A regulator operates on pressure instead of volume. This means a flowmeter will always be more accurate than a regulator for flow volume. A flowmeter is an outer tube with another glass tube inside, marked for flow in CPH and liters per minute or LPM. A small ball bearing inside indicating the flow.
While gas is in the distribution system, regulators control the flow from higher pressure to lower pressure. Regulators sense if the pressure in a line drops below a specified set point and opens, allowing more gas to flow.