Fundamentals of Vacuum Technology (1248463), страница 63
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The starting properties of the sputter-ion (triode)pumps manufactured by LEYBOLD are so good that no problems will beencountered when returning the units to service, even after a longer periodin storage.When the sputter-ion pumps are installed one should ensure that themagnetic fields will not interfere with the operation of other devices(ionization vacuum gauges, partial pressure measurement units, etc.).Mounting devices for the sputter-ion pumps may not short circuit theinductance flow and thus weaken the air gap inductance and pumpingspeed.If the ultimate pressure which can be attained is not satisfactory eventhough the apparatus is properly sealed, then it will usually be sufficient tobake out the attached equipment at about 200 to 250 ¡C. If the pressurehere rises to about 1 á 10-5 mbar when this is done, then the sputter ionpump will become so hot after evacuating the gases for two hours that it willnot be necessary to heat it any further in addition.
It is also possible to heatthe pump by allowing air to enter for 2 hours at 10-5 mbar before the otherapparatus is then subsequently baked out. If the ultimate pressure is stillnot satisfactory, then the pump itself will have to be baked out for a fewhours at 250 to 300¡C (but not higher than 350 ¡C!). The pump shouldwithout fail remain in operation throughout this period! If the pressure risesabove 5 á 10-5 mbar it will be necessary either to heat more slowly or toconnect an auxiliary pump. Before airing one should allow the hot sputterion pump enough time to cool down at least to 150 ¡C.8.4Information on working withvacuum gauges8.4.1 Information on installing vacuumsensorsHere both the external situation in the immediate vicinity of the vacuumapparatus and the operating conditions within the apparatus (e.g.
workingpressure, composition of the gas content) will be important. It is initiallynecessary to ascertain whether the measurement system being installed issensitive in regard to its physical attitude. Sensors should only be installedvertically with the vacuum flange at the bottom to keep condensates, metalflakes and filings from collecting in the sensor or even small componentssuch as tiny screws and the like from falling into the sensorand themeasurement system. The hot incandescent filaments could also bend anddeform improperly and cause electrical shorts inside the measurementsystem.
This is the reason behind the following general rule: If at allpossible, install sensors vertically and open to the bottom. It is alsovery important to install measurement systems if at all possible at thosepoints in the vacuum system which will remain free of vibration duringoperation.The outside temperature must be taken into account and above all it isnecessary to avoid hot kilns, furnaces or stoves or other sources of intenseradiation which generate an ambient temperature around the measurementsystem which lies above the specific acceptable value.
Excessive ambienttemperatures will result in false pressure indications in thermal conductivityvacuum sensors.145HomeInstructions for equipment operation8.4.2 Contamination at the measurementsystem and its removalThe vacuum gauges used in vacuum technology for pressure measurementwill certainly work under ÒdirtyÓ conditions. This is quite understandablesince a vacuum device or system does not serve simply to produce lowpressures but rather and primarily have to run processes in chemistry,metallurgy or nuclear physics at low pressures.
Here, depending on thenature of the process, considerable quantities of gases or vapors will beliberated either continuously or intermittently; these can pass into themeasurement systems provided for pressure measurement and installed inthe vacuum system and Ð due to surface reactions or through simpledeposits Ð can falsify the pressure measurements considerably. This is truefor all types of vacuum gauges whereby, of course, high-sensitivity, highaccuracy measurement systems are particularly susceptible to soilingresulting from the causes named. One can attempt to protect themeasurement systems against contamination by providing suitableshielding.
This, however, will often lead to the pressure registered by themeasurement system Ð which is indeed clean Ð deviating considerablyfrom the pressure actually prevailing in the system.It is not fundamentally possible to keep the measurement system in avacuum gauge from becoming soiled. Thus it is necessary to ensure that• the influence of the contamination on pressure measurement remainsas small as possible and that• the measurement system can readily be cleaned.These two conditions are not easy to satisfy by most vacuum gauges inpractice.Dirt in a compression vacuum gauge will cause an incorrect anduncontrollable pressure indication. Dirty THERMOVAC sensors will show apressure which is too high in the lower measurement range since thesurface of the hot wire has changed. In Penning vacuum gaugescontamination will induce pressure readings which are far too low since thedischarge currents will become smaller.
