When an additional gauge liquid is used in a
U-tube manometer, a large difference in meniscus levels may be obtained for a very small pressure difference.
Fig 4.9 A Micromanometer
The equation of hydrostatic equilibrium at PQ can be written as
where and are the densities of working fluid, gauge liquid and manometric liquid respectively.
From continuity of gauge liquid,
(4.6)
(4.7)
If a is very small compared to A
(4.8)
With a suitable choice for the manometric and gauge liquids so that their densities are close
a reasonable value of y may be achieved for a small pressure difference.
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are the densities of working fluid, gauge liquid and manometric liquid respectively. 
a reasonable value of y may be achieved for a small pressure difference. 
