Gas Actuated Thermometer vs. Bimetal Thermometer

Gas-actuated thermometers and bimetal thermometers are both commonly used temperature measurement instruments in multiple industries due to their reliability and precision. However, they operate on different principles. No matter what industry you work in, if you require accurate temperature monitoring, understanding these thermometers can enhance both efficiency and safety in your operations. 

As the temperature product manager at Ashcroft – an industry leader in pressure and temperature instrumentation – I am often asked which of these instruments will be most appropriate for different applications. The answer, of course, depends on several factors. 

Read this article to compare the mechanics, applications, and advantages of gas-actuated and bimetal thermometers. Links to additional resources will also provide relevant information to help deepen your knowledge of temperature measurement. When you are finished reading, you will be in a better position to select the right instrument for your specific needs. 

Industries that use gas-actuated and bimetal thermometers.

Industrial temperature measurement instruments like the Ashcroft® Duratemp Gas-Actuated Thermometer and the Ashcroft® Bimetal Thermometer are used to measure the temperature of air, gas or liquid in harsh indoor and outdoor environments. These thermometers are a good choice when you need a reliable, durable and easily readable indication of temperature that can be seen from a distance to help keep operators safe from hazardous areas. They can also serve as a reference to confirm electronic monitoring.

Figure 1. Industries that use gas-actuated and bimetal thermometers.

How a gas-actuated thermometer works. 

The mechanics of a gas-actuated system include a Bourdon tube, capillary line and temperature-sensing bulb. The entire thermometer, including the bulb, head assembly and capillary tubing (on remote types), is filled with an inert gas, which offers fast response and clear scale markings.

Bulb styles include:

1. Union-connected bulbs. These include threaded swivel nut fittings, which are ideal for ensuring a reliable and sturdy connection in high-pressure or corrosive settings when used with a thermowell.
2. Plain bulbs. These bulbs have no fittings and are well-suited for measuring temperatures in open tanks, sinks or vats. 

Note: Thermowells can be used to protect these thermometers and make it easier to remove thermometers without shutting the system down.

Figure 2. How gas-actuated thermometers work.  

Ashcroft's gas-actuated thermometers can have optional electrical contacts to manage process temperatures. These contacts enhance the control and safety in temperature-sensitive applications and are offered in multiple configurations

Features of gas-actuated thermometers.

Gas-actuated thermometers are available in both direct and remote-mount options and offer numerous benefits:

1. Wide temperature range.  Temperature coverage spans from -320 ° F to 1,200 ° F ( -200 ° C to 650 ° C).
2. Bourdon tube design. With a "movementless" system and no gears to wear out, these thermometers offer strong resistance to shock and vibration and have minimized pointer flutter.
3. Carbon-based molecular sieve. Allows for the lowest internal system pressures to maximize system life.
4. Direct and remote mount options. Gives you the option to use up to 120' capillary line to isolate remote-mount instruments for added safety. 
5. Filled with inert gases. Prevents danger if capillary gets damaged.
6. No head or elevation error. Temperature accuracy will remain the same, regardless of elevation.

How a bimetal thermometer works. 

A bimetal thermometer uses a mechanical strip made of two different metals that are bonded together and then coiled into a spiral or helix. Each of these metals expands at a different rate when heated. As the process temperature changes, the differential expansion of the metals causes the coil to wind or unwind.

This motion is converted into a temperature reading by mechanically connecting it to the pointer which glides across a precisely calibrated scale. This straightforward yet efficient mechanism enables the thermometer to deliver direct temperature readings without the use of external power sources.

Figure 3. How bimetal thermometers work.  

Features of a bimetal thermometer.

Bimetal thermometers have a basic design and are only available for direct-mount installations. Features include

• Hermetically sealed case. Prevents moisture from entering the instrument to protect the interior from corrosion and the coil from freezing.
• Maxivision® dial. Ensures precise readings (minimizes parallax reading errors).
• Silicone-dampened coil. Reduces pointer flutter on high-vibration applications and provides improved response times.
• External adjustment. You can adjust or recalibrate the pointer position on the scale. 
• Customized dial printing. Allows you to select multiple or alternate scales, add zone indicators, company branding and more. 
• Everyangle™ connection. Enables the dial to be rotated 360º.

Gas actuated and bimetal thermometer differences. 

While both gas-actuated and bimetal thermometers are accurate and durable options for measuring air, gas or liquid temperature in challenging environments, some differences will help you determine which one is best for your unique needs. 

1. Temperature range. For more extreme temperatures, the gas-actuated thermometer offers wider temperature coverage.

Figure 3. Bimetal thermometer vs. gas-actuated thermometer temperature coverage. 

2. Mounting options.

• Bimetal thermometers come in direct-mount options only. 
• Gas-actuated thermometers have the option of direct-mount or remote-mount. 

3. Cost.

• Bimetal thermometers have a cost range of about $20 - $200, depending on the dial size and other factors. 
• Gas-actuated thermometers are significantly more expensive due to the type of inert gas that is used and the type and length of the capillary. For example, you may spend as low as $250 and upwards of $1,500 or more based on the different variables. If cost is the only consideration, bimetal is more cost-effective.

4. Electric contacts.

• Option for gas-actuated thermometers only.

5. Dial sizes.

• Bimetal thermometers typically have a 2-inch, 3-inch and 5-inch dial size.
• Gas-actuated thermometers are typically 4 1/2 -inch, 6-inch and 8 1/2 inch.
  

Ready to learn more?

Now that you understand the features, benefits and uses of gas-actuated and bimetal industrial thermometers, and have compared their differences, you may still have questions. After all, we just scratched the surface of industrial temperature instruments and realize you may still have questions. The following are a few related articles that may interest you on the subject. 

• How temperature is measured
• How Much Do Industrial Bimetal Thermometers Cost?
• How Does Temperature Affect Pressure Gauge Performance?
• When to use an RTD or Thermocouple

You can also reach out to our product experts if you have questions about temperature measurements or instrumentation. In the meantime, download our guide to learn about RTD and Thermocouple Temperature Probes.