VACUUM GAUGEINTRODUCTIONFor pressure measurement below atmosphere or vacuum, different gauges are available. Manometers and bell gauges can go up to 0.1 Torr. Diaphragm gauges are usable up to a pressure of 10-3 Torr. For pressure below this value, electrical gauges like Pirani or Ionization Gauges are used. Vacuum measurement is broadly classified into Mechanical Type, Thermal Type, Ionization Type, and Radiation Vacuum Gauge. A standard manometric type mechanical gauge that is used down to 10-4 Torr is the McLeod Gauge. Thermal type vacuum gauge can be further divided into Pirani Gauge and Thermocouple Type Vacuum Gauge. Ionization Gauge can be divided into Hot Cathode Type and Cold Cathode Type.Pressures below 1 torr are best measured by gages which infer the pressure from the measurement of some other property of the gas, such as thermal conductivity or ionization. The thermocouple gage, in combination with a hot- or cold-cathode gage (ionization type), is the most widely used method of vacuum measurement today.Other gages used to measure vacuum in the range of 1 torr or below are the McLeod gage, the Pirani gage, and the Knudsen gage. The McLeod gage is used as an absolute standard of vacuum measurement in the 1010-4 torr (10310-2 Pa) range.The Knudsen gage is used to measure very low pressures. It measures pressure in terms of the net rate of transfer of momentum (force) by molecules between two surfaces maintained at different temperatures (cold and hot plates) and separated by a distance smaller than the mean free path of the gas molecules.A McLeod gauge is a scientific instrument used to measure very low pressures, down to 10-4 Torr. It was invented in 1874 by Herbert McLeod. McLeod gauges were once commonly found attached to equipment that operates under a vacuum, such as a lyophilizer. Today, however, these gauges have largely been replaced by electronic vacuum gauges.The design of a McLeod gauge is somewhat similar to a that of a mercury column manometer. Typically it is filled with mercury. If used incorrectly, this mercury can escape and contaminate the vacuum system attached to the gauge.

McLeod manometer symbol according to ISO 3753-1977 (E)McLeod gauges operate by taking in a sample volume of gas from a vacuum chamber, and then compressing it by tilting, and infilling with mercury. The pressure in this smaller volume is then measured by a mercury manometer, and, by knowing the compression ratio, the pressure of the original vacuum can be determined. Boyle's law is then used to find the initial pressure from the final pressure, and the initial and final volumes.This method is fairly accurate for non-condensible gases, such as oxygen and nitrogen. However, condensible gases, such as water vapour, ammonia, carbon dioxide, and pump oil vapors may be in gaseous form in the low pressure of the vacuum chamber, but will condense when compressed by the McLeod gauge. The result is an erroneous reading, showing a pressure much lower than actually present. It has the advantage that it is simple to use and that its calibration is nearly the same for all non-condensable gases.The device can be manually operated and the scale read visually, or the process can be automated in various ways. For example, a small electric motor can periodically rotate the assembly to collect a gas sample. If a fine platinum wire is in the capillary tube, its resistance indicates the height of the mercury column around it.Modern electronic vacuum gauges are simpler to use, less fragile, and do not present a mercury hazard, but their reading is highly dependent on the chemical nature of the gas being measured and their calibration is unstable. For this reason McLeod gauges continue to be used as a calibration standard for electronic gauges.Ionization gaugeIonization gauges are the most sensitive gauges for very low pressures (also referred to as hard or high vacuum). They sense pressure indirectly by measuring the electrical ions produced when the gas is bombarded with electrons. Fewer ions will be produced by lower density gases. The calibration of an ion gauge is unstable and dependent on the nature of the gases being measured, which is not always known. They can be calibrated against a McLeod gauge which is much more stable and independent of gas chemistry.Thermionic emission generate electrons, which collide with gas atoms and generate positive ions. The ions are attracted to a suitably biased electrode known as the collector. The current in the collector is proportional to the rate of ionization, which is a function of the pressure in the system. Hence, measuring the collector current gives the gas pressure. There are several sub-types of ionization gauge.Useful range: 10-10 - 10-3 torr Most ion gauges come in two types: hot cathode and cold cathode. A third type that is more sensitive and expensive known as a spinning rotor gauge exists, but is not discussed here. In the hot cathode version, an electrically heated filament produces an electron beam. The electrons travel through the gauge and ionize gas molecules around them. The resulting ions are collected at a negative electrode. The current depends on the number of ions, which depends on the pressure in the gauge. Hot cathode gauges are accurate from 103Torr to 1010Torr. The principle behind cold cathode version is the same, except that electrons are produced in the discharge of a high voltage. Cold Cathode gauges are accurate from 102Torr to 109Torr. Ionization gauge calibration is very sensitive to construction geometry, chemical composition of gases being measured, corrosion and surface deposits. Their calibration can be invalidated by activation at atmospheric pressure or low vacuum. The composition of gases at high vacuums will usually be unpredictable, so a mass spectrometer must be used in conjunction with the ionization gauge for accurate measurement.

Medium- High VacuumMedium to high vacuums need to be measured using thermal and molecular devices. Thermocouple gauges measure changes in the thermal conductivity of a residual gas within a gauge tube. The pressure readings for this deviceare dependent on the type of gas.Thermocouple gaugesinclude a filament, power supply for the filament, and moving coil meter for displaying the pressure. The amount of heat lost depends on the gas pressure. There are several designs of the Pirani gauge.One design includes using two plates with different temperatures. The amount of power spent for heating is the measure of gas pressure. Another design uses a single plate to measure the thermal conductivity of gas by heat loss to the surrounding area.

Pirani gauge (using singl plate)ThePirani gaugeis a robustthermal conductivitygauge used for the measurement of thepressuresin vacuum systems.It was invented in 1906 byMarcello Pirani.

The Pirani gauge consists of a metal filament (usuallyplatinum) suspended in a tube which is connected to the system whose vacuum is to be measured. Connection is usually made either by a ground glass joint or aflangedmetal connector, sealed with ano-ring. The filament is connected to an electrical circuit from which, after calibration, a pressure reading may be taken.Mode of operationAheated metal wire(also called a filament) suspended in a gas will lose heat to the gas as its molecules collide with the wire and remove heat. If the gas pressure is reduced the number of molecules present will fall proportionately and the wire will lose low heat more slowly. Measuring the heat loss is an indirect indication of pressure.The electrical resistance of a wire varies with its temperature, so the resistance indicates the temperature of wire. In many systems, the wire is maintained at a constant resistanceRby controlling the currentIthrough the wire. The resistance can be set using a bridge circuit. The power delivered to the wire isI2R, and the same power is transferred to the gas.[3]The current required to achieve this balance is therefore a measure of the vacuum.The gauge may be used for pressures between 0.5Torrto 104Torr. The thermal conductivity and heat capacity of the gas may affect the readout from the meter, and therefore the apparatus may need calibrating before accurate readings are obtainable. For lower pressure measurement other instruments such as aPenning gaugeare used.