Pressure measurement in instrumentation
Pressures Measurement in instrumentation
What is Pressure?
The force per unit area is called pressure.
Three main methods are used to measure pressure, absolute pressure, gauge pressure, and differential pressure.
Pressure varies depending on the altitude of the site where we are measuring pressure.
Weather, pressure fronts, and other conditions also affect the measurement of pressure in a process.
The measurement of pressure is relative to all these factors of weather and climate, and pressure measurements are carried out in two main scales
1. gauge pressure
2. absolute pressure.
Gauge pressure
Gauge pressure is the scale considered in everyday life (like pressure in the tire is in rating indicated as gauge pressure).
Gauge pressure and differential pressures are referenced to another pressure such as the ambient pressure or pressure in an adjacent vessel or any other reference pressure of some value.
Instruments designed to measure a Gauge pressure will indicate zero pressure below atmospheric pressure, so the gauge pressure is referenced to atmospheric pressure. When a device is completely vented down it will indicate zero reading.
Gauge pressure is written as (g) at the end of the pressure unit like K Pa (g), bar (g), mmws (g).
Absolute pressure
Absolute pressure has the total effect of atmospheric pressure and gauge pressure.
It is written as (a) with the pressure unit like as K Pa (a), Bar (a), mmWs (a)
Absolute Pressure is a measurement referenced to the pressure in a vacuum,
An absolute pressure will indicate atmospheric pressure when completely vented down to the atmosphere. This means it will not indicate zero, it will indicate zero when atmospheric pressure is also zero.
Absolute Pressure
= Gauge Pressure + Atmospheric Pressure
Total vacuum
Total vacuum is a condition when there is no pressure or zero pressure or lack of pressure, we can experience a total vacuum in outer space.
Vacuum
A vacuum is a pressure measurement made between total vacuum and normal Atmospheric pressure (14.7 psi).
negative pressure or vacuum can not be less than -1 bar
Atmospheric pressure
We can define Atmospheric pressure as the pressure on the surface of the earth due to the weight of the gases in the atmosphere and atmospheric pressure can be usually expressed at sea level as 14.7 psi (pounds per square inch).
It is dependant on atmospheric conditions, altitude, and the climate where we are measuring the pressure.
The pressure decreases above sea level.
So if we go towards height it will decrease gradually at an elevation of 5000 feet atmospheric pressure drops to 12.2 psi (14.7-2.5=12.2) it is a drop of 2.5 psi. The atmospheric pressure at different elevations will be different.
Differential pressure
Differential pressure is the pressure measured with respect to another pressure and is expressed as the difference between the two values of pressure.
delta P = P1 - P2
DP or delta pressure is proportional to FLOW, so by measuring DP we can measure the flow.
Pressure unit
Pa (Pascal) SI unit of pressure
Bar
PSI (Pounds per Square Inch)
mmWs (millimeter of water scale)
atm (atmospheric)
torr
kg/cm2
There are also other units of pressure
Pressure conversion units
100KPa = 1Bar
10 bar= 1MPa
100000 Pa = 1 Bar
100pa= 1mbar
1Bar =1000 m Bar
1 Bar = 14.5 PSI
100000mmws =1 bar
100mmws=1mbar
1KPa
=0.294 IN MERCURY
=7.5 mm MERCURY
=4.02 IN WATER
= 102 mm WATER
1 atm
= 101.3 ´ 10³ N/m²
= 101.3 kN/m²
= 1.013 bar
= 760 mmHg
= 14.696 psi
1 bar
= 100 ´ 10³ N/m²
= 100 kN/m²
= 0.987 atm
= 750.006 mmHg
= 14.504 psi
1mm Hg ´ 10³
= 133.3 ´ 10³ N/m²
= 133.3 kN/m²
= 1.333 bar
= 1.316 atm
= 19.337 psi
1 psi ´ 10³
= 6.895 ´ 106 N/m²
1psi = 27.74 " H2O
1 Kg/cm2 = 14.223 psi
1 Bar = 14.504 psi
1 Kpa = 0.145 psi
1 Kg/cm2 = 10.000mm of
H20
1 Bar = 1.0197 Kg/cm2
1 Kg/cm2 = 0.98 Bar
1 Torr = 1 mm of Hg.
Pressure sensors
Types of pressure sensors
1. Bourdon tube Pressure gauge
Type of Bourdon tubes
'C ' type bourdon tube
Helical Bourdon tube
Spiral Bourdon tube
Pressure gauge Parts
A: Pressure connection:
Receiver part or connection socket
This part joins the pressure inlet to the fixed end of the Bourdon tube.
