Vacuum measuring systems gauges

The best-known design of a diaphragm vacuum gauge is a barometer with an aneroid capsule as the measuring system. It contains a hermetically sealed, evacuated, thin-walled diaphragm capsule made of a copper-beryllium alloy. As the pressure drops, the capsule diaphragm expands. 

The variable C is the vacuum gauge constant of the measuring system. For... 

Fig. 3.16 Schematic drawing of the ebctrode arrangement of various bnization vacuum gauge measuring systems...

It is not fundamentally possible to keep the measurement system in a vacuum gauge from becoming soiled. Thus it is necessary to ensure that... 

The easiest way to see a quantitative vacuum measurement of a system is to look at a dial and read it. This direct reading can be done with mechanical gauges. The surface of mechanical gauges that come in contact with gases within a vacuum system can be made out of metal or glass (borosilicate or quartz), both of which can be fairly impervious to chemical attack. ... 

It is very important to be able to measure the pressure in a vacuum system, particularly when carrying out a distillation. For low vacuum measurement a simple manometer, such as that shown in Fig. 8.3a, is commonly used and the pressure is taken by subtracting the heights of the mercury levels. Dial gauges are also useful for in-line measurements and they are particularly valuable when used with rotary evaporators. For high vacuum... 

Absolute pressure is pressure measured relative to a perfect vacuum, an absolute 2ero of pressure (2). Like the absolute 2ero of temperature, perfect vacuum is never reali2ed in a real world system but provides a convenient reference for pressure measurement. The acceptance of strain gauge technology in the fabrication of pressure sensors is resulting in the increased use of absolute pressure measurement in the CPI (see Sensors). The pressure reference... 

Vacuum in process systems refers to an absolute pressure that is less than or below the local barometric pressure at the location. It is a measure of the degree of removal of atmospheric pressure to some level between atmospheric-barometer and absolute vacuum (which cannot be attained in an absolute value in the real world), but is used for a reference of measurement. In most situations, a vacuum is created by pumping air out of the container (pipe, vessels) and thereby lowering the pressure. See Figure 2-1 to distinguish between vacuum gauge and vacuum absolute. 

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). 

The kinetics of many decompositions are conveniently studied from measurements of the pressure of the gas evolved in a previously evacuated and sealed constant volume system. It is usually assumed, and occasionally confirmed, that gas release is directly proportional to a, so that the method is most suitable for reactants which yield a single volatile product by the irreversible breakdown of a substance that does not sublime on heating in vacuum. A cold trap is normally maintained between the heated reactant and the gauge to condense non-volatile products (e.g. water vapour) and impurities. The method has found wide application, notably in studies of the decomposition of azides, permanganates, etc., and has been successfully developed as an undergraduate experiment [114—116]. 

Although a pressure gauge is more commonly used to measure the pressure inside a laboratory vessel, a manometer is sometimes used (Fig. 4.5). It consists of a U-shaped tube connected to the experimental system. The other end of the tube may be either open to the atmosphere or sealed. For an open-tube manometer (like that shown in Fig. 4.5a), the pressure in the system is equal to that of the atmosphere when the levels of the liquid in each arm of the U-tube are the same. If the level of mercury on the system side of an open manometer is above that of the atmosphere side, the pressure in the system is lower than the atmospheric pressure. In a closed-tube manometer (like that shown in Fig. 4.5b), one side is connected to a closed flask (the system) and the other side is vacuum. The difference in heights of the two columns is proportional to the pressure in the system. 

In addition to the conventional ionization gauge, whose electrode structure resembles that of a common triode, there are various ionization vacuum gauge systems (Bayard-Alpert system, Bayard-Alpert system with modulator, extractor system) which more or less suppress the two effects, depending on the design, and are therefore used for measurement in the high and ultrahigh vacuum range. Today the Bayard-Alpert system is usually the standard system. 

In all vacuum processes the pressure in the system must be constantly checked and, if necessary, regulated. Modern plant control additionally requires that all measured values w/hich are important for monitoring a plant are transmitted to central stations, monitoring and control centers and compiled in a clear manner. Pressure changes are frequently recorded over time by recording equipment. This means that additional demands are placed on vacuum gauges ... 

General The measurement of the absolute pressure in a vacuum system is important to vacuum engineers but generally not for chemists. Very few commercially available gauges are suitable for installation directly onto... 

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