A mains electricity (AC-powered) laboratory instrument designed for intermittent/periodic measurements of oxygen (O2) content in a breath and/or respiratory gas specimen. It usually requires manual aspiration of a quantity of gas into a sampling chamber and may operate according to one of several basic principles (e.g., paramagnetism, polarography). The device is used in pulmonary function tests and for measurements in critically ill patients, such as infants in incubators and patients breathing air with supplemental oxygen.