To facilitate service, testing or replacement, all pressure switches and mainline gauges downstream of the source valve shall be provided with a ________________.

The requirement for providing a demand check fitting downstream of the source valve relates to ensuring that any service, testing, or replacement of pressure switches and mainline gauges can be performed safely and without the risk of backflow. A demand check fitting allows for the isolation of the section of the system where maintenance is occurring while maintaining the integrity of the overall gas system.

In medical gas systems, maintaining a sterile and safe environment is critical. The demand check fitting effectively prevents any contamination from the atmosphere during maintenance, ensuring that the gaseous supply remains uncontaminated and consistent. This is especially important in medical settings, where the gas involved could be used for life-supporting functions.

While options like pressure relief valves, regular hose fittings, and isolation valves have their own functionalities in a gas system, they do not specifically address the need for isolation during service activities in the same way that demand check fittings do. Isolation valves can indeed be used for maintenance but do not allow for pressure testing and service simultaneously while keeping the line safely sealed. Therefore, the use of a demand check fitting is specifically designed for the operational requirements of maintaining systems in a medical environment.