DATASHEET SEARCH SITE | WWW.ALLDATASHEET.COM

Document overview

  • Manufacturer or author: Provided By ALLDATASHEET.COM(FREE DATASHEET DOWNLOAD SITE)
  • PDF pages: 8

Technical content

Bayard-alpert type IonIzatIon GauGe tuBe WWW.MKSInST.COM Pressure & Vacuum Measurement Solutions

Figure 1 — Operating Range for Bayard-Alpert Ionization Gauge Tube

Description

Bayard-Alpert (hot cathode) gauges provide pressure measurement based on the ionization of gas molecules by a constant flow of electrons from a filament. The number of ions created is directly proportional to the density of molecules. The ions are collected and measured by a vacuum gauge controller and converted to pressure based on the sensitivity of the gauge. The Series 274 Bayard- Alpert gauges consist of 3 types: glass tubulated, nude gauge with electron bombardment degas (EB), and the nude gauge with I 2R degas (resistive). Gas Species Eff ects Bayard-Alpert ionization gauges have different relative sensitivities for different gas species. As a result, pressure readout provides a direct reading only for the gas for which the gauge is specified ( n itrogen). This is called a readout of n itrogen equivalent pressure. A simple mathematical conversion of the direct pressure readout to the pressure of the non standard gas can be made. This conversion is made using the relative gauge sensitivity for the specific gas. These sensitivities are tabulated in reference material and are contained in the Granville-Phillips ® instruction manual for Series 274. Hot filament type ionization vacuum gauges work best in given pressure ranges for individual gauge designs. The following is a brief outline of the considerations relevant to selection of the proper gauge. A glance at the chart (Figure 1) will quickly indicate the gauge or gauges suitable for a given application. The useful range of most of these gauges starts at 2 x 10 Torr. Variations in design permit operation to various orders of vacuum. The nude type 274022, 274023, 274041 and 274042 are suitable to 2 x 10 -11 Torr. Operating Principle n egative electrons are emitted at a well-controlled, selectable rate from a heated filament and are accelerated toward a positively charged wire grid (anode). Electrons pass into the space between the grid and a collector wire at ground. In this space the electrons collide with gas molecules from the vacuum system, producing positive ions. The positive ions are then collected by the grounded collector wire that is located along the axis of the cylindrical grid. At a constant filament to grid voltage and electron emission current, the rate that positive ions are formed is directly proportional to the density of molecules (pressure) in the space for pressures below 1 x 10 -3 Torr. The strength of the ion current is then indicated on a microammeter that is calibrated in units of pressure. The low end of the operating range of a Bayard-Alpert gauge is determined by the X-ray limit of this type of gauge. The X-ray limit varies with different gauge designs. Features & Benefits

  • Good v acuum pressure measurement f or economical cost Operates on industr y standard electrode v oltages and wide r ange of electron emission currents (10 mA to 10 mA) Well-known perf ormance char acteristics in v acuum industry
  • Ion collector electrode w ell-shielded from leakage currents
  • Available with b urn-out resistant filaments and standard v acuum connections
  • All units can be degassed b y electron bombardment; some can be resistance degassed X-r ays are produced when the electrons emitted by the filament impact the grid and support wires. Because of the geometry of the Bayard-Alpert gauge, only a small fraction of the emitted X-rays are intercepted by the ion collector. When the X-rays strike the collector wire they cause electrons to be photoelectrically ejected from the collector. This X-ray current limits the pressures that can be measured, and is equivalent to a pressure reading in the 10 -10 or 10-11 Torr ranges, depending upon the gauge model. The X-ray limit refers to the lowest pressure indication that may be obtained in a gauge when all the output current is due to X-ray induced photoemission and there is an absence of gas. The X-ray limit will be increased as a result of hydrocarbon contamination of the electrodes, since contaminated surfaces release more secondary electrons under X-ray bombardment. Such contamination can generally be removed by thorough degassing of the electrodes.

