3 Guages

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Introduction

 

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.

 

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.

 

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.

 

1)   Mechanical Gauges:

 

These gauges read values ranging from atmospheric pressure down to 100 mTorr.

 

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.

 

 

2)      U-Tube Manometers:

 

These gauges read values ranging from Atmospheric pressure to 200 mTorr.

 

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.

 

Let us assume that the cross-section area of the tube is a constant A,

 

 

LIMITATIONS:

 

  • As Mercury has fairly large vapor pressure (~ 1.2 mTorr at 20°C), thus the assumption that the closed tube manometer relies on the pressure above the closed tube being 0 Torr.
  • Capillarity, surface tension, and adhesion between the liquid and the tube are also a concern. Once the mercury gets dirty it tends to stick erratically to the glass. These forces were neglected in computing the sumof forces above.
  • Contamination of the system because of the Mercury or oil vapors unless vapor traps are used.
  • A manometer is bulky, heavy and breakable.
  • Inaccuracy in readings, especially at low pressure, as both Mercury and glass are fairly sticky.

 

3) McLeod Gauge

 

These gauges read values ranging from 10 Torr to 10-6Torr.

 

McLeod gauge is a moderation of a manometer that can record absolute pressure of gases quite accurately, and is used to calibrate other gauges.

 

A fixed volume of gas from the vacuum is trapped by the McLeod gauge and then is compressed by volume. This builds the pressure to a point where it can be easily read. TheMcLeod gauge measures pressure periodically rather than continuously.

 

 

As indicated by black ink, vacuum is created with the mercury level at the original level. In order to monitor the pressure, a volume V0 of gas is trapped raising the mercury level at the vacuum pressure Pv. The volume V0 is the total combined volume of the bulb A and the capillary B.The mercury is then raised until the level inthe tube connected to vacuum is equal to thetop of the sealed capillary, indicated by the dashed line. The compressed gas trapped in the capillary is at a higher pressure, andkeeps the mercury at a lower level, as shown in gray. The height difference between thetwo final mercury levels is h and the capillary has a cross-sectional area A. Calling the final volume Vf = hA, by Boyle’s Law the final pressure Pf is

 

Pf = P0(V0/Vf)

 

Assuming that the vacuum is at a low pressure,

 

Pf – Pv  ~ Pf = h Torr

 

and using Vf = hA we can write

 

Pv= (A/V0)h2

 

This type of gauge makes a direct readingof pressure and is easily related to the basicdefinition of pressure. It is unaffected by thecomposition of the gas except for condensable vapors. If a condensable vapor such aswater vapor is present it may condense uponcompression and give a false reading of thepressure.

 

The gauge is not a continuous reading gauge,and it involves considerable operator intervention to make a reading. Vapors from thegauge can contaminate the system and thusthe gauge must be carefully trapped. At the higher pressures in the capillary some vapors present in the system may be condensedleading to an incorrect reading. The gauge isbulky and breakable and contains mercury, ahazardous material. This gauge is primarilyused for calibration of other gauges.

 

Capacitance Manometer:

 

These gauges read values that range from 10+4Torr – 10-5Torr.

 

A capacitance manometer uses a metal diaphragm as one plate of a capacitor. If the pressure differs on the two sides of the diaphragm, the diaphragm will move and will change the capacitance. Typically, the difference between two capacitors is used, with the diaphragm serving as a plate in both of the capacitors. Absolute pressure units use a sealed chamber on one side that, through the use of getters,can be maintained at about 10-6Torr. Differential manometers use chambers that are both open. A given sensor is only sensitive for a range of about 5 decades, although this may be chosen from the nine decades of operation, 10+4Torr to below 10-5Torr. At any given time two or three digits are displayed on the readout.

 

The gauge is easy to use and reads pressure directly. It is insensitive to the type of gas being used. The diaphragm is made of an inert metal and is not subject to rapid corrosion. It is a rugged gauge and can safely be exposed to atmospheric pressure while on.It covers a pressure range more nicely thanmost other competing gauges, and it can bemade part of a control loop for process control.

 

In an absolute unit the sealed chamberis prone to long-term changes in pressure which will change the calibration of the unit.The diaphragm is exposed to the gases in thesystem that may lead to long-term corrosionof the unit. The unit is quite temperaturedependent, so the gauge head is temperature controlled. The zero of the unit tends to drift and must periodically be reset.

Range 100 Torr|0.1 mTorr.

 

Both Pirani and thermocouple gauges work on the same principle: a wire carrying anelectric current will heat up until it reachesan equilibrium temperature. Joule heat inthe amount i2R is produced by a currentpassing through a wire of resistance R. The wire will increase in temperature until anequal amount of heat is removed from thewire. The heat losses from the wire can occur via convection, conduction, or radiation. At high pressures convection dominates, and the wire stays cool; the convection is not very pressure sensitive. Inthe pressure range of about 200 Torr to 0.1mTorr, conduction dominates and the heattransfer is very pressuresensitive. It is mostcommon to use the gauge to measure pressure in this range. At low pressures the heatloss is due to radiation that is pressure insensitive.

