344 Vacuum Gauge Controller Manual (344005)

Transcript

Series 344 Granville-Phillips® Series 344 Vacuum Gauge Controller Instruction Manual Instruction manual part number 344005 Revision B - January 2015 16 Series 344 Granville-Phillips® Series 344 Vacuum Gauge Controller This Instruction Manual is for use with the following GranvillePhillips catalog numbers: 344001, 344002, 344003, 344011, 344012, 344013, 344014, 344015, 344016, 344017, 344018 Customer Service/Support For Customer Service or Technical Support 24 hours per day, 7 days per week, every day of the year including holidays: Phone: +1-800-227-8766 or +1-303-652-4691 MKS, Granville-Phillips Division 6450 Dry Creek Parkway Longmont, CO 80503 USA Phone: FAX: Email: 1-303-652-4691 or 1-800-776-6543 1-303-652-2844 [email protected] Corporate Office MKS Instruments, Inc. 2 Tech Drive, Suite 201 Andover, MA 01810 USA Phone: 1-978-645-5500 www.mksinst.com Instruction Manual © 2015 MKS Instruments, Inc. All rights reserved. Granville-Phillips® and Stabil-Ion® are registered trademarks of MKS Instruments, Inc. All other trademarks and registered trademarks are the properties of their respective owners. RECEIVING INSPECTION On receipt of your equipment, inspect all material for damage. Confirm that the shipment includes all items ordered. If items are missing or damaged, submit a claim as stated below for a domestic or international shipment, whichever is applicable. If materials are missing or damaged, the carrier that made the delivery must be notified within 15 days of delivery, or in accordance with Interstate Commerce regulations for the filing of a claim. Any damaged material including all containers and packaging should be held for carrier inspection. Contact our Customer Service Department, 6450 Dry Creek Parkway, Longmont, Colorado 80503, telephone (303) 652-4400, if your shipment is not correct for reasons other than shipping damage. INTERNATIONAL SHIPMENT Inspect all materials received for shipping damage and confirm that the shipment includes all items ordered. If items are missing or damaged, the airfreight forwarder or airline making delivery to the customs broker must be notified within 15 days of delivery. The following illustrates to whom the claim is to be directed. • If an airfreight forwarder handles the shipment and their agent delivers the shipment to customs, the claim must be filed with the airfreight forwarder. • If an airfreight forwarder delivers the shipment to a specific airline and the airline delivers the shipment to customs, the claim must be filed with the airline. Any damaged material including all containers and packaging should be held for carrier inspection. If your shipment is not correct for reasons other than shipping damage, contact our Customer Service Department, 6450 Dry Creek Parkway, Longmont, Colorado 80503, U.S.A., or telephone (303) 652-4400. LIMITED WARRANTY These Granville-Phillips products are warranted against defects in materials and workmanship for one year from the date of shipment provided the installation, operating and preventive maintenance procedures specified in this instruction manual have been followed. Granville-Phillips will, at its option, repair, replace or refund the selling price of the product if Granville-Phillips determines, in good faith, that it is defective in materials or workmanship during the warranty period, provided the item is returned to GranvillePhillips together with a written statement of the problem. Defects resulting from or repairs necessitated by misuse or alteration of the product or any cause other than defective materials or workmanship are not covered by this warranty. GRANVILLE-PHILLIPS EXPRESSLY DISCLAIMS ANY OTHER WARRANTY, WHETHER EXPRESSED OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. UNDER NO CIRCUMSTANCES SHALL GRANVILLE-PHILLIPS BE LIABLE FOR CONSEQUENTIAL OR OTHER DAMAGES RESULTING FROM A BREACH OF THIS LIMITED WARRANTY OR OTHERWISE. WARNING READ THIS INSTRUCTION MANUAL BEFORE INSTALLING, USING, OR SERVICING THIS EQUIPMENT. IF YOU HAVE ANY DOUBTS ABOUT HOW TO USE THIS EQUIPMENT SAFELY, CONTACT THE GRANVILLE-PHILLIPS CUSTOMER SERVICE DEPARTMENT AT THE ADDRESS LISTED ON THIS MANUAL. DANGER, HIGH VOLTAGE 180V is present in the power supply, on the cable, and at the ion gauge tube when the tube is turned on. EXPLOSIVE GASES Do not use the 344 Vacuum Gauge Controller to measure the pressure of explosive or combustible gases or gas mixtures. Ionization gauge filaments operate at high temperatures. IMPLOSION AND