Electronic equipment mounted in an enclosure gets warm. If the cooling inside the enclosure is inadequate, the equipment will get hot, and this can lead to early failure. If you are designing a forced-air cooling system, how can you be sure that the air will be moving fast enough to keep the equipment cool?

Air velocity can be sensed using a pitot tube and a differential pressure sensor. A pitot tube is a device consisting of a metal or plastic tube with a 90 degree bend at one end. The pitot tube is connected to the + port of a low pressure transducer with flexible plastic tubing. The open end of the tube (having the 90 degree bend) is inserted into the enclosure and used to probe the enclosure for air currents.

When moving air impinges on the open end of the pitot tube, a small pressure is developed. This pressure is proportional to the velocity of the air. The pressure transducer senses the pressure and produces a signal that indicates the relative strength of the air flow at that point. As the probe is moved about the enclosure, an air velocity profile can be constructed that can be used to determine if the cooling will be sufficient at all critical points.

In some enclosures the air flow may be gentle, and the low velocity will not generate a very large pitot tube pressure. It is important that a very sensitive transducer be used in order to detect these low velocities accurately.