U-tube vacuum gauges

U-tube vacuum gauges fiiied with mercury are the simpiest and most exact instruments for measuring pressure in the rough vacuum range (1013 to a... 

An example of such an instrument is the U-tube vacuum gauge, with which the measurement of the pressure in the measurement capillary is based on a measurement of the weight over the length of the mercury column. 

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. 

S.6. Choice of gauges For the general operation of a vacuum system, a vacuum gauge is usually not required, but it may be useful, especially to the less experienced operator. For general monitoring purposes the small U-tube manometers and the Vacustat -type mini-McLeod gauge are adequate. 

The mechanical phenomena gauges measure the actual force exerted by the gas. They include a U-tube, a capsule dial, a strain, a capacitance manometer, a McLeod gauge, etc. Vacuum is measured according to the displacement of an elastic material or by measuring the force required to compensate its displacement. The measurement ranges from atmospheric pressure to 102 Pa in rough vacuum conditions. 

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. 

Figure 6-20. Minimum-component vacuum distillation apparatus. (A) pump, (B) and (C) Variacs, (D) support plate, (E) terminal strip, (F) stirring motor, (G) heating mantle, (H) r. b. flask, (I) Vigreux distilling column, (J) distilling head, (K) thermometer, (L) Liebig condenser, (M) manostat, (N) stopcock, (O) stopcock, (P) bleeding valve, (Q) drying tower, (R) U-tube manometer, (S) trap, (T) Dewar flask, (U) support plate, (V) r. b. flasks, (W) cow, (X) vacuum adapter, (Y) stopcock, (Z) Hg overflow, (AA) trap, (BB) Dewar flask, (CC) McLeod gauge, (DD) cork rings.

Low vacuum P>3kPa U-tube manometer, capsule gauge... 

A Reaction flask (100 ml). B Filling tube. C Funnel with stopcock. D Pressure guage. E, H Floating valves with a vacuum source. F U-tube. G Pressure gauge. K Gas-penetrable valve. I Valve tube. 

---