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Vacuum measuring gauges are instruments used to measure the level or degree of vacuum in a system. They are commonly used in various industries and applications, such as HVAC systems, scientific research, manufacturing processes, and automotive diagnostics.
Here are some common types of vacuum measuring gauges:
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Bourdon Gauge: This is a mechanical gauge that uses a curved tube (Bourdon tube) that straightens or bends in response to changes in pressure. It converts the pressure into a mechanical displacement, which is then displayed on a dial. Bourdon gauges are relatively inexpensive and provide accurate readings for a wide range of vacuum levels.
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Capacitance Manometer: This type of gauge uses the principle of capacitance to measure the pressure. It consists of two parallel plates, with one plate fixed and the other movable. As the pressure changes, the distance between the plates changes, altering the capacitance. By measuring this capacitance, the pressure can be determined. Capacitance manometers are highly accurate and often used in scientific and high-precision applications.
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Ionization Gauge: Ionization gauges use the principle of ionization to measure vacuum levels. They contain a filament that emits electrons, and these electrons collide with gas molecules, causing them to ionize. The ions are collected on an electrode, producing an electrical current proportional to the pressure. Ionization gauges are used for measuring high vacuums and ultra-high vacuums.
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Thermocouple Gauge: Thermocouple gauges use the thermal conductivity of gas to determine the pressure. They consist of two wires of different metals joined at one end. One wire is heated, and the other acts as a temperature sensor. As gas molecules collide with the heated wire, they transfer heat away, causing a temperature difference between the two wires. By measuring this temperature difference, the pressure can be determined. Thermocouple gauges are commonly used in vacuum systems and are reliable and cost-effective.
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Mcleod Gauge
It is normally regarded as the standard gauge from which other gauges are calibrated. The basic principle of these gauge is that it takes a known volume of gas at a pressure that’s too low to be measured by direct measurement then compresses it in a known ratio until the pressure becomes large enough to be measured by ordinary manometer .
Vacustat
This is a specialized reading portable type of McLeod gauge. Its normally kept horizontally when it is not used and turning it vertically when taking readings. The mercury inside traps and compresses the gas from the system in a closed capillary. The height which mercury rises indicates the degree of vacuum. Vacustat covers a pressure range from 1 torr – 10-3 torr.
The Pirani Gauge
It is also one of the most used gauges that measures within the range of medium to high vacuum. Its action depends upon the variation in thermal conductivity of gas with pressure.
A metal filament heated by an electric current reaches an equilibrium temperature as heating generated by an electric current is balanced by the heat loss due to conduction , convection and radiation .
At higher pressure , the number of gas molecules striking the heated filament in unit diameter are large due to the intermolecular conduction . While at low pressure, the mean free path is comparable with the size of the vessel and therefore few gas molecules strike the heated filament hence the measurement of the electric resistance of the filament provides a means of determining the pressure.
Pirani gauge consist of a glass or metallic envelope containing a heated filament made of a material of high temperature coefficient of resistance e.g. Platinum or tungsten.
The resistor of the filament is used as one arm of a whetstone bridge inserted in a vacuum system. The bridge is balanced initially at atmospheric pressure or at fixed low pressure (<10-3 torr) and any change of filament resistance will cause a deflection of the null indicator which can be calibrated in unit pressure.
Cold Cathode Ionization Gauge or Penning Gauge
This depends upon ionization of gas molecules for its operation. The gauge head consist of a pair of cathode in form of a wire loop placed between them. A 2KV direct current (DC) voltage is applied between the anode and cathode. The cathode will emit electrons, which are attracted towards the anode.
A magnetic field (provided by a horseshoe magnet) perpendicular to the cathodes prevents the straight-line motion of electrons from cathodes to the anodes and instead cause a helical path to be taken. The electrons pass through the anode loop and are repelled by the cathode many times before they eventually strike the anode. This action greatly increases the path which electrons take, thus increasing the number of ions formed because of collision with the gas molecules in the vacuum system.
The collision of the ions at the cathode gives a current that is large enough to be measure on a micrometer. The ion current is proportional to the number of collisions and is thus a measure of the pressure in the system. The meter can be calibrated directly in units of pressure. The cold cathode gauge can measure from 10-2 –10-6 torr
These are just a few examples of vacuum measuring gauges. The selection of a gauge depends on the specific application, the desired accuracy, and the range of vacuum levels to be measured.