U-Tube Manometer


Manometer is a device used to measure pressure at a single or multiple points in a single or multiple pipelines, by balancing the fluid column by the same or another column of fluid.
Manometers work by the principle that a column of fluid in a tube will rise or fall until its weight is in equilibrium with the pressure differential between the two ends of the tube. For a pressure difference P is the height difference h between the level of liquid in the two halves of the tube A and B, is given by the equation P = pgh, where p is the density of the fluid in the tube.

U-Tube Manometer

“A U-tube manometer, in which differential pressure is measured as the difference 'h' between the high-pressure reading and the low-pressure reading, multiplied by the density of the liquid in the tube.”
The difference in height of the liquid represents the applied pressure.
In U-tube manometer we measure the pressure as:
P= sp wt*height( fluid)- sp wt*height( Hg)


A U-shaped tube is half-full of liquid, one side of which is connected to the region of interest and the other side to the reference atmospheric pressure is applied to the other. The inclined and vertical u-tube manometers are inexpensive and common in the differential pressure measurement with flow meters like pitot tubes, orifices and nozzles Pressure measuring devices using liquid columns in vertical or inclined tubes are called manometers. One of the most common is the water filled u-tube manometer used to measure pressure difference in pitot or orifices located in the airflow in air handling or ventilation system. In the figure bellow illustrates the water levels in an u-tube where the left tube is connected to a point with higher pressure than the right tube example: the left tube may be connected to a pressurized air duct when the right tube is open to the ambient air.

Vertical U-Tube Manometer

The pressure difference measured by a vertical U-Tube manometer can be calculated as

pd = γ h 
    = ρ g h

pd = pressure (Pa, N/m^2, lb/ft^2)
γ = ρ g = specific weight of liquid in the tube (kN/m^3, lb/ft^3 )
ρ = U-tube liquid density (kg/m^3, lb/ft^3)
g = acceleration of gravity (9.81 m/s2, 32.174 ft/s^2)
h = liquid height (m fluid column, ft fluid column)

Inclined U-Tube Manometer

A common problem when measuring the pressure difference in low velocity systems - or systems with low density fluids like air ventilation systems are low column heights and accuracy. Accuracy can be improved by inclining the u-tube manometer. The given  figurer indicates a u-tube where the left tube is connected to a higher pressure than the right tube. Note that the left and the right tube must in the same declined plane for the angle to the horizontal plane to be correct.

The pressure difference in a inclined u-tube manometer can be expressed as
pd = γ h sin(θ)

h = length, difference in position of the liquid column along the tube (mm, ft)
θ = angle of column relative the horizontal plane (degrees)
Inclining the tube manometer increases the accuracy of the measurement.

U-tube Manometer Principle

The principle of the manometer is that the pressure to be measured is applied to one side of the tube producing a movement of liquid, as shown in figure

It can be seen that the level of the filling liquid in the leg where the pressure is applied, i.e. the left leg of the tube, has dropped, while that in the right hand leg as risen. A scale is fitted between the tubes to enable us to measure this displacement.

Let us assume that the pressure we are measuring and have applied to the left hand side of the manometer is of constant value. The liquid will only stop moving when the pressure exerted by the column of liquid, H is sufficient to balance the pressure applied to the left side of the manometer, i.e. when the head pressure produced by column ” H ” is equal to the pressure to be measured.

Knowing the length of the column of the liquid, H, and density of the filling liquid, we can calculate the value of the applied pressure.
The applied Pressure = ρ × g × h
y suitable choice of filling liquid, various low ranges of gauge pressure can be measured from about 500 Pa to 1.5 bar.

Typical filling liquids commonly used in manometers and their densities.
Water ( ρ = 1000 kg m-3 )
Oil ( ρ can be between 800 and 950 kg m-3 )
 Mercury ( ρ = 13560 kg m-3 )

  • U-tube manometer is a simple mano metric device used to measure pressure at a point in a fluid, by balancing the fluid column by the same or another column of fluid.
  • It has a glass tube bent in “U” shape with some amount of same or other type of fluid, called mano metric fluid like mercury, oil, water.