How can I tell if my expansion vessel membrane is gone?

[vice]

Hi. Great forum, hope someone can advise-
I have 2 external expansion vessels - one for heating one for hot water system.
I'm trying to figure out if they need replacing or not - if the membrane has gone.
But how can I figure that out?

If a membrane has gone wouldn't the 4bar air just sit there in the top of the vessel anyway?
Ideas welcome before I have to strip, drain and have the fun of yucky water suddenly shoot out everywhere

 

[Jackrae]

Maybe, but the water will gradually absorb, or is it absorp, the air which will cause a reduction in pressure. One simple test it to release the air pressure via the air pressurisation valve and see if water eventually comes out of the valve. If it doesn't then bladder is OK, if it does then .....