Pressure Measurement Manometers Sensors, Transducers Ashish J. Modi Lecturer, Dept. of Mech.Engg., Shri S.V.M. inst. Of Technology, Bharuch
Pressure Pressure is a force per unit area exerted by a fluid perpendicular (say normal) to a boundary (as opposed to shear stress, which is exerted parallel to the boundary.) A more precise term is hydrostatic pressure. Since almost all pressure sensors measure a pressure difference, we need to distinguish between absolute and gauge pressures.
Units of Measure System Length Force Mass Time Pressure MKS Meter Newton Kg Sec N/M 2 = Pascal CGS CM Dyne Gram Sec D/CM 2 English Inch Pound Slug Sec PSI
STANDARDS
Terminology Atmospheric Pressure (P at ): This is the pressure exerted by envelope of air surrounding the earth s surface. Usually determined by Mercury column barometer. P at P vp gh But Mercury has a low vapour pressure ( 1.6 10-6 kg/cm 2 at 20ºC ) P gh at P at
Absolute Pressure (P abs ): Pressure has been defined as the force per unit area due to interaction of fluid particles amongst themselves. A zero pressure intensity will occur when molecular momentum is zero. Such situation can occur only when there is a perfect vacuum. Pressure intensity measured from this state of vacuum or zero pressure is called absolute pressure.
Gauge Pressure (P g ) and Vacuum (P vac ): When the unknown pressure is more than atmospheric pressure, the pressure recorded by the instrument is called gauge pressure. A pressure below the atmospheric pressure is known as vacuum, negative pressure.
How is pressure generated? Collision of molecule with wall Momentum is mass x velocity Change of momentum is double Collision is isothermal = perfectly elastic Sum collisions over area to get force
How is pressure measured? Absolute Vs. relative pressure Manometry Bourdon Aneroid Strain gauge
The hydrostatic pressure at any point in a fluid is related to the pressure at any other point by the weight of the fluid between the two points.
Pascal s Law Two points at the same elevation in a continuous fluid at rest are at the same pressure, called Pascal s law, Pressure applied to a confined fluid increases the pressure throughout by the same amount. In picture, pistons are at same height: F F F A P1 P2 A A F A 1 2 2 2 1 2 1 1 Ratio A 2 /A 1 is called ideal mechanical advantage
Pressure Reference Instruments Reference instruments are those which utilize directly the weight of a column of fluid. Pressure can be calculated from the geometry of the instrument and the specific weight of the fluid. They can be made quite precise but do not generally provide an electrical output. The most common instrument is the manometer, which measures a difference between two pressures
Manometers can be used to measure gauge, differential, atmospheric and absolute pressure e.g., Piezometer U-tube manometer (single & Double column) U-tube differential manometer U-tube single reservoir manometer U-tube double reservoir manometer Inclined tube manometer
Piezometer Tube 1) The pressure in the container has to be greater than atmospheric pressure. 2) The measurement of pressure must be of a liquid. p o Closed End Container p A (abs) Note: p A = p 1 because they are at the same level Force balance : p A(abs) = p o + 1 h 1 p Rearranging: A o 1 p 1 h Gage Pressure Then in terms of gage pressure, the equation for a Piezometer Tube: Gage Pressure,
The Manometer P P 1 2 P P gh 2 atm An elevation change of z in a fluid at rest corresponds to P/ g. A device based on this is called a manometer. A manometer consists of a U-tube containing one or more fluids such as mercury, water, alcohol, or oil. Heavy fluids such as mercury are used if large pressure differences are anticipated.
The static sensitivity K can be controlled within limits by selecting the manometer fluid, although the density of most gases at atmospheric pressure is much less than that of any liquid.
U-Tube Manometers Examples of U- Tube Manometers are as follows:
How can you measure the pressure?
Manometer
Manometer measures contained gas pressure U-tube Manometer
We can also increase the static sensitivity by using a micromanometer, which improves the precision of measuring H.
Another technique which increases static sensitivity is inclining the sensing tube.
