PLANT ENGINEERING

Fine vacuum measurementPirani sensor with high chemical and mechanicalresistance

For vacuum measurement of e.g. rotary vane pumps the VSP 3000 has a

chemical and mechanical robustness which is unequalled by conventional Pirani

sensors up to now, lts ceramic protective covering of the metal wire and asplash-watertight plastic housing make it a reliable fine vacuum gauge, whichis easy to use and offers high life expectancy. The application possibilities rangefrom the fine vacuum measurement and regulation up to continuously precise

measurements of rough vacuum down to fine vacuum.

Many lab applications like residual drying,lyophilization or short-path distillationneed ultimate vacuum and measurementin the fine vacuum range (0.001 to l mbar)where, for example, rotary vane pumpsand Pirani gauges are combined. In par-ticular, vacuum processes with chemicallyaggressive gases and vapors typically leadto corrosion of the Pirani filaments and to abuild up of deposits on their surface fromthe oil vapours coming from the rotaryvane pump and process products. As a con-sequence of this corrosion and depositbuild up the measurement becomes inac-curate. It can even lead to a complete fai-lure of the sensor.

Even the careful choice of chemically resist-ant materials for the metal wire or its coat-ing cannot prevent the problem of deposi-tion. The influence of depositions can beestimated roughly from the relation of de-position layer thickness to wire diameter.Because the metal wire is very thin (typical0.01 mm) the relative measuring error is al-

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PBT- housing

ready very big even with slight deposi-tions.To solve these problems Vacuubrand hasdeveloped the Pirani sensor VSP 3000. Atthe heart of this technological innovation isa ceramic measuring element whose con-struction and impact fulfils at the sametime several important functions:

. As a gasproof ceramic covet it protectsthe metal wire against corrosion.

. As an isolating protective coating it sep-arates the metal wire from conductivedepositions and prevents a change of theelectric properties of the wire.

. The special surface quality of the ceramicmaterial reduces the formation of deposi-tions on the protective covering.

o The ceramic cover increases the diameterof the measuring element by a factor of100 and thus reduces the influence of a

possible deposition layer on the measur-ing result.

The breaking of the thin metal filament inconventional Pirani sensors is also a well-known problem. The delicate filamentsreact very sensitively to quick pressurechanges or to mechanical stress which ap-pear, for example, with quick venting or vi-bration of the vacuum pump. With the VSP

3000 the ceramically enclosed metal wireis no longer exposed to the gas and vaporstream, so it becomes very stable againstsudden pressure changes. Additionallythe ceramic protective covering enclosesthe metal wire completely and improvesits mechanical stability. A1l together thisstabilisation results in a vastly improvedlife expectancy of the sensor.Inaccuracies in the indicated measuringvalues based on depositions and corrosionappear more quickly and frequently, thehigher the concentration is of the gases or

vapors to be measured. Whilst thefine vacuum range is not 1 et reach-ed, the fiiament must be protected

in conventional sensors against highconcentrations of gases and vapors.Therefore, traditionally Pirani sensorsare separated from the application

using valves. The VSP 3000 with its gas

proof protective covering negates the needfor this additional valve protection.Cleaning of the filament is necessary for allPirani sensors for reliable pressuremeasurements. Usually this process carrieswith it a high risk of destroying or damag-ing the fragile metal filament. The VSP 3000

with its protective covering and open geo-metry allows for easy optical control andtrouble-free cleaning.The VSP 3000 covers the measuring rangefrom 0.001 to 1000 mbar. Its precision, reso-lution and reproducability of measurementand response time to pressure changes isequal or even better to that of conventionalPirani sensors. The gauge set assemblywith the VSP 3000 can be combined with anadditional capacitive vacuum sensor (VSK3000) which allows for the continuousprecise measurement from atmosphericpressure a1l the way down to fine vacuum.

Online-lnfowww. cp p- n et. com/2 309406

spläsh-wäter proofplastic housing

electronics

ceramic encapsulatedmetal wire

The VSP 3000 (sectional

drawing at the left) can be

used in combination withthe vacuum controller CVC

3000 (above) providing ad-

ditionally for vacuumregulation in the finevacuum range