Pressure measurement errors

In the holes between the static pressure and total pressure differential pressure is created, and differential pressure measurement error is 0.75%. Set to a normal distribution, k = 2, pitot tube relative standard uncertainty u (AP) = 0.375%. 

Disturbances upstream of the probe can cause large errors, in part because of the turbulence generated and its effect on the static-pressure measurement. A calming section of at least 50 pipe diameters is desirable. If this is not possible, the use of straightening vanes or a honeycomb is advisable. 

With nonnewtouiau fluids the pressure measured at the wall with non-flush-mounted pressure gauges may be in error (see subsection Static Pressure ). 

Flow Low mass flow indicated. Mass flow error. Transmitter zero shift. Measurement is high. Measurement error. Liquid droplets in gas. Static pressure change in gas. Free water in fluid. Pulsation in flow. Non-standard pipe runs. Install demister upstream heat gas upstream of sensor. Add pressure recording pen. Mount transmitter above taps. Add process pulsation damper. Estimate limits of error. 

As well as measurement errors due to the pressure measurement instrument itself, other errors related to pressure measurements must be considered. In ventilation applications a frequently measured quantity is the duct static pressure. This is determined by drilling in the duct a hole or holes in which a metal tube is secured. The rubber tube of the manometer is attached to the metal tube, and the pressure difference between the hole and the environment or some other pressure is measured. 

Gee ° has applied this method to the determination of the interaction parameters xi for natural rubber in various solvents. Several rubber vulcanizates were used. The effective value of VelV for each was determined by measuring its extension under a fixed load when swollen in petroleum ether. Samples were then swollen to equilibrium in other solvents, and xi was calculated from the swelling ratio in each. The mean values of xi for the several vulcanizates in each solvent are presented in Table XXXVI, where they are compared with the xi s calculated (Eq. XII-30) from vapor pressure measurements on solutions of unvulcanized rubber in some of the same solvents. The agreement is by no means spectacular, though perhaps no worse than the experimental error in the vapor pressure method. 

Physical methods involve the measurement of a physical property of the system as a whole while the reaction proceeds. The measurements are usually made in the reaction vessel so that the necessity for sampling with the possibility of attendant errors is eliminated. With physical methods it is usually possible to obtain an essentially continuous record of the values of the property being measured. This can then be transformed into a continuous record of reactant and product concentrations. It is usually easier to accumulate much more data on a given reaction system with such methods than is possible with chemical methods. There are certain limitations on physical methods, however. There must be substantial differences in the contributions of the reactants and products to the value of the particular physical property used as a measure of the reaction progress. Thus one would not use pressure measurements to follow the course of a gaseous reaction that does not... 

During normal operation of a chemical plant it is common practice to obtain data from the process, such as flowrates, compositions, pressures, and temperatures. The numerical values resulting from the observations do not provide consistent information, since they contain some type of error, either random measurement errors or gross biased errors. This means that the conservation equations (mass and energy), the common functional model chosen to represent operation at steady state, are not satisfied exactly. 

Gas law experiments generally involve pressure, volume, and temperature measurements. In a few cases, other measurements such as mass and time are necessary. You should remember that AP, for example, is NOT a measurement the initial and final pressure measurements are the actual measurements made in the laboratory. Another common error is the application of gas law type information and calculations for non-gaseous materials. Typical experiments involving these concepts are numbers 3 and 5 in the Experimental chapter. 

The pressure profiles with rotation shown in Fig. 12.2 are ideal. In practice the pressure profiles contain a level of measurement error and unsteady-state behavior. Pressure in an actual channel operating at a screw speed of 30 rpm for an ABS resin is shown in Fig. 12.3. 

Measurement of pressures in the rough vacuum range can be carried out relatively precisely by means of vacuum gauges with direct pressure measurement. Measurement of lower pressures, on the other hand, is almost always subject to a number of fundamental errors that limit the measuring accuracy right from the start so that it is not comparable at all to... 

The Bayard-Alpert system with modulator (see Fig. 3.16 d), introduced by Redhead, offers pressure measurement in which errors due to X-ray and ion desorption effects can be quantitatively taken into account. In this arrangement there is a second thin wire, the modulator, near the anode in addition to the ion collector inside the anode. If this modulator is set at the anode potential, it does not influence the measurement. If, on the other hand, the same potential is applied to the modulator as that on the ion collector, part of the ion current formed flows to the modulator and the current that flows to the ion collector becomes smaller. The indicated pressure p, of the ionization gauge with modulator set to the anode potential consists of the portion due to the gas pressure pg and that due to the X-ray effect pg ... 

One sees that the ion flow caused by a gas is proportional to the partial pressure. The linear equation system can be solved only for the special instance where m = g (square matrix) it is over-identified for m> g. Due to unavoidable measurement error (noise, etc.) there is no set of overall ion flow Ig (partial pressures or concentrations) which satisfies the equation system exactly. Among all the conceivable solutions it is now necessary to identify set 1 which after inverse calculation to the partial ion flows 1, will exhibit the smallest squared deviation from the partial ion currents i actually measured. Thus ... 

Argon at liquid-nitrogen temperature exhibits an equilibrium pressure of 187 torrs. It offers the advantage of a lower vapor pressure than nitrogen, which will reduce the void volume error while retaining ease of pressure measurements. However, the cross-sectional area of argon is not well established and appears to vary according to the surface on which it is adsorbed. 

When pore sizes are analyzed above the usual BET range of relative pressures, the error associated with saturated pressure measurements is insignificant when compared to the many other assumptions which are made. 

Crowell and Young (15) compare their differential heats of adsorption with those of Jura and Criddle (38) and conclude that there were errors in the low surface coverage region of the latter investigation due to the effect of thermal transpiration on pressure measurement. 

Similarly, Thurmond (150) and Arthur (151) found that the interaction coefficients obtained from a fit of the experimental liquidus or vapor pressure in the arsenide and phosphide systems did not produce the same temperature dependence. Panish et al. (142, 154) pointed out that these discrepancies may be due to (1) errors resulting from the assumed values for AH/j and the approximation ACp[ij] = 0 in 0, (2) deviations from simple-solution behavior, or (3) uncertainties in the interpretation of the vapor pressure data, because some of the quantities necessary in the calculations are not accurately known (e.g., reference-state vapor pressures for pure liquid As and P). Knobloch et al. (184, 185) and Peuschel et al. (186, 187) have obtained excellent agreement between calculated and experimental activities and vapor pressures with the use of Krupkowski s asymmetrical formalism for activity coefficients, whereas Ilegems et al. (Ill) demonstrated that satisfactory agreement between liquidus and vapor pressure measurements exists when an accurate expression for the liquidus is used. 

Comparison of experimental points with theoretical points is shown in Figure 10. Agreement is good at low pressure (i.e., p < lOOkPa), and may differ by as much as 10J at the higher pressures. This error is of the same order as that claimed by the existing, more empirical, models in the literature. A systematic agreement in shape is noted, and the actual error may be in the experimental data where "flotation" effects are serious for gravi-metic measurements at elevated pressures. 

Note that Q(23)=e1JQ + a. A measurement on an individual specimen can be expected to deviate from this estimate because of measurement error,and because of the specimen effect embodied in the S. Values of Q(23) and 3 obtained from the analysis of the permeance and the time-lag data are given in Tables I and II respectively. Since the time-lag for carbon dioxide depends upon the upstream pressure it is necessary to multiply the estimates obtained from equation 8 by a term of the form ... 

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