Here is a bit of background of my Thermal expansion problems.
Here in Scottsdale, my water pressure was hitting about 100 during a 24 hour period.
I have been seeing a bit of water on my garage floor from the over pressure relief on a daily basis. ( I think since they changed out my meter to a new style, I assume this new one has a check valve?
The 100psi was the norm and I would see the highest indicator being over 140 150.
So I sweated a pressure regulator on my line and dialed it down to 70 psi.
I still get stagnate pressures over 120 psi, after turning on the tap it drops to about 55 and then settles at 70. Then it builds again from there.

So, now I still see the water about noon time on the floor.
I have a 80 gallon tank with a SunEarth solary system heating it. I do not use the Electrical back up 11 months out of the year.
So, my tank probably cycles a temperature delta of about 70 deg. With the highest here in May, 160 F.. Meaning after a family of 4 takes showers, and dishes, clothes ect. it drops to 70-90 F. When it builds temp the pressure of course increases.
I have a mixing valve that adds cold to the hot water to drop it to about 130 F.

I think my old meter allowed the pressure back into the citys network.

So I added a Watts Thermal expansion tank. A Plt 12 last week.
I followed some info from
And punched in my info from Watts web site.
That tells you how to size it.
I set the air pressure to 70psi, to match my water pressure.

I still get a bit of water on the floor. The engineering link I posted says after the pressure valve has been exercised a few times it becomes leaky. I think I believe him as it seems to blow about 120 psi.

So, after my long story> I have a question or two.
I understand water does not compress.
How does this expansion vessel work? I know it's designed to compress the Air on the other side of the bladder.
So, I got the pre charge set to 70, and the water set to 70.
Lets say the water is 100 F. and it temps to 140F.
As the water wants to expand, the water has a place to go, (not the city this time) and pushes into my vessel.
So, I monitor the pressure on the clothes washer hot water spigot > I think I should not see the pressure rise on this location.
I think it should instead compress the air.

My new Watts PL12 seems to be a dud, as I see the pressure rise on the hot water and pop the over pressure relief at about 120psi.
I know it's suppose to pop at 150psi, so I will address that later.

I have not checked it< But if it was working correctly I would expect the air pressure to rise from 70 to the 100's

How do you check for a dud?

Any thoughts.