Measurable pressure range
With a U-shaped vertical manometer, in order to limit the reading error to less than 2%, differences in heights less than 50 mm cannot be measured since the accuracy of the naked eye in reading the height of liquid in each tube is limited to 0.5 mm; the presence of the meniscus does not recommend the use of systems of magnification. On the other hand it is impractical to build manometers with arms longer than 5 or 6 meters.
The minimum and maximum differential pressures measurable by U-tube manometers using water or mercury can therefore be calculated approximately, using Equation (1.1):
Water Apmm = 0.5 [kPa] Ap^ = 60 [kPa]
Mercury Apmin = 6.6 [kPa] Apmax = 810 [kPa]
For low pressure, alcohol is preferred to water because it has a surface tension which is 30% that of water and the meniscus is negligible, but its density must be controlled because it changes rapidly since it absorbs moisture from the environment. The other liquids listed in Table
1.1 and having an intermediate density are corrosive and used more rarely.