Measurement of pressure difference

The absolute, barometric pressure is not normally required in ventilation measurements. The air density determination is based on barometric pressure, but other applications are sufficiently rare. On the other hand, the measurement of pressure difference is a frequent requirement, as so many other quantities are based on pressure difference. In mass flow or volume flow measurement using orifice, nozzle, and venturi, the measured quantity is the pressure difference. Also, velocity measurement with the Pitot-static tube is basically a pressure difference measurement. Other applications for pressure difference measurement are the determination of the performance of fans and air and gas supply and e. -haust devices, the measurement of ductwork tightness or building envelope leakage rate, as well as different types of ventilation control applications. 

For reactions in which one or more reactants or products is a gas, manometry (the measurement of pressure differences) can provide a convenient means for monitoring the course and kinetics of the reaction Thus, enzymes that can be assayed with this method include oxidases, urease, carbonic anhydrase, hydrogenase, and decarboxylases. For example, bacterial glutamate decarboxylase is readily assayed by utilizing a Warburg flask and measuring the volume of gas evolved at different times using a constant-pressure respirometer. ... 

The measurement of pressure difference across the systems revealed that the pressure drop due to HRS is more than 10 times higher than that when the conventional recuperative system is in operation. The pressure drop was 1.7 mbar and 2.8 mbar in cases CRS-3 and CRS-4, respectively, while it was 36 mbar and 30 mbar for HRS-3 and HRS-4, respectively. 

While simple mechanical fiHing-level meters such as dipsticks and inspection glasses are reliable and accurate, they are not suitable for transmitting measured values. A cheap and reliable method is bubble-through measurement (Figure 2.8-4), which in principle measures the hydrostatic pressure and thus transforms the measurement of level into the measurement of pressure difference, which can readily be... 

A mechanical resistance is introduced into the flow and the resulting pressure drop is measured. Thus, flow measurement is transformed into the measurement of pressure difference. An orifice guage or standard orifice plate is generally used (Figure 2.8-5). Other types of throttle are the standard jet, the standard Venturi jet, and the spiral tube, which have the advantage over the standard orifice plate of a lower permanent pressure drop. This measurement principle is applicable over a wide measuring range, requires... 

Electrical manometers have developed during the last 30 years. Modern electrical manometers are well suited for ventilation applications, both in the laboratory and in the field. The advantage of this type of instrument is fhat they are sensitive enough to measure small pressure differences with electrical output, enabling monitoring. A convenient feature, especially in the field is that the instrument is hand-held and there is no need for leveling on a bench, as for fluid manometers. The conversion of the pressure difference into an electrical signal can be based on several different phenomena. 

One of the best and most convenient methods of measuring the flow in the terminal is to use the terminal characteristic pressure difference. This requires that the manufacturer of the terminal provide calibration curves, where the flow rate is expressed as the function of the characteristic pressure difference. Some devices have integrated pressure measurement tappings, and the user has only to attach a manometer to measure the pressure difference. 

It may be noted that the pressure measuring devices (a) to (e) all measure a pressure difference AP(— Pj — P ). In the case of the Bourdon gauge (0, the pressure indicated is the difference between that communicated by the system to the tube and the external (ambient) pressure, and this is usually referred to as the gauge pressure. It is then necessary to add on the ambient pressure in order to obtain the (absolute) pressure. Even the mercury barometer measures, not atmospheric pressure, but the difference between atmospheric pressure and the vapour pressure of mercury which, of course, is negligible. Gauge pressures are not. however, used in the SI System of units. 

Continuous Polymerizations As previously mentioned, fifteen continuous polymerizations in the tubular reactor were performed at different flow rates (i.e. (Nj g) ) with twelve runs using identical formulations and three runs having different emulsifier and initiator concentrations. A summary of the experimental runs is presented in Table IV and the styrene conversion vs reaction time data are presented graphically in Figures 7 to 9. It is important to note that the measurements of pressure and temperature profiles, flow rate and the latex properties indicated that steady state operation was reached after a period corresponding to twice the residence time in the tubular reactor. This agrees with Ghosh s results ). 

The usual laboratory gas flow meter is essentially a manometer which measures the pressure difference across a capillary tube through which the gas flows, as in Figure 74,1, showing a design described by W. Barkas (1939). This is intended to prevent a sudden change in pressure from forcing the liquid in the manometer into the rest of the... 

The ideal variable to measure is one that can be monitored easily, inexpensively, quickly, and accurately. The variables that usually meet these qualifications are pressure, temperature, level, voltage, speed, and weight. When possible the values of other variables are obtained from measurements of these variables. For example, the flow rate of a stream is often determined by measuring the pressure difference across a constriction in a pipeline. However, the correlation between pressure drop and flow is also affected by changes in fluid density, pressure, and composition. If a more accurate measurement is desired the temperature, pressure, and composition may also be measured and a correction applied to the value obtained solely from the pressure difference. To do this would require the addition of an analog or digital computer to control scheme, as well as additional sensing devices. This would mean a considerable increase in cost and complexity, which is unwarranted unless the increase in accuracy is demanded. 

