Hi, I wanted to tell you what I finally did.
As my PC has a relief valve which closes the hole using its own weight, I put one coin on it and after waiting for 10 minutes and lifting the relief valve (for me to be able to open the PC sooner), the rice was very well cooked. I guess with 8 minutes would have been enough.
I am happy that somehow we managed to reduce the total time needed.
Before I used to wait for 15 minutes + another 15 before opening the PC, but now what I did was to wait for 10 minutes + 2 minutes (lifting the relief valve), and I could open the PC inmediately.
At first I put four coins, but the result was dramatic, at some point I lifted the four coins and A LOT of foam and steam started coming out very fluently, I felt my PC could exploit
, I got a bit scared and put the fire out, after that I left just one coin.
By the way, the people near me who have PC don't use it to cook rice, that's why I couldn't get much information from them. I do live in Oruro, where we are at 3735 meters above the sea level and gravity is 9.76 [m/s^2] instead of 9.81 [m/s^2] at sea level.
I will continue trying to understand the physical reasons behind all the behaviour of this things. I very very much thank your help and opinions. Cheers.