part. For example, industries depend on precise temperature measurement for food processing, manufacturing of paper, etc. The bimetallic thermometer consists of two metal strips that change temperature into mechanical when fastened together.

The two metal strips are heated at different rates that create a bending effect that measures temperature. The heating of the metal strips enlarges them at different rates. The change compares the temperature through the needle on the scale.

2. What is the origin of a bimetallic thermometer?

The word thermometer is derived from the Greek words thermos meaning hot, and meter mean measuring. Thus, the bimetallic thermometer is the device that measures temperature through two main elements, a temperature sensor, and converts it into a numerical valve.

The history of the bimetallic thermometer goes back to the Eighteenth Century. The clockmaker John Harrison was the one who made the invention. He joined two individual metal strips by rivets. He also invented the technique of fusing s brass onto a steel substrate.

3.  How does a bimetallic thermometer work?

The bimetallic thermometer works on the thermal expansion principle of the metal. Thermal expansion is the tendency of a metal to change its volume with the temperature change. Every metal has a varying temperature coefficient.

One metal should be given exposure to low heat sensitivity, and the other is given high heat sensitivity. When given the heat, the two metals expand differently, and the bimetallic strip bends toward the lower coefficient of thermal expansion. The strip points to the calibrated scale show the temperature.

The metal is made of copper, steel, or brass. The temperature coefficient shows the relationship between the changes in the physical dimension of the metal with the temperature changes.

The expansion and contraction of metal depending on the temperature coefficient. At the same level of temperature, metals go through various changes in the physical dimension. The bimetallic thermometer is a reliable and cost-effective way of measuring temperature.

4. What is the working principle of a bimetallic thermometer?

The bimetallic thermometer focuses on the two main properties of the metal. First, the metal expands and contracts to depend on the temperature. Metal has the property of thermal expansion. The temperature coefficient of all types of metal is not the same. The metal expands or contracts differently at the same temperature.

The bimetallic thermometer is made up of a bimetallic strip with two thin strips of metal. The two metals do not bond in proportion with one another. Usually, the high expansion metal is brass, and the low expansion is invar when the temperature on the strip increases, the length of the metal will change with the expansion rate.

The metal strip will bend towards the low coefficient of thermal expansion of the metal. The deflection is directly proportional to the length of the metal strip and inversely proportional to metal thickness. It has two metal strips that are joined together through welding. The welding is made in such a way as to ensure there is no relative motion between them. The change in temperature changes the physical dimension of metals.

5. What are the types of bimetallic thermometers?

There are two types of bimetallic thermometers; one is a helix strip bimetallic thermometer, and the other is a spiral strip bimetallic thermometer.

Helix strip bimetallic thermometer

The helix strip bimetallic thermometer has a bimetal strip that is wrapped into a helical coil. This bimetal connects at one end of the measuring tube, and a metal rod connects at the other end. When heat is given to the measuring tube, the helical bimetal turns the metal rod, and temperature is shown on the calibrated scale.

When the temperature decreases, the metal with a lower coefficient of thermal expansion rotates the shaft by shrinking, and lower temperature shows up in the dial. Therefore, the helix strip bimetallic thermometer is suitable for applications with high temperature and pressure requirements.

Spiral strip bimetallic thermometer

The spiral strip measures the temperature in this type of bimetallic thermometer. When temperature increases, the two metal strips expand differently, which creates the bonding effect. At the outer side of the arc, the metal with a higher coefficient of expansion is formed.

When the temperature decreases, the metal with a lower coefficient forms the inner layer of the arc. The pointer shows the media’s temperature. The spiral strip bimetallic thermometer is suitable for measuring ambient temperature because of its sensitivity to low-temperature variation.

6. What are the considerations before selecting a bimetallic thermometer?

It would help if you considered specific criteria before selecting a bimetallic thermometer for your application:

The bimetallic thermometer should be within the limits of lower and higher temperature ranges. When exposed to extremely high temperatures, metal reaches the expansion limit that causes damage to the thermometer.