EXPLOSION Glass ionization gauges if roughly handled may implode under vacuum causing flying glass which may injure personnel. If pressurized above atmospheric pressure, glass tubes may explode. A substantial shield should be placed around vacuum glassware to prevent injury to personnel. Danger of injury to personnel and damage to equipment exists on all vacuum systems that incorporate gas sources or involve processes capable of pressurizing the system above the limits it can safely withstand. For example, danger of explosion in a vacuum system exists during backfilling from pressurized gas cylinders because many vacuum devices such as ionization gauge tubes, glass windows, glass bell jars, etc, are not designed to be pressurized. Install suitable devices that will limit the pressure from external gas sources to the level that the vacuum system can safely withstand. In addition, install suitable pressure relief valves or rupture discs that will release pressure at a level considerably below that pressure which the system can safely withstand. Confirm that these safety devices are properly installed before installing the 344 Vacuum Gauge Controller. In addition, check that (1) the proper gas cylinders are installed, (2) gas cylinder valve positions are correct on manual systems, and (3) the automation is correct on automated systems. WARNING Operation of the 344 Vacuum Gauge Controller with line voltage other than that selected by the power supply voltage selector switch can cause damage to the instrument and injury to personnel. WARNING Do not attach cables to glass gauge pins while the gauge is under vacuum. Accidental bending of the pins may cause the glass to break and implode. Cables once installed should be secured to the system to provide strain relief for the gauge tube pins. WARNING Safe operation of vacuum equipment, including the 344 VGC, requires grounding of all exposed conductors of the gauges and the controller and the vacuum system. LETHAL VOLTAGES may be established under some operating conditions unless correct grounding is provided. Research at Granville-Phillips has established that ion producing equipment, such as ionization gauges, mass spectrometers, sputtering systems, etc., from many manufacturers may, under some conditions, provide sufficient conduction via a plasma to couple a high voltage electrode to the vacuum chamber. If conductive parts of the chamber are not grounded, they may attain a potential near that of the high voltage electrode during this coupling. Potentially fatal electrical shock could then occur because of the high voltage between these chamber parts and ground. During routine pressure measurement using ionization gauge controllers from any manufacturer, about 160V may become present on ungrounded chambers at pressures near 10-3 Torr. All isolated or insulated conductive parts of the chamber must be grounded to prevent these voltages from occurring. Grounding, though simple, is very important! Please be certain that the ground circuits are correctly utilized, both on your ion gauge power supplies and on your vacuum chambers, regardless of their manufacturer, for this phenomenon is not peculiar to Granville-Phillips equipment. Refer to Safety Instructions and Section 2.4, Installation, for additional information. If you have questions, or wish additional labels or literature, please contact one of our technical personnel. Table of Contents Page Chapter 1 Introduction 1-1 Chapter 2 VGC Installation and Operation 2.1 AC Line Voltage 2.2 Mounting 2.3 Ionization Gauge Types and Installation 2.4 System Ground Test Procedure 2.5 Control and Status Connector 2.6 Degas 2.7 Emission Current Adjustment 2.8 Ion Gauge Cables 2.9 Electrometer Operation 2.10 Protective Shutdowns 2-1 2-2 2-2 2-3 2-4 2-4 2-7 2-8 2-9 2-12 Chapter 3 VGC Calibration 3.1 Introduction 3.2 Relative Gas Sensitivities 3.3 Adjusting Emission Current 3.4 Electrometer Calibration 3-1 3-3 3-3 3-4 Chapter 4 Theory of Operation 4.1 Ion Gauge Theory 4.2 Vacuum Gauge Controller 4-1 4-2 Chapter 5 Specifications 5-1 Chapter 1 Introduction General Description The 344 Vacuum Gauge Controller (VGC) provides power, control, biasing and ion current measurement for up to eight hot filament ionization gauges. All gauges can be operated simultaneously while one gauge at a time can be degassed using resistance 2 grid heating (I R). Control and status of the gauges are electrically isolated logic inputs/outputs and pressure is monitored via analog outputs. All signals are through a 40 pin header connector on the rear panel. The 344 VGC is packaged in a 19 inch x 5.25 inch high rack mount chassis. The chassis is a card cage accommodating one I/O (input/output) module and a maximum of eight IG modules. The IG modules may be removed through the front panel without removal of the chassis from the instrument rack. Blank modules are available for unused spaces. Components 344001 - Vacuum Gauge Controller Basic. This is comprised of the chassis, power transformer, bus circuit board and I/O module. It requires one IG module for each ion gauge to be operated. 