presure measurement
TRANSCRIPT
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PRESSURE SENSOR
TECHNOLOGY
priyatmadi
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Pressure Definition
Static Pressure. Pressure, P, is defined as
force, F, per unit area, A:
P = F/A
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Pressure in open tankA container filled with a liquid has a pressure (due to the
weight of the liquid) at a point in the liquid of:
P = F/A
P = W/A
P = gV/A
P = ghA/AP = gh
P = pressure
F = force
A = Area
W = weight of the liquid
V = volume above the Area
g = gravitation
= mass density
h = distance from the surface
A
h
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Static pressure of atmosfeer
Gases differ from liquidsin two respects: they arevery compressible, andthey completely fill any
closed vessel in whichthey are placed.
The nonlinear airpressure variation withaltitude shown in the
figure is an example ofthe effect of thecompressibility of gases.
The compressibility of gases is
illustrated by nonlinear atmosphere
pressure as a function of altitude
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Types of Pressure
measurements Absolute pressure is measuredrelative to a perfect vacuum
(psia)
Gauge pressure is measured
relative to ambient pressure(psig)
Differential pressure is the
difference in pressure between
two points of measurement.
(psid). Note that the same sensor may
be used for all three types; only
the reference is different.
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Dynamic Effects.
Static pressure is measured under steady-state or equilibriumconditions, but most real-life applications deal with dynamic orchanging pressure.
For example, the measurement of blood pressure usually gives thetwo steady-state values of systolic and diastolic pressure.
There is much additional information in the shape of the bloodpressure signal, however, which is the reason for the monitors usedin critical-care situations.
To measure changing pressures, the frequency response of thesensor must be considered. As a rough approximation, the sensorfrequency response should be 5-10 the highest frequencycomponent in the pressure signal.
Another issue is the remote measurement of pressure where a liquidcoupling medium is used. Care must be taken to purge all airbecause its compressibility will corrupt the waveform.
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Pressure in Moving FluidsBernoulli's theorem states that for horizontal
flow the following relation holds:
PS = PO + PI
PS = stagnation (or total) pressure
PO = static pressurePI = Impact pressure due to moving fluid
PI = Vo /2
Where V0 = the velocity of the fluid
Hence we can measure the velocity if we knowthe pressure
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Pressure Units
1 psi = 51.714 mmHg
= 2.0359 in.Hg
= 27.680 in.H2O
= 6.8946 kPa
1 bar = 14.504 psi
1 atm = 14.696 psi
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Pressure Units
pascal
(Pa)
bar
(bar)
technical
atmosphere
(at)
atmosphere
(atm)
torr
(Torr)
pound-force per
square inch
(psi)
1 Pa 1 N/m2 10
51.019710
59.869210
67.500610
3145.0410
6
1 bar 100 106 dyn/cm2 1.0197 0.98692 750.06 14.5037744
1 at 98,066 0.980665 1 kgf/cm2 0.96784 735.56 14.223
1 atm 101,325 1.01325 1.0332 1 atm 760 14.696
1 torr 133 1.3332103
1.3595103
1.3158103
1 Torr;
1 mmHg19.33710
3
1 psi 6,894 68.948103 70.307103 68.046103 51.715 1 lbf/in2
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Pressure Sensing
Pressure is sensed by mechanicalelements such as plates, shells, andtubes that are designed andconstructed to deflect when pressureis applied.
This is the basic mechanismconverting pressure to physicalmovement.
Next, this movement must betransduced to obtain an electrical or
other output. Finally, signal conditioning may be
needed, depending on the type ofsensor and the application. Figure 8illustrates the three functional blocks.
Pressure
Signal
Conditioner
Sensing
Element
Transduction
element
displacement
electric
V or I output
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Sensing Elements
The main types ofsensing elements areBourdon tubes,diaphragms, capsules,
and bellows All except diaphragms
provide a fairly largedisplacement that isuseful in mechanical
gauges and for electricalsensors that require asignificant movement
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Potentiometric Pressure Sensors
Potentiometric pressuresensors use a Bourdon tube,capsule, or bellows to drive awiper arm on a resistiveelement.
For reliable operation the wipermust bear on the element withsome force, which leads torepeatability and hysteresiserrors.
These devices are very low
cost, however, and are used inlow-performance applicationssuch as dashboard oilpressure gauges
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Inductive Pressure Sensors
Several configurations based onvarying inductance or inductivecoupling are used in pressuresensors. They all require ACexcitation of the coil(s) and, if a
DC output is desired,subsequent demodulation andfiltering. The LVDT types have afairly low frequency responsedue to the necessity of drivingthe moving core of the
differential transformer The LVDT uses the moving core
to vary the inductive couplingbetween the transformer primaryand secondary.
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Capacitive Pressure Sensors.
Capacitive pressure sensors typically use a thindiaphragm as one plate of a capacitor.
Applied pressure causes the diaphragm to deflect andthe capacitance to change.
This change may or may not be linear and is typically onthe order of several picofarads out of a total capacitanceof 50-100 pF.
The change in capacitance may be used to control the
frequency of an oscillator or to vary the coupling of anAC signal through a network.
The electronics for signal conditioning should be locatedclose to the sensing element to prevent errors due tostray capacitance.
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Capacitive Pressure Sensors
Capacitive Pressure Sensors
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Piezoelectric Pressure Sensors.