B: Stationary socket
C: Bourdon tube
D: Pointer
E: Gear
a spur gear that amplifies the motion of the tube, before passing it to the needle.
F: Hairspring
Hairspring to preload the gear train and eliminate gear lash and hysteresis.
To avoid backlash error by eliminating any play into linkages and control torque
G: Link
H: Sector and pinion
I: Indicator needle axle.
Applications of Bourdon tube pressure Gauge:
It is used in a system having medium pressure up to a system having very high-pressure applications.
we can use it for air, water, oil, steam pressure measurement, and it's also good for corrosive fluids as well.
Advantages of the Bourdon tube pressure Gauge:
1. Accuracy
accuracy of the bourdon tube pressure gauge is up to +2% of span and it can be increased to accuracies ranging 0.1% of full scale.
2. Bourdon tube gauge is a very simple construction.
3. Better to use for high-pressure systems, Bourdon tube gauge is Safe for high-pressure measurement.
4. Portable device and bourdon tube device installation is very easy and quick.
5. its low cost is the main advantage bourdon tube pressure gauge.
Limitations of Bourdon Gauge:
It is not recommended for dynamic pressure measurements because its response time to changing pressures is slow, so it's mainly used to measure static pressure.
If pressure is varying rapidly bourdon tube may give the wrong value.
It is Temperature sensitive so not good for high-temperature applications.
Its reading is not very precise so it is not preferred for high precision measurements.
It is Sensitive to shock and vibrations so use in a location where vibration is minimum.
These types of gauges are subjected to hysteresis.
Materials for bourdon tube
Phosphor (For non-corrosive process).
Brass (For low-pressure measurements).
Beryllium Copper (For medium-range process).
SS316 (For corrosive fluids and high-pressure measurements).
Some other materials can also be used for the construction of the Bourdon tube, depending upon pressure ratings and fluid type. These materials may be;
monel,
tantalum,
titanium,
various other SS grades.
2. McLeod Gauge
Working principle of McLeod gauge
Mcleod gauge working principle sI explained here in simple words
In McLeod gauge, a known volume of gas is compressed to a small volume and the final value of that compressed gas indicates the applied pressure, so by this method pressure is measured. The gas used to be compressed must obey Boyle's law
boyle's law is given by ;
P1V1=P2V2
Where,
P1 = Pressure of gas at initial condition (applied pressure).
P2 = Pressure of gas at final condition.
V1 = Volume of gas at initial Condition.
V2 = Volume of gas at final Condition.
Initial Condition = Before Compression.
Final Condition = After Compression.
The pressure to be measured is denoted as (P1) is applied to the top side of the column of the McLeod Gauge. so the mercury level in the gauge is raised by operating the piston to fill the volume in the tube.
The applied pressure fills the bulb and the capillary tube.
Now again the piston is operated so that the mercury level in the gauge increases more. When the mercury level reaches the cutoff point, a known volume of gas denoted by (V1) is trapped in the bulb and measuring capillary tube. The mercury level is further raised by operating the piston so the trapped gas is compressed in the bulb and measuring capillary tube.
This phenomenon is done until the mercury level reaches the Zero reference Point.
In this condition, the volume of the gas in the measuring capillary tube is shown on a scale.
That is, the difference in height H of the measuring capillary and the reference capillary becomes a measure of the
volume (V2) and
pressure (P2)
of the trapped gas.
Now as V1, V2, and P2 are known, the applied pressure P1 can be calculated using Boyle s Law given by;
P1V1 = P2V2
Let the volume of the bulb from the cutoff point up to the beginning of the measuring capillary tube = V
Let the area of the cross-section of the measuring capillary tube = a
Let the height of the measuring capillary tube = hc.
Therefore, the Initial Volume of gas
V1 = V+ahc.
the final volume of the gas
V2 = V +ah.