344002 - Ion Gauge Module. The IG Module houses circuitry to control the emission current and grid voltage of the gauge as well as the electrometer circuit for measuring ion current. Protective shutdowns for filament over-power, grid over-current and overpressure also are on the IG module. The filament protection fuse is on the front panel while the Ion Gauge connector is on the rear. 344003 - I/O Module. The Input/Output Module circuits electrically isolate external logic signals from internal logic levels and ground. Logic decoding and line voltage selection are also provided here. The front panel holds the grid and degas fuses. The I/O connector to the system control is on the rear panel. 344004 - Blank Module. This module fills a space not occupied by an IG Module. It provides a cosmetic function as well as a safety barrier when fewer than eight IG Modules are used. Chapter 2 VGC Installation and Operation 2.1 Line Voltage Selection The 344 Vacuum Gauge Controller (VGC) is designed to operate from 115 VAC nominal line voltage, 50 to 60 Hertz. Verify power source before connecting the controller. WARNING Operation of the VGC with incorrect line voltage applied can cause damage to the equipment and injury to personnel. 2.2 Mounting The 344 VGC can be easily rack mounted in a standard 5.25 inch space using supplied hardware. The chassis is ventilated by vents on each side. These areas must be free of obstructions and access should be provided to the connectors and fuse holder on the rear panel and to the fuse holders on the front. Ambient air temperature should not exceed o o 50 C (122 F). 2.3 Ionization Gauge Types And Installation WARNING Do not attach cables to glass gauge pins while the gauge is under vacuum. Accidental bending of the pins may cause the glass to break and implode. Cables, once installed, should be secured to the system to provide strain relief for the gauge tube pin. The 344 VGC is designed to operate 8 Bayard-Alpert type or equivalent ionization gauges. Coated iridium filament type gauges are recommended since at higher pressures iridium filaments provide longer operating life and greater burnout resistance. When installing your ion gauge, note that if placed near the pump, the pressure in the gauge may be considerably lower than in the rest of the system. If placed near a gas inlet or source of contamination, the pressure in the gauge may be higher. If an unshielded gauge is placed near an electron beam evaporation source or used in a sputtering system, spurious electrons or ions may disturb the measurement. Screens or other shielding should be placed between the gauge and the system if spurious charged particles or severe electromagnetic interference is present. Consideration should also be given to electrostatic shielding of glass tubulated gauges when measuring pressures near their x-ray limits. 2.4 System Ground Test Procedure (Refer to the Safety Instructions for further information) Procedure: Physically examine the grounding of both the 344 VGC and the vacuum chamber. Is there an intentional heavy duty ground connection to all exposed conductors on the vacuum chamber? There should be. Note that an "O" ring or "L" ring gasket, without metal clamps, can leave the chamber on one side electrically isolated. Power can be delivered to mechanical and diffusion pumps without any ground connections to the system frame or chamber. Water line grounds can be lost by a plastic or rubber tube interconnection. What was once a carefully grounded vacuum system can, by innocent failure to reconnect all ground connections, become a very dangerous device. Use the following procedure to test each of your vacuum systems which incorporates an ionization gauge. This procedure uses a conventional Volt-Ohm Meter (VOM) and Resistor (10 ohm, 10 watt). 1. With the gauge controller turned off, test for both dc and ac voltages between the metal parts of the vacuum chamber and the power supply chassis. 