What dictates the A320's 'emergency ram air inlet' operation? Why does the emergency ram air inlet flap open when delta p is greater than or equal to 1 psi on the A320?

Below is the extract I was reading from the FCOM. Why has the figure of 1 psi been set for opening or closing the flap?


  The emergency ram air inlet opens
  ‐ If Δp ≥ 1 PSI: The outflow valve control remains normal. No emergency ram air flows in
  ‐ If Δp &lt; 1 PSI: The outflow valve opens to about 50 % when under automatic control. It does not automatically open when it is under manual control. Emergency ram airflow is directly supplied to the mixer unit.
(wikimedia.org)

You have misinterpreted the text. Given that the DITCHING pb is not pushed, the RAM AIR pb opens the emergency ram inlet.

The differential pressure (DP) being above or below 1 psi is for the outflow valve (not the inlet). If the DP &lt; 1 psi, the outflow valve opens—this is the opposite to the former title (now the first line) of the question.

This serves two purposes:


Clear smoke from the airplane
Bring in fresh air if the packs fail.


When the system operates in this mode, there is no fine tuning for the outflow valve. By testing, they would have figured out with the fixed ~50% open position, any DP above 1 psi results in rapid decompression.

Note: the protection is via a check valve, so no system logic is involved. It's mechanical.