Piezoelectric elements are bi-directional transducerscapable of converting stress into an electric potentialand vice versa.
One important factor to remember is that this is a
dynamic effect, providing an output only when theinput is changing.
This means that these sensors can be used only forvarying pressures.
The piezoelectric element has a high-impedanceoutput and care must be taken to avoid loading theoutput by the interface electronics. Somepiezoelectric pressure sensors include an internalamplifier to provide an easy electrical interface.
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Piezoelectric Pressure Sensors.
Piezoelectric sensors convert stress into an electric potential and vice versa.
Sensors based on this technology are used to measure varying pressures.
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Strain Gauge Pressure Sensors
Strain gauge sensors originally used a metaldiaphragm with strain gauges bonded to it.
the signal due to deformation of the material is
small, on the order of 0.1% of the base resistance Semiconductor strain gauges are widely used,
both bonded and integrated into a silicondiaphragm, because the response to applied
stress is an order of magnitude larger than for ametallic strain gauge.
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Strain Gauge Pressure Sensors
When the crystal lattice structure of silicon isdeformed by applied stress, the resistancechanges. This is called the piezoresistive effect.
Following are some of the types of strain gaugesused in pressure sensors.
Deposited strain gauge. Metallic strain gauges canbe formed on a diaphragm by means of thin film
deposition. This construction minimizes the effectsof repeatability and hysteresis that bonded straingauges exhibit. These sensors exhibit the relativelylow output of metallic strain gauges.
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Strain Gauge Pressure Sensors
Bonded semiconductor strain gauge. A silicon bar may
be bonded to a diaphragm to form a sensor with
relatively high output. Making the diaphragm from a
chemically inert material allows this sensor to interface
with a wide variety of media
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Piezoresistive Integrated
Semiconductor IC processing is used to form
the piezoresistors on thesurface of a silicon wafer
There are four piezoresistorswithin the diaphragm area onthe sensor. Two are subjectedto tangential stress and two toradial stress when thediaphragm is deflected.
They are connected in a four-element bridge configuration(see Figure 22) and providethe following output:
VOUT/VCC = R / R
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Piezoresistive Integrated
Semiconductor
IC processing is used to form the piezoresistors
on the surface of a silicon wafer to fabricate
an integrated piezoresistive pressure sensor.
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Piezoresistive Integrated
Semiconductor
Integrated silicon pressure sensor
measures 0.52 in. long by 0.44 in. wide
by 0.75 in. high, including the port.
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Pressure Switches
Pressure switches, combining adiaphragm or other pressure measuringmeans with a precision snap switch, can
provide precise single-point pressuresensing.
Alternatively, simple electronic switchesmay be combined with electrical sensorsto construct a pressure switch with anadjustable set point and hysteresis.
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Electrical Interfacing
Care must be taken to avoid corrupting the signal by noise of 60/50Hz AC pickup. If the signal must be run some distance to theinterface circuitry, twisted and/or shielded wire should beconsidered. A decoupling capacitor located at the sensor andconnected from the supply to ground will also filter noise, as will acapacitor from output to ground.
For long runs, a current output sensor should be considered. Thesedevices have a 2-wire interface and modulate the supply current inresponse to applied pressure. Obviously, wire resistance has noeffect and noise must change the loop current, not simply impress avoltage on the signal. The industry standard interface is:
PL = 4 mA PH = 20 mA
PL= low pressure range limitPH = high pressure range limit
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Calibration
Dead-Weight Tester.A dead-weight tester uses calibratedweights that exert force on apiston which then acts on a
fluid to produce a testpressure.
For high pressures (>500 psi),oil is typically used for lowerpressures, pneumatic air
bearing testers are availableand are much moreconvenient as well as lessmessy to use.
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Manometer
A mercury manometer is
a simple pressure
standard and may be
used for gauge,
differential, and absolute
measurements with a
suitable reference. It is
useful mainly for lower
pressure work becausethe height
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Selection Considerations
Selection of a pressure sensor involves consideration ofthe medium for compatibility with the materials used inthe sensor, the type (gauge, absolute, differential) ofmeasurement, the range, the type of electrical output,and the accuracy required.
Manufacturer's specifications usually apply to aparticular temperature range. If the range of operation ina given application is smaller, for example, the errorsshould ratio down.
Total error can be computed by adding the individual
errors (worst-case) or by computing the geometric sumor root sum of the squares (RSS). The latter is morerealistic since it treats them as independent errors thattypically vary randomly.
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Selection Considerations Following is a comparison of the two methods. Given the following error terms:
Linearity = 1% F.S.
Null calibration = 1% F.S.
Sensitivity calibration = 1% F.S. Temperature errorsare sometimes given as coefficients per Creferenced to 25C. Simply multiply the coefficientby the temperature range of the application to obtainthe total error.
Temperature error = 0.5% F.S. Repeatability and hysteresis = 0.1% F.S.
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Industrial ApplicationsFluid level in a tank:A gauge pressure sensor located
to measure the pressure at the bottom of a tank canbe used for a remote indication of fluid level usingthe relation:
h = P/g
Fluid flow:An orifice plate placed in a pipe sectioncreates a pressure drop. This approach is widelyused to measure flow because the pressure dropmay be kept small in comparison to some othertypes of flowmeters and because it is impervious to
clogging, which may otherwise be a problem whenmeasuring flow of a viscous medium or onecontaining particulate matter. The relation is: