What is a major concern when using a gas meter to measure gas flow?

A major concern when using a gas meter to measure gas flow is indeed the challenge of ensuring that all appliances and equipment on the gas line are turned off. If any appliance or device is still consuming gas while measurements are being taken, the readings can be misleading. This can lead to inaccurate assessments of the system's pressure and flow rates, which can complicate diagnostics and maintenance.

Additionally, confirming that the gas line is completely isolated is critical for safety reasons. If devices are inadvertently left on, there is a risk of leaks or other safety hazards, which is why this concern stands out as a significant one when measuring gas flow with a gas meter.

The other options, while they may have some relevance, do not highlight the same level of immediate importance to safe and accurate measurement as the necessity to ensure all devices on the line are turned off.