Hi Paul/ Everyone.
Hope everyone is doing good. Paul - I did check the tank and I believe its almost to the end of its life. The top portion make a sound indicating water is in there. Midway of the tank makes the same sound. Past the rings, and a little further down after half, I get a different sound - indicating its empty (Air occupied). Its not much space between the bottom of the tank to there I think the seal - separating water and the air - I would say 4-5 inches.
Do you recommend switching the tank?
With diaphragm style tanks the Tap-To-Test method doesn't work well. Neither does feeling for a temperature difference. Those methods work great for non-diaphragm tanks most often found on older hydronic and very large, industrial potable & process water systems.
Your tank might be good, but low on air due to oxygen migration- a very normal thing. On potable water systems, the tank shell usually rusts before the diaphragm goes bad. (Opposite on hydronic systems)
Why Tap-To-Test Isn't Reliable:
In day-to-day use, the diaphragm is crowned at or near the top of the tank. When the water heats & has no place to go,
only if the pressure rises above the air charge pressure, the diaphragm moves down to suit. The diaphragm on most tanks won't move more than an inch or two- even if the air charge is lost. Even with no air pressure, the diaphragm will be at or near the top of the tank when water isn't pushing on it. So, sounds can be confusing, indeed!
No water is indicated by your Magic Cap, so I'm guessing your tank's diaphragm is A-OK.
Before spending on a new tank:
Perhaps order an isolation valve with drain or weep port & have it on hand. Then, when the water's cool to a safe temperature, remove the tank.
***It may be quite heavy if lots of water is in it***.
Then tank must be empty and off of the system to properly test:
Dump the water out of the tank & check the air charge. A quick indication of a leaking diaphragm is if air comes out of the water connection when you pump it up. (If you can't pump and feel for air at the same time, a balloon over the water inlet is handy for that check.) Note that a tiny leak might not be noticeable at the water connection while you pump.
If you put the isolation valve in where the tank was connected now, you can turn the house water back on & leave the tank off for a while to see if the pressure holds. Allow about a quarter to half pound of pressure drop when you re-check to account for the compressed air cooling down after pumping.
Here's Hoping Your Tank's Good!
Paul
PS: A place called SupplyHouse Com sells both types of expansion tank isolation valves. Usually orders arrive the next day. Also, plumbing supply houses and heating supply houses usually have one type or another in stock.