What dictates the A320's 'emergency ram air inlet' operation?

Question

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 < 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.

Answer 1

(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 < 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:

1. Clear smoke from the airplane
2. 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.

Answer 2

Please read carefully ... Extract taken from my FCOM, Info accurate at January 2020:

An emergency ram air inlet ventilates the cockpit and cabin to remove smoke, or if both packs fail. The emergency ram air inlet valve is controlled by the RAM AIR pushbutton on the AIR COND panel. This pushbutton opens the ram air valve, provided that ditching is not selected.

When the RAM AIR pushbutton is ON : The outflow valve opens about 50%, provided that it is under automatic control and ΔP is less than 1 PSI. The outflow valve does not automatically open if it is under manual control, even if ΔP is less than 1 PSI. If ΔP is greater than 1 PSI, the check valve located downstream the ram air door will not open, even if the ram air door has been selected open. No airflow will then be supplied.

So based on that, the ‘door flap’ (ram air inlet), will open at any time, is the CHECK VALVE located downstream, the one that won’t open if the ditching pb or &P >1 psi...

The outflow will always be open (a little, in auto mode), that makes perfect sense, in order to &P becoming lower, and then that check valve (and the outflow), will open and the plane can be ventilated (smoke, fumes removal).