Feindosiertes Vakuum

Faced with the task of generating arbitrary vacua within a recipient and recording calibration values of examined sensors a new motor driven fine control valve in connection with a LabVIEW software was utilized for the automated pumping station control and recording of measurement series. This new combination allows for proprietary calibrations and inspections of vacuum gauges as well as customerspecific measurements in the range from atmospheric pressure down to 10^?7 mbar while operating resource‐ and time‐saving.     

Kompakt-Vakuum-Meßgeräte mit PC-Anschluß – Windows-Software zur Prozeß-Auswertung

Personal computers, which are almost omnipresent nowadays, offer an economic way for storage, analysis and documentation of measurement data. In the following paragraph a new concept of vacuum gauges together with a corresponding Windows®®-software is presented, that enables the user to analyze various pressure measurements, such as pump diagrams and other process curves. The first group of instruments contains digital vacuum transmitters with an integrated display unit (VTA). They cover pressure ranges from 1600 mbar down to 10^?5 mbar. The second group consists of digital vacuum transmitters with logging capability (VTS). These pressure loggers have appropriately sealed housings and are suitable for being directly inserted into the evacuation volume, e. g. into vacuum packs. After the evacuation process has finished, the pressure data, that have been stored internally by the gauge, can be easily read out and analyzed via PC.     

Vacuum System Evacuation to Base Pressure

A study has been made to compare the times taken to evacuate a turbomolecular and scroll pumped chamber to a base, high vacuum, pressure. A small test header (26.7 litre) was evacuated via two different paths: through a fore-line, switching paths once ‘cross-over’ pressure was reached, and directly through a turbomolecular pump (TMP) where the TMP was not operational until cross-over pressure was reached. When starting the TMP/switching paths at 0.1 mbar, pump-down times were significantly slower through the TMP; the time taken to reach 0.1 mbar increased by between 2 and 8 minutes dependent upon the TMP model (from a 24% increase in time with nEXT300 to a 232% increase for the nEXT240). For pressures below 8 · 10⁻⁶ mbar there was no significant difference in required pumpdown time between the two methods. The variation between methods being less than the ±30% uncertainty in pressure gauge measurement. The increase in pumpdown times to achieve pressures down to 10”⁵ mbar when evacuating through the TMP was seen for all TMP models tested, however a greater difference was seen for a TMP configured with Holweck drag stages than those containing Siegbahn drag stages. The best pumpdown time for pressures down to 10⁻⁵ mbar was achieved when pumping through the TMP whilst allowing the TMP to ‘start’ operation from atmospheric pressure.     

Kennfeldanalyse einer Schraubenvakuumpumpe

Operating Performance of Screw Vacuum Pumps This paper presents detailed analyses of the operating performance of a dry‐running screw vacuum pump. The characteristic parameters, suction speed and final attainable pressure ‐ which primarily define the operating performance of screw vacuum pumps ‐ are explored in experimental and theoretical investigations. Experiment and simulation in combination are used to show the correlation between the main physical and technical characteristics and the operating performance of screw vacuum pumps. This basic knowledge is essential for understanding the specific machine physics of positive displacement vacuum pumps, especially for screw vacuum pumps, and is useful in view of further design and optimization processes. The experiment covers measurements of the operating performance of the investigated isochoric screw vacuum pump working against ambient pressure. As operating parameters intake pressure (1000mbar to 10^‐3mbar) and rotor speed are varied over a wide range. The theoretical analysis of the operating performance contains simplifying models as well as simulations of the thermodynamic processing. The impact of external leakages, clearance vacuum flows and further losses on operating performance are described in detail.     

Modifiziertes Bayard‐Alpert‐ Ionisationsmanometer für Druckmessungen von 10‐9 ... 10+1 mbar

Generally, the requirements for the design and the operating parameters of hot filament ionisation gauges for high and low pressure measurement are incompatible. Nevertheless it is possible to construct an ionisation gauge with a wide measuring range. For that purpose the electrode configuration and its operating parameters of a suitable gauge have to be optimised in this way that the contrary requirements are met with minor restrictions. With the help of computer simulations and experimental investigations a vacuum gauge was developed which combines the main features of both different types of ionisation gauges in one gauge construction. According to our ideas a gauge geometry similar to the ordinary Bayard Alpert gauge enables to measure the pressure in an extended operation range by using not only the pressure dependence of the formation of ions but additional the pressure dependence of the ion drift velocity in an electric field in the upper pressure range too. The upper pressure limit of this ionisation gauge can be shifted up to the rough vacuum range without changing the lower pressure limit of 10^‐9 mbar as usual for Bayard‐Alpert gauges. Thus, the operation range of this modified gauge cover nearly ten decades of pressure.     

