A method of measuring the gas flow of an incondensable gas in a flowing mixture of said incondensable gas and a condensable gas comprising the steps of calibrating a leak detector; injecting a known quantity of gauge gas at a known mass flow rate into said flowing mixture of gases; drawing a sample of the said gauge mix off at a sample point; passing said sample through a condenser so as to remove said condensable gas from the gauge mix, resulting in a test mix; passing the said test mix into the leak detector so as to measure the concentration of the gauge gas within the test mix; and calculating the mass flow rate of said incondensable gas within said flowing mixture. There is also provided a method of measuring the rate of ingress of an incondensable gas at an ingress point along a flow path of a flowing mixture of said incondensable gas and a condensable gas.