{"id":40,"date":"2018-11-29T05:01:35","date_gmt":"2018-11-29T05:01:35","guid":{"rendered":"http:\/\/msp12.epgpbooks.inflibnet.ac.in\/?post_type=chapter&p=40"},"modified":"2019-05-01T08:48:20","modified_gmt":"2019-05-01T08:48:20","slug":"guages","status":"publish","type":"chapter","link":"https:\/\/ebooks.inflibnet.ac.in\/msp12\/chapter\/guages\/","title":{"rendered":"Guages"},"content":{"raw":"
\"epgp<\/a>\r\n<\/span><\/div>\r\n
\r\n\r\n \r\n\r\nIntroduction<\/strong>\r\n\r\n \r\n

Vacuum gauges or pressure gauges are the instruments used to measure and display pressure as an integral unit inside a vacuum chamber. Generally, gauges measure pressure lower than the ambient atmospheric pressure, set as the zero point. A combination of gauges is usually used to measure the vacuum pressures produced, as most of the single gauges are insufficient to read entire range of vacuum pressure. The pressure measured by the gauge may slightly differ from the actual pressure inside the vacuum chamber due to several reasons.<\/p>\r\n \r\n

The position of the gauge in the system is an important point for measuring vacuum. Usually, the pressure in the gauge would not be exactly equal to the pressure inside the vacuum chamber. This is because of the conductance of the connecting pipes. Some gauges function as sources of gas due to excessive out-gassing while other gauges function as pumps. Most of the gauges behave in different ways to different gases, therefore it is essential to know the composition of gas inside chamber in prior in order to know the exact pressure. Also, the characteristics of the gauges may be changed due to their exposure to a variety of atmospheres in a system.<\/p>\r\n \r\n

Gauges can be divided into different categories. Some gauges evaluate the pressure directly (using the formula: force per unit area) or based on the force it applies. These include the liquid and capacitance manometers. Other gauges operate by measuring a quantity that is sensitive towards pressure such as viscosity or thermal conductivity. These include the thermocouple gauge and the viscometer gauge. Further, few of the gauges ionize the gas and measure the amount of ionization which is proportional to the pressure. These include hot and coldcathode ionization gauges and the Residual Gas Analyzer.<\/p>\r\n\r\n<\/div>\r\n\"\"\r\n\r\n\"\"\r\n

\r\n\r\n \r\n\r\n1)<\/strong>\u00a0\u00a0 Mechanical Gauges:<\/strong>\r\n\r\n \r\n\r\nThese gauges read values ranging from atmospheric pressure down to 100 mTorr.\r\n\r\n \r\n

In mechanical gauges, pressure difference is used to create macroscopic movement of a pointing needle. Following figure shows an example of Bourdan gauge, which is a type of mechanical gauge. Evacuation of the tube in the gauge lets the tube to coil up and this leads to movement of the needle on the dial. Such dials are rough and measure continuously. Mechanical gauges are useful for measuring rough vacuums.<\/p>\r\n\r\n<\/div>\r\n\"\"\r\n\r\n \r\n

\r\n\r\n \r\n\r\n2)\u00a0\u00a0\u00a0\u00a0\u00a0 U-Tube Manometers:<\/strong>\r\n\r\n \r\n\r\nThese gauges read values ranging from Atmospheric pressure to 200 mTorr.\r\n\r\n \r\n

Figure below depicts a U-shaped tube which is filled with a liquid and one of its sides is connected to the vacuum. The other end of the tube is opened to air, while the other end is evacuated to high vacuum. When the pressure above the two surfaces varies, a difference in the height of the liquid contained in the manometer comes up. By monitoring the difference in height between the two columns of a liquid. In such gauges, it is assumed that the forces due to surface tension effect and capillarity action are negligible in comparison to the forces that appear because of the pressure difference.<\/p>\r\n\"\"\r\n\r\n \r\n\r\nLet us assume that the cross-section area of the tube is a constant A,\r\n\r\n\"\"\r\n\r\n \r\n\r\n \r\n\r\nLIMITATIONS:<\/strong>\r\n\r\n \r\n