# What's a good way to equalize pressure between 2 spaces without allowing gas exchange under normal circumstances?

I have a reaction chamber where gasses are injected to maintain a certain mixture. The container is sort of like a bell, where the bottom is submerged in water to make a seal.

The issue I have is that when injecting new gas into the chamber the excess has nowhere to go, so the water level is forced downward. I would like to maintain the water level difference inside and outside the chamber of <0.1in.

This rules out most kinds of check valves, because the pressure differential is too low. What I'm currently doing, is intentionally leaving a small hole open in the reaction chamber to allow pressure to equalize. Gas is injected at a very slow rate, so this works ok, but unwanted diffusion is always occurring through the hole.

Does anyone know of a mechanism that will allow the pressure to equalize, but which will not allow significant gas exchange once the pressure is equal? I'm thinking something similar to a water trap, but with even lower pressure drop.

• raise or lower the bell to compensate for the amount to be added. May 16, 2019 at 19:12
• Are you trying to keep the pressure constant, or running this in a shallow pan so you can't move the bell, or what? How about a valve, either hand- or automatically actuated, that opens when the water level differential gets too high? May 16, 2019 at 19:14
• @TimWescott: Yes, both of those things. I am essentially trying to maintain atmospheric pressure in a "closed" space, into which I need to inject gasses. A valve is an interesting idea, but it would have to be a powered ball valve, since a solenoid would not operate well with such a low pressure differential. I'm hoping for a small passive device, but that's one option.
– Drew
May 16, 2019 at 21:48
• Your problem is that any passive device will have to be powered by the pressure differential between your dome and the outside. What's the pressure differential needed by your water trap compared to ordinary fluctuations in barometric pressure in your lab? May 16, 2019 at 23:18