berkeman said:
Pretty cool. How did you do that?
Like this.
This is the simple version:
The air is removed from the sealed fish tank to the sealed box on the right.
An air pump in the sealed box pumps the air out. I have now found a fish tank air pump that can either pump air into or out of airline tubing, so the box can be eliminated.
Removing the air draws a weak vacuum on the tank atmosphere.
It is regulated by providing an adjustable air inlet to the tank. When the vacuum in the tank (relative to the outside atmosphere) exceeds the the pressure difference needed to suck the water level out of the tube until it reaches the bottom of the tube. When it reaches the bottom of the tube, air enters the tank's internal atmosphere, reducing the pressure. This will happen under a steady state is reached where air being pumped out is made up by the air entering the tank through the pressure relief tube.
There are tables for converting inches of water pressure to more normal measures of atmospheric pressure like psi or mm Hg. These can be used to set the desired pressure (off set from the local atmospheric pressure).
I have put electronic barometers into the tank it measure the presses I get.
With the proper electronics you could just measure the internal pressure and use that info to determine when to turn the air pump on and off, but I found the plumbing approach easy.
I have now elaborated on this design to add a bunch of useful capabilities:
- Air pump in box replaced by a sucking air version of the air pump.
- the top of the tank is held by snap clamps, tight to a gasket on the top.
- air relief tube is in a different sealed (tall and skinny) container so a greater range of pressure can be achieved, connected to the tank.
- There is an airlock for introducing dry food to the tank.
- There is a syringe fitting for introduce liquid foods (like brine shrimp).
- I also can up with a way to exchange water (both water going in and out), based on similar principles of different heights of water levels controlling the out flow without depleting the vacuum.
- Ways to get electrical power into the tank (for pumps and heaters, if wanted).
As long as the amount of air going out exceeds the volumes of stuff coming in (air, food items, water), this kind of system works pretty well.
Here is drawing of a more complicated version, but it got messed up in being converted between different file formats.
The big tube in which air is let into the tank atmosphere can be at any level.
The overflow from the water outflow tube determines the water level in the tank.
The distance from the top of the bubble chamber to the bottom of the (adjustable) air inlet tube determines the off-set from local atmospheric pressure in inches (or cm) of water.