2. If no voltages exist, measure resistance. The resistance should not exceed 2 ohms. Two ohms, or less, implies commonality of these grounds that should prevent the plasma from creating a dangerous voltage between them. This test does not prove that either connection is earth ground, only that they are the same. If more than 2 ohms is indicated, check with your electrician. 3. If ac or dc voltages exist and are less than 10 volts, shunt the meter with a 10 ohm, 10 watt resistor. Repeat the voltage measurement. With the shunt in place across the meter, if the voltage remains at 83% or more of the unshunted value, commonality of the grounds is implied. Repeat the measurements several times to be sure that the voltage ratio is not changing with time. If Voltage(shunted) = .83moremore, Voltage(unshunted) this should prevent the plasma from creating a dangerous voltage between these grounds. If more than 10 volts exists between grounds, check with your electrician. 4. If the voltage change in step 3 is greater than 17% due to the placement of the shunt, it complicates the measurement. The commonality of the grounds may be satisfactory and the coupling poor, or the commonality could be poor! Your electrician should be asked to check the electrical continuity between these two ground systems. The placement of a second ground wire (dashed line in the illustration below) between the vacuum chamber and the ion gauge controller chassis is not a safe answer, for large currents could flow through it. Professional help is recommended. 2.5 Control and Status Connector The 344 VGC is intended as a slave device to a programmable controller, system computer or manual switch panel. All inputs and outputs are electrically isolated from internal circuitry, thus an external power supply must provide 24 Vdc to implement the control inputs and status outputs. Connections to the I/O of the 344 VGC are via a 40 pin header connector on the rear panel of the I/O Module. The logic states shown are positive true logic and the voltages are with reference to the 24 volt Logic Power Supply negative terminal. 2.6 Degas One gauge tube may be degassed at a time. The gauge is selected with the three Degas Select inputs and the Degas Enable input according to the table below. Again, the logic is positive true and voltages reference to the Logic Power Supply negative. Note that Degas Enable must go to logic 1 state (no gauge selected) when switching degas among gauges to avoid ambiguous logic combinations. Also, the 344 VGC requires that the IG filament be on in order to degas the gauge. Pressure within the gauge is measured by the electrometer during degas. It is strongly -5 recommended that the pressure be below 5 x 10 Torr when degas is switched on. This pressure is sufficiently low to pump degassed contaminants from the gauge elements as they are heated. Function Filament On/Off (Input) Emission Current Select (Input) Degas Select (Input) (See Fig. 2.5) Pin IG1 IG2 IG3 IG4 IG5 IG6 IG7 IG8 1 3 5 7 9 11 13 15 IG1 IG2 IG3 IG4 IG5 IG6 IG7 IG8 4 8 12 16 19 23 20 24 Bit 3 Bit 2 Bit 1 26 27 25 Most Significant Bit (4) (2) Least Significant Bit (1) 28 Low = Enable Must go to Hi while Degas Select data is changing. + - 40 39 24 Vdc ± 10% Isolated from internal supplies and analog ground IG1 IG2 IG3 IG4 IG5 IG6 IG7 IG8 2 6 10 14 17 21 18 22 Degas Enable (Input) (see Fig. 2.5) Logic Power Supply (Input) Gauge Status (Output) Analog Ground Electrometer Analog Outputs Comments 29,30 IG1 IG2 IG3 IG4 IG5 IG6 IG7 IG8 31 32 33 34 35 36 37 38 Low = 0 V or short to Logic Supply negative terminal (pin 39) Hi = 24 V or open circuit (internal passive pull-ups provided) Low = On Low = 1 mA Emission Hi = 0.1 mA Emission Low = On *Important* This Must Be Isolated From Logic Supply For Input/Output Isolation -v Pressure = 10 Refer to Fig. 2.9, 2.10, 2.11 Function Degas Sel 3 Degas Sel 2 Degas Sel 1 Degas Enable Pin # 26 27 25 28 Degas Selected IG1 IG2 IG3 IG4 IG5 IG6 IG7 IG8 None 1 1 1 0 0 0 0 1 X 1 0 0 1 1 0 0 1 X 0 1 0 1 0 1 0 1 X 0 0 0 0 0 0 0 0 1 0 = Low Input = 0 Vdc or short to logic supply negative terminal 1 = Hi Input = 24 Vdc or open circuit (internal passive pull-ups are provided. X = Don't care = 0 or 1 NOTE: IG filament must be on to enable degas for that gauge. NOTE: Degas enable must go to logic 1 (no gauge selected) when changing Degas Select inputs to avoid ambiguous logic combinations. 