Das Drehscheiben-Vakuummeter als primäres Totaldruckmeßgerät im Hochvakuum

An absolute vacuum gauge exploiting the phenomenon of molecular drag between coaxial discs has been developed. The device consists of a silicon disc 92 mm in diameter suspended on a calibrated torsion fibre above a rotor of matching size which rotates at speeds up to 51000 rpm. Molecules leaving the rotor communicate a torque to the suspended disc by molecular drag. The angular deflection thus produced is sensed optically and is about 2° at 10^?6 mbar. The factors relevant to absolute measurement such as geometry, edge effects and tangential momentum accommodation are discussed in this article. Experimental comparisons with a spinning rotor gauge demonstrate that the rotating disc gauge has a universal application for primary total pressure measurement in high vacuum with an associated uncertainty in measurement of ±3% in the 10^?6 mbar range.     

Sicher zündende,langzeitstabile Kaltkathode

Long term stable cold cathode with reliable ignition Cold cathodes gauges have been used in various areas of vacuum technology for decades. Their unique properties make them the method of choice even despite the residual magnetic field, poor ignition characteristics in the UHV‐region and long term stability. In other applications they do not significantly shorten the maintenance intervals. However, especially in the UHV or XHV region these properties lead to an extinguishing of cold cathode discharge. Considering the underlying processes and their correlation in this contribution an improved cold cathode will be presented. The measured data show a residual magnetic field of 1 mT in close vicinity to the gauge and an extended operating pressure range down to the 10^?12 mbar region. Furthermore the ignition characteristics were improved to < 60 s at 1×10^?9 mbar.     

Innovativer Bayard‐Alpert Sensor mit ausgetrickster Röntgengrenze

Innovative Bayard‐Alpert gauge beats the X‐ray limit In this contribution typical problems that arise when using Bayard‐Alpert (BA) vacuum gauges at UHV and XHV chambers regarding measurement range, accuracy, and reproducibility are discussed. Besides others BA gauges deliver a minor residual current that is inherent to the design of the sensor and that defines the lower measurement limit of the gauge. An investigation of the basic physical processes that contribute to the residual currents in BA gauges yields a novel approach to minimize the residual current in BA gauges directly in the sensor. Experimental results of a new BA gauge with a compensation of the residual current (BARION® XHV) show an extended lower measurement limit by two decades downto XHV and a significantly increased accuracy in the pressure range 10^?9…10^?11 mbar. Due to a respective design, furthermore, an improved reproducibility is ensured.     

Kurze Wege für große Moleküle

Short‐Path distillation, often also called incorrectly molecular distillation guarantees the advantages of a distillation at considerably reduced pressure and thus reduced evaporation temperature with the distillation rates of 100–200 kg per square meter of evaporator surface area and hour which can be achieved in the fine vacuum range, i.e. at a pressure range of 10⁻³ to 1 mbar and which are economical. Short‐Path Distillation is being applied whenever all other distillation methods lead to thermal product damages, or if the vapor pressures of the components to be separated are so small that an evaporation at atmospheric pressure or rough vacuum would required extremely high temperature. The article below describes the history and the basics of Short‐Path Distillation and is giving an typical example of nowadays applications.     

Hydrogen pumping at 20 K by molecular sieves 5A

Molecular sieves bonded to the condensing surface of a continuous flow cryopump serve as the adsorbent of a cryosorption pump. A study of cool down and pumpdown characteristics of the pump shows that it can be used to pump hydrogen in a chamber to high vacuum (10^?6 mbar) even from an initial filling pressure in the region of 10 mbar.     

