What types of gases may be used for leak checks in piping systems?

The correct response indicates that only fuel gas in systems previously tested is deemed acceptable for leak checks. This approach underscores a critical safety practice in maintaining piping systems. Fuel gases, when utilized correctly, can help identify leaks by creating a pressure differential that is measurable.

Prior testing is a vital requirement because it ensures that the piping system has been evaluated and is deemed capable of safely handling fuel gases without risk of further leaks or complications. If a system has been rigorously tested for integrity and found acceptable, using fuel gas in that context becomes a responsible choice for leak detection.

On the other hand, options suggesting the use of any explosive gas or regular air do not take into account the safety and effectiveness needed during leak detection procedures. Similarly, test gases that have not been specifically approved or checked for safety may pose risks and potentially lead to undetected leaks or hazardous situations. It's critical for safety and regulatory compliance to use only gases that are known to be safe and appropriate for the specific conditions of the piping system in question.