What is the measurement principle of a vacuum gauge?

Here, the measurement principles of three types of vacuum gauges are introduced. Vacuum measurement refers to the determination of vacuum level, which denotes the degree of thinness of the gas below atmospheric pressure. Expressing vacuum level in terms of pressure is a practice that has been carried over from history and is not entirely reasonable. High pressure means low vacuum level ； Conversely, low pressure corresponds to high vacuum. 1. The U-tube manometer is the simplest instrument for measuring pressure; it is typically made of glass tubes, and there are various working fluids available, with mercury being the most common one. One end of the tube is connected to the vacuum chamber under test for pressure measurement, while the other end is sealed or open to the atmosphere; the degree of vacuum is indicated by the difference in liquid levels at the two ends of the U-tube. The measurement range of the U-tube vacuum gauge is 105–10 Pa. It is an absolute vacuum gauge. (1) Open U-tube vacuum gauge: An appropriate amount of working fluid (such as mercury) is filled in the U-tube; one end of the tube is open to the atmosphere (i.e., the ambient atmospheric pressure p0), while the other end is connected to the vacuum system under test (the pressure to be measured, p). The formula for calculating the pressure is as follows: p = p0 – ρgh (1), where p represents the pressure to be measured, p0 is the ambient atmospheric pressure, h is the height difference between the two liquid surfaces, ρ is the density of the working fluid, and g is the acceleration due to gravity. (2) For a closed U-tube vacuum gauge: the tube is first evacuated to a pressure of less than 10-1 Pa, after which the working fluid (such as mercury) is poured into the tube; the open end of this tube is connected to the vacuum system whose pressure is to be measured. Before the vacuum system is evacuated, the pressure inside it is equal to the ambient atmospheric pressure; at this point, the working fluid fills the closed end, resulting in a maximum liquid level difference of h0 ； At a certain moment when the system is evacuated, and the liquid levels at both ends are in equilibrium due to the hydrostatic pressure, the pressure value to be measured can be calculated using the following formula (ignoring the effect of the pressure inside the closed end on the liquid level): p = ρgh (2). 2. Elastic-element vacuum gauges: Vacuum measuring instruments that utilize the principle of elastic deformation of elastic elements under a pressure difference are known as elastic-element vacuum gauges. It is similar in structure and appearance to industrial pressure gauges, and is generally used for measuring rough vacuums (102–105 Pa). Based on the classification of deformable elastic elements, these types of vacuum gauges usually include diaphragm tube type, bellows type, and diaphragm type. Vacuum gauges with elastic elements feature stable performance; their measurement range is generally 102–105 Pa, and the accuracy levels available are 0.5 grade, 1.5 grade, and 2.5 grade. In industrial production, some equipment requires the measurement of both positive pressure (above atmospheric pressure) and negative pressure (below atmospheric pressure, that is, in a vacuum state). Therefore, the pressure-vacuum gauges made with elastic elements have markings for both positive pressure and vacuum levels on the same dial. The main features of the elastic-element vacuum gauge are as follows: (1) The measurement result is the total pressure of gases and vapors, and it is independent of the type, composition, and properties of the gases ； (2) During measurement, the suction and exhaust of air by the instrument are minimal; moreover, there are no high-temperature components inside the instrument, so oil vapors will not decompose ； (3) High measurement accuracy ； (4) The reaction speed is relatively fast ； (5) Robust structure; appropriate materials are used to enable the measurement of corrosive gases ； (6) is an absolute vacuum gauge; gauges of class 0.5 or higher can be used as standard gauges. 3. Compression vacuum gauge: The compression vacuum gauge is a significant improvement over the U-tube vacuum gauge; it is based on Boyle’s law for ideal gases. Since it was first proposed by McLaughlin, this type of vacuum gauge is also known as a McLaughlin vacuum gauge (abbreviated as Mc gauge). The compression vacuum gauge is a practical absolute vacuum gauge for measuring pressures below 1 Pa, and it has been used as the main instrument for calibrating other vacuum gauges since 1874.