Betz (or projection) manometer
A hydrostatic manometer that is easier to be read, if not more accurate, is the projection or Betz manometer. The operating principle is illustrated in Figure 1.4. The difference in level between the liquid in the cylinder C and the reservoir R is indicated by a scale suspended to a float. The scale directly shows the difference in level (the decrease in the level of R is accounted for in the calibration of the scale).
An enlarged image of the scale located on the plane XX is projected onto a frosted glass on which a reference line is marked. The scale can be engraved in order to show centimeters of difference in level; the fractions of a centimeter can be appreciated by comparison with a scale etched on the frosted glass.
The Betz manometer was used primarily to control the operating speed of a wind tunnel: the large internal volume makes it too slow for the measurement of unsteady pressures, as in turbulent boundary layers, and the quite high cost does not permit the use of many Betz manometers each connected to a pressure tap.