Manometer

A Manometer by definition is a device used for measuring the pressure of a fluid by balancing it against a pre-determined column of fluid.The most common type of manometer which almost all us might have seen is sphygmomanometer. It is the device which doctors use to measure your blood pressure.

A manometer is can be used at any applicative process where the pressure of a fluid needs to be measured.

The five basic types of manometers are:

1. U-Tube Manometer:

Manometer

This type of manometer is U shaped, with one end open to atmosphere.It is used for measuring suction. It has the capability of measuring both positive and negative suction pressures. It utilizes the principle of specific gravities for measuring the suction pressure. This type of manometer generally has a liquid whose specific gravity is greater than that of the fluid whose suction pressure is to be measured.

Also Read: Dynamometer: Introduction, Types & Working

2. Differential U-Tube Manometer:

Manometer

This type of manometer is slightly different from the normal U Tube manometer. Here the open end is also closed, what that means is both the ends of this type of manometer are closed.It really becomes a tool of importance where the pressure is to be measured directly.This manometer type has fluid or liquid filled at different pressures on both ends.

The equation for this is as follows:

3. Inverted U – Tube manometer:

This type of manometer as the name suggests has an inverted U-shaped tube. It is used to measure differences in low pressure between two points, where the high level of accuracy is desired.It is filled with liquid on the ends. The space between liquids is filled with air.This air can be expelled or admitted into the apparatus using a tap.This is done to adjust the pressure difference to obtain an accurate reading.

 

Manometer

The equation for the same (source:http://www.msubbu.in/ln/fm/Unit-I/InvertedUtubeMano.htm)

Equating the pressure at the level XX'(pressure at the same level in a continuous body of static fluid is equal),

Also Read: Dynamometer: Introduction, Types & Working

On the left-hand side:

Px = P1 – rg(h+a)

On the right-hand side:

Px = P2 – (rga + rmgh)

Since P= Px’

P1 – rg(h+a) = P2 – (rga + rmgh)

P1 – P2 = (r – rm)gh

If the manometric fluid is chosen in such a way that rm << r then,

P1 – P2 = rgh.

4. Micro Manometer:

Micromanometer is a great tool used in fluid statics. It is a modified form of a simple manometer. It has one limb which is larger in cross-section. It is a device which is used to measure very minute pressure differences with high accuracy.

Manometer

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,

If a is very small compared to A

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.

Also Read: Dynamometer: Introduction, Types & Working

5. Inclined Manometer:

This a high precision and high accuracy type of manometer. It is used to measure the minuscule amount of pressures with high accuracy.Its configuration is inclined and this makes it more accurate than other types of manometers.It is used where manometric properties of liquids are very close.

Manometer

If the manometer, instead of being vertical, is set at an angle θ to the horizontal, then a pressure difference corresponding to a vertical difference of levels x gives a movement of the meniscus s = x/sinq along the slope.

If θ is small, a considerable magnification of the movement of the meniscus may be achieved.

Angles less than 50 are not usually satisfactory because it becomes difficult to determine the exact position of the meniscus.One limb is usually made very much greater in cross-section than the other. When a pressure difference is applied across the manometer, the movement of the liquid surface in the wider limb is practically negligible compared to that occurring in the narrower limb. If the level of the surface in the wider limb is assumed constant, the displacement of the meniscus in the narrower limb needs only to be measured, and therefore only this limb is required to be transparent.

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