The pitot tube is a device for measuring v(r), the local velocity at a given position in the conduit, as illustrated in Fig. 10-1. The measured velocity is then used in Eq. (10-2) to determine the flow rate. It consists of a differential pressure measuring device (e.g., a manometer, transducer, or DP cell) that measures the pressure difference between two tubes. One tube is attached to a hollow probe that can be positioned at any radial location in the conduit, and the other is attached to the wall of the conduit in the same axial plane as the end of the probe. The local velocity of the streamline that impinges on the end of the probe is v(r). The fluid element that impacts the open end of the probe must come to rest at that point, because there is no flow through the probe or the DP cell this is known as the stagnation point. The Bernoulli equation can be applied to the fluid streamline that impacts the probe tip ... 

A manometer is a device employing the change in liquid levels to measure gas pressure differences between a standard and an unknown system. For example, a typical mercury manometer consists of a glass tube partially filled with mercury. One arm is open to the atmosphere and the other is connected to a container of gas. When the pressure of the gas in the container is greater than atmospheric pressure, the level of the mercury in the open side will be higher and... 

The results of interfacial tension measurements on BLM formed from five different lipid solutions are given in Table I. One of the immediate questions is whether the measured values represent the true bifacial tension of BLM. It is implicitly assumed in order to apply equation 3 that yb is a characteristic property of BLM and should be independent of the extension of the BLM area. It is generally recognized that if the BLM also possessed elastic properties, the measured yb would be different when it is stretched. To answer this question, yb was measured during both expansion and contraction of the membrane. A typical trace of pressure difference vs. time in which the membrane was being expanded and contracted is shown in Figure 3. The symmetric nature of the curve indicates that little hysteresis was present during inflation and deflation of the BLM. Therefore, it seems safe to conclude that for BLM formed from lipid materials alone the membrane does not appear to possess appreciable elastic properties. 

Elastic elements such as diaphragms and capsules are used as sensing elements in combination with different displacement transducers (e.g. inductive, potentiometric, etc.) for low/medium vacuum measurements down to about 1 torr. Elastic elements have been employed in conjunction with capacitive transducers for the measurement of pressures as low as 10 5 torr. 

Notice that a positive AP means the system has a pressure higher than atmospheric pressure, whereas a negative AP means the object has a pressure lower than atmospheric pressure. Manometers become inconvenient for measuring gauge pressures of greater than about 1 atm because of the large column of mercury that must be contained. Less dense liquids, such as water, can be used in a manometer to measure smaller pressure differences. 

Bourdon gauges are used on gas cylinders and are also considered a type of aneroid gauge. These devices have a coiled tube (shown in Figure 3.5) and are used to measure the pressure difference between the pressure exerted by the gas in a cylinder and the atmospheric pressure. The coiled tube is mechanically coupled to a pointer (shown in red). As a gas at a pressure above atmospheric pressure enters the coiled tube, it causes it to slightly uncoil, kind of like those New Year s Eve paper noisemakers. This causes the pointer to move over a numerical scale, thereby indicating the gauge pressure in the tank. 

A Venturi tube measures the pressure difference between two tubes of different diameter through which a fluid is flowing. The pressure difference can be used to calculate the flow rate. 

It is also interesting to briefly consider online measurements of variables different from temperature [5], Since pressure is defined as the normal force per unit area exerted by a fluid on a surface, the relevant measurements are usually based on the effects deriving from deformation of a proper device. The most common pressure sensors are piezoresistive sensors or strain gages, which exploit the change in electric resistance of a stressed material, and the capacitive sensors, which exploit the deformation of an element of a capacitor. Both these sensors can guarantee an accuracy better than 0.1 percent of the full scale, even if strain gages are temperature sensitive. 

Conductance may be calculable for fairly simple geometries but with complex vacuum components such as baffles, filters, valves, etc., it is not possible to calculate conductances analytically. Conductances of such objects are usually determined by measuring the pressure difference (A/ ) across the object at a known throughput (qpV) ... 

In the bridge design, a holdup reservoir in front of the reference capillary (C4) ensures that only pure mobile phase flows through the reference capillary when the peak passes the sample capillary (C3). This design offers considerable advantages The detector actually measures the pressure difference AP at the dif-... 

In the present case it is different, since chlorine and iodine both exist in the vapour, and partly also combined analysis as well as measurement of pressure is thercfor( necessary. Both results will be stated in order. 

Figure 3.4-4a shows an open-end manometer one end is exposed to a fluid whose pressure is to be measured, and the other is open to the atmosphere. Figure 3.4-4i> shows a differential manometer, which is used to measure the pressure difference between two points in a process line. Figure 3.4-4c shows a sealed-end manometer, which has a near-vacuum enclosed at one end. (Some of the enclosed fluid will vaporize into the empty space, thereby preventing the existence of a perfect vacuum.) If the open end of a sealed-end manometer is exposed to the atmosphere (Pi = Patm) functions as a barometer. 

A slurry contains crystals of copper sulfate pentahydrate [CuS04-5H20(s), specific gravity = 2,3] suspended in an aqueous copper sulfate solution (liquid SG = 1.2). A sensitive transducer is used to measure the pressure difference, A/ (Pa), between two points in the sample container separated by a vertical distance of h meters. The reading is in turn used to determine the mass fraction of crystals in the slurry, Xc(kg crystals/kg slurry). 

Homogenous lactam polymerizations usually proceed with a decrease of volume. At atmospheric pressure the value of the pAV term is negligible and the heat of polymerization can be considered as a measure of the difference in internal energy between the linear and cyclic monomer unit. At very high pressures, however, the effect of volume contraction during polymerization on the monomer-polymer equilibrium cannot be neg-... 

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