Where,
h = height of the compressed gas in the measuring capillary tube
P1 = Applied pressure of the
gas unknown.
P2 = Pressure of gas at a final condition,
that is, after compression = P1+h We have,
P1V1 = P2V2 (Boyle s Law) Therefore, P1V1= (P1+h)ah
P1V1 = P1ah + ah^2
P1V1-P1ah = ah^2
P1 = ah^2/(V1-ah)
Since ah is very small when compared to V1, it can be neglected.
Therefore,
P1 = ah^2/V1
Thus the applied pressure is calculated using the McLeod Gauge
Advantages of the McLeod Gauge:
Limitations of McLeod Gauge:
The measured gas should obey Boyle‟s law ·
Moisture traps are required to avoid any vapor entry into the gauge.
It measures only on a sampling basis.
It cannot give a continuous output
3. Liquid column manometer
Manometers are a very simple instrument used to measure pressure.
Now the use of manometer is reduced due to advancement in instrumentation technology, now smaller, more rugged, and easier-to-use pressure sensors are available.
Manometer & Its Types
There are different types of manometers.
Three main types of manometer are discussed here
1. U tube manometer
2. Inclined manometer
3. Well manometers
1. U tube manometers
In U type or U tube manometers, U-shaped glass tubes partially filled with a liquid are used as a manometer to measure pressure.
When there are equal pressures on both sides of P and Pre, the liquid levels will correspond to the zero points on a scale.
The scale is graduated in pressure units.
When applied pressure at P side of the U tube is higher than the Pre side, the liquid rises higher on the lower pressure side, so that the difference in the height h of the two columns of liquid compensates for the difference in pressure,
The pressure difference is given by
PH − PL = g × difference in height of the liquid in the columns
where
PH is a high-pressure column
PL is a low-pressure column
g is the specific weight of the liquid in the manometer.
2. Inclined manometers
These manometers were designed for low-pressure measurement
The low-pressure arm of the manometer is inclined so that the fluid has a longer distance to travel than in a vertical U tube manometer for the same pressure change. As a result, there is a magnified scale for low pressure
3. Well manometers
Well, manometers are alternatives to inclined manometers for measuring low pressures. Well, manometer uses low-density liquids to measure low pressure. In the well manometer, one leg has a much larger diameter than the other leg.
When there is no pressure difference in the liquid levels will be at the same height to indicate a zero reading.
An increase in the pressure in the larger leg will cause a larger change in the height of the liquid in the smaller leg. The pressure across the larger area of the well must be balanced by the same volume of liquid rising in the smaller leg.
4. Diaphragm
The diaphragm is used to measure pressure. Diaphram sensor is made up of metal sheet membrane in a circular shape of precise dimensions, it can either be flat or corrugated.
The mechanism of transmission is connected with the diaphragm, the transmission system amplifies a small deflection of the diaphragm, and the pointer moves with this mechanics.
Advantages of diaphragm:
It has Good Linearity
Its size is small
its cost is moderate
diaphragm Seals the process
so no leakage problems in diaphragm applications
Drawbacks:
Limited to low pressure.
Difficult to repair.
Susceptible to cracking.
Applications:
Processes where purity is important.
Diaphragm isolates the sensor so metal parts or corrosive fluids like Food, pharmaceuticals, chemicals, etc, don't directly contact the sensor.
It is used in pneumatic processes where proper sealing is necessary.
5. Bellows
Bellows are constructed of tubular membranes that are convoluted around the circumference. The membrane is attached at one end to the source of pressure and the other end is connected to an indicator measuring device or instrument.
The bellows can apply a long stroke to the indicator or sensor and pressure is applied at the input of bellows.
6. Capsule
Capsules made up of several diaphragms connected with a more circular shape diaphragms are welded together to form a pressure capsule.
The pressure is applied to the inlet of the capsule, if we apply only air it will expand as a balloon expands.
The capsule can in both directions. Capsules are mostly made of stainless steel.
In some constructions of capsules, both diaphragms are connected the internal space is filled with viscous oil. Mostly Silicone oil is filled between the diaphragms to facilitate the even transmission of pressure.
The advantage of the capsule can withstand high static pressure of up to 2000 psi.
