LMSC
whilst i’m not the best teacher, here’s my Derbyshire version!!
when its working properly, lets assume pi 2 and shot start 4. as we all know on the 2nd and 3rd “top up” with the lever down fully, the end pressure isnt an exact science. so assuming with the lever down the pressure never goes above 3.99 bars then all is well…… but , if it hits 4 bars (which it can easily do) the timer starts even with the lever down so as it passes back through below 4 bars (a few seconds later) the shot will dump even though it never actually started.
Having the shot start pressure at 9 bars avoids this. if you hit 9 bars during preinfusion, something is a miss!!
But you will always go past 9 bars when you lift the lever unless you forgot to tamp!! so no issues.
No point in my mind having the shot start pressure set below 8 bars.
