I have a level gauge that is measuring water level in a pressurised bladder tank  (air in bladder, bladder hanging from the top of the tank), my problem is that when the water level goes down in the tank it will continue to fall until the bladder is fully expanded, which is fine, however, in the level gauge the level can continue to fall until the liquid level reaches the bottom connection.
At this point all the air in the bladder escapes around the level gauge into the line.

I can't change the centres of the level gauge, but what I could do is to put in a ball type float check valve with say a polypylene ball that floats when the water level is high, but then falls back onto a seat to seal when it drops, thereby stopping the air going around the level gauge and escaping the vessel.

Does anyone know where to buy such an item, I can't find anything on the net after much looking.

Floating ball steamtraps are built to do just what you describe: keeping gas (steam) inside, letting liquid fluid
(condensate) pass, and not depending on temperature.

The problem is however capacity, most likely you will have problems finding one with high enough capacity, but you could try. (Gestra, Spirax and a larger number of other suppliers).

If you do not find a solution here, you could perhaps try to find a sensing device mounted outside the tank, detecting the difference between water-filling and air-bladder filling of the tank at a given level, giving alarm or shutting down the outlet. (Ultrasound???).

Another idea is to use a pressure switch as signal giver on the bladder, given that there is a significant pressure level difference in the air bladder by expanding past a certain point. Actually I do not really 100% believe in this. I have not looked into the details but believe that both the difference in pressure when filling the bladder and the perhaps too small difference in pressure when expanding might give problems - but there is many really good, small and cheap pressure switches on the market, if you can control the parameters mentioned.

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