What is Manometer?
A Manometer is a device to measure pressures. A common simple manometer consists of a U-shaped tube of glass filled with some liquid. Typically, the liquid is mercury because of its high density.
However, unless otherwise qualified, the term “manometer” most often refers specifically to a U-shaped tube partly filled with fluid. You can easily build this type of manometer as part of a laboratory experiment to demonstrate the effect of air pressure on a liquid column.
Building a Manometer
A simple manometer can be built by partially filling a clear plastic tube with a colored liquid to allow the fluid level to be easily observed. The tube is then bent into a U-shape and fixed in an upright position.
The levels of the fluid in the two vertical columns should be equal at this point, as they are currently exposed to the same pressure. This level is therefore marked and identified as the zero points of the manometer.
Measurement of Pressure
The manometer is placed against a measuring scale to allow any difference in the height of the two columns. This height differential can be used directly to make relative comparisons between different test pressures. This type of manometer can also be used to calculate the absolute pressure when the density of the liquid in the manometer is known.
How Does Manometer Works?
One end of the tube is connected with a gas-tight seal to a test pressure source. The other end of the tube is left open to the atmosphere and will therefore be subjected to a pressure of approximately 1 atmosphere (atm).
If the test pressure is greater than the reference pressure of 1 atm, the liquid in the test column is forced down the column. This causes the fluid in the reference column to rise by an equal amount.
Manometer atmospheric pressure diffrence:
Case – 1
In the figure to the right, we show such a U-shaped tube filled with a liquid. Note that both ends of the tube are open to the atmosphere. Thus, both points A and B are at atmospheric pressure. The two points also have the same vertical height.
Case 2
Now the top of the tube on the left has been closed. We imagine that there is a sample of gas in the closed end of the tube.
- The right side of the tube remains open to the atmosphere. The point A, then, is at atmospheric pressure.
- The point C is at the pressure of the gas in the closed end of the tube.
- The point B has a pressure greater than atmospheric pressure due to the weight of the column of liquid of height h.
- The point C is at the same height as B, so it has the same pressure as B. And we have already seen that this is equal to the pressure of the gas in the closed end of the tube.
Thus, in this case, the pressure of the gas that is trapped in the closed end of the tube is greater than atmospheric pressure by the amount of pressure exerted by the column of liquid of height-h.
Case 3
Now we show another possible arrangement of the manometer with the top of the left side of the tube closed. Perhaps the closed end of the tube contains a sample of gas as before, or perhaps it contains a vacuum.
- Point A is at atmospheric pressure.
- Point C is at whatever pressure the gas in the closed end of the tube has, or if the closed-end contains a vacuum the pressure is zero.
- Since point B is at the same height as point A, it must be at atmospheric pressure too. But the pressure at B is also the sum of the pressure at C plus the pressure exerted by the weight of the column of liquid of height h in the tube.
- We conclude that pressure at C, then, is less than atmospheric pressure by the amount of pressure exerted by the column of liquid of height h.
If the closed end of the tube contains a vacuum, then the pressure at point C is zero, and atmospheric pressure is equal to the pressure exerted by the weight of the column of liquid of height h. In this case, the manometer can be used as a barometer to measure atmospheric pressure.
Calculating the Pressure
We conclude with a discussion of the units for pressure measurements. Recall that pressure is defined as the force per area. The SI unit for pressure is the pascal, which is one newton per square meter.
The pressure exerted by a column of fluid can be given by the equation P = hgd. In this equation, P is the calculated pressure, h is the height of the fluid, g is the force of gravity and d is the density of the liquid.
Because the manometer is measuring a pressure differential rather than an absolute pressure, we use the substitution P = Pa – P0. In this substitution, Pa is the test pressure and P0 is the reference pressure.
Types of Manometers
- U-Tube Manometer.
- Differential U-Tube Manometer.
- Inverted U-Tube Manometer.
- Small Manometer.
- Inclined Manometer.
1. U-Tube Manometer
A U-tube manometer is the simplest pressure measurement device. Its name comes from the U-shape formed when the two ends of a flexible tube full of liquid are raised to keep the liquid from coming out of the ends. A U-tube manometer is a ‘liquid’ balance.
A spring balance used in the kitchen weighs a load by matching the force produced by the weight of the load with the force produced from the tension of the balance spring. The change in length of the spring is a measure of the load’s weight and is shown on a graduated scale by a pointer attached to the spring.
Similarly, a U- tube manometer is used to balance the weight of the liquid in one leg of the ‘U’ against the pressure introduced into the other leg. The difference in height between the two legs of liquid represents the pressure pushing the liquid down one leg and up the other. The height difference is measured on a graduated scale.
2. Differential U-Tube Manometer
A U tube differential manometer is a type of differential manometer that is used to measure the difference of pressure between the two points of the pipes. U tube manometer’s connected pipes may be at the same level and the different level let’s see both one by one.
Whenever we want to require the determine the difference of pressure between the two points in a pipe or in a different pipe then we use the differential manometer or we may say that it is used to determine the measure the pressure difference between the two points in a pipe.
Differential manometer consists of u tube filled with manometric fluid (this manometric fluid’s specific gravity should be higher than the fluids which pressure have to measure) whose both ends are connected to the points whose pressure is to be measured.
As we connect both ends of the differential manometer to the points of the pipe at which the pressure difference is to be measured then after the pressure variation the heavy liquid/manometric fluid moves and after the equilibrium, we get the pressure head “h” and by using the hydrostatic law and balancing the column method we may calculate the pressure difference between the points
3. Inverted U-tube manometer
An inverted U-tube manometer is used for measuring pressure differences in liquids. The space above the liquid in the manometer is filled with air which can be admitted or expelled through the tap on the top, in order to adjust the level of the liquid in the manometer.
In this type of manometer, the U-tube is inverted and contains a light liquid. The two ends of the tube are connected to the points whose pressure difference is to be measured.
It is used for measuring the difference in low pressures. The figure shows an inverted U-tube a differential manometer connected to the two points A and B. Let the pressure at point A is more than the pressure at point B.
4. Small Manometer
The micro-manometer could be a special kind of liquid column manometer that is predicated on the principle of an inclined tube manometer. It is used for the measuring of very tiny variations of pressure or terribly low-pressure variations.
One will say micro-manometer is that the changed kind of an easy manometer whose one limb is formed of larger cross-sectional space. It observes so low-pressure variations with high exactness.
5. Inclined Manometer
An inclined manometer is employed for the measuring of tiny pressures and is Observe a lot of accurately than the vertical tube kind manometer. Because of inclination, the gap affected by the Liquid in the manometer is a lot of associate inclined manometer could be a slightly curved tube with a liquid within, usually a kind of oil mixture. On the tube’s middle portion are graduations. The graduations are normally hundredths of an in., looking on the manometer’s manufacturer.
Advantages of Manometers
Following are the main advantages of manometer:
- It is simple to construct.
- It has great accuracy.
- Used to measure pressure, temperature, flow and other process variables.
Disadvantages of Manometers
Following are the main disadvantages of manometer:
- The manometer has a smaller dynamic response.
- They are fragile and therefore provide low portability.
- They have small operational limits which are on the order of 1000 kN/m2.
- The density of manometric fluid depends on temperature. Therefore, errors may occur due to change in temperature.
Application of Manometers
Following are the main application of the manometer:
- Used in the maintenance of heating, ventilation and air conditioning (HVAC) systems, and gas systems.
- It is used to construct bridges, swimming pools and other engineering purposes.
- Used in climate forecasting.
- In clinical applications such as blood pressure measuring and physiotherapy.