2.7 Emission Current Adjustment The 344 VGC is factory calibrated to operate gauges with a sensitivity of 10/Torr for nitrogen gas. This calibration is valid for both glass tubulated (encapsulated) and nude 2 Bayard-Alpert ionization gauges with I R degassable grids. (The grids of these gauges are typically configured as a bifilar helix). When using a gauge with a sensitivity different than 10/Torr or when measuring a gas other than N2 consult paragraphs 3.1 through 3.3 of this manual to compensate for this difference. The emission adjust potentiometer is located at the top edge of the IG Module. It is accessed by either connecting the Module to the chassis with an extender board or by removing the top cover of the chassis. 2.8 Ion Gauge Cables Ionization gauge connections are accessed at the rear panel on four pin, twist lock connectors, AMP #206430-1. Granville-Phillips can supply cables for glass tubulated or nude ion gauges in lengths of 10 to 60 feet. In special installations as where the cable must route through bulkheads or conduit, it may be necessary to build a custom cable to suit your needs. The illustrations below show the gauge tube connections and cable wiring for standard Bayard-Alpert type gauges. The coaxial cable from the gauge tube collector to the VGC electrometer is bundled within the gauge cable (in Granville-Phillips cables). If necessary the coax could be routed separately; however, it is advisable to route the collector cable parallel with the gauge cable to minimize inductively coupled ground currents which may cause pressure reading errors. Cable for Glass Tubulated Ion Gauge (Bayard-Alpert) Cable for Nude Ion Gauge 2.9 Electrometer Operation The electrometer circuitry of the IG Module is a logarithmic amplifier which produces an output voltage proportional to the logarithm (base ten) of the gauge pressure. For a 10/Torr gauge with emission current of 1.0 mA V out = - log10 P oreP = 10V out For a 10/Torr gauge with emission current of 0.1 mA (1-V out ) V out = 1 - log10 P oreP = 10 The electrometer analog output is accessed on the rear panel connector of the I/O Module. Refer to figures 2.3 and 2.4. The illustrations below show Vout versus pressure for 1 mA and 0.1 mA emission currents in tabular and graphical formats. If emission current is other than 1 or 0.1 mA or if gauge sensitivity is not 10/Torr, refer to section 3.1 to calculate resultant ion current. Analog output voltage is related to ion current by: V out = -(2 + LogI + ) EMISSION CURRENT (I-) = 1 mA PRESSURE (TORR) Gauge Off -9 1 x 10 -8 1 x 10 -7 1 x 10 -6 1 x 10 -5 1 x 10 -4 1 x 10 -3 1 x 10 -3 2 x 10 ION CURRENT (I+) (AMPERES) 0 -11 1 x 10 -10 1 x 10 -9 1 x 10 -8 1 x 10 -7 1 x 10 -6 1 x 10 -5 1 x 10 -5 2 x 10 VOUT (VOLTS) _10.000 9.000 8.000 7.000 6.000 5.000 4.000 3.000 2.699 Electrometer Response, I- = 1 mA, S = 10/Torr EMISSION CURRENT (I-) = 0.1 mA PRESSURE (TORR) Gauge Off -8 1 x 10 -7 1 x 10 -6 1 x 10 -5 1 x 10 -4 1 x 10 -3 1 x 10 -2 1 x 10 -2 1 x 10 ION CURRENT (I+) (AMPERES) 0 -11 1 x 10 -10 1 x 10 -9 1 x 10 -8 1 x 10 -7 1 x 10 -6 1 x 10 -5 1 x 10 -5 1 x 10 VOUT (VOLTS) _10.000 9.000 8.000 7.000 6.000 5.000 4.000 3.000 2.699 Electrometer Response, I- = 0.1 mA, S = 10/Torr 2.10 Protective Shutdowns The 344 VGC provides three detection circuits which limit damaging conditions in the ion gauge tube. Any of these protective circuits will shut off the affected gauge but will not affect operation of the remaining seven gauges. The protective circuitry is located on the IG Module. Overpressure Shutdown This circuit compares the electrometer output with a fixed reference voltage. Thus the overpressure shutdown is at a fixed ion (collector) current (100 µA). The gauge pressure for this ion current is dependent on the selected emission current since P= 1 I+ S I- where P = Pressure (Torr) S = Gauge sensitivity (1/Torr) I+ = Ion current (Amperes) I- = Emission current (Amperes) -2 For an emission current of 1 mA (10/Torr gauge), the gauge will shut off at 1 x 10 Torr. -1 For an emission current of 0.1 mA, the gauge will shut off at 1 x 10 Torr. Filament Overvoltage Shutdown This circuit indirectly measures the RMS voltage applied to the gauge filament and shuts off the gauge if this exceeds a limit. This situation can occur if the gauge is not connected to the VGC or if the pressure in the gauge is so high that no emission current can flow (e.g., atmospheric pressure) or if the emissive coating on the filament has eroded. Since the filament power control circuit responds relatively slowly (to prevent thermal oscillation) the overvoltage shutdown may take several seconds to respond to an error condition. Grid Overcurrent