Flow measurements accuracy

Belt-conveyor scales determine the amount of material being conveyed on a belt. A section of belt is weighed by placing the belt support rollers on a scale the belt speed is also measured. Weight and speed data are suppHed to a controller which integrates them to arrive at a material flow rate, often stated in tons per hour. The controller may display a flow rate, shut the conveyor down when a predeterrnined amount of material has passed, or it may be used to maintain a specified flow rate. Accuracy is limited because of the number of detrimental influences involved, eg, variable belt tension. 

Flow measuring equipment must generally be wet caHbrated to attain maximum accuracy, and principal flow meter manufacturers maintain extensive facihties for this purpose. In addition, a number of governments, universities, and large flow meter users maintain flow laboratories. 

Vortex-shedding flow meters typically provide 1% of flow rate accuracy over wide ranges on Hquid, gas, and steam service. Sizes are available from 25 to 200 mm. The advantages of no moving parts and linear digital output have resulted in wide usage in the measurement of steam, water, and other low viscosity Hquids. 

Electromagnetic flow meters ate available in essentially all pipe si2es, ie, 1 mm to 3 m, and provide measurement accuracy of 1% of rate or better over wide ranges. The meters are obstmctionless, have no moving parts, and are extremely mgged. Pressure loss is that of an equivalent section of pipe. 

The flow capacity of the transducer can be increased bv adding a booster relav like the one shown in Fig, 8-7.3/ , The flow capacity of the booster relav is nominally fiftv to one hundred times that of the nozzle amplifier shown in Fig, 8-7.3 3 and makes the combined trans-diicer/booster suitably responsive to operate pneumatic actuators. This type of transducer is stable into all sizes of load volumes and produces measured accuracy (see Instrument Society of America [ISA]-S5l, 1-1979, Process Instrumentation Terminology for the definition of measured accuracy) of 0,5 percent to 1,0 percent of span. 

These techniques have been described previously in the pressure measurement section. Usually, one of the flow-measuring devices and the required instrumentation is incorporated as a part of the plant piping. The choice of technique depends on the allowable pressure drop, flow type, accuracy required, and cost. 

Mass flow measurement has been shown to be efficient as a mole ratio control, even where S03 mass flow was not measurable with sufficient accuracy in the diluted gas stream. 

This chapter has the following structure in Sect. 3.2 the common characteristics of experiments are discussed. Conditions that are needed for proper comparison of experimental and theoretical results are formulated in Sect. 3.3. In Sect. 3.4 the data of flow of incompressible fluids in smooth and rough micro-channels are discussed. Section 3.5 deals with gas flows. The data on transition from laminar to turbulent flow are presented in Sect. 3.6. Effect of measurement accuracy is estimated in Sect. 3.7. A discussion on the flow in capillary tubes is given in Sect. 3.8. 

Wintermark M, Reichhart M, Thiran JP, Maeder P, Chalaron M, Schnyder P, Bogous-slavsky J, MeuU R. Prognostic accuracy of cerebral blood flow measurement by perfusion computed tomography, at the time of emergency room admission, in acute stroke patients. Ann Neurol 2002 51 417-432. 

In some manufacturing process analysis applications the analyte requires sample preparation (dilution, derivatization, etc.) to afford a suitable analytical method. Derivatization, emission enhancement, and other extrinsic fluorescent approaches described previously are examples of such methods. On-line methods, in particular those requiring chemical reaction, are often reserved for unique cases where other PAT techniques (e.g., UV-vis, NIR, etc.) are insufficient (e.g., very low concentrations) and real-time process control is imperative. That is, there are several complexities to address with these types of on-line solutions to realize a robust process analysis method such as post reaction cleanup, filtering of reaction byproducts, etc. Nevertheless, real-time sample preparation is achieved via an on-line sample conditioning system. These systems can also address harsh process stream conditions (flow, pressure, temperature, etc.) that are either not appropriate for the desired measurement accuracy or precision or the mechanical limitations of the inline insertion probe or flow cell. This section summarizes some of the common LIF monitoring applications across various sectors. 

Sample ports are also a key issue. While the EPA accepts five pipe diameters before and two pipe diameters downstream of the sample port, experience has shown that the recommended eight pipe diameters before and two diameters after the port improves testing accuracy. The proper lengths are important to flow measurement, but they are also critical to obtaining representative dust samples. Turbulence in gas flow will result in mass emission test results that are not representative. The particulate matter will be maldistributed after an elbow and the heaviest particles will be biased to the outside wall. Even if appropriate gas rates are collected, the amount of dust may be biased to the outside wall but collected at too small a rate. 

