The Dry Leg in instrumentation is used to protect process instruments by containing what?

• A. A device that measures humidity in the process line.
• B. A purge line that uses liquid nitrogen to prevent freezing.
• C. An impulse line filled with non-condensing gas to protect process instruments.
• D. A sensor that detects moisture in air.

Answer: (C)

The idea behind a Dry Leg is to keep the impulse line dry and free from condensate by using a short section that is filled with a non-condensing gas. Impulse lines carry the process pressure to a transmitter, and if moisture or steam condenses inside, it can lead to measurement errors, freezing, or blockages. Filling that leg with a gas that remains a gas over the operating range (often nitrogen) creates a barrier that prevents liquid water from reaching the instrument. This non-condensing gas stays dry even as temperatures vary, preserving accurate readings and protecting the transmitter.