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Continuous Pressure Measurement

In many ways, pressure is the primary variable for a wide range of process measurements. Many types of industrial measurements are actually inferred from pressure, such as:

   • Flow (measuring the pressure dropped across a restriction)

   • Liquid level (measuring the pressure created by a vertical liquid column)

   • Liquid density (measuring the pressure difference across a fixed-height liquid column)

   • Weight (hydraulic load cell)

Even temperature may be inferred from pressure measurement, as in the case of a fluid-filled chamber where fluid pressure and fluid temperature are directly related. As such, pressure is a very important quantity to measure, and measure accurately. This section describes different technologies for the measurement of pressure.

Manometers - A very simple device used to measure pressure is the manometer: a fluid-filled tube where an applied gas pressure causes the fluid height to shift proportionately. This is why pressure is often measured in units of liquid height (e.g. inches of water, inches of mercury). Click here to read the whole sub-article about Manometers.

Mechanical Pressure Elements - Mechanical pressure-sensing elements include the bellows, the diaphragm, and the bourdon tube. Each of these devices converts a fluid pressure into a force. If unrestrained, the natural elastic properties of the element will produce a motion proportional to the applied pressure. Learn more about Mechanical Pressure Elements here.

Electrical Pressure Elements - Several different technologies exist for the conversion of fluid pressure into an electrical signal response. These technologies form the basis of electronic pressure transmitters: devices designed to measure fluid pressure and transmit that information via electrical signals such as the 4-20 mA analog standard, or in digital form such as HART or FOUNDATION Fieldbus. Expand your knowledge about Electrical Pressure Elements here.

Force-Balance Pressure TransmittersAn important legacy technology for all kinds of continuous measurement is the self-balancing system. A “self-balance” system continuously balances an adjustable quantity against a sensed quantity, the adjustable quantity becoming an indication of the sensed quantity once balance is achieved. Click here to read more.

Differential Pressure Transmitters - One of the most common, and most useful, pressure measuring instruments in industry is the differential pressure transmitter. This device senses the difference in pressure between two ports and outputs a signal representing that pressure in relation to a calibrated range. Differential pressure transmitters may be based on any of the previously discussed pressure-sensing technologies, so this
section focuses on application rather than theory. Learn more about Differential Pressure Transmitters by clicking here.

Pressure Sensor Accessories - Multiple accessories exist for pressure-sensing devices to function optimally in challenging process environments. Sometimes, we must use special accessories to protect the pressure instrument against hazards of certain process fluids. Read more by clicking here.

Process / Instrument Suitability - On a fundamental level, pressure is universal. Regardless of the fluid in question; liquid or gas, hot or cold, corrosive or inert, pressure is nothing more than the amount of force exerted by that fluid over a unit area... Read the whole text about Process / Instrument Suitability here.

 

References

Beckerath, Alexander von; Eberlein, Anselm; Julien, Hermann; Kersten, Peter; and Kreutzer,

Jochem, WIKA-Handbook, Pressure and Temperature Measurement, WIKA Alexander Wiegand GmbH & Co., Klingenberg, Germany, 1995.

“Digital Sensor Technology” (PowerPoint slideshow presentation), Yokogawa Corporation of America.

Fribance, Austin E., Industrial Instrumentation Fundamentals, McGraw-Hill Book Company, New York, NY, 1962.

Kallen, Howard P., Handbook of Instrumentation and Controls, McGraw-Hill Book Company, Inc., New York, NY, 1961.

Lipt´ak, B´ela G., Instrument Engineers’ Handbook – Process Measurement and Analysis Volume I, Fourth Edition, CRC Press, New York, NY, 2003.

Patrick, Dale R. and Patrick, Steven R., Pneumatic Instrumentation, Delmar Publishers, Inc., Albany, NY, 1993.

Technical Note: “Rosemount 1199 Fill Fluid Specifications”, Rosemount, Emerson Process Management, 2005.

Go Back to Lessons in Instrumentation Table of Contents


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