Approx. 60" of clear 3/8" flexible plastic (vinyl) tubing.
Staples or tie straps to hold tubing to back plate.
6" x 16" wood board or sheet metal.
1.) Attach the plastic tubing to the back board using staples or tie straps in a "U" shape 12" to 14" in both directions as per the below drawing.
2.) At about the midway point of the tubing, mark the back board with a zero location.
3.) From the zero location, place marks every 1/2" above and below. Number the marks every one inch. Place a target arrow mark at 5 1/2" on top and bottom. This target will equal 11 inches of water column.