Velocity Profile Effects Many variables can influence the accuracy of specific flow measurement methods. For example, the velocity profile in a closed conduit affects many types of flow-measuring devices. The velocity of a fluid varies from zero at the wall and at other stationary solid objects in the flow channel to a maximum at a distance from the wall. In the entry region of a conduit, the velocity field may approach plug flow and a constant velocity across the conduit, dropping to zero only at the wall. As a newtonian fluid progresses down a... 

Flow Rate Accuracy. One of the key performance requirements for the pump module is the ability to maintain accurate and consistent flow of the mobile phase. This is necessary to provide stable and repeatable interactions between the analytes and the stationary phase [8,9]. Poor flow rate accuracy will affect the retention time and resolution of the separation. The flow-rate accuracy of the pump can be evaluated simply by calculating the time required to collect a predetermined volume of mobile phase at different flow rate settings. For example, the flow-rate accuracy at 2 mL/min can be verified by using a calibrated stopwatch to measure the time it takes to collect 25 mL of effluent from the pump into a 25-mL volumetric flask. A calibrated flow meter can be used to determine the flow rate as well. The typical acceptance of the flow rate accuracy is listed in Table 11.1. A steady backpressure may be required, depending on the requirement of the system. 

Measurement method Pressure range bar Allowable flow-range Accuracy Additional notes... 

The accuracy of multiple dilutions fades as more and more dilutions are made because of the added errors of additional flow measurements. In the double dilution above, four flow measurements are needed, two for each dilution. Fortunately, however, multiple dilutions are used to produce low concentrations where perhaps analysis accuracy of 10% would be acceptable. 

Thermal Mass Flowmeters The trend in the chemical process industries is toward increased usage of mass flowmeters that are independent of changes in pressure, temperature, viscosity, and density. Thermal mass meters are widely used in semiconductor manufacturing and in bioprocessing for control of low flow rates (called mass flow controllers, or MFCs). MFCs measure the heat loss from a heated element, which varies with flow rate, with an accuracy of 1 percent. Capacitance probes measure the dielectric constant of the fluid and are useful for flow measurements of slurries and other two-phase flows. 

With regard to multiphase flow measurement, two of the most commonly used flow meter devices in industry are the Coriolis flow meter and electromagnetic flow meter. The Coriolis flow meter is widely used owing to its high accuracy (Tavoularis, 2005) but it assumes... 

The accuracy of orifice-based flow measurements can be increased by the use of several orifices in parallel and by opening or closing the pipes to some of them to keep the flow in the active paths within 80-90% of full flow. Another less expensive choice is to use two (or more) transmitters, one for high (10-100%) pressure drop and the other for low (1-10%), and to switch their outputs depending on the actual flow. 

Solids flow measurement is more important in the control and optimization of coal-fired power plants than in alternative energy processes. The mass flow of solids can be detected by impact flowmeters, which are relatively low-accuracy devices (1-2% FS). Better accuracy and rangeability are provided by belt-type gravimetric feeders (0.5% AF over a 10 1 range), which measure both the speed and loading on the moving belt, as shown in Figure 3.89. 

There are many other flow measurement devices including Onlicc/Venturi meters, turbine meters, and more sophisticated instruments using ultrasonic, magnetic, and Coriolis effect techniques. Orifice/Venturi type meters have a restriction causing a pressure drop related to the flow rate of liquid. Such meters are popular because of their low cost however, their accuracy can be compromised by upstream elbows and valves. Turbine meters are designed so that rotation speed varies linearly with the... 

Several effects can play a role in the temperature measurement accuracy. Due to the small channel length, the temperature difference between the channel outlet and inlet can be as small as the sensor sensitivity. Thermocouples can have a size comparable to the channel dimensions and where is measured the temperature is questionable. Moreover, the heat flow rate through the thermocouple itself can be not negligible. The importance of these effects must be appreciated. 

The flow volume exiting of the detector was measured using water of chromatographic quality. The eluent volume was measured over a 10 min period, in a graduated manometer and for different flow rates (0.5, 1.0 and 2.0 ml min x) the time elapsed was measured with a precision chronometer. The flow rate was then calculated. This test was repeated every 10 days to evaluate the reproducibility of the flow. An accuracy of up to 3% of the theoretical value was recorded. The observed variations should be to the critical parts of the pump (fundamentally the valves). 

Flow rate accuracy max. 0.1 mlmin (set value minus flowmeter display). 0 Flow precision max. 0.02 ml min (highest minus lowest measured value). 

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