Shutdown Each IG Module employs a circuit to turn off power to the gauge if the grid current exceeds a fixed limit of 15 milliamperes. Such a condition implies that a short circuit or shunt has occurred in the gauge or the cable. The circuit responds very quickly to an overcurrent condition removing grid and filament voltage. Chapter 3 VGC Calibration 3.1 Introduction The 344 VGC is factory calibrated to measure pressure of nitrogen gas using gauge tubes with sensitivity of 10/Torr. To conveniently read the pressure in gauges of a different sensitivity or of other gases, it is necessary to adjust the emission current in the gauge. The adjustment is described in paragraph 3.3. Calibration of the electrometer circuitry is a delicate procedure requiring precision laboratory instruments. It is unlikely that this calibration will change over time; however, the procedure is detailed in paragraph 3.4. Gauge Sensitivity An ionization gauge determines pressure by measuring the ratio of two currents in the gauge where: Pressure = 1 I+ 1 collector current = S emission current S I- S = gauge sensitivity constant Sensitivity, S, is a value determined by design and measured empirically under controlled conditions. A typical Bayard-Alpert ionization gauge, such as the Granville-Phillips 274, measuring nitrogen gas has a sensitivity of 10/Torr. Several companies, in addition to the National Institute of Science and Technology, offer the service of measuring the sensitivity of a gauge. This may be necessary for precise low pressure measurement although manufacturer's specified sensitivity is sufficient for the vast majority of applications. Since the electrometer measures the collector current, I+, of the above equation with a fixed gain, variation of gauge sensitivity may be compensated by changing the emission current, I-, or by mathematically manipulating the electrometer output voltage to know the pressure. The examples below illustrate these two methods. Example: In using the 344 VGC, I would like to measure pressure with a gauge which has a sensitivity of 8/Torr instead of the calibrated 10/Torr. I have two ways to accomplish this: 1) For the electrometer to have the same output voltage as for a 10/Torr gauge (i.e., indicate the same pressure): Collector current (10/Torr) = Collector current (8/Torr) or: I + (10) = I + (8)at Pressure = P where I+ (10) = collector current for a 10/Torr gauge. From the ion gauge equation: P= 1 I + (8) 1 I + (10) = 8 I - (8) 10 I - (10) re-arranging: I - (8) = 1.25 I -(10) Therefore, to make the 8/Torr gauge read the same electrometer output voltage as a 10/Torr gauge at any pressure, I can adjust the emission current to 1.25 · 1.0 mA = 1.25 mA. Pretty easy, huh? 2) If I wish to read pressure for the 8/Torr gauge from my system control computer I can do so without adjusting emission current by applying some simple software math. In this case at pressure, P: P= 1 I + (8) 1 I + (10) = as above 8 I - (8) 10 I - (10) where, I + (10) = collector current for a 10/Torr gauge In this case, though, I will leave the emission current the same, i.e., I- (8) = I- (10), so, 1 I + (8) 1 I + (10) = 8 I - (10) 10 I - (10) re-arranging: I + (10) = 10 = 1.25 8 I + (8) So the collector current from the 10/Torr gauge is 1.25 times the collector current from a 8/Torr gauge at any pressure. Since the voltage output of the electrometer -V is a logarithmic relationship: (for I+ = 1.0 mA) P = 10 , the pressure in an 8/Torr -V gauge as output from a 10/Torr calibrated electrometer is simply: P = 1.25 x 10 . Hence, I will put a gauge calibration factor of 1.25 in my pressure calculation software. 3.2 Relative Gas Sensitivities Sensitivity depends on the gas being measured as well as the type of IG tube. The table below lists the relative gauge sensitivities for common gases. These values are from NASA Technical Note TND 5285, "Ionization Gauge Sensitivities as Reported in the Literature", by Robert L. Summers, Lewis Research Center, National Aeronautics and Space Administration. Refer to this technical note for further definition of these average values and for the gauge sensitivities of other gases. To calculate the sensitivity Sx for gas type x: S x = R x . SN 2 Where SN2 is the gauge sensitivity for N2 and Rx is: GAS He Ne D2 H2 N2 Air 02 Rx 0.18 0.30 0.35 0.46 1.00 1.00 1.01 GAS H20 N0 Ar C02 Kr SF6 Xe Rx 1.12 1.16 1.29 1.42 1.94 2.50 2.87 To correct for change in sensitivity due to gas composition, apply the corrections as shown in the preceding example. 