All gauges listed in this brochure have either tungsten or thoria coated iridium cathodes. The relative advantages and disadvantages of each are tabulated in Figure 2. Sensitivity The sensitivity is defined as follows: This definition makes the sensitivity relatively independent of the electron current and dependent only on gauge construction. In practice, it is often necessary to determine the lowest pressure that can be read with a given gauge tube and controller. To determine this, the lowest readable current sensitivity of the electrometer portion of the controller must be known. Knowing the sensitivity of the gauge tube from the manufacturer’s data, the relationship for ionization current vs. pressure can be determined. Figure 2 — Filament Selection Guide Current measured in amperes. Pressure in Torr. Degassing To reduce the outgassing in a gauge to a negligible level, the process of degassing is employed. For attainment of the lowest pressure levels the glass envelope gauges should be baked at 400°C for 1 hour. The electrodes are degassed by heating to a temperature of 900°C nominally for approximately 15 minutes after baking. The electrode heating is accomplished by either electron bombardment (EB) or by passing a heavy current (I 2R) at low voltage through the grid. In general, gauges equipped with b uilt up g rids; i.e squirrel cage or g rids w elded at m ultiple intersecting components, are degassed only by electron bombardment. The collector may be degassed only by the electron bombardment method. Grids consisting of a helix or double helix, in which both ends of the helix have external connections, are degassed by the heavy current method. All gauges with helical grids may also be degassed by means of electron bombardment. The 274022, 274023, 274041, 274042, 274050, 274057, and 274058 gauges utilize a squirrel cage grid and must use electron bombardment for degassing. To provide a nude gauge suitable for I 2R degassing, the 274028, 274043, and 274053 gauges were developed. Electrical Leakage The accuracy of pressure measurement, which depends on the measurement of currents as low as nanoamperes and below, is affected by leakage. These paths may be classified as either internal or external. Internal and external leakage is held to the lowest possible value in designs such as the tubulated Bayard-Alpert. The collector terminal of the gauge is at the opposite end of the envelope from the grid and cathode terminals. Designs which bring the collector out as one of a series of pins in a base or header are more susceptible to leakage problems than the tubulated design. Internal leakage usually results from the evaporation of tungsten or thoria vapor from the cathode. This is controlled by means of a shield where the collector lead passes through the stem or header insulator. Great care must be exercised in insulating and shielding the lead to the collector terminal on the gauge from the controller. phenomenon tungsten Filament t horia-Iridium Filament Accidental Exposure to Atmosphere Do n ot Use High Tolerance High Oxygen Partial Pressure Easily Oxidized Relatively Immune to Oxidation Chemical Reaction with Gas to be Measured Higher Filament Temperature, High Reaction Rate Lower Filament Temperature, Lower Reaction Rate Corrosive Gas Tolerance Higher Lower

Figure 3 — Operating Characteristics for Series 274 Ionization Gauge Tubes Pumping The ionization gauge exhibits a certain pumping capacity. This is due to both chemical and electrical effects. Chemical pumping is due, in general, to the affinity of gases for very clean surfaces. As the surface becomes saturated, the pumping action is diminished and reaches a steady state value. Readings for a typical glass tabulated gauge will be 20 to 50 percent lower during the period of chemical pumping. The duration of the pumping may be of the order of four hours at 1 x 10 -9 Torr. Electronic or ionic pumping saturates after pumping approximately three months at 1 x 10 -9 Torr. The most common remedy for pumping effects is to provide a passage of large conductance between the gauge and the vacuum system. A third mechanism of pumping in gauges involves chemical reactions with the hot cathode. A number of gases such as oxygen, nitrogen, water vapor, and hydrogen have been shown to react with the carbon present in tungsten. The ratio of these reactions are dependent on the cathode temperature, and are low enough that serious errors in measurement can be avoided when high-conductance connections are used. In small systems, however, the change in gas composition may be significant. Several gauge configurations are available to cope with the pumping phenomenon. The glass envelope gauge equipped with 3/4-inch tubulation, such as the 274002, has adequate conductance for use down to the 10 -8 Torr range. A gauge for use down to the 10-10 Torr range is available with 1-inch tubulation (274015). To compensate for the high electronic pumping speed, all internally shielded gauges should be specified with 1-inch tubulations. n ude gauges are the best solution to severe ionization gauge pumping problems. With the nude geometry, the gauge elements can be positioned directly into the vacuum chamber, thereby eliminating the pressure differential normally associated with a tubulated gauge. The time response of this gauge system is greatly reduced as compared to the tubulated gauge. Compar ing the requirements of a given application to the items above will permit the selection of the most appropriate gauge for the purpose.

r esistance Heated (I2r) d egassable n ude Ionization Gauge Model Benefits • Non-sag bifilar g rid