The length and diameter of the stem in a bimetallic thermometer are essential to consider for different applications. For instance, you need to determine the immersion length of the tank where a thermometer is required.

A thermowell is an essential part of the bimetallic thermometer that protects the temperature sensors in industrial applications. It has a cylindrical tube that protects the thermometer from any damage. In applications with high temperature, high pressure, and viscosity, it is essential to use thermowell. In addition, it is a cost-effective and low-maintenance tool.

You have to select the type of bimetallic thermometer based on your application. It can be a helix strip or spiral strip. The helix strip is suitable for industrial applications such as refineries, while the spiral strip is applicable in thermostats due to low-temperature sensitivity.

7. What are the advantages of a bimetallic thermometer?

The bimetallic thermometers are mechanical thermometers that are used widely in various industries for controlling temperature. Following are the key benefits of a bimetallic thermometer:

• The bimetallic thermometer is cost-effective, robust, and has a simple structure.
• It is easy to use and can use it for a temperature range of up to 500°C.
• The installation and maintenance of a bimetallic thermometer are easy.
• It provides accurate temperature measurement and is a reliable tool.
• As a bimetallic thermometer is entirely mechanical, it does not require any power source to operate.
• It provides a linear response to temperature change.
• You can use a bimetallic thermometer for wide temperature ranges.

8. What are the disadvantages of a bimetallic thermometer?

Apart from the number of advantages, the bimetallic thermometer has certain disadvantages as well:

• The bimetallic thermometer is not suitable to use for high temperatures.
• For low-temperature applications, you may not get accurate readings as the expansion of metals is smaller. Due to this, the device becomes insensitive to low temperatures.
• The frequent calibration of bimetallic thermometers is required from time to time.
• It is suitable for local mounting only.
• The use of a bimetallic thermometer is limited to applications with manual readings.

9. What are the specifications of a bimetallic thermometer?

The bimetallic thermometer is an important temperature measuring instrument with a rugged yet straightforward structure. It is suitable for measuring at a temperature ranging from -50°C to 600°C. Following are the specifications of a bimetallic thermometer:

• The bimetallic thermometer provides high repeatability with an accuracy of ±1% FSD.
• The case and bezel material is cast aluminum and stainless steel 304.
• The pointer has micro zero adjustments and balanced aluminum.
• The stem material is made of SS 304, SS 316, and brass chrome plated.
• The stem length has a range from 50mm to 500mm.

10. What are the performance features of a bimetallic thermometer?

 

• The external parts of the bimetallic thermometer are corrosion resistant that makes it suitable for harmful chemicals. In addition, the parts are manufactured in maximum strength to meet the requirements of heavy industrial applications.
• The case and bezel are manufactured with precise and high-quality tooling that is approved by OSHA stamping equipment.
• The case of a bimetallic thermometer is filled with silicon for giving additional dampening. Thus, it ensures reliable performance in high humidity environments.
• The pointer has the black painted aluminum that is precisely assembled to the bimetal stem.
• The threaded connection has the 304 stainless steel standard and 316 stainless steel optional.
• In the external reset of the bimetallic thermometer, the stainless steel pinion has the sealing of silicone O ring that helps maintain the hermetic seal’s integrity. There are models without that feature of external reset available.
• The dial has a concave design that increases its readability.
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11. What are the safety considerations of a bimetallic thermometer?

The bimetallic thermometer is suitable for corrosive fluid and high-velocity applications. If you use thermowell in bimetallic thermometer, you can remove thermometer for calibration check without shutting the whole process. During installation, make sure to install thermometers in locations with fewer chances of injury or damage in case of an accidental stem breach.

In the case of a silicone-filled thermometer, it has the limit for use at a temperature ranging from -50°F to 500°F. Therefore, it is better to avoid using silicone-filled bimetallic thermometers in applications where potent oxidizing agents such as nitric acid, oxygen, and hydrogen peroxide are present.

