Which statement best describes the Dry Leg in instrumentation?

• A. Non-condensing gas in an impulse line to protect process instruments.
• B. Liquid nitrogen in a purge line.
• C. A device that measures temperature.
• D. A method to vent gas to atmosphere.

Answer: (A)

A Dry Leg is a purge setup that uses a non-condensing gas in the impulse line to protect process instruments from moisture and condensation. By filling the leg with dry gas (often nitrogen or dry air), a barrier is created so that any condensate in the main process line doesn’t travel down into the instrument. This keeps the impulse line dry, prevents liquid water from reaching the transmitter, and helps avoid measurement errors or freezing that can occur when moisture enters the line. The other options don’t describe this protective, dry-gas barrier: one describes introducing a cryogenic liquid, another isn’t a measuring device, and a venting-only approach wouldn’t reliably keep the line dry.