Die Kalibrierung von Wärmeleitungsvakuummetern. The Calibration of Thermal Conductivity Vacuum Gauges

In many applications in the rough and fine vacuum thermal conductivity gauges are utilized. In order to determine the accuracy of the measuring values modern quality assurance systems require a regular calibration. The calibration laboratory of the German Calibration Service (DKD) at VACUUBRAND GMBH + CO KG features calibration equipment and procedures to calibrate vacuum gauges in the pressure range from 10^‐3 to 1000 mbar. The measuring techniques and calibration procedures are explained for thermal conductivity gauges with digital readouts. Besides general information on the calibration, particularities on thermal conductivity gauges and their effects on the calibration procedure are pointed out. The meaning of measuring deviation and measuring uncertainty and the interpretation of the calibration certificate for the user are discussed.     

Thermal conductivity measurement with a free floating molecule detector

“Zero‐pressure” elimination with Pirani principle for vacuum measurement and increase of the measurement range limit of thermal gas‐sensors and flow sensors Measurement of pressure in the medium vacuum range has been done via thermal conductivity. The literature on the Pirani principle has defined a lower range limit — the so‐called “zero pressure” until now [1]. The minimum power needed to hold a sensor up to operating temperature exceeds the useful signal, due to the conductivity of the sensor mounting and the resulting heat loss through the suspension. The author was able to eliminate the “zero pressure”, thereby expanding the measurement range of Pirani sensors significantly downwards. Measurement results confirmed an extension of the measurement range of two decades downwards with the coiled Pirani. Also with other sensors that use thermal conductivity and heatentrainment effects, e.g. gas sensors and sensors for flow measurement, the new principle can be applied and thus lower the lower range limit.     

Gasflußmessung mit Blenden und Düsen

The amount of gas flowing through a duct can be measured by inserting a constriction and detecting the pressure drop at the constriction. In the case of ideal flow conditions, the relation between gas flow and pressure drop can be calculated from fundamental principles. In practical operation, however, disturbing effects have to be considered. The various types of constrictions (orifice, short nozzle, Venturi nozzle) exhibit different characteristics. In the laboratory for vacuum technology of the Fachhochschule in Giessen several rigs for measuring the gas throughput by means of the pressure drop principle have been installed. The characteristics of these rigs have been determined by calibration using primary methods. The apparatus allows the measurement of gas throughput in the range 10^?7 bis 10⁵ mbar · l/s.     

Dichtheitsprüfung

Leakage Tests — Flexible packaging in food processing and pharmaceutical industry Extended storage life for food products or better protection against environmental influences increases the demand for more accurate leak detection methods. Flexible, and therefore more fragile packaging is even more complicated when it comes to the commonly required non destructive leak testing. The main focus when testing flexible packaging is on welded seams. They represent the weakest point in flexible packaging. Today, there are several leak detection methods used to perform tests on flexible packaging. The most common ones are waterbath, pressure/vacuum decay and tracer gas based leak detection. All of which have their strength and weaknesses wheras the desired reject leak rate is mostly influencig the decision process. To measure very small leaks (<10 μ oder 10^‐5 mbar l/s) or to enable very short test cycles, trace gas leak detection is the way to go. On top of high accuracy, tracer gas leak detection also allows the precise localization of the leak. Nevertheless, the mechanical stress applied to the flexible packaging due to the vacuum required for tracer gast testing is not an easy task to cope with.     

Stabile Vakuumprozesse mit Dreh‐/Schiebedurchführungen

Stable vacuum processes despite rotary/sliding feedthroughs – Effects of motion speed and direction using the MeTraVac system as an example Components for ultra‐high (p < 1 · 10^?7 mbar) and extreme high vacuum (p < 1 · 10^?11mbar) conditions must have very clean surfaces with low outgassing rates. The determination of outgassing characteristics for static components is common practice. However, it is unusual to investigate the influence of the outgassing rates of dynamic components when they move in a vacuum. Various experiments with a self‐designed and built rotary/sliding feed‐through were carried out. The aim of these investigations was to monitor the pressure increases during movements and thereby draw conclusions on the quality of the vacuum conditions. One rotary and two translational movement types were selected exemplarily for practical vacuum applications. Each experiment was performed in an unconditioned and conditioned (in‐situ bake‐out) state. For the experiments a heatable test setup was assembled. The rotary motion does not affect the process vacuum perceivable. Two translational movements were carried out, which differed in the speed. The pressure increases during slower and faster translatory movements show that the speed of movement has no significant influence on the vacuum. Nevertheless, a significant change in pressure was observed with translational movements. By bake‐out, the amount of water released per movement will significantly be reduced. Most kind of vacuum‐sided movements do not affect the process pressure at all or only to a small extent.     

