Abstract: A gas leakage detector includes a housing, an inlet coupling, internal fluid flow path and an outlet coupling enabling the housing to be installed and coupled permanently in-line with a gas supply line. The housing includes a cutoff valve, a gas pressure indicator and a charging gas source coupling enabling a one way flow of optional use charging gas to enter the housing downstream of the valve and to pass into and through the gas supply line. Existing line pressure or charging gas pressure is monitored at the indicator to indicate the presence of any leaks downstream of the valve. The system may be used to continuously monitor gas pressure.