When to Use Gas-Actuated Thermometers

This article was originally published on December 19, 2022, and updated on October 8, 2025.

In industries where temperature extremes, vibration, or hazardous process environments are part of daily operations, accurate and reliable temperature measurement becomes a serious challenge. In these environments, operators need instruments that deliver accurate readings from a safe distance, reducing the need to approach hazardous areas.

Gas-actuated thermometers are an effective solution for these applications. Their robust design, wide measurement range, large dial size  and remote mounting capabilities make them ideal when both safety and accuracy are priorities.

As a global authority in pressure and temperature instrumentation, Ashcroft has helped manufacturers and process operators improve reliability and reduce downtime for more than a century. In this article, you’ll learn:

• How gas-actuated thermometers work
• When and where they are most effective
• Why remote mounting and thermowell selection matter
• Which design options can extend instrument life in extreme conditions

When are gas-actuated thermometers used?

Gas-actuated thermometers are best used when applications involve:

• Extreme temperature ranges. Common in refineries and power generation plants, where process media can swing from cryogenic to several hundred degrees and standard thermometers cannot operate reliably.
  
  
• Vibration or shock. Found in offshore oil rigs or pulp and paper mills, where rotating machinery or pumps can cause continuous vibration. The “movementless” Bourdon tube design helps minimize wear and eliminates pointer flutter.
  
  
• Remote monitoring requirements. Typical in chemical and petrochemical facilities, where operators must read temperatures from safe control areas away from high-pressure or corrosive processes. Capillary lengths up to 80 feet or 100 meters allow operators to mount the dial in a safe, accessible location while the sensing bulb is near the process source.
  
  
• Hazardous or confined environments. Seen in food, beverage, and pharmaceutical manufacturing, where sanitary requirements or confined equipment layouts make remote mounting safer and more practical. Remote mounting lets personnel take readings from control panels or safe areas, away from heat, obstructions, or caustic media.
  
  
• Outdoor or wide-area systems. This can include water and wastewater treatment sites, where thermometers may need to be installed far from monitoring stations.

How do gas-actuated thermometers work?

Gas-actuated thermometers use a Bourdon tube system filled with an inert gas to sense and indicate temperature changes. As the temperature rises, the gas expands within the sealed system, creating pressure inside the Bourdon tube that moves the pointer across a calibrated scale.

In remote-mount designs, the gas pressure travels through a capillary line that connects the sensing bulb to the dial, allowing the indicator to be installed at a safe or more accessible distance.

Because the system is completely sealed and filled with gas, it delivers fast response, linear accuracy and stable readings across a wide range—from approximately –320 °F to 1,200 °F (–200 °C to 650 °C).

This design makes gas-actuated thermometers ideal for both direct-mount installations and remote applications where the sensing bulb and dial must be separated for visibility, accessibility, or operator safety.

Electric contact switches can be added to the thermometer for process alarm or control.

Why is remote mounting important for gas-actuated thermometers?

Remote-mounted thermometers separate the dial indicator from the temperature-sensing bulb, connected by a gas-filled capillary line. This configuration helps:

• Reduce vibration damage by isolating the sensitive indicator from pumps, compressors, or piping.
• Protect operators from hot or hazardous process zones.
• Simplify readings by allowing installation on panels or easily visible control stations.

Ashcroft’s Duratemp® 600A and 600H-45 Gas-Actuated Thermometers, for example, offer bendable extension bulbs with adjustable union connections that can be freely positioned for the best insertion depth. The armored capillary provides added mechanical protection.

How do bulb and thermowell configurations affect performance?

The bulb acts as the sensing element, while the thermowell shields the bulb from pressure, corrosion, and velocity effects. Two common bulb types are:

• Plain bulbs. Best for air or liquid measurement in open tanks or low-pressure environments.
• Union-connected bulbs. Threaded fittings that secure the bulb into a thermowell, making them ideal for pressurized or corrosive processes

Using a thermowell not only extends the thermometer’s service life but also enables instrument interchange or recalibration without process shutdown—a best practice across high-reliability industries.

Ready to learn more?

Now that you understand the advantages of gas-actuated thermometers and when it’s appropriate to use one, you have the information you need to improve both operational safety and system uptime for your operation. To learn more about temperature measurement instruments, here are some related articles:

• Choosing a Bimetal Thermometer: 5 Factors to Consider
• Fitting a Thermowell to Bimetal Thermometers or RTDs/Thermocouples
• What is a Thermowell Wake Frequency Calculation?
• How Much Do Industrial Bimetal Thermometers Cost?
• How Much Do Thermowells Cost? 5 Factors that Influence Price

Contact us to speak to a product expert for any additional questions you have about temperature measurements or instrumentation. In the meantime, download our guide for more information about RTDs and Thermocouple temperature instruments.