What is the nominal orifice size drilled in unions with noncorrosive diaphragms allowed in place of check valves in sensing lines?

The nominal orifice size drilled in unions with noncorrosive diaphragms that are allowed in place of check valves in sensing lines is 1/8". This specification is crucial for ensuring proper fluid flow and pressure sensing within the system.

Using an orifice size of 1/8" facilitates adequate flow while preventing the backflow that check valves are designed to stop. In applications where check valves may not be suitable, or where space is a concern, the drilled orifice serves as a reliable alternative, allowing the system to maintain its operational integrity.

The other options are either too large or too small, which would not provide the desired flow characteristics or may lead to inadequate pressure sensing in the system. Proper sizing is essential to maintain the balance and functionality of the sprinkler system, hence why the 1/8" orifice is the accepted standard for this application.