This device is really simple to make, and the results were for me reassuring. I made it even more simple by just marking a horizontal line (for no pressure applied) and then one mark five and a half inches above and the same below. My simplified way of attaching the tubing was to drill a number of strategically-place holes and tie the tubing to the board with zip ties. Black ones.
I specifically wanted to know the pressure at the Dometic refrigerator
, and there is a plug on it specifically designated for such tests. The fitting between the tubing and the threaded hole was readily available at our local hardware store, no problem. Connected it, turned on the gas, and bingo, 11" of difference in the levels.
As long as you have the board vertical and the initial level along the horizontal line, the device is quite accurate and trouble-free. The only real controversy is centered around what shade of food coloring to add to the water. I chose a deep red, but I'm sure I'll be flamed for this radical choice.