In the case of ionization vacuumgauges with hot cathodes, electrodes and the tube walls can be soiledwhich, under certain circumstances, will result in a reduction of dielectricstrengths. Here, however, the measurement systems can usually be bakedout and degassed by passing a current through or by electronbombardment, quite aside from the fact that ionization vacuum gauges areoften used in the ultrahigh vacuum range where it is necessary to ensureclean operating conditions for other reasons.is necessary to ensure in particular that the required high insulation valuesfor the high-voltage electrodes and ion traps also be maintained duringoperation and sometimes even during bake-out procedures.
Insulationdefects may occur both in the external feed line and inside themeasurement system itself. Insufficient insulation at the trap (detector) leadmay allow creep currents Ð at low pressures Ð to stimulate overly highpressure value readings. The very low ion trap currents make it necessaryfor this lead to be particularly well insulated. Inside the measurementsensors, too, creep currents can occur if the trap is not effectively shieldedagainst the other electrodes.An error frequently made when connecting measurement sensors to thevacuum system is the use of connector piping which is unacceptably longand narrow. The conductance value must in all cases be kept as large aspossible. The most favorable solution is to use integrated measurementsystems.
Whenever connector lines of lower conductance values are usedthe pressure indication, depending on the cleanliness of the measurementsensors and the connector line, may be either too high or too low. Heremeasurement errors by more than one complete order of magnitude arepossible! Where systems can be baked out it is necessary to ensure thatthe connector line can also be heated.8.4.4 Connectors, power pack,measurement systemsThe measurement cables (connector cables between the sensor and thevacuum gauge control unit) are normally 2 m long.
If longer measurementcables must be used Ð for installation in control panels, for example Ð thenit will be necessary to examine the situation to determine whether thepressure reading might be falsified. Information on the options for usingover-length cables can be obtained from our technical consultingdepartment.8.4.3 The influence of magnetic andelectrical fieldsIn all those measurement instruments which use the ionization of gasmolecules as the measurement principle (cold-cathode and hot-cathodeionization vacuum gauges), strong magnetic leakage fields or electricalpotentials can have a major influence on the pressure indication. At lowpressures it is also possible for wall potentials which deviate from thecathode potential to influence the ion trap current.In vacuum measurement systems used in the high and ultrahigh regimes it146HomeTables, Formulas, Diagrams9. Tables, formulas, nomograms and symbolsUnitN á mÐ2, Pa 2) mbar1 N á mÐ2 (= 1 Pa)1 mbar1 bar1 Torr 3)11001 á 1051331 á 10Ð211 á 1031.33barTorr1 á 10Ð51 á 10Ð311.33 á 10Ð37.5 á 10Ð30.7575011) The torr is included in the table only to facilitate the transition from this familiar unit to thestatutory units N á m-2, mbar and bar.
In future the pressure units torr, mm water column, mmmercury column (mm Hg), % vacuum, technical atmosphere (at), physicalatmosphere (atm),atmosphere absolute (ata), pressure above atmospheric and pressure below atmosphericmay no longer be used. Reference is made to DIN 1314 in this context.2) The unit Newton divided by square meters (N á m-2) is also designated asPascal (Pa): 1 N á m-2= 1 Pa.Newton divided by square meters or Pascal is the SI unit for the pressure of fluids.3) 1 torr = 4/3 mbar; ß torr = 1 mbar.Table I: Permissible pressure units including the torr 1) and its conversionAbbrev.GasC* = λ á p[cm á mbar]H2HeNeArKrXeHgO2N2HClCO2H2ONH3C2H5OHCl2AirHydrogenHeliumNeonArgonKryptonXenonMercuryOxygenNitrogenHydrochloric acidCarbon dioxideWater vaporAmmoniaEthanolChlorineAir12.00 á 10Ð318.00 á 10Ð312.30 á 10Ð36.40 á 10Ð34.80 á 10Ð33.60 á 10Ð33.05 á 10Ð36.50 á 10Ð36.10 á 10Ð34.35 á 10Ð33.95 á 10Ð33.95 á 10Ð34.60 á 10Ð32.10 á 10Ð33.05 á 10Ð36.67 á 10Ð3Table III:1 ↓ = ...
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