A singular sealed apparatus and method suitable for confirming pressure measurements in a rebreather prior to use of the rebreather. The singular sealed apparatus may have a canister lid, gas sensors and a processor, all from a rebreather, as well as a pressure sensor, an input device, a processor, an indicator and a pod with a pressure measurement outlet, a gas supply inlet, a relief valve and a gas exit valve. Alternatively, the singular sealed apparatus may have a gas sensor from a rebreather and an analyzer, a pressure sensor, an input device, a processor, an indicator, a lid and a pod with a pressure measurement outlet, a gas supply inlet, a relief valve and a gas exit valve. The inventive method is characterized by assembling the singular sealed apparatus, inputting a concentration of a gas present in a gas mixture, admitting the gas mixture into the singular sealed apparatus, expelling ambient gas present in the singular sealed apparatus, measuring absolute pressure of the gas mixture, calculating partial pressure of the gas, indicating the calculated partial pressure of the gas, measuring partial pressure of the gas, indicating the measured partial pressure of the gas and comparing the calculated partial pressure of the gas to the measured partial pressure of the gas.