Kalibrierdruckerzeugung durch statische Gasexpansion

The method of generating well known calibration pressures by means of static expansion of gases is described and the required mathematical equations are derived in general form for real gases. Temperature effects caused by the expansion process are estimated and compared with experimental data obtained at an expansion apparatus. Thereafter, the experimental measurement of the expansion ratio and the generation of calibration pressures over a wide pressure range are discussed for a specific apparatus. Controlling the temperature by immersing the apparatus in a circulated water bath, the uncertainty contribution due to temperature effects was substantially reduced as compared to conventional apparatuses. The uncertainty budget reveals that this apparatus allows the generation of a pressure of, e.g., 0.1 mbar for various gases with a small relative uncertainty (2σ) of 1 . 10^‐3.     

Kolbenmanometer mit Kraftmesszelle zur Kalibrierung sehr kleiner Drücke

Piston gauges are very precise pressure gauges. In the conventional design, the force exerted by the pressure balances the gravitational force on a piston or cylinder. Due to the mass of piston or cylinder, the smallest measurable pressure is limited to a few kPa (a few 10 mbar). The company DH Instruments, Inc. has developed a new piston gauge in which the pressure‐ induced force on the piston is measured by a mass comparator. Thereby, the measuring range is substantially extended towards smaller pressures. The instrument offers a measuring range from 1 Pa to 15 kPa (0,01 mbar to 150 mbar) for gauge and absolute pressure. The resolution is 1 mPa (10^‐5 mbar) and the uncertainty (5 mPa + 3·10^‐5 p). The piston gauge is calibrated through the determination of mass and piston‐cylinder effective area. An automated pressure controller is included in the instrument. Integrated software allows multi‐point calibrations with another device to be run unattended. The system has been used to perform an automatic test of capacitance diaphragm gauges in a variety of ranges.     

The Micro Pirani™: A solid-state vacuum gauge with wide range

For measuring pressures in the range 1 × 10^?2 to 100 mbar, thermal conductivity gauges are commonly used. The widespread Pirani gauge with a thin wire suffers from problems with temperature drift, low accuracy, requirement for adjustment, large measuring volume, filament contamination, and sensitivity to mounting position. Recently, Wenzel Electronics has introduced the Micro Pirani?, which is based on a newly developed solid-state sensor element and which eliminates or significantly reduces the problems. A temperature compensation improves the stability and extends the useful range of operation to the high vacuum range at 1 × 10^?5 mbar. The geometry of the basic sensor element, close to “infinite slab geometry”, facilitates the introduction of a fairly accurate mathematical model of the sensor. The sensor is insensitive to mounting position since no convection can take place in the sensor due to its small dimensions. Furthermore, the internal volume (“dead volume”) has been significantly reduced as compared to traditional Pirani sensors. The conversion from transmitter output voltage to pressure is achieved by a simple and fast algorithm. The selection of a particular gas species is simple, since the algorithm contains only 3 gas-dependent constants. An excellent reproducibility in production eliminates the need for an individual calibration of individual sensors in most applications.     

Der Alleskönner der Messtechnik

The all‐rounder of measurement technology – Monitoring of vacuum properties The Novion is an all‐rounder of vacuum measurement technology. The newly developed sensor is based on a novel ion source and an integrated TOF mass spectrometer. The new vacuum measuring instrument is versatile: precise total pressure measurement over a wide range and simultaneously determination of helium partial pressure and residual gas analysis. By using the patented ion source, the instrument also achieves a high dynamic range without the use of a cost‐intensive electron multiplier, down to the UHV range. The Novion comes with a compact design, easy maintenance and an intuitive operation. The article covers the fields of application of this multi‐method sensor and explains its functionality in detail.     

Method for measuring a vacuum in an environment at liquid helium temperatures

The experimental results of a new method to measure a vacuum in an environment at liquid helium temperatures are presented. The method uses a thin superconducting wire suspended in the vacuum vessel, the wire is heated by a current pulse > l_c. The cool down time, which depends on the heat transfer into the rest gas and on the axial heat conductance of the wire, is a measure of the vacuum and it is detected by the recovery of the wire to superconductivity. The results exhibit good resolution in the range of pressures from 5 × 10^?3 to 5 Pa (5 × 10^?5 to 5 × 10^?2 mbar).