When do you start the 30-second timer when draining LPG provers?

Starting the 30-second timer when draining LPG provers is crucial for ensuring accurate measurement and verification. The correct timing aligns with specific operational conditions of the prover, which ensures that the draining process is monitored accurately.

In the case of dry bottom provers, the timer starts when the pump changes RPM, indicating that a flow change is occurring. This is essential because it signals that the liquid level is changing, which affects the volume measured during the draining process. For wet bottom provers, the timer commences once the liquid reaches the bottom of the sight glass. This point is significant as it marks the transition to a stable flow rate and ensures that any fluctuations that might impact the measurement are accounted for.

By starting the timer at these precise moments, operators can ensure that the data obtained is reliable and reflects the true conditions of the measurement process. This systematic approach to timing also helps maintain consistency in operations, contributing to better accuracy in LPG measurements. The other options do not provide the same operational precision or measurement reliability, making this choice the most appropriate for the procedure.