Vacuum pressure indication

Wrong The chief engineer failed to realize that the vacuum pressure indicator was not equipped with a barometric pressure compensator. An ordinary vacuum pressure indicator or pressure gauge reads the pressure difference between the vacuum system and atmospheric pressure. When ambient temperatures drop, the barometer rises or ambient pressure goes up. An ordinary vacuum pressure gauge or indicator would then read an improved vacuum. But in reality, the vacuum has not changed. 

In practice, a mixture of actinide dioxide and graphite powder is first pelletized and then heated to 2275 K in vacuum in a graphite crucible until a drop in the system pressure indicates the end of CO evolution. The resulting actinide carbide is then mixed with tantalum powder, and the mixture is pressed into pellets. The reduction occurs in a tantalum crucible under vacuum. At the reduction temperature, the actinide metal is vaporized and deposited on a tantalum or water-cooled copper condenser. 

The fact that the pressure reading at vacuum gauges (see Section 3.3) is sensitive to the type of gas involved can, to a certain extent, be utilized for leak detection purposes. Thus it is possible to brush or spray suspected leaks with alcohol. The alcohol vapors which flow into the device - the thermal conductivity and ionizablity of which will vary greatly from the same properties for air - will affect and change pressure indication to a greater or lesser extent. The availability of more precise, easy-to-use helium leak detectors has, however, rendered this method almost completely obsolete. 

The outside temperature must be taken into account and above all it is necessary to avoid hot kilns, furnaces or stoves or other sources of intense radiation which generate an ambient temperature around the measurement system which lies above the specific acceptable value. Excessive ambient temperatures will result in false pressure indications in thermal conductivity vacuum sensors. 

In all those measurement instruments which use the ionization of gas molecules as the measurement principle (cold-cathode and hot-cathode ionization vacuum gauges), strong magnetic leakage fields or electrical potentials can have a major influence on the pressure indication. At low pressures it is also possible for wall potentials which deviate from the cathode potential to influence the ion trap current. 

Figure 7.8. Is spectra of vacuum-cleaved, clean flat MgO(lOO) and various water-dosed MgO(lOO) surfaces (3 min exposures at the pressures indicated). One (clean surface) and two component (water-dosed surfaces) fits are shown, along with the fit residual. The incident photon energy was 620 eV. (from [144])...

Figure 3.5. Vacuum control with steam jet ejectors and with mechanical vacuum pumps, (a) Air bleed on PC. The steam and water rates are hand set. The air bleed can be made as small as desired. This can be used only if air is not harmful to the process. Air bleed also can be used with mechanical vacuum pumps, (b) Both the steam and water supplies are on automatic control. This achieves the minimum cost of utilities, but the valves and controls are relatively expensive, (c) Throttling of process gas flow. The valve is larger and more expensive even than the vapor valve of case (a). Butterfly valves are suitable. This method also is suitable with mechanical vacuum pumps, (d) No direct pressure control. Settings of manual control valves for the utilities with guidance from pressure indicator PI. Commonly used where the greatest vacuum attainable with the existing equipment is desired.

Figure 5 summarizes the low pressure end of water vapor adsorption isotherms obtained on quartz A. Bound water loss, above the surface condition resulting from 400° C. outgassing in vacuum, is indicated by displacement of the adsorption origin to reflect total water content (bound water plus adsorbed water) as a function of water vapor relative pressure. Vapor desorption curves show that... 

PP piston pump VP vacuum pump CP circulation pump CT1, CT2 cold traps V5 three-way valve V9, VI3 six-way valves VII, V15 expansion valves PI 1, PI2 pressure indicators TI temperature indicator VI vacuum indicator... 

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