a device for measuring the presence or time - average amounts of at least one selected gas in a mixture thereof which comprises an enclosure having therewithin at least one substance which interacts with the gas to be monitored . attenuating means associated with the enclosure regulate gas movement within said enclosure to insure that an essentially placid layer of gas exists between the attenuating means and the reactive substance insuring that the amount of selected gas available to react with the interactive substance is basically a function of the concentration of the gas or gases being measured and its diffusion through the placid layer . the amount of selected gas measured is substantially independent of the velocity and impinging angle of the gas mixture at the interface of the enclosure with the ambient gas mixture . the device is capable of controlling mass uptake and response time without losing velocity and angle independence . the control of mass uptake can be accomplished independent of the concentration of detectable gas .