Category: Education
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20.4 Displacement and Displacer Level Instrumentation
Displacer level instruments exploit Archimedes’ Principle to detect liquid level by continuously measuring the weight of an object (called the displacer) immersed in the process liquid. As liquid level increases, the displacer experiences a greater buoyant force, making it appear lighter to the sensing instrument, which interprets the loss of weight as an increase in level and transmits a…
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20.3 What is Hydrostatic Pressure?
A vertical column of fluid generates a pressure at the bottom of the column owing to the action of gravity on that fluid. The greater the vertical height of the fluid, the greater the pressure, all other factors being equal. This principle allows us to infer the level (height) of liquid in a vessel by…
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20.2 Continuous Float Level Transmitters
Perhaps the simplest form of solid or liquid level measurement is with a float: a device that rides on the surface of the fluid or solid within the storage vessel. The float itself must be of substantially lesser density than the substance of interest, and it must not corrode or otherwise react with the substance. Floats…
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Chapter 20 Introduction to Continuous Level Measurement
Many industrial processes require the accurate measurement of fluid or solid (powder, granule, etc.) height within a vessel. Some process vessels hold a stratified combination of fluids, naturally separated into different layers by virtue of differing densities, where the height of the interface point between liquid layers is of interest. A wide variety of technologies exist to…
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19.7 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: It should come as no surprise, then, that the common mechanical sensing elements for measuring pressure…
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19.6 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. One such hazard is pressure pulsation, for example at the discharge of a piston-type (positive-displacement) high-pressure pump. Pulsating pressure can quickly damage mechanical sensors such as…
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19.5 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…
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19.4 Force-balance pressure transmitters
An 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. A common manual-balance system is the type of scale used in laboratories to measure mass: Here,…
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19.3 Electrical pressure sensing 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. A brief…
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19.2 Mechanical pressure sensing 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. Bellows resemble an accordion constructed from metal instead of fabric. Increasing pressure inside a bellows unit causes it…
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Chapter 19 Introduction To Continuous pressure measurement-Manometers
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…
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Chapter 18 Basic Principles of Instrument Calibration and Ranging
Every instrument has at least one input and one output. For a pressure sensor, the input would be some fluid pressure and the output would (most likely) be an electronic signal. For a loop indicator, the input would be a 4-20 mA current signal and the output would be a human-readable display. For a variable-speed motor drive, the…
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16.6 H1 FOUNDATION Fieldbus segment troubleshooting
Feedback obtained from industrial users of FF reveal a common pattern: Fieldbus is a powerful and reliable technology, but only if it is properly installed. Poor installations, usually driven by a desire to minimize capital expenses, will cause numerous problems during commissioning and operation. One relatively easy way to avoid problems caused by short-circuits in…
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16.5 H1 FOUNDATION Fieldbus device configuration and commissioning
Fieldbus devices require far more attention in their initial setup and commissioning than their analog counterparts. Unlike an analog transmitter, for example, where the only “configuration” settings are its zero and span calibration adjustments, a FF transmitter has a substantial number of parameters describing its behavior. Some of these parameters must be set by the…
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16.4 FF function blocks
Data-processing modules within FF systems are known as function blocks. Sometimes these blocks serve merely to catalogue data, while in other instances the blocks execute specific algorithms useful for process measurement and control. These “blocks” are not physical entities, but rather abstract software objects – they exist only as bits of data and instructions in computer…