The bimetallic thermometer should not be exposed to a temperature over 800°F as exceeding this temperature limit can damage the bimetal element. There are operating conditions that should be kept in mind for safety reasons. For glass lenses, the temperature of the thermometer should not exceed 200 °F. The limit is 150°F for plastic lenses and silicone-filled thermometers.

There are different lens materials available for different applications. The acrylic plastic lens is suitable for low temperatures and provides good chemical resistance. Polycarbonate plastic has poor chemical resistance and has a higher temperature limit. The laminated safety glass has high corrosion resistance, but it will deteriorate above 200°F.

12. How to calibrate a bimetallic thermometer using the ice-point method?

 

For precision and accuracy, it is necessary to calibrate the bimetallic thermometer from time to time. This method is used to check the stability specifications of the instrument. When you have used the bimetallic thermometer for hot or cold food, it is essential to calibrate it after using it. For calibration using the ice-point method, you need to take a clean Styrofoam cup. Fill half of the cup with ice cubes and the other half with water to make ice water slush.

Dip/ insert the tip of the thermometer into the cup and make sure that it does not touch the bottom and the sides of the cup. Wait for four to five minutes until the needle is steady. If the needle does not read 32°F, turn the nut under the dial until it read 0°C. Clean and sanitize the thermometer for next use. Calibrate it regularly to ensure its accuracy.

13. What are the applications of bimetallic thermometers?

The bimetallic thermometer is the temperature measuring tool widely used in various industries, including chemical and petrochemical plants, wastewater pressure control, refineries, ovens, power, pharmaceutical, food, and beverages. The bimetallic thermometer is suitable for residential as well as commercial purposes. The spiral bimetallic thermometer is ideal for AC thermostats, while the helix type is used in refineries and oil burners.

They are also suitable in control devices, air conditioning thermostats, and in-process applications such as oil, refinery, oil burners, etc. You can also customize the specifications of the bimetallic thermometer, and its usage is also widespread in heavy industrial plants and machinery.

14. What is a bimetallic thermometer with a rear connection?

The bimetallic thermometers with a rear connection have the stems behind the cabinet to start the process connection. The stem designs vary in terms of width, length, and connection type. The temperature ranges of bimetallic thermometers range from -30 to 500°C that makes them suitable for high-temperature applications.

The bimetallic thermometer with a rear connection has a stem made of aluminum or stainless steel. The aluminum stems do not need thermowells for connecting or protecting them from system pressure. The length of the stem ranges from 40 to 600mm, and the cabinet diameter ranges from 40 to 200mm. The bimetallic thermometer with plastic and aluminum material is suitable for operating pressure of up to 6 bar while stainless steel can withstand the pressure of up to 25 bar.

15. How to carry out maintenance of bimetallic thermometer?

A bimetallic thermometer is an essential tool for measuring the temperature in various industrial sectors. It provides accurate measurements, so proper maintenance of a bimetallic thermometer is very necessary. Maintenance also enhances the service life of the thermometer and reduces the failure rate.

To maintain a bimetallic thermometer, it is essential to do a regular inspection of the thermometer probe. If there have been any contamination jams or plastic wrapping, you need to go through the clean processing not to affect the standard measurement. Also, clean the thermometer by regularly removing the surface dust, regular inspection, and calibration to ensure accuracy of the equipment.

16. What are the technical details of a bimetallic thermometer?

The bimetallic thermometer provides accurate temperature measurement for a variety of applications. It has the measuring element of fast response bimetallic helix. The two strips of metal provide a different coefficient of expansion that acts as a function of temperature. The operating pressure of thermowell is at the maximum of 25 bar. The dial angle is approximately 270°C.

The dial face has an aluminum matt finish with fine graduation. The immersion of the bimetallic thermometer is a minimum of 2″ in liquid and 4″ inches in gas. In some cases, the limit exceeds 4″ in liquid and 5″ in gas.

