d. Well Type Manometer Principle.
(1) The well type manometer used the principle of balancing an unknown
pressure with pressure exerted by an unknown pressure with pressure
with pressure exerted by a quantity of liquid whose density is known.
When a manometer well and tube A (A in Figure 20) are exposed to equal
pressures, P1, the balance created causes the surface of the liquids in
the tube and well to rise to equal levels.
Well type Manometer Schematic
(2) When a pressure such as P2 in Figure 20B, is applied to the manometer
well, the combined pressures P1 and P2 cause the liquid level in the
well to drop from level A to level C. The combined pressures cause the
liquid level in the tube to rise from the original level A to level B.
In the tube, the pressure at level C is equal to P1 plus the weight of
the column of liquid above C divided by the cross-sectional area of the
tube. In the well at level C, the pressure equals P1 plus P2. Since
these pressures are at the same level (c), they are equal, and the
basic equation for the manometer is:
Because h = h1 + h2 is the level change in the well and therefore
difficult to determine directly, h is calculated. Proper compensation
is made in the graduation of the scale which measures the level change
in the tube (h1).
(3) If P2 is less than P1, the level changes are in the opposite direction
so that C is above B. In practical operation, when P2 is less than P1,
the lower pressure is applied to the tube. The height is read from the
scale in the usual manner.
When using the equation P2 = hd, h and d
must be in consistent units.
If h is measured in inches, d is in
pounds per cubic inch and P2 is in