I would think that that the most sensible substitute for inches of
water would be centimeters of water.

Water is water and -- keeping with the tradition of using a manometer
in which pressure is simply read by measuring the height of the column
of water -- it makes the most sense to use a similar scale but in
metric distance measurement. To me that makes a lot more sense than
atmospheres, bars or femtopascales, etc.

Mercury is too dense to use for measuring pressure in pneumatic
instruments, making reading of the scale difficult.

Spencer Chase
Northern California