When the stem or bulb of the bimetallic thermometer is exposed to pressure, corrosion, velocity, or abrasion, you should use thermowell. Thermowell plays an important part is you can remove the thermometer without losing the contents of the process.

17. What are the features of a bimetallic thermometer?

• It is a high-quality, low friction tool that provides accurate temperature measurement.
• The measuring point sealing of the bimetallic thermometer is fast and accurate with roughened thread.
• It has a solid, strong case that acts as an anti-resistant to corrosion.
• The vibration effects are minimum because of the ruggedized protected bimetallic element.
• It has a short temperature damping time with the adaptation of a thermowell.
• The bimetallic thermometers provide accurate measurements at higher temperatures since their accuracy tends to decline with low temperatures.
• The design can be customized as recording thermometers by affixing a pen to the pointer. The location of the pen is placed in such a way that it makes recordings.
• The bimetallic strips often come in long sizes. They are coiled into spirals that make them compact.

18. What are the instructions for installing a bimetallic thermometer?

The bimetallic thermometer requires proper care to install without damaging any parts. When you remove it from the packing box, handle it by the case assembly. Do not hold the thermometer by the stem, and also avoid bending the stem. It can cause misalignment of internal parts and damage the thermometer.

The bimetallic thermometer should be mounted at a safe location with average temperature variations. For tightening the thermometer with the apparatus, use a wrench located outside the case. Tighten it in the same manner as a pipe fitting until the scale is in the desired position. Next, locate two or three inches of the free end stem to measure the average temperature. While installing the bimetallic thermometer, keep the maximum case temperature below 200°F.

If you install a bimetallic thermometer with a thermowell, install thermowell into the apparatus first. Then, coat the stem of the thermometer with heat-conducting media, such as glycerin or graphite. If you use the bimetallic thermometer to measure the material that builds up an insulating layer on the stem, it is essential to clean the stem occasionally.

19. What are the components of a bimetallic thermometer?

The bimetallic thermometer measures the temperature using two metals. The components of the bimetallic thermometer consist of a stem, dial with pointer, a bimetallic strip, and instrument connection. The stem has a 6-9.6mm diameter and a 63-1000mm length and has the bimetal helix facing away from the case. The helix has a length of approximately 50mm. The stem should not be exposed to pressure without installing thermowell.

The bimetallic thermometer has a dial with a pointer that is designed in compliance with the EN1390 standards. The instrument connection has a location on the bottom that can be executed as a threaded connection. The bimetallic strip is the most important component of the bimetallic thermometer that has two metal parts with varying coefficients.

20. What are the main specifications while purchasing a bimetallic thermometer?

Four general specifications must be met initially while choosing the bimetallic thermometer for your application. These four components include:

• Model type
• Temperature range
• Mounting thread
• Stem length

Most industries need non-standard specifications; however, these four specifications must be met in every application to achieve maximum functionality of your bimetallic thermometer. The temperature ranges for a bimetallic thermometer start from -100°F to a maximum capacity of 1000°F. You must specify the operating temperature of the process. The operating range should be read mid-scale, between 10 and two O clock on the dial face.

The standard and non-standard stem length of the bimetallic thermometer starts at 2″ and is available up to 80″, depending on the temperature range. The stem has stainless steel material, and it is ideal for matching the diameter and stem length. The standard mounting thread is ½” NPT, the other standard, and metric threads are ¼’’NPT and ½” BSPP. The back connection and bottom connection are different forms of the bimetallic thermometer. Back connection is the simplest and most economical form, and the bottom connection is helpful on top of the tanks.

Conclusion

A bimetallic thermometer is a reliable tool for temperature measurement that has been widely used worldwide for almost a century. It is suitable for critical applications and industrial sectors because of its economic, easy-to-read, and extended product life features. It comprises two metals bonded together to form a helical coil.

At Instruthink, we provide a high-quality and reliable bimetallic thermometer to our customers at affordable rates. Our products are well inspected and comply with international standards. You can contact our team of professionals to get the right equipment for your specific application.