What is Pressure
The pressure is the ratio of force to the area over which that force is distributed, it is the force per unit area applied in a direction perpendicular to the surface of an object. An action of one force against another force can be called as pressure, the pressure is force applied to or distributed over a surface.
Why do we measure Pressure
The measurement and control of fluid pressure is important in process industries a pressure sensor measures pressure, typically of gases or liquids. Pressure sensor acts as a transducer it generates a signal as a function of the pressure imposed and the signal will be electrical. Pressure sensors can be used to indirectly measure other variables such as fluid/gas flow, speed, water level, and altitude.
What are the types of Pressure
Atmospheric pressure
It is the pressure that an area experiences due to the force exerted by the atmosphere.
Gauge pressure
Gauge pressure is the pressure relative to the atmospheric pressure, it can be described as, how much above or below is the pressure with respect to the atmospheric pressure
Vacuum pressure
Vacuum pressure is the pressure below atmospheric pressure it is measured by using vacuum gauges which will indicate the difference between the atmospheric pressure and the absolute pressure.
Absolute pressure
Measured above total vacuum or zero absolute. Zero absolute represents total lack of pressure
Differential pressure
It can be defined as the difference in magnitude between some pressure value and some reference pressure. Absolute pressure can be considered as a differential pressure with a total vacuum or zero absolute as the reference, gauge pressure could be considered as differential pressure with atmospheric pressure as reference.
Static and dynamic pressure
Static pressure is uniform in all directions, so pressure measurements are independent on the direction in an immovable fluid flow. If pressure is applied on the surface perpendicular to the flow direction while having a little impact on the surface parallel to flow direction and this directional component which is present in a moving fluid can be called as dynamic pressure
What are the types of pressure measurement
Liquid column method – These type of devices are those in which the pressure being measured is balanced against the pressure exerted by a column of liquid and if the density of the liquid is known the height of the liquid column is a measure of pressure
Manometer
A manometer is based on liquid column method it can be used to measure the pressure of the fluids and this device is based on the principle of balancing the column of liquid by the same or other columns of liquid they are of two types simple and differential manometer. Simple manometers are those which measure pressure at a point in a fluid containing in the pipe or a vessel, differential manometer measures the difference of pressure between any two points in a fluid contained in a pipe or vessel. Properties of manometers are they have high chemical stability, low viscosity, low capillary constant, low volatility, and low vapor pressure.
Elastic element method
Elastic element pressure measuring devices are those in which the measured pressure deforms some elastic material within its elastic limit and the magnitude of deformation is approximately proportional to the applied pressure
Diaphragm type
Diaphragm elements can be classified into two types first one is those which utilize the elastic characteristics of the diaphragm and the second one is those that are opposed by spring or other separate elastic elements. The first one consists of one or more capsules each composed of two diaphragms bonded together by soldering, brazing, or welding, commonly used metal in diaphragm elements are brass, phosphor bronze, and stainless steel. The second type of diaphragm is used for containing the pressure and exerting a force on the opposing elastic elements and the diaphragm will be flexible. Movement of the diaphragm is opposed by a spring that determines the deflection for a given pressure.
Advantages and applications of diaphragm type
It is used for the measurement of extremely low pressure, vacuum or differential pressure. They are commonly used in very corrosive environments. Their advantages are it is very sensitive and they can measure fractional pressure differences over a very minute range and they only needs less space.
Bourdon pressure gauge
The idea behind the device is that cross-sectional tubing when deformed in any way will regain its circular form under the action of pressure. The tube is generally bent into C shape or arc length of about 27 degrees. Bourdon tubes can be used for very high range of differential pressure measurement. Bourdon gauges can also be constructed in the form of a helix or spiral for better linearity and large sensitivity bourdon tube material must have good elastic or spring characteristics
Advantages of bourdon pressure gauge
- Low cost and simple construction
- Wide variety of ranges are available
- High accuracy
Disadvantages of bourdon pressure gauge
- Low spring gradient
- Susceptibility to hysteresis, shock, and vibrations
Bellows
Bellows are used to measuring pressure and they can be made of cascade capsules. It is basically manufactured by fastening together many individual diaphragms. The bellows element is a one-piece expansible, collapsible and axially flexible member. It can be manufactured from a single piece of thin metal. The commonly used bellow elements are by rolling a tube, by hydraulically forming a drawn tubing, by turning from a solid stock of metal. Liquid filled bellows can function in a number of transducer applications
Advantages of bellows
- Moderate cost
- Delivery of high force
- It is good in the low to moderate pressure range
Disadvantages of bellows
- Unsuitable for high pressure
- Ambient temperature compensation needed
Electric methods
Strain gauge pressure transducer
This is a passive type of resistance pressure transducer and its electrical resistance will change when it is stretched or compressed. A strain gauge is a wire which will change its resistance when mechanically strained, due to physical effects. Strain gauge is connected to the diaphragm and when the diaphragm flexes due to the applied pressure, the strain gauge will stretch or compress and because of this variation in its cross-sectional areas and its resistance will change due to this and this change is converted into voltage by linking two or four similar gauges similar to Wheatstone bridge so that the output can be maximized and reduces sensitivity to errors.
Advantages of strain gauge pressure transducer
- Simple to maintain and easy to install
- Good accuracy and stability
- Fast speed of response
- Wide range of measurements
- No moving parts and high output signal strength over range capacity
Disadvantages of strain gauge pressure transducer
- Temperature compensation and constant voltage supply is needed
- The electrical readout is necessary
Piezoelectric pressure transducer
Piezoelectricity is the ability of some materials mostly crystals to generate an electric potential in response to applied mechanical stress. In this transducer, the piezoelectric effect is used in certain materials such as quartz to generate rapid electrical signals and measuring the strain upon the sensing mechanism due to pressure. Common types of piezoelectric pressure transducers are charge mode type and low impedance voltage-mode type
Advantages of a piezoelectric pressure transducer
- It has good frequency response and no need for external power
Disadvantages of a piezoelectric pressure transducer
- Changes in temperature will affect the output and it cannot measure static pressure
Piezoresistive transducers
Piezoresistivity is the change in materials resistance because of a change in stress in the material. Piezoresistive gauge factor decreases as temperature increases, a transducer which uses this effect is a silicon-based MEMS pressure sensor and it has various applications like blood pressure sensing and tire pressure sensing
