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/s 2 , 32.174 ft/s 2 )
h = liquid height (m fluid column, ft fluid column)
The specific weight of water , which is the most commonly used fluid in u-tube manometers, is 9.81 kN/m 3 or 62.4 lb/ft 3 .
Note! - the head unit is with reference to the density of the flowing fluid. For other units and reference liquid - like mm Water Column - check Velocity Pressure Head .
A water manometer connects the upstream and downstream pressure of an orifice located in an air flow. The difference height in the water column is 10 mm .
The pressure difference head can calculated from (1) as
p d = (9.8 kN/m 3 ) (10 3 N/kN) (10 mm) (10 -3 m/mm)
= 98 (N/m 2 , Pa)
9.8 (kN/m 3 ) is the specific weight of water in SI-units.
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 figure bellow 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
p d = γ h sin(θ) (2)
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.
We use the same data as in the example above, except that the U-Tube is inclined 45 o .
The pressure difference head can then be expressed as:
p d = (9.8 kN/m 3 ) (10 3 N/kN) (10 mm) (10 -3 m/mm) sin(45 o )
= 69.3 N/m 2 (Pa)
This calculator can be used to calculate the differential pressure measured with an U-tube manometer.
(9.8 kN/m 3 , 62.4 lb/ft 3 default values for water)
Flow metering principles - Orifice, Venturi, Flow Nozzles, Pitot Tubes, Target, Variable Area, Positive Displacement, Turbine, Vortex, Electromagnetic, Ultrasonic Doppler, Ultrasonic Time-of-travel, Mass Coriolis, Mass Thermal, Weir V-notch, Flume Parshall and Sluice Gate flow meters and more.
The study of fluids - liquids and gases. Involving velocity, pressure, density and temperature as functions of space and time.
Systems for ventilation and air handling - air change rates, ducts and pressure drops, charts and diagrams and more.
A limited comparison of flowmeter principles - regarding service, rangeability, pressure loss, typical accuracy, upstream pipe diameters, viscosity and relative costs.
An introduction to density, specific weight and specific gravity.
The hydraulic grade line and the energy line are graphical presentations of the Bernoulli equation.
Introduction to accuracy in flow measurement devices.
Turndown ratio (Rangeability) can be used to compare flow measurement devices like orifices, venturi meters etc.
An introduction to the different types of fluid flowmeters - Orifices, Venturies, Nozzles, Rotameters, Pitot Tubes, Calorimetrics, Turbine, Vortex, Electromagnetic, Doppler, Ultrasonic, Thermal, Coriolis.
The orifice, nozzle and venturi flow rate meters makes the use of the Bernoulli Equation to calculate fluid flow rate using pressure difference through obstructions in the flow.
Pitot tubes can be used to measure fluid flow velocities by measuring the difference between static and dynamic pressure in the flow.
Flow rate or discharge in an open conduit, channel or river can be calculated with the velocity-area principle.
Definitions, online calculator and figures and tables with water properties like density, specific weight and thermal expansion coefficient of liquid water at temperatures ranging 0 to 360°C (32 to 680°F).
Add standard and customized parametric components - like flange beams, lumbers, piping, stairs and more - to your Sketchup model with the Engineering ToolBox - SketchUp Extension - enabled for use with the amazing, fun and free SketchUp Make and SketchUp Pro . Add the Engineering ToolBox extension to your SketchUp from the Sketchup Extension Warehouse!
We don't collect information from our users. Only emails and answers are saved in our archive. Cookies are only used in the browser to improve user experience.
Some of our calculators and applications let you save application data to your local computer. These applications will - due to browser restrictions - send data between your browser and our server. We don't save this data.