Pump-gauge devices

Pump-gauge devices are derived from amperometric and coulometric devices. They consist of a pumping part, to which a variable current can be applied, and a gauge part, which is used for measuring the resulting voltage. Only oxygen sensors are based on this principle. 

Benammar and Maskell have recently proposed a more sophisticated sensor to limit the parasitic phenomena. Their device is similar to the coulometric one, but it works in a pump-gauge tracking mode. As in the case of the amperometric sensor proposed by Maskell et an alternating current I = Fsin cot is applied to the pumping part, with a low... 

Fig. 9. (a) Experimental device for the study of gas-solid reaction imder constant oxygen pressure using a pump-sensor device, G vacuum gauge, LN liquid nitrogen trap (b) Variation of the stoichiometry ratio of Ce02-x at 1273 K. 

Vacuum system. Components associated with lowering the pressure within a mass spectrometer. A vacuum system includes not only the various pumping components but also valves, gauges, and associated electronic or other control devices the chamber in which ions are formed and detected and the vacuum envelope. 

In practice, it is often necessary to take readings from hot-filament ionization gauges or other devices. Figure 5 gives pump-down curves for six different types of pumping equipment on the same vacuum chamber (23). The shape of curve 1 indicates that a real leak could be responsible for the zero slope demonstrated by the Bayard-Alpert gauge (BAG). The shape of the other curves could result from a combination of real and virtual leaks. 

Figure 2, Block diagram of a liquid chromatograph. A, solvent reservoir B, filter C, pump D, pulse dampener (optional) E, pre-column (used only in liquid-liquid chromatography) F, pressure gauge G, infector H, column I, detector J, fraction collector K, recorder or oth readout device.

When the sputter-ion pumps are installed one should ensure that the magnetic fields will not interfere with the operation of other devices (ionization vacuum gauges, partial pressure measurement units, etc.). Mounting devices for the sputter-ion pumps may not short circuit the inductance flow and thus weaken the air gap inductance and pumping speed. 

Some questions will require the identification of various mechanical tools or devices. Some of the types of mechanical devices that may appear on the exam— and covered in this chapter—include hand tools, gears, pulleys, levers, fasteners, springs, valves, gauges, and pumps. In addition to individual mechanical devices, the exam may test your knowledge of various systems, or combinations of mechanical devices. A common example of a mechanical system is the internal combustion engine of an automobile. 

Mercury is directly below cadmium in the periodic table, but has a considerably more varied and interesting chemistry than cadmium or zinc. Elemental mercury is the only metal that is a liquid at room temperature, and its relatively high vapor pressure contributes to its toxicological hazard. Mercury metal is used in electric discharge tubes (mercury lamps), gauges, pressure-sensing devices, vacuum pumps, valves, and seals. It was formerly widely used as a cathode in the chlor-alkali process for the manufacture of NaOH and Cl2, a process that has been largely discontinued, in part because of the mercury pollution that resulted from it. 

Figure 1. Schematic diagram of the static plasma grafting device. Key 1, glass reactor 2, copper electrodes connected to HF generator 3, device for polymer sustaining 4, fabrics to be treated 5, outlet to vacuum pump 6, gas supply 7, outlet to vacuum gauge.

Second only to the mechanical gauge as the easiest device to measure and read a vacuum (and decidedly easiest in construction) is the liquid manometer (see Fig. 7.37). A well-made mercury manometer, kept very clean, can measure vacuums of up to 10 3 torr. This sensitivity can be increased by up to 15 times if a liquid with less density, such as diffusion pump oil, is used. However, diffusion pump oil is far more difficult to keep clean and can require either (a) a very tall (and thereby impractical) column or (b) a manometer of very limited range. In addition, because of the strong surface tension between diffusion pump oil and glass, long waiting periods between readings are required as the oil settles into place. 

Two serial reactors Ri and R2 with an internal volume of 8.3 ml, are fed with C02 by an air driven pump (2) through a preheating coil (3). The content of the reactors are mixed by two stirring devices (4) a pressure gauge (5) and a water bath (6) allow a precise setting of the P and T experimental conditions. By-passing of Rl can be achieved through an adequate combination of valves Vi to V4 A decompression needle valve Vs adjusted in combination with the pump, allows to set the levels of both flow rate and pressure within the apparatus. 

Rotary oil pumps will provide a reliable vacuum down to about O.OlmmHg (at the apparams) and are thus one of the most valuable pieces of equipment in the lab. Ideally every research worker should have his/her own high vacuum pump, but in many cases cost prohibits this and pumps are shared. If a pump is shared, it is common to have it mounted on a trolley which has all the ancillary devices (traps, gauge etc.) mounted on it. Alternatively, a shared pump may be fixed in one place, but attached to a communal manifold, distillation set-up or other piece of apparatus. A good two-stage pump is suitable for most high vacuum requirements in an organic chemistry lab. 

The international unit of pressure, the Pascal, Pa, is seldom used in this country at this time, mainly because it has inconvenient numbers that are not related to the common measuring devices. A person can visualize 10 mm of mercury, but not 1330 Pascals. Normally, mm Hg is used with coarse gauges, such as the U-tube manometer. For pressures obtained with mechanical pumps, the term torr is common (1 torr = 1 mm Hg), and the term micron x 10 atmosphere) is used with diffusion pumps. We must eventually switch to Pascals, so the pressure in Pascals or kilo Pascals (kPa) will be included, where convenient, in parentheses. A person s lungs produce a vacuum of about 300 torr, and the tentacles of an octopus can attain 100 torr. 

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