Pressure, absolute measurement

Bellows Element The bellows element is an axially elastic cylinder with deep folds or convolutions. The bellows may be used unopposed, or it may be restrained by an opposing spring. The pressure to be measured may be applied either to the inside or to the space outside the bellows, with the other side exposed to atmospheric pressure. For measurement of absolute pressure either the inside or the space outside of the bellows can be evacuated and sealed. Differential pres-... 

Example 1 Compare Wet-Bulb and Adiabatic-Saturation Temperatures For tne air-water system at atmospheric pressure, the measured values of dry-bulh and wet-hulh temperatures are 85 and 72 F respectively. Determine the absolute humidity and compare the wet-bulb temperature and adiabatic-saturation temperature. Assume that h /k is given by Eq. (12-4). 

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. 

It would appear that measurement of the integrated absorption coefficient should furnish an ideal method of quantitative analysis. In practice, however, the absolute measurement of the absorption coefficients of atomic spectral lines is extremely difficult. The natural line width of an atomic spectral line is about 10 5 nm, but owing to the influence of Doppler and pressure effects, the line is broadened to about 0.002 nm at flame temperatures of2000-3000 K. To measure the absorption coefficient of a line thus broadened would require a spectrometer with a resolving power of 500000. This difficulty was overcome by Walsh,41 who used a source of sharp emission lines with a much smaller half width than the absorption line, and the radiation frequency of which is centred on the absorption frequency. In this way, the absorption coefficient at the centre of the line, Kmax, may be measured. If the profile of the absorption line is assumed to be due only to Doppler broadening, then there is a relationship between Kmax and N0. Thus the only requirement of the spectrometer is that it shall be capable of isolating the required resonance line from all other lines emitted by the source. 

Three general test procedures used to measure the permeability of plastic films are the absolute pressure method, the isostatic method, and the quasi-isostatic method. The absolute pressure method (ASTM D 1434, Gas Transmission Rate of Plastic Film and Sheeting) is used when no gas other than the permeant in question is present. Between the two chambers a pressure differential provides the driving force for permeation. Here the change in pressure on the volume of the low-pressure chamber measures the permeation rate. 

Vacuum pressure Gauge pressure in psi (gpsi) is the amount by which pressure exceeds the atmospheric pressure of 14 psi (negative in the case of vacuum). The absolute pressure (psia) is measured with respect to zero absolute vacuum [29.92 in. (101 kPa) Hg], In a vacuum system it is equal to the negative gage pressure subtracted from the atmospheric pressure. (Gauge pressure + atmospheric pressure = absolute pressure) (1 in. Hg = 0.4912 psi of atmosphere on a product) (1 psi = 2.036 in. Hg). 

Ratio scales possess not only order and interval characteristics but also have meaningful origins. Examples of properties that can be measured on ratio scales are mass, length, pressure (absolute, not guage), and volume. In each case the origin (zero point) on the ratio scale signifies that none of the property is present. 

Exclusively absolute pressures are measured and used for calculations in vacuum technology. 

Pressure is defined as the force per unit area. Pressure devices measure with respect to the ambient atmospheric pressure The absolute pressure is the pressure of the fluid (gauge pressure) plus the atmospheric pressure. 

PRESSURE. If a body of fluid is at rest, the forces are in equilibrium or the fluid is in static equilibrium. The types of force that may aci on a body are shear or tangential force, tensile force, and compressive force. Fluids move continuously under the action of shear or tangential forces. Thus, a fluid at rest is free in each part from shear forces one fluid layer does not slide relative to an adjacent layer. Fluids can be subjected to a compressive stress, which is commonly called pressure. The term may be defined as force per unit area. The pressure units may be dynes per square centimeter, pounds per square foot, torr. mega-Pascals, etc. Atmospheric pressure is the force acting upon a unit area due to the weight of the atmosphere. Gage pressure is the difference between the pressure of the fluid measured (at some point) and atmospheric pressure. Absolute pressure, which can be measured by a mercury barometer, is the sum of gage pressure plus atmospheric pressure. 

The eventual products in reaction (1) have been identified as SO and MSA from experiments involving the steady photolysis of mixtures of DMS and a photolytic precursor of OH (4-91 Absolute measurements of lq have been obtained using the discharge-flow method with resonance fluorescence or electron paramagnetic resonance (EPR) detection of OH (10-141. and the flash photolysis method with resonance fluorescence or laser induced fluorescence (LIF) detection of OH (14-181. Competitive rate techniques where Iq is measured relative to the known rate constant for a reaction between OH and a reference organic compound (18-211 have also been employed to determine k at atmospheric pressure of air. 

The method proposed allows direct absolute measurement of local concentration at the instant of the laser pulse in a low pressure flame. We believe that this method could be applied to higher pressure flames by the use of ultrashort duration laser pulses with the new mode locked dye laser technique. But until the detector technology allows such short time resolutions we think that collisional lifetimes studies must be pursued to obtain more precise evaluation of the fluorescence efficiency, and to have a better understanding of the redistribution phenomena involved in optical pumping. For this purpose we are now studying the decay of resolved fluorescence lines. 

Xenon Adsorption Experiments. Gaseous xenon was co-adsorbed onto the samples on a vacuum manifold the xenon equilibrium pressure was measured by an absolute-pressure transducer (MRS Baratron) capable of measuring pressure with accuracy 0.1 torr. The adsorption isotherms of the co-adsorbed xenon in the samples were measured volumetrically at 22 °C. 

The accurate determination of rate constants for the reactions of 19F atoms is often hampered by the presence of reactive F2 and by the occurrence of side reactions. The measurement of the absolute concentration of F atoms is sometimes a further problem. The use of thermal-ized 18F atoms is not subject to these handicaps, and reliable and accurate results for abstraction and addition reactions are obtained. The studies of the reactions of 18F atoms with organometallic compounds are unique, inasmuch as such experiments have not been performed with 19F atoms. In the case of addition reactions, the fate of the excited intermediate radical can be studied by pressure-dependent measurements. The non-RRKM behavior of tetraallyltin and -germanium compounds is very interesting inasmuch as not many other examples are known. The next phase in the 18F experiment should be the determination of Arrhenius parameters for selected reactions, i.e., those occurring in the earth s atmosphere, since it is expected that the results will be more precise than those obtained with 19F atoms. 

Example 1.3 A dead-weight gauge with a 1-cm-diameter piston is used to measure pressures very accurately. In a particular instance a mass of 6.14 kg (including piston and pan) brings it into balance. If the local acceleration of gravity is 9.82 m s 2, what is the gauge pressure being measured If the barometric pressure is 748(torr), what is the absolute pressure ... 

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