| | Re: Passive CO2 diffuser
Your CO2 diffuser device has its own built-in pressure gauge; just measure the depth of the CO2/water interface in the pipe below the surrounding water. Each inch of water depth displaced is equal to 0.036 psi greater than ambient pressure.
Edit: I'm assuming the device is inside the aquarium, so the aquarium water level is pushing back against the CO2-enriched gas pressure in the tube. If this is all outside the tank, then info above may not apply.