3.3 Adjusting Emission Current To set emission current in the 344 VGC, extend the appropriate IG Module on an extender board or remove the chassis top cover by loosening the two screws on the back panel at the top. Locate the emission adjust potentiometer. Connect a voltmeter (2 volts full scale) across CR6 located to the left of the pot. 1.0 volt across the diode is equivalent to 0.1 mA emission with the Emission Current Select input (of I/O connector) in the logic high or open circuit state. 1.0 volt across the diode is equivalent to 1.0 mA emission with the Emission Current Select input in the low state. Adjust the potentiometer to give the emission current required. The range of adjustment is 0.85 to 1.35 volts. 3.4 Electrometer Calibration - The electrometer of the IG Module is calibrated at the factory for accuracy between 1 x 10 9 Torr and 1 x 10-3 Torr. Normally, no further adjustment is required; however, the following procedure may be used to re-calibrate. The electrometer has three potentiometers to adjust: OFFSET, SCALE, and ZERO. These pots are labeled on the circuit board silkscreen as R32, R33 and R34. Equipment required: DC picoampere current source - 10 pA to 10 µA (e.g., Keithley 220) Cable for current source to electrometer input Digital voltmeter, 4-1/2 digit Extender card for IG Module Adjusting screwdriver Procedure: 1) Connect cable from current source to the IG coax input. 2) Connect DVM to IG analog output and ground (refer to Fig. 3.3) of the I/O module connector. 3) Connect an ion gauge simulator or an ion gauge to the IG gauge connector and turn on the gauge. 4) -8 Set current source to 1 x 10 Amp (10 nA). 5) Adjust IG OFFSET pot (R32) to give a DVM reading of 6.000 V ± 0.002 V. 6) -6 Set current source to 1 x 10 Amp (1 µA). 7) Adjust IG SCALE pot (R33) to give a DVM reading of 4.000 V ± 0.002 V. 8) -11 Set current source to 1 x 10 Amp (10 pA). Note: Reduce the current slowly in the picoampere range to avoid saturating the amplifier. 9) Adjust IG ZERO pot (R34) to give a dvm READING OF 9.00 v ± 0.010 v. The response of the logarithmic amplifier is slow in the picoamp range. 10) Set the current source to even decade values (1 x 10 , 1 x 10 , etc.) and verify that the analog output responds -1 volt per decade (8.000, 7.000, etc.). Refer to the Emission Current Tables in Chapter 2 for input currents versus output voltages. -10 Input IG1 IG2 IG3 IG4 IG5 IG6 IG7 IG8 Analog Output* Pin 31 Pin 32 Pin 33 Pin 34 Pin 35 Pin 36 Pin 37 Pin 38 *Output voltages relative to analog ground (pins 29 and 30) -9 Chapter 4 Theory of Operation 4.1 Ion Gauge Theory The functional parts of a typical ionization gauge are the filament (cathode), grid (anode) and ion collector. These electrodes are maintained by the gauge controller at +30, +180, and 0 volts, relative to ground, respectively. The filament is heated to a high temperature so that electrons are emitted and accelerated toward the grid by the potential difference between the grid and filament. All the electrons eventually collide with the grid, but many first traverse the region inside the grid one or more times. When an energetic electron collides with a gas molecule, an electron may be dislodged from the molecule, leaving it with a positive charge. Most ions are then attracted to the collector. The rate at which electron collisions with molecules occur is proportional to the density of gas molecules, and hence the ion current is proportional to the gas density (or pressure, at constant temperature). The amount of ion current for a given emission current and pressure depends on the ion gauge design. This gives rise to the definition of ion gauge "sensitivity", denoted in this manual by "S". S = ion current / (emission current x pressure) Bayard-Alpert type gauges typically have sensitivities of 10/Torr when used with nitrogen or air. Sensitivities for other gases are given in Chapter 3. The ion gauge controller varies the heating current to the filament to maintain a constant electron emission, and measures the ion current to the collector. The pressure is then calculated from these data. 2 2 Ion gauge degas is accomplished by resistance heating (I R). During I R degas, a large current is passed through the grid structure, raising its temperature and driving off 2 contaminants. Note that some ion gauge designs do not allow I R degas. 