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| Date post: | 13-Apr-2017 |
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Pressure Methods
ManometerElastic TransducersMeasuring VacuumBalancing Force on known areaElectrical Pressure Transducers
Manometers
Manometers measure a pressure difference by balancing the weight of a fluid column between the two pressures of interest. Large pressure differences are measured with heavy fluids, such as mercury (e.g. 760 mm Hg = 1 atmosphere). Small pressure differences, such as those experienced in experimental wind tunnels or venturi flowmeters, are measured by lighter fluids such as water (27.7 inch H2O = 1 psi; 1 cm H2O = 98.1 Pa).
Liquid/Gas Pressure
0PrGage essure p p p ghr= = - =V
Where
p is unknown pressure
Po is Ambient pressure
is fluid density
g is gravitational force
h is fluid column
1 atm = 760 torr = 101 kPa. AND Hg = 13.5951 g cm-3
Manometers
http://www.chm.davidson.edu/vce/GASLAWS/Pressure.html
Pressure by Elastic Transducers
BourdonBellowsDiaphragm
Pressure Measurement Pressure Measurement
Bourdon gaugeA Bourdon gauge uses a coiled tube, which, as it expands due to pressure increase causes a rotation of an arm connected to the tube. In 1849 the Bourdon tube pressure gauge was patented in France by Eugene Bourdon.
Pressure MeasurementDiaphragm
A second type of aneroid gauge uses the deflection of a flexible membrane that separates regions of different pressure. The amount of deflection is repeatable for known pressures so the pressure can be determined by using calibration. The deformation of a thin diaphragm is dependent on the difference in pressure between its two faces. The reference face can be open to atmosphere to measure gauge pressure, open to a second port to measure differential pressure, or can be sealed against a vacuum or other fixed reference pressure to measure absolute pressure. The deformation can be measured using mechanical, optical or capacitive techniques. Ceramic and metallic diaphragms are used.
Diaphragms
BellowsThe bellows pressure sensor is made of a sealed chamber that has multiple ridges like the pleats of an accordion that are compressed slightly when the sensor is manufactured. When pressure is applied to the chamber, the chamber will try to expand and open the pleats. which uses a spring to oppose the movement of the bellows and provides a means to adjust the amount of travel the chamber will have when pressure is applied
Pressure MeasurementPiezoresistor
For typical stress values in the MPa range the stress dependent voltage drop along the resistor Vr, can be considered to be linear. A piezoresistor aligned with the x-axis as shown in the figure may be described by:
Where:
Vr is Voltage across piezoresistor
R0 is unstrained value of Piezoresistor
I is applied current
L, T are transverse and longitudinal coefficient
xx, yy, and zz are tensile components
Pressure MeasurementStrain Gauge
00
900
1800
2700
Pressure MeasurementCapacitiveIn a capacitance-type
pressure sensor, a high-frequency, high-voltage oscillator is used to charge the sensing electrode elements. In a two-plate capacitor sensor design, the movement of the diaphragm between the plates is detected as an indication of the changes in process pressure.
Pressure MeasurementPotentiometer
The potentiometric pressure sensor provides a simple method for obtaining an electronic output from a mechanical pressure gauge. The device consists of a precision potentiometer, whose wiper arm is mechanically linked to a Bourdon or bellows element.
The movement of the wiper arm across the potentiometer converts the mechanically detected sensor deflection into a resistance measurement, using a Wheatstone bridge circuit
Pressure Measurement
The resonant-wire pressure transducer was introduced in the late1970s. In this design (Figure 3-7), a wire is gripped by a static member at one end, and by the sensing diaphragm at the other.
An oscillator circuit causes the wire to oscillate at its resonant frequency. A change in process pressure changes the wire tension, which in turn changes the resonant frequency of the wire. A digital counter circuit detects the shift. Because this change in frequency can be detected quite precisely, this type of transducer can be used for low differential pressure applications as well as to detect absolute and gauge pressures.
Resonant of wire
Applications
AUTOMOTIVE MEDICAL DEVICES ERGONOMICS ROBOTICS
AutomotiveMeasuring tire footprints and their distributed contact pressures under motion is no easy feat---but PPS has a solution. Our industrial-grade sensors are proven to have the speed, resolution, and ruggedness for this demanding application. PPS tire footprint sensors feature auxiliary analog inputs capable of reading position encoder or accelerometer data to correlate the type of motion with the tactile data. And they can be configured to fit tread widths for nearly any size vehicle.
Tire Tread Measurement
AutomotiveDynamic Brake Pad Measurement System
Medical Devices
The osteopathic approach to medicine
Medical Devices
State-of-the-Art Foot Scanning
