Look an the table below. Discuss with a partner different types of gauges.
(Displacement of a wall)
Read and translate the text
Background and History of Vacuum Gages
To make measurements in the vacuum region, one must possess a knowledge of the expected pressure range required by the processes taking place in the vacuum chamber as well as the accuracy and/or repeatability of the measurement required for the process. Typical vacuum systems require that many orders of magnitude of pressures must be measured. One gage will not give reasonable measurements over such large pressure ranges. Over the past 50 years, vacuum measuring instruments (commonly called gages) have been developed that used transducers (or sensors) which can be classified as either direct reading (usually mechanical) or indirect reading (usually electronic). When a force on a surface is used to measure pressure, the gages are mechanical and are called direct reading gages, whereas when any property of the gas that changes with density is measured by electronic means, they are called indirect reading gages.
Direct Reading Gages
A subdivision of direct reading gages can be made by dividing them into those that utilize a liquid wall and those that utilize a solid wall. The force exerted on a surface from the pressure of thermally agitated molecules and atoms is used to measure the pressure.
Liquid Wall Gages
The two common gages that use a liquid wall are the manometer and the McLeod gage. The liquid column manometer is the simplest type of vacuum gage. It consists of a straight or U-shaped glass tube evacuated and sealed at one end and filled partly with mercury or a low vapor pressure liquid such as diffusion pump oil. In the straight tube manometer, as the space above the mercury is evacuated, the length of the mercury column decreases. In the case of the U-tube, as the free end is evacuated, the two columns approach equal height. The pressure at the open end is measured by the difference in height of the liquid columns. If the liquid is mercury, the pressure is directly measured in mm of Hg (torr). The manometer is limited to pressures equal to or greater than ~1 torr (133 Pa). If the liquid is a low density oil, the U-tube is capable of measuring a pressure as low as ~0.1 torr. This is an absolute, direct reading gage but the use of mercury or low density oils that will in time contaminate the vacuum system preclude its use as a permanent vacuum gage. Due to the pressure limitation of the manometer, the McLeod gage was developed to significantly extend the range of vacuum measurement. This device is essentially a mercury manometer in which a volume of gas is compressed before measurement. This can be used as a primary standard device when a liquid nitrogen trap is used on the vacuum system.
Solid Wall Gages
There are two major mechanical solid wall gage types: capsule and diaphragm.
The capsule-type gages depend on the deformation of the capsule with changing pressure and the resultant deflection of an indicator. Pressure gages using this principle measure pressures above atmospheric to several thousand psi and are commonly used on compressed gas systems. This type of gage is also used at pressures below atmospheric, but the sensitivity is low. The Bourdon gage, is used as a moderate vacuum gage.
If compensated capsule or diaphragm mechanisms are combined with sensitive and stable electronic measuring circuits, performance is improved. One such gage is the capacitance diaphragm gage (also referred to as the capacitance manometer).
A flexible diaphragm forms one plate of a capacitor and a fixed probe the other. The flexible diaphragm deforms due to even slight changes in pressure, resulting in a change in the capacitance. The capacitance is converted to a pressure reading. The sensitivity, repeatability, and simplicity of this gage enables this type of direct reading gage to be a standard from 10–6 torr to atmospheric pressure, provided multiple heads designed for each pressure range are used. A single head can have a dynamic range of 4 or 5 orders of magnitude. In this case, deformation of the diaphragm causes a proportional output from the attached strain gage. Sensitivities and dynamic range tend to be less than those of the capacitance diaphragm gage, but the price of the strain gage type diaphragm gage is usually lower. Both of these gages are prone to errors caused by small temperature changes due to the inherent high sensitivity of this gage type. Temperature-controlled heads or correction tables built into the electronics have been used to minimize this problem. Other sources of error in all solid wall gages are hysteresis and metal fatigue.