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If your avionics includes any systems that use barometric altimeters (common on most deployment and tracking systems), you will need to vent and equalize the pressure of your avionics bay, so that the ambient pressure matches that inside the bay. To do this, a series of holes need to be drilled into the outside wall of the bay. The more holes there are, the better the airflow will be. The sum of the surface areas of the holes need to be greater than a threshold that can be calculated. For a reference table, and the formulas required, visit: http://www.vernk.com/AltimeterPortSizing.htm.
There is also a nice port size calculator available here.
When placing these pressure equalization holes, it is important that they are not immediately over top of one of the barometric sensors. The flowing air escaping the hole can trick the sensor and cause it to deploy charges at random times.
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