from:
http://www.sensorsmag.com/sensors/pressure/fundamentals-pressure-sensor-technology-846
Types of Pressure Measurements
Absolute pressure is measured relative to a perfect vacuum. An example is atmospheric pressure. A common unit of measure is pounds per square inch absolute (psia).
Differential pressure is the difference in pressure between two points of measurement. This is commonly measured in units of pounds per square inch differential (psid).
Gauge pressure is measured relative to ambient pressure. Blood pressure is one example. Common measurement units are pressure per square inch gauge (psig). Intake manifold vacuum in an automobile engine is an
example of a vacuum gauge measurement (vacuum is negative gauge pressure).
Pressure Units
Pressure is force per unit area and historically a great variety of units have been used, depending on their suitability
for the application. For example, blood pressure is usually measured in mmHg because mercury manometers were used originally. Atmospheric pressure is usually expressed in in.Hg for the same reason. Other units used for atmospheric pressure are bar and atm. The following conversion factors should help in dealing with the various 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
Pressure Sensing
Pressure is sensed by mechanical elements such as plates, shells, and tubes that are designed and constructed to deflect when pressure is applied. This is the basic mechanism converting pressure to physical movement. Next,
this movement must be transduced to obtain an electrical or other output. Finally, signal conditioning may be needed, depending on the type of sensor and the application.
Pressure Sensor Technologies
Potentiometric Pressure Sensors; Inductive Pressure Sensors; Capacitive Pressure Sensors; Piezoelectric Pressure Sensors; Strain Gauge Pressure Sensors; Piezoresistive Integrated Semiconductor.
A piezoelectric sensor is a device that uses the piezoelectric effect, to measure changes in pressure, acceleration, temperature, strain, or force by converting them to an electrical charge (electric potential).
Coriolis Mass Flowmeter Technology
from http://www.flowmeters.com/coriolis-mass-technology
How Coriolis Mass Flowmeters Work
Coriolis mass flowmeters measure the force resulting from the acceleration caused by mass moving toward (or away from) a center of rotation. This effect can be experienced when riding a merry-go-round, where moving toward the center will cause a person to have to “lean into” the rotation so as to maintain balance. As related to flowmeters, the effect can be demonstrated by flowing water in a loop of flexible hose that is “swung” back and forth in front of the body with both hands. Because the water is flowing toward and away from the hands, opposite forces are generated and cause the hose to twist. They represent about 21% of all flowmeters sold.
In a Coriolis mass flowmeter, the “swinging” is generated by vibrating the tube(s) in which the fluid flows. The amount of twist is proportional to the mass flow rate of fluid passing through the tube(s). Sensors and a Coriolis mass flowmeter transmitter are used to measure the twist and generate a linear flow signal.
Plusses and Minuses
This technology has high accuracy, can handle sanitary applications, is approved for custody transfer and is highly reliable and low maintenance. Mass flow is more important than volume for fluids intended for the production of energy. These include petroleum liquids and natural gas both compressed and liquefied. The cost is high, especially for line sizes above four inches. Pressure drop can be a consideration for “U” shaped tube designs and high viscosity fluids.
When two wires composed of dissimilar metals are joined at both ends and one of the ends is heated, there
is a continuous current which flows in the thermoelectric circuit. Thomas Seebeck made this discovery in 1821.
All dissimilar metals exhibit this effect. For small changes in temperature the Seebeck voltage is linearly proportional
to temperature:
∆eAB = α∆T
where α, the Seebeck coefficient, is the constant of proportionality.