Obstruction meters

The simplest and most common device for measuring flow rate in a pipe is the orifice meter, illustrated in Fig. 10-7. This is an obstruction meter that consists of a plate with a hole in it that is inserted into the pipe, and the pressure drop across the plate is measured. The major difference between this device and the venturi and nozzle meters is the fact that the fluid stream leaving the orifice hole contracts to an area considerably smaller than that of the orifice hole itself. This is called the vena contracta, and it occurs because the fluid has considerable inward radial momentum as it converges into the orifice hole, which causes it to continue to flow inward for a distance downstream of the orifice before it starts to expand to fill the pipe. If the pipe diameter is D, the orifice diameter is d, and the diameter of the vena contracta is d2, the contraction ratio for the vena contracta is defined as Cc = A2/A0 = (d2/d)2. For highly turbulent flow, Cc 0.6. 

As for the other obstruction meters, when the continuity equation is used to eliminate the upstream velocity from Eq. (10-11), the resulting expression for the mass flow rate through the orifice is... 

Three classes of problems involving orifices (or other obstruction meters) that the engineer might encounter are similar to the types of problems encountered in pipe flows. These are the unknown pressure drop, unknown flow rate, and unknown orifice diameter problems. Each... 

Other devices can be used to determine the flow rate from a single measurement. These are sometimes referred to as obstruction meters, since the basic principle involves introducing an obstruction (e.g., a constriction) into the flow channel and then measuring the pressure drop across this obstruction, which depends on the flow rate. Two such devices, the venturi meter and the nozzle, are illustrated in Eigures 5.14 and 5.15, respectively. In both cases, the pressure drop from a point upstream of the meter to a point in a plane with the minimum flow area (A ) is related to the velocity V2 by the Bernoulli equation ... 

TABLE 6.4 Permanent Pressure Loss of Obstruction Meters as a Function oi P (Holman, 2001)... 

Any obstruction inserted into a duct or pipe that creates a measurable pressure difference can be used as a flow meter. The three basic standardized flow measurement devices presented above are perhaps more suitable for laboratory work than installation as permanent ductwork instruments in ventilation applications. They are sensitive to flow disturbances, relatively expensive, require considerable space, and have a narrow measurement range and a high permanent pressure loss. For these reasons, numerous attempts have been made to develop instruments without these drawbacks. Some of them, like the... 

Filling pipes should be as short as possible and free from sharp bends. The terminal should be in a convenient position to allow easy coupling of the vehicle hose connection, wherever possible within 5 meters (15 ft) of the hard standing for vehicle delivery. The most suitable height for a filling pipe is about 1 m (3 ft) above ground level and clear of all obstructions. 

When a bluff body is interspersed in a fluid stream, the flow is split into two parts. The boundary layer (see Chapter 11) which forms over the surface of the obstruction develops instabilities and vortices are formed and then shed successively from alternate sides of the body, giving rise to what is known as a von Karman vortex street. This process sets up regular pressure variations downstream from the obstruction whose frequency is proportional to the fluid velocity, as shown by Strouai. 9. Vortex flowmeters are very versatile and can be used with almost any fluid — gases, liquids and multi-phase fluids. The operation of the vortex meter, illustrated in Figure 6.27, is described in more detail in Volume 3, by Gjnesi(8) and in a publication by a commercial manufacturer, Endress and Hauser.10 ... 

Some physicians recommend epinephrine metered-dose inhalers as an alternative to epinephrine autoinjectors. While a few inhalations might relieve mild or moderate respiratory symptoms, for relief of life-threatening airway obstruction or shock, adults need to inhale 20-30 puffs and children need to inhale 10-20 puffs, which is hard to do [35]. Epinephrine metered-dose inhalers contain chlorofluorocarbon propellants. For environmental reasons, they might not be manufactured in the future. 

Objective measurements of airflow obstruction with a home peak flow meter may not necessarily improve patient outcomes. The NAEPP advocates use of PEF monitoring only for patients with severe persistent asthma who have difficulty perceiving airway obstruction. 

The AEGL-1 concentration was based on a 1-hour (h) no-effect concentration of 8,000 parts per million (ppm) in healthy human subjects (Emmen et al. 2000). This concentration was without effects on pulmonary function, respiratory parameters, the eyes (irritation), or the cardiovascular system. Because this concentration is considerably below that causing any adverse effect in animal studies, an intraspecies uncertainty factor (UF) of 1 was applied. The intraspecies UF of 1 is supported by the absence of adverse effects in therapy tests with patients with severe chronic obstructive pulmonary disease and adult and pediatric asthmatics who were tested with metered-dose inhalers containing HFC-134a as the propellant. Because blood concentrations in this study approached equilibrium following 55 minutes (min) of exposure and effects are determined by blood concentrations, the value of 8,000 ppm was made equivalent across all time periods. The AEGL-1 of 8,000 ppm is supported by the absence of adverse effects in experimental animals that inhaled considerably higher concentrations. No adverse effects were observed in rats exposed at 81,000 ppm for 4 h (Silber and Kennedy 1979) or in rats exposed... 

