There can be a wide margin between "good practice" and "usable". The precharge SHOULD be at or below the system pressure, but it it is not, the system will compress it to the proper point. it is when it is too high and the air cannot adjust itself to the system pressure that it only works when the pressure rises above that level that it becomes a problem. When the tank is "empty" the bellows is all the way against the "inlet opening". When it is full, but no pressure, it against the Shraeder valve end. Between those two points the bellows folds to accomodate the volume differences. If it were under ANY stress it would be either when it is empty or no pressure, not under any other condition. But I would not be dogmatic and tell the customer to check the pressure every month to make sure it is proper, because he could not do it unless the system was drained. When the system is pressurized the air pressure will ALWAYS be at the system pressure.
I never suggested checking the pressure every month. I also understand that the system must be drained to accurately test the TET's pressure. Symptoms and actions of the water system will let you know when there is a thermal expansion issue. But again, somebody on this forum has tried to explain away the simple and practiced advice that I gave when there is nothing wrong or faulted with it. My experience as well as the application of items dictated by the manufacturers have proved that by not following the basic installation methods will cause premature failure of the product. Explaining to me what I already know doesn't change that what I said to the OP was absolutely correct and didn't need any negative or devaluing comments. It's as if you didn't answer my question but just talked in a circle around it while still trying to sound justified.
This has happened on the posts that I've made regarding vacuum relief valves on hot water tanks and another thread where I recommended not turning down/off water heaters for days so that bacteria and legionella are not an issue when returning home after days away.
On those threads, as well as this one, somebody jumps into make it sound like I'm giving inaccurate or hack advice when in fact it is part of the basics that licensed plumbers practice when in somebody's home.
I'm sure you can find the threads from my profile, if you are interested, but doubt that many members here would be willing to say "oops, I was wrong, sorry..."
It would be explained away again that I must be too sensitive or speaking in hyperbole.
It's starting to remind me of some egos in the PZ.