Abstract: Apparatus measures airflow when used with a hood to control the flow of air through the hood. The apparatus measures air velocity when used with a pitot tube, a probe, or a grid. Differential pressure is measured, and absolute pressure is also measured. A temperature probe and a humidity probe are used to obtain temperature and humidity measurements, and the measurements are used to correct airflow and air velocity measurements for the effects of temperature, humidity, and absolute pressure. Back pressure is compensated for when the hood apparatus is used for airflow measurements by utilizing a pair of movable flaps that are mechanically inserted into and removed from the airflow. A differential pressure circuit includes a voltage controlled oscillator which is linearized over a specific operating range, and the specific operating range depends on the differential pressure being measured.