U-tube Manometer http://www.armfield.co.uk/images/h12.gif Mercury Water Manometer http://hyperphysics.phy-astr.gsu.edu/hbase/fluids/flupic/bern5.jpg Air Water Manometer Applications: air pressure, pipe pressure, etc.
Advantages and Disadvantages of Manometers Advantages: Linear No calibration required Good precision Relatively Inexpensive Disadvantages: No electrical output Poor Sensitivity Poor Dynamic Response.
The Barometer P gh P P C atm gh atm Atmospheric pressure is measured by a device called a barometer; thus, atmospheric pressure is often referred to as the barometric pressure. P C can be taken to be zero since there is only Hg vapor above point C, and it is very low relative to P atm. Change in atmospheric pressure due to elevation has many effects: Cooking, nose bleeds, engine performance, aircraft performance.
Barometer (a) (b) (c)
Barometer It measures the difference between ambient pressure and the vapor pressure of mercury (at 25o C, the vapor pressure is.002 mmhg, which is about 10-4 atm). If ambient temperature is measured, the result can be corrected for the vapor pressure as well as the dependence of specific weight on temperature.
Dead Weight Tester They are used to calibrate other pressure transducers. They are null rather than deflection instruments.
The primary error in their use comes from friction between the piston and cylinder. This can be minimized by spinning the piston during the measurement (dynamic friction is less than static) and by proper design. The clearance between piston and cylinder must be small enough to prevent leakage but large enough to limit friction. The pressure is therefore,
Pressure Transducers Almost all pressure transducers operate on the basis of elastic deflection - the applied pressure causes some component to deform or deflect.
Pressure Measuring Devices Bourdon Gage: http://www.cpigauges.com/images/gauges/weldgagestlcsbm400psi.jpg http://www.hydraulicspneumatics.com/fpe/images/sensors1_1.jpg Principles: change in curvature of the tube is proportional to difference of pressure inside from that outside the tube Applications: tire pressure, pressure at the top or along the walls of tanks or vessels
The Bourdon Tube The Bourdon Tube forms the basic element of most dial pressure gauges. Static sensitivity can be controlled by varying the dimensions of the tube. It is possible, though not common, to convert the mechanical output to an electrical signal by bonding strain gages to the tube. Bourdon tube gages can be made quite precise by using large diameter dials.
Typical pressure sensing Element
The Bourdon Tube
The Bourdon Tube
Bellow and Capsule Gauge Bellows and capsule elements are frequently used when an electrical output is desired. Bellow type Capsule type
Bellow Gauge
Other Electrical Pressure Transducers What is a pressure transducer? A pressure transducer is a device that converts a pressure signal to an electric signal. What type of devices can be used to create a pressure transducer? Strain gage devices Capacitance devices Piezoelectric devices
Pressure Transducer Strain gage device Strain gage is attached to a flexible membrane. When the membrane is deflected the resistance in the strain gage changes.
Strained Gauge Sensors A strain gauge measures the strain in a material subjected to applied stress Signal due to deformation of the material is small, on the order of 0.1% of the base resistance. Used a metal diaphragm with strain gauges bonded to it.
Semiconductor strain gauges Widely used, both bonded and integrated into a silicon diaphragm, because larger (one order of magnitude) response than metallic strain gauge. Piezoresistive effect: Change of resistance, when the crystal lattice structure of silicon is deformed by applied stress.
Electrical readout Cross-coupling with temperature Apply several gauges and use Wheatstone bridge Signal enhanced, Temperature effect strongly reduced compression tension Two more gauges on the other side
Diaphragm Gauges Diaphragm elements are the most commonly used transducers with an electrical output.
Diaphragm Gauges
Diaphragm Gauges
Pressure Transducer Capacitance device The capacitance between two plates changes with the distance between the plates.
Pressure Transducer Piezoelectric device Piezoelectric device take advantage of the electrical properties of crystals such as quartz. These crystals generate an electrical charge when they are strained.