4.2 Vacuum Gauge Controller The 344 VGC contains the following circuit blocks to correctly bias and control a hot filament ionization gauge and measure ion current. Low voltage supply - provides ± 12 volts for logic and op amp power and biasing within the power supply. The common of this supply is "Analog Ground" which is isolated from the +24 volt logic input. This supply consists of two full wave, center-tapped rectifiers, filter capacitors and fixed linear regulators. Grid bias supply - provides +180 Vdc for biasing the grids of the ionization gauges. It consists of a full wave rectifier, filter capacitor and series pass regulator. Additionally, each gauge has an overcurrent sense circuit which couples to the filament logic circuit to switch off the gauge in a fault condition and a relay to isolate the grid from its supply when the filament is off. Degas supply/control - selects the gauge to be degassed based on the "degas Select" and "Degas Enable" logic inputs from the user. The circuit isolates these logic inputs and decodes the bit pattern to select none or one of the IG's. A relay of the selected gauge then applies 9 volts AC to the grid of the tube causing the temperature of the gauge to increase thus desorbing impurities from gauge components. Please note that degas control relies on the user's control equipment to determine that the pressure within the gauge is sufficiently low to preclude damage. The gauge filament must be on to enable degas. Filament logic - switches the gauge on or off according to various inputs. The "Filament on/off" signal from the I/O connector is optically isolated and drives the set input of an R-S flip-flop. The reset input comes from three sources: The grid overcurrent sensor, the electrometer overpressure detector, and the filament overvoltage comparator. The latter determines if the filament drive circuit has exceeded specified bounds. When a shut-down signal is received by the filament logic, the filament control circuit is forced to the off state, the grid relay is switched off and the filament status output goes into open collector state. Emission control - circuit senses the emission current flowing from grid to filament in the gauge. This signal is the feedback for the amplifier which controls filament heating power thus precise control of emission current is obtained. The output of the emission control amplifier is compared with the AC line frequency. The resultant output switches a triac to pulse width modulate the AC heating power to the filament. Excessive power to the tube is obviated by the overvoltage comparator sensing the output of the control amplifier and shutting off the triac drive signal. Electrometer - is a logarithmic current input - voltage output amplifier. As with most log amplifiers, the electrometer uses a p-n junction as a feedback impedance with a matching junction for temperature compensation. A subsequent amplifier adds an offset to produce the -1 volt per decade of input current. Chapter 5 Specifications 344001 Vacuum Gauge Controller Physical Width Height Depth Weight 18.95 in. 5.2 in. 9.0 in. 34 lbs. Electrical Voltage Frequency Power Primary Fuse Environmental 105-125 Vac 50 to 60 Hz 400 watts maximum 4.0 A S.B. o 0 to 50 C operating temperature 0 to 80% RH non-condensing Ion Gauge Emission Current 0.085 to 0.130 mA, or 0.85 to 1.30 mA, adjustable Collector Potential Grid Potential Filament Potential Degas 0 Vdc* +180 Vdc* +30 Vdc* 2 I R (resistance heating) 10 V, 6 A Logic Input/Output Logic Supply (input 24 Vdc ± 10% isolated from internal supplies and analog ground. Filament Control (input) 8 inputs, active low (0 V), optically isolated. Low = filament on. Current: Low 1.7 mA (maximum); High 0 mA. Degas Control (inputs) 4 inputs, active low, optically isolated. Refer to Fig. 2.6 for logic. Current: Low 1.7 mA (maximum); High 0 mA. Emission Select (input) 8 inputs, active low, magnetically isolated. Low = 1 mA emission range. Current: Low 12 mA (maximum); High 0 mA. Filament status (output) 8 outputs, active low, open collector, optically isolated. Low = filament on. Current: Low 10 mA (maximum) VCE = 1.0 V (max) @ IC = 2 mA. High 0 mA, V(max) = 30 Vdc. Electrometer Input/Output Coaxial inputs Signal outputs 10 pA to 1 mA 0 to 9 Vdc Logarithmic, -1 volt per decade of pressure. 9 V = 1 -9 -3 x 10 Torr...3 V = 1 x 10 Torr. (10/Torr gauge at 1 mA emission). Electronic accuracy ± 3% of reading at 25 C ± 5 C (typical). AC Line 4.0 A, slow blow, 250 V, 3 AG Littelfuse, 313004 0.15 A, slow blow, 250 V, 3 AG, Littelfuse, 313.150 6 A, normal blow, 250 V, 3 AG, Littelfuse, 312006 10 A, slow blow, 250 V, 3 AG, Littelfuse, 313010 o o Fuses Grid Filaments Degas *Voltages referenced to analog ground.