Category: General
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22.9 Change-of-quantity flow measurement
Flow, by definition, is the passage of material from one location to another over time. So far this chapter has explored technologies for measuring flow rate en route from source to destination. However, a completely different method exists for measuring flow rates: measuring how much material has either departed or arrived at the terminal locations…
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22.8 Weighfeeders
A special type flowmeter suited for powdered or granular solids is the weighfeeder. One of the most common weighfeeder designs consists of a conveyor belt with a section supported by rollers coupled to one or more load cells, such that a fixed length of the belt is continuously weighed: The load cell measures the weight of…
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22.7 True mass flowmeters
Many traditional flowmeter technologies respond to the volumetric flow rate of the moving fluid. Velocity-based flowmeters such as magnetic, vortex, turbine, ultrasonic, and optical generate output signals proportional to the speed of fluid molecules and nothing else. This means that if the fluid flowing through one of these flowmeter types were to suddenly become denser (while still…
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22.5 Positive displacement flowmeters
A positive displacement flowmeter is a cyclic mechanism built to pass a fixed volume of fluid through with every cycle. Every cycle of the meter’s mechanism displaces a precisely defined (“positive”) quantity of fluid, so that a count of the number of mechanism cycles yields a precise quantity for the total fluid volume passed through the flowmeter. Many positive…
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22.4 Velocity-based flowmeters
The Law of Continuity for fluids states that the product of mass density (ρ), cross-sectional pipe area (A) and average velocity (v) must remain constant through any continuous length of pipe: If the density of the fluid is not subject to change as it travels through the pipe (a very good assumption for liquids), we…
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22.3 Variable-area flowmeters
An variable-area flowmeter is one where the fluid must pass through a restriction whose area increases with flow rate. This stands in contrast to flowmeters such as orifice plates and venturi tubes where the cross-sectional area of the flow element remains fixed. 22.3.1 Rotameters The simplest example of a variable-area flowmeter is the rotameter, which uses a solid object…
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22.2 Laminar flowmeters
A unique form of differential pressure-based flow measurement deserves its own section in this flow measurement chapter, and that is the laminar flowmeter. Laminar flow is a condition of fluid motion where viscous (internal fluid friction) forces greatly overshadow inertial (kinetic) forces. A flowstream in a state of laminar flow exhibits no turbulence, with each fluid molecule…
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Chapter 22 Continuous fluid flow measurement
The measurement of fluid flow is arguably the single most complex type of process variable measurement in all of industrial instrumentation1 . Not only is there a bewildering array of technologies one might use to measure fluid flow – each one with its own limitations and idiosyncrasies – but the very nature of the variable itself…
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21.5 Non-contact temperature sensors
Virtually any mass above absolute zero temperature emits electromagnetic radiation (photons, or light) as a function of that temperature. This basic fact makes possible the measurement of temperature by analyzing the light emitted by an object. The Stefan-Boltzmann Law of radiated energy quantifies this fact, declaring that the rate of heat lost by radiant emission from a…
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21.4 Thermocouple Types, Junctions, Connector and Tip Styles
RTDs are completely passive sensing elements, requiring the application of an externally-sourced electric current in order to function as temperature sensors. Thermocouples, however, generate their own electric potential. In some ways, this makes thermocouple systems simpler because the device receiving the thermocouple’s signal does not have to supply electric power to the thermocouple. It also…
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21.3 Thermistors and Resistance Temperature Detectors (RTDs)
One of the simplest classes of temperature sensor is one where temperature effects a change in electrical resistance. With this type of primary sensing element, a simple ohmmeter is able to function as a thermometer, interpreting the resistance as a temperature measurement: Thermistors are devices made of metal oxide which either increase in resistance with increasing…
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21.2 Filled-bulb temperature sensors
Filled-bulb systems exploit the principle of fluid expansion to measure temperature. If a fluid is enclosed in a sealed system and then heated, the molecules in that fluid will exert a greater pressure on the walls of the enclosing vessel. By measuring this pressure, and/or by allowing the fluid to expand under constant pressure, we…
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Chapter 21 Introduction to Continuous Temperature Measurement
Temperature is the measure of average molecular kinetic energy within a substance. The concept is easiest to understand for gases under low pressure, where gas molecules randomly shuffle about. The average kinetic (motional) energy of these gas molecules defines temperature for that quantity of gas. There is even a formula expressing the relationship between average kinetic…
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18.10 Calibration Standards to Produce and Measure Physical Quantities
As previously defined, calibration refers to the checking and adjustment of an instrument so that its output faithfully corresponds to its input throughout a specified range. In order to calibrate an instrument, we must have some means of knowing the input and/or output quantities associated with the instrument under test. A substance or device used as a…