 

As the heat balance changes, the temperature of the wire changes, and hence the resistance of the wire also changes. By measuring either the temperature of the wire orthe resistance of the wire we can determinethe pressure.

 

The Pirani gauge measures the resistance(usually at constant current) while the thermocouple gauge, measures thetemperature of the wire. Both readingsare quite non-linear with pressure, and thegauges are calibrated by the manufacturer.In the constant current mode of operationthe useful range is 1 Torr to 10-4Torr.By using a Pirani gauge and keeping the temperature(resistance) constant, the higherpressure limit can be raised to about 100Torr. A few gauges use the convectioncooling properties of gases and claim accurate readings at pressures up to atmospheric.

 

Continuous readings are possible, and vapors are measured as well as gases. No vapors are introduced by the gauge. The gaugecan safely be exposed to atmospheric pressure.The electronics must be calibrated. Thegauge characteristics will change over timesince the wire may become contaminated bygases in the system. The gauge is sensitiveto the type of gas as is shown in the diagramabove. This is due to the effect of convectionespecially at higher pressures.

 

Viscometer Gauge:

 

Range 10-1Torr to 10-4Torr.

 

The viscosity of a gas varies with pressure ina well-defined way that can be derived fromkinetic theory. Viscometer gauges use viscosity to measure pressure. One approachuses a wire in a tube with very little clearance between the wire and the tube. By using the wire as a torsion pendulum and observing damping of the oscillations, the viscous force can be measured and can be related to pressure. A variation uses a rotatingball in a spherical cavity.

 

The viscometer gauge is used to calibrateother gauges. It is quite accurate downto very low pressures, however it is quitedelicate and cannot withstand bumps andexcessive vibration. It is an expensivegauge.

 

Thermionic (Hot Cathode) Ionization Gauge:

 

Range 0.1 Torr to 10    11 Torr.

 

Pressure is proportional to the number ofmolecules in the system. If we can count themolecules we can compute the pressure. Thethermionic ionization gauge uses a heated filament to produce electrons (thermionicemission) which are then accelerated by anelectric field and cause ionization of themolecules in the system. The positive ions are collected and theircurrent is measured.

 

The ion current is proportional to the number of ions in the chamber which is proportional to the number of molecules andhence the pressure. Lower currents at lowerpressures can be amplified for detection,and so we can change the sensitivity of thegauge.

 

Very low pressures can be read. The scale islinear, and easy to read. Since the pressuresignal is a current, it is easy to interface thisinto a control system.The gauge must not be exposed to high pressures when it is on. If it is exposed tohigh pressure the filament may oxidize andbreak. Usually there are safety interlocks which turn off the gauge when the ionization current becomes too high, but even sothe filament is stressed.The gauge is very sensitive to the composition of the gas. Lighter gases (He, H2, H2O)will read low while heavier gases (N2 CO2,Ar) will read high. The determining factoris the ionization energy of the gas species.

 

Penning (Cold Cathode)Ionization Gauge:

 

Range 10-3Torr to 10-13Torr

 

Electrons are emitted from a cold cathode (room temperature) when a large enough voltage is applied. If the electrons are accelerated with the same field as was used to pull them out, we can have energetic electrons capable of ionizing the gas in the system. We increase the mean free path of the electronsby applying a magnetic field. This increasesthe rate of ionization per electron, and weobtain an ion current that is large enough tomeasure even without amplification.

 

The emission of electrons from the cathodecan be hampered by the formation of an insulating layer such as a thick oxide. Periodically therefore the cathode should becleaned. The gauges I have used can becleaned simply with a _ne emery paper followed by an acetone or alcohol rinse to remove grit.

 

The gauge is rugged, and can be exposedto atmosphere without harm. The electronics is easier to design than that for other gauges.

 

A high voltage source is needed. The gaugeis sensitive to the composition of the gas inthe same fashion as the thermionic ionization gauge. You must not allow metal particles to fall into the gauge, especially magnetic materials!

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REFERENCES

  1. A. Chambers, Basic Vacuum Technology, pp. 100-102, CRC Press, 1998.
  2. John F. O’Hanlon, A User’s Guide to Vacuum Technology, pp. 92-94, John Wiley & Sons, 2005.
  3. Robert M. Besançon, ed. (1990). “Vacuum Techniques”. The Encyclopedia of Physics (3rd ed.). Van Nostrand Reinhold, New York. pp. 1278–1284.
  4. Beckwith, Thomas G.; Roy D. Marangoni & John H. Lienhard V (1993). “Measurement of Low Pressures”. Mechanical Measurements (Fifth ed.). Reading, MA: Addison-Wesley. pp. 591–595.
  5. F M Penning, Physica, 1937, IV, 71 (DOI: 10.1016/S0031-8914(37)80123-8).
  6. F M Penning, Philips Technical Review, 1937, 2, 201.