SM0486

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.

Figure 20.

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

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