  • Burn-out resistant filament or dual tungsten filaments
  • Tubulation— 3/4-inch or 1-inch diameters in K ovar®, Pyrex or flanged
  • I 2R or electron bombardment degas
  • Mates with standard electr ic connector
  • Cage g rid
  • Burn-out resistant filament or dual tungsten filaments Electron bombardment degas Mates with standard flanged connector
  • Replaceable filament Non-sag bifilar g rid
  • Burn-out resistant filament I2R or electron bombardment degas
  • Mates with standard electr ic connector
  • Replaceable filament physical data Tubulation 3/4 in. (19mm) or 1 in. (25 mm) dia. x 2 1/4 in. (57 mm) long, Kovar, Pyrex, or flanged n /A n /A Envelope n onex 7720 glass, 2 1/4 in. (57 mm) dia. x 5 in. (127 mm) long n ude n ude Mounting Position Any, Vertical preferred Any Any Filament Dual tungsten or one thoria coated iridium Dual tungsten or dual thoria coated iridium on replaceable assembly Thoria coated iridium on replaceable assembly Grid Refractory Metals Refractory Metals Refractory Metals Overall Length 6 in. (152 mm) 4 1/8 in. (105 mm) 4 1/8 in. (105 mm) Insertion Length n /A 3 in. (76mm) 3 in. (76mm) operating data Sensitivity for n2 10/Torr 25/Torr 10/Torr Typical Accuracy ±20% ±20% ±20% X-ray Limit About 3 x 10-10 Torr About 2 x 10-11 Torr About 4 x 10-10 Torr Electron Bombardment Degas 100 watts max. 40 watts max. 100 watts max. 70 watts nominal Resistance Heated Degas (l 2R) 6.3 to 7.5 VAC at 10 A n /A 6.3 to 7.5 VAC at 10 A Bakeout 450°C 450°C 450°C Filament - Heating Current 4 to 6 A 2.5 to 3.5 A 4 to 6 A Filament - Heating Voltage 3 to 5 V 3 to 5 V 3 to 5 V Filament - Voltage Potential +30 VDC +30 VDC +30 VDC Collector Potential 0 V 0 V 0 V Grid Potential +180 VDC +180 VDC +180 VDC Specifications

Series 274 Tubulated (Glass) Gauges Connector for Tubulated Gauges Series 274 Nude Gauges Base view of an I2R degassable gauge Base view of an EB degassable gauge Base view of a thoria-iridium type gauge Base view of a dual tungsten type gauge Grid Grid Grid CollectorCollector Filament Not used Filament #1 Filament #1 Filament #2 Grid I2R & EB A C D EH FilamentFilament #2 GridGridGrid Filament #2Filament #1 Grid I2R Filament Common Filament No Connection Collector This could be a Separate Cable Pin Guard With Pin Guard WithOut Pin Guard Cable Connections — Series 274 tubulated glass and nude gauges shown

Single thoria-coated iridium filament 3/4 inch Pyre x glass inlet port 274002 3/4 inch K ovar metal inlet port 274003 1 inch Pyre x glass inlet port 274005 1 inch K ovar metal inlet port 274006 15 mm Pyre x glass inlet port 274036 n W25KF flange with 1 inch port 274032 1.33 inch ( n W16CF) non-rotatable, 3/4 inch port 274020 2.75 inch ( n W35CF) non-rotatable, 3/4 inch port 274007 2.75 inch ( n W35CF) non-rotatable, 1 inch port 274008 Dual tungsten filaments 3/4 inch Pyre x glass inlet port 274012 3/4 inch K ovar metal inlet port 274013 1 inch Pyre x glass inlet port 274015 1 inch K ovar metal inlet port 274016 15 mm Pyre x glass inlet port 274037 1.33 inch ( n W16CF) non-rotatable, 3/4 inch port 274021 2.75 inch ( n W35CF) non-rotatable, 3/4 inch port 274017 2.75 inch ( n W35CF) non-rotatable, 1 inch port 274018 Nude Gauges Single thoria-coated iridium filament (resistive or electron bombardment degas) n W40KF flange with pin guard/locking strain relief 274053 2.75 inch ( n W35CF) non-rotatable 274028 2.75 inch ( n W35CF) non-rotatable, pin guard/locking strain relief 274043 Replacement filament 274029 UHV Nude Gauges Dual thoria-coated iridium filament (electron bombardment degas only) n W40KF flange with pin guard/locking strain relief 274058 2.75 inch ( n W35CF) non-rotatable 274023 2.75 inch ( n W35CF) non-rotatable, pin guard/locking strain relief 274042 Replacement filaments 274025 Dual tungsten filaments (electron bombardment degas only) n W40KF flange 274050 n W40KF flange, pin guard/locking strain relief 274057 2.75 inch ( n W35CF) non-rotatable 274022 2.75 inch ( n W35CF) non-rotatable, pin guard/locking strain relief 274041 Replacement filaments 274024 Accessories Glass tub ulated test gauge with thoria-coated iridium filament, pumped down to appro ximately 10-5 Torr and sealed off. nOT to be used as a pressure reference. 274031 Ordering Inf ormation

© 2014-2018 MKS Instruments, Inc. All rights reserved. MKS products provided subject to the US Export Regulations. Diversion or transfer contrary to US law is prohibited. Specifications are subject to change without notice. Granville-Phillips® is a registered trademark, and mksinst™ is a trademark of MKS Instruments, Inc. ConFlat ® is a registered trademark of Varian Associates. Kovar® is a registered trademark of Carpenter Technology Corporation. MKS Instruments, Inc. Pr essure & Vacuum Measurement Solutions

6450 Dry Creek Parkway

Longmont, CO 80503 Tel: 303.652.4400 MKS Instruments, Inc. Global Headquarters

2 Tech Drive, Suite 201

Andover, MA 01810 Tel: 978.645.5500 T el: 800.227.8766 (in USA) Web: www.mksinst.com