Mechanical Engineering - Hydraulics and Fluid Mechanics - Discussion

The liquid used in manometers should have a high density so that the height of the column can be reduced. Because the manometers are used for high-pressure measurement, it is important to make the column height within a practical limit.

Most of case, for gauge pressure measurements, mercury is widely used as manometric fluid because it has non-evaporating quality under normal conditions, sharp meniscus and stable density. For some pressure differences and low-level vacuum, water can be considered as the working fluid in the manometer.

Important Points:

Manometric fluid should have a low surface density to have higher accuracy in the reading.
∴ Low surface tension is one of the desirable properties but the most important property is the property of being highly densified.

@All.

Some people say high surface tension is the correct option. No, it is incorrect as it will reduce the efficiency of the manometer.

Manometric fluids should have the following properties:

* High density ( for high-pressure measurement the rise of liquid will be within reasonable limits).
* Low Viscosity (As high viscosity will slow down the sensitivity or responsiveness of fluid movement).
* Low Vapour Pressure (Because, if Vapour pressure is greater than the fluid pressure to be measured then the problem of cavitation will arise).
* High Boiling point.
* Low Freezing Point.
* Low Surface tension.
* Chemically Inert.
* Distinct Interface.

Manometric fluids should have the following properties

* High density (for high-pressure measurement the rise of liquid will be within reasonable limits)
* Low Surface tension.
* Low Viscosity (As high viscosity will slow down the sensitivity or responsiveness of fluid movement).
* Low Vapour Pressure (Because, if Vapour pressure is greater than the fluid pressure to be measured then the problem of cavitation will arise.).
* High Boiling point.
* Low Freezing Point.
* Chemically Inert.
* Distinct Interface.

Yes, 100 % correct @Pvdatt.

One thing, I would like to add is if the manometer liquid has high surface tension then the liquid surface in a tube will be in a shape of the dome it wouldn't be flat so we can't measure a correct reading that's why liquid should have low surface tension.

@Nadim.

The diameter of the tube is large enough to avoid the capillary effect. The Surface tension of the liquid in large diameter tube has nothing to do with the capillary effect.

Density of fluid selection depends upon the pressure of the liquid system. If we choose low-density fluid we can measure small changes in pressure. If we choose high density we can measure larger pressure difference.

Although the increase in density leads to the reduction in accuracy of the manometer.

Low surface tension is a desirable property.

Desirable properties of good manometric liquid should have;
-> Low-freezing point.
-> High boiling point.
-> Non-wetting characteristics.
-> Low surface tension.
-> Chemically inert.
-> Clear visible interface.
-> Ability to maintain density at various temperatures.

Surface tension should be high because higher surface tension gives low vapor pressure (i.e. No vapor formation takes place of manometric fluid) and also high surface tension prevents the unnecessary rise of fluid in Fluid column due to adhesion during pressure measurement.

Desirable properties of good manometric liquid should have;

-> Low-freezing point.
-> High boiling point.
-> Non wetting characteristics.
-> Low surface tension.
-> Chemically inert.
-> Clear visible interface.
-> Ability to maintain density at various temperatures.

Characteristics of liquid used in U-tube Manometer:

1. Low viscosity.
2. Low surface tension.
3. Liquid should have a high boiling point and should not get vaporized.
4. The liquid should stick to the walls of the tube.

Should have high density so that the height of column can be reduced. Because the manometers are used for high pressure measurement so it is important to make the column height in practical limit.