To determine if there is air in the system, first apply
pressure as described in 5.1.2 to raise the diaphragm 9 mm (3⁄8
in.) above the top of the diaphragm plate and hold for 1 min.
Any air trapped between the diaphragm and the fluid will show
up as a white spot under the surface of the diaphragm. If this
occurs, reinstall the diaphragm.