A calibrated test gas kit for sensor inclusively testing Carbon Monoxide (CO) alarms manufacturered to BS EN 50291(Europe) and UL 2034(USA). Low cost, safe and easy to use, this test kit ensures CO alarm sensors are still sensing CO. Sensors have a limited and underminable life and can be effected by airborne pollutants such a cigarette smoke, cooking fumes etc. Each kit contains a patented enclosure shroud which holds the test gas at the required level for a fixed time to facilitate a field sensor test. It contains sufficient test gas to carry out 11 tests per can.