Manometer types and working principle

What does a manometer measure?

A manometer gauge is an extremely basic but very effective device which is used to measure pressure. In the majority of cases this will relate to a gauge which consists of a U-shaped glass tube which is filled with mercury or some other liquid. Traditionally one end of the manometer tube is left open, susceptible to atmospheric pressure, while a manometer hose is connected via a gas tight seal to an additional pressure source. While normally associated with gas pressures a manometer gauge can also be used to measure the pressure exerted by liquids.

As the manometer pressure gauge has no mechanical parts it requires minimal maintenance and is extremely accurate.

What is the working principle of a manometer?

The principle behind a manometer gas or liquid pressure gauge is extremely simple. Hydrostatic equilibrium shows that the pressure when a liquid is at rest is equal at any point. For example, if both ends of the U-tube are left open to the atmosphere then the pressure on each side will be equal. As a consequence the level of the liquid on the left-hand side will be the same as the level of the liquid on the right-hand side – equilibrium. However, if one end of the U-tube is left open to the atmosphere and the other connected to an additional gas/liquid supply this will create different pressures.

If the pressure from the additional gas/liquid supply is greater than the atmospheric pressure this will exert a downward pressure on the measuring liquid. As a consequence, the liquid will be pushed down on one side with the greater pressure causing the liquid to rise on the side with the lesser pressure. The opposite would occur if the additional gas/liquid supply creates a lesser pressure than the atmospheric pressure. In this instance the liquid would fall on the side of the open section of the U-tube and rise on the side connected to the additional gas/liquid supply.

Perhaps the easiest way to explain this is the movement of a playground see-saw. If both parties are the same weight and exerting no additional pressure on the see-saw it will balance perfectly. However, if the weight or the pressure exerted on one end of the see-saw is greater than the other, the heavier side will lower and the lighter side will rise.

What are the types of manometers?

There are many variants on the market. However we will focus on the five basic types which are:

	U-Tube manometer The traditional U-Tube device is the most common with one end of the partially liquid filled tube open to the atmosphere and the other connected to an outside source. By measuring the different heights of liquid on the left and the right hand side of the U-tube it is possible to calculate the pressure from the outside source in relation to atmospheric pressure.
	Differential U-tube manometer A differential U-tube is closed and both ends are filled with different liquid/gas at different pressures. This tends to be used where the pressure needs to be measured directly, not based on an outside pressure.
	Inverted U-tube manometer The inverted U-tube design is used to measure low-pressure between two separate points to a relatively high level of accuracy. Liquid is introduced to the U-tube at either end with a volume of air separating the different liquids. The use of a tap allows air to be expelled from or admitted into the inverted U-tube device. Thereby adjusting the pressure difference and giving an accurate reading
	Micro manometer The micro-design is a modified version of the basic U-tube except one side of the tube has a larger cross-section. The manometer definition of measuring pressure is still the same but due to the structure of the U-tube it is able to measure minute pressure differences.
	Inclined manometer As the name suggests, an inclined manometer involves a gradual incline in the design. This allows for the measurement of minuscule pressure to extremely high levels of accuracy. It is used where the manometric properties of liquids are similar. Again, it is the simplicity, lack of maintenance and no moving parts make it extremely efficient and easy to use.

What is difference between a manometer and a barometer?

While these two devices are used to measure air pressure there are subtle differences. If we look at the original mercury barometers they were simply a glass tube filled with the mercury and part-vacuum. The height of the mercury column would rise and fall in tandem with differing atmospheric pressure. This was calibrated against a scale allowing the measurement of actual atmospheric pressure. The main point being that the tube was one column and both ends were closed.

A manometer allows the measurement of additional liquid/gas sources against atmospheric pressure or against other liquid/gas sources. There are various types which allow you to adjust the pressure on one side to give a more accurate reading. It is also the option to have closed/open ends and the U-tube design which differentiates these two pressure measurement devices.

Even though great strides have been made in technology, the basic design and structure of a manometer has remained intact. No moving parts, no maintenance and extremely accurate readings – what more could you ask for?