Direct reading tritium-in-air-monitors are widely used for immediate observation of discharges or working area levels where significant sources of tritium are held. These monitors require initial and periodic calibration certification at the levels likely to be encountered. We describe a purpose built gas handling apparatus which enables various activity levels of tritiated gas (3H + N2) to be precisely metered and circulated in a closed loop through the monitor under calibration at a wide range of activity levels.