This is what I use to determine how much HHO gas is being produced per minute with my Hydrogen Generator.
Step 1: The 2 Liter Design
The 2nd 2 liter bottles center is cut out to make the 1st bottle longer.
Step 2: Simple 1 Liter bottle
A 1 liter bottle with just the cap removed.
Step 3: Base of the unit
Simply, it is a 2 - 2 liter bottles with the bottom cut off and glued to the top of the other one to form the base of the unit. Sand is placed in the base to allow it to stand upright.
Step 4: The inside of the 2 Liter bottle showing the PEX tube.
The air or gas flows through this into the 1 Liter bottle and you just calculate the time it takes to lift the bottle till the first bubble comes out.