Limitation of capsules
Capsules are sensitive up to only a few hundred kPa of differential pressure. Differential pressure that is significantly higher than the capsule range may damage the capsule.
7. Strain Gauge
The strain gauge is a device that can be affixed to the surface of an object to detect the force applied to the object.strain gauges are also used in load cell of belt weigh feeder
One form of the strain gauge is a metal wire of a very small diameter that is attached to the surface of a device being monitored. . For a metal, the electrical resistance will increase as the length of the metal increases or as the cross-sectional diameter decreases.
When cross-sectional, the overall length of the wire tends to increase while the cross-sectional area decreases.
The amount of increase in resistance is proportional to the force that produced the change in length and area. The output of the strain gauge is a change in resistance that can be measured by the input circuit of an amplifier.
Strain gauges can be bonded to the surface of a pressure capsule or to a force bar positioned by the measuring element. Strain gauge that is bonded to a for elements inside the DP capsule. The change in the process pressure will cause a resistive change in the strain gauges, which is then used to produce a standard signal of 4-20 mA.
8. Piezoelectric
All piezoelectric sensors are self-generating and develop electrical potential (charge) inproportion to applied stress.
Piezoelectric-based transducers rely on the piezoelectric effect, which occurs when a crystal is under stress forming an internal polarization.
This polarization results in the generation of charge on the crystal face that is proportional to the applied stress
Quartz, tourmaline, and several other naturally occurring crystals exhibit a piezoelectric effect. An electric charge proportional to the
the applied force is generated when a piezoelectric material is stressed by being coupled to an appropriate force summing device. Specially formulated ceramics can be artificially polarized to be piezoelectric with sensitivities 100 or more times higher than found in natural crystals.
Advantages
Piezoelectric sensors require no external excitation.
These sensors exhibit high output impedance
piezoelectric sensor exhibit low signal levels
Limitation
Special low-noise coaxial cable is required for low signal
Also, charge amplifiers are needed in the measurement chain of a piezoelectric sensor.
Capacitive sensor
Differential Pressure Transmitter used to measure flow contains a free-floating variable capacitance sensing element, the Differential capacitance between the sensing diaphragm and the capacitor plates on both sides of the sensing diaphragm is converted electronically to a two-wire 4-20 mA dc signal or a three-wire, 1-5 V dc signal.
10. Inductive
In this type of sensor inductance of the cell is varied with applied pressure and by measuring the inductance we can measure the applied pressure to at the sensor.
Pressure Transmitters
A pressure transmitter takes pressure as input and converts it into a standard signal of 4 to 20 mA.
Pressure Switches
Any pressure measuring sensor is connected to a switch, so when the pressure reaches a specific point contact of the pressure switch changes its position from open to close contact or vice versa. so at the output voltage changes and gives a digital signal which indicates an alarm etc . we can use a normally open or normally close contact as per our system requirements.
Dry Leg Condition
Low-side transmitter piping will remain empty if gas above the liquid does not condense.
This is a dry leg condition. Range determination calculations are the same as those described for bottom-mounted transmitters in open vessels.
Wet Leg Condition
Condensation of the gas above the liquid causes the low side of the transmitter piping to fill slowly with liquid. The pipe is purposely filled with a convenient reference fluid to eliminate this potential error. This is a wet leg condition.
The reference fluid will exert head pressure on the low side of the transmitter. Zero elevation of the range must then be made.
Common faults of pressure transmitter
* no power
* impulse line leak or block
* transmitter faulty
* mA cable faulty
* calibration not ok
* analog channel
Calibration of pressure transmitter
Remove impulse line and adjust zero if required. Apply span range pressure adjust span if required.
The following steps are to be taken which calibrating:
1. Adjust zero of the Transmitter.
2. Static pressure test: Give equal pressure on both sides of the transmitter. Zero should not shift. If it is shifting carry out static alignment.
3. Vacuum test: Apply equal vacuum to both sides. The zero should not shift.
4. Calibration Procedure:
Give 24Vdc supply to the transmitter.
Vent the L.P. side to the atmosphere.
Connect the output of the Instrument to a standard test gauge or Multimeter and adjust zero.
5. Apply the required span pressure to the high-pressure side of the transmitter and adjust the span.
6. Adjust zero again if necessary.
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