In instrumentation, a dry leg is used to protect an instrument from which problem?

• A. Impulse line that is filled with a noncondensing gas used to protect an instrument
• B. A bypass line around a process instrument
• C. A leg of pipe that carries condensate away
• D. A return line to the reservoir

Answer: (A)

The key idea is using a dry leg to keep the impulse line gas-filled so condensation never reaches the instrument. In pressure measurement systems, moisture or condensate in the impulse line can cause erroneous readings, liquid slugging, or even damage to the sensing element. By filling a portion of the impulse line with a noncondensing gas, such as nitrogen, any potential condensate is prevented from reaching the instrument, because the gas acts as a barrier and the recognizing element stays exposed only to a dry, stable pressure. This gas-filled leg is designed to trap any condensate away from the sensing path, maintaining measurement accuracy and protecting the equipment. It’s not meant to bypass the instrument, drain condensate elsewhere, or return flow to a reservoir, which is why the other ideas don’t fit as the protective mechanism.