Ultrasonic meters are finding increasing application because of their ability to measure clear and dirty liquids in difficult situations. They are usually non-intrusive and present little or no obstruction to the flow. They are effective also in measuring flow in open channels (Section 6.2.5) and in partially filled pipes. They are, however, highly sensitive to flow conditions and should be calibrated with care. 

To avoid swirl, elbows should be well separated and have large radii of curvature. If this is not possible then the flowmeter should be sited at least 40 pipe diameters downstream of fittings causing asymmetric flow only and a minimum of 100 pipe diameters downstream when swirl is likely to occur0 S). There should also be at least 10 pipe diameters allowed downstream of the meter free of any obstruction or fitting. If the flow is laminar then these distances should be doubled. 

Inhalation aerosols have been used for the delivery of drugs to the respiratory system since the mid-1950s. The most common dosage form for inhalation is the metered-dose inhaler (MDI), by which the drug is delivered from a pressurized container using a liquefied gas propellant. Medication delivered via this dosage form has allowed for a quick therapeutic response to the symptoms of asthma, emphysema, and chronic obstructive pulmonary disease (COPD), and has resulted in an improvement in the quality of life for millions of asthma sufferers. 

Ram FS, Brocklebank DM, Muers M, et al. Pressurised metered-dose inhalers versus all other handheld inhalers devices to deliver bronchodilators for chronic obstructive pulmonary disease. Cochrane Database Syst Rev. 2002 CD002170. 

From the previous discussion, it can be gleaned that the meter measures rates of flow proportional to the cross-sectional area of flow. Rectangular weirs are therefore area meters. In addition, when measuring flow, the unit obstructs the flow, so the meter is also called an intrusive flow meter. 

Another flow meter is the nutating disk meter. This is widely used to measure the amount of water used in domestic as well as commercial consumption. It has only one moving element and is relatively inexpensive but accurate. This element is a disk. As the water enters the meter, the disk nutates (wobbles). A complete cycle of nutation corresponds to a volume of flow that passes through the disk. Thus, so much of this cycle corresponds to so much volume of flow which can be directly calibrated into a volume readout. A cycle of nutation corresponds to a definite volume of flow, so this flow meter is called a volume flow meter. Nutating disk meters are intrusive meters, because they obstruct the flow of the liquid. 

The last flow meter that we will address is the rotameter. This meter is relatively inexpensive and its method of measurement is based on the variation of the area through which the liquid flows. The area is varied by means of a float mounted inside the cylinder of the meter. The bore of this cylinder is tapered. With the unit mounted upright, the smaller portion of the bore is at the bottom and the larger is at the top. When there is no flow through the unit, the float is at the bottom. As liquid is admitted to the unit through the bottom, the float is forced upward and, because the bore is tapered in increasing cross section toward the top, the area through which the liquid flows is increased as the flow rate is increased. The calibration in rates of flow is etched directly on the side of the cylinder. Because the method of measurement is based on the variation of the area, this meter is called a variable-area meter. In addition, because the float obstructs the flow of the liquid, the meter is an intrusive meter. 

The first MDI products were developed by Riker Laboratories and marketed in 1956, using a newly patented design of metering valve. In most countries the MDI is now established as the principal dosage form of inhalation drug therapy for bronchial asthma and chronic obstructive pulmonary disease (COPD). Since its introduction, MDI technology has evolved steadily. However, with the phase-out in the commercial use of chlorofluorocarbon (CFC) propellants, which have been the mainstay of pharmaceutical MDIs, the pace of MDI technology development has accelerated with the transition to hydrofluorocarbon (HFC) propellants. ... 

Pulmonary delivery of drugs is the administration route of choice in respiratory diseases such as chronic obstructive pulmonary disease and asthma. Different devices are available, including metered-dose inhalers, dry powder inhalers, and nebulizers, and nearly 80% of asthmatic patients worldwide use metered dose inhalers (1). Chlorofluorocarbons have been used as an aerosol propellant in metered-dose inhalers however, they deplete the ozone layer and are being replaced by more environment-friendly propellants, even though the contribution of aerosols of this type to the total global burden of chlorofluorocarbons is less than 0.5%. The first chloro-fluorocarbon-free metered-dose inhaler for asthma treatment was approved by the FDA in 1996 (2) and the European Union has set 2005 as a target date for the withdrawal of all chlorofluorocarbon-based inhalers (1). In the USA, prescriptions for chlorofluorocarbon-free medications rose from 16.4 million in 1996 to 33.8 million in 2000 (2). Most of the chlorofluorocarbon-free medications were steroids for nasal use (27.2 million). However, chlorofluorocarbon-containing medications stiU represented two-thirds of all prescriptions and increased from 63.0 to 67.6 million dispensed (2). 

In a randomized, double-blind study, 943 patients with chronic obstructive pulmonary disease were treated over 12 weeks with either inhaled salmeterol 42 micrograms bd via a metered-dose inhaler or oral modified-release theophylline bd, titrated to serum concentrations of... 

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