Electricity: measuring and testing – Magnetic – Fluid material examination
Patent
1986-12-11
1988-12-27
Strecker, Gerard R.
Electricity: measuring and testing
Magnetic
Fluid material examination
73 27A, G01N 2774, G01R 3312
Patent
active
047943343
DESCRIPTION:
BRIEF SUMMARY
The invention relates to devices that detect gases which have a strong paramagnetic susceptibility, such as oxygen, relative to most other common gases which have a weak diamagnetic susceptibility.
Such devices utilise a test body such as a dumb-bell suspended in a strong non-uniform magnetic field by a metallic strip and have a mirror on the test body which is part of an optical lever comprising a light source and a pair of opposing photocells. This optical system enables detection of the slightest movement of the test body from a reference position, when a reference gas such a nitrogen is present in the chamber displaced by a sample gas containing a constituent which is paramagnetic such as oxygen. The movement of the test body is inhibited by arranging a single turn coil round it and energising it by a current derived from an amplifier network which is driven from the photocells. The value of the current required to keep the test body in its initial reference position against the magnetic force induced by the paramagnetic component in the sample gas is linearly proportional to the partial pressure of that gas in the cell chamber.
Devices based on this principle have been described in U.K. Pat. Nos. 703,240 and 746,778 where the test body is made of quartz spheres filled with nitrogen. The dumb-bell, the mirror, the single turn coil: that is the components of the test body; are usually cemented together by adhesives such as an epoxy resin. The pole pieces and other cell components are also cemented together. The cell chamber is sealed using an epoxy resin.
The test body/optical lever system is very sensitive and thus can detect the small forces produced by the impingement of the incoming gas sample on it. This introduces noise into the measurement.
This problem was partially alleviated by adopting the design described in UK Pat. No. 829,444. This required however, in addition to the especially designed nozzles, an accurate and elaborate process of testing the induced flow errors, adjusting the angle of the nozzles and so on till the required performance is met. The inlet pipes had to be sealed gas tight with epoxy resin and keep their set position.
The deterioration of the epoxy resins used as cements and for gas tight seals, due to the presence of corrosive gases, solvent vapours and water vapour in the sample gas, results in gas leaks, output signal drift and changes in the temperature coefficient and sometimes in the collapse of the test body.
In an attempt to reduce this, UK Pat. No. 1,366,227 describes a test body which is assembled without the use of cements. Despite the great improvements brought by this design, the need to seal the cell and the inlet and outlet pipes gas tight with epoxy, the need to set and test the angle of the test body relative to the pole pieces axis so as to achieve workable spread in cell parameters, and the need to lock the test body at that angle with cement, left a large number of problems untackled.
From one aspect, the present application provides a cell structure which excludes the use of cements and epoxy. Preferably, a gas flow regime is provided which requires no setting of the gas pipes and whereby flow errors are minimal.
From another aspect, the present invention provides specially designed pole pieces which enables the replacement of the soft magnetic material used under normal conditions by a strong magnetic material if an integral magnet construction is required.
From a still further aspect, the present invention provides specially designed pole pieces the shape of the magnetic field which they produce ensures one equilibrium position only and produces a narrow spread in the initial position for the normal manufacturing spread in test bodies.
The present invention provides gas testing apparatus comprising a main body provided with a chamber, a hole extending from a first surface of the main body into the chamber, a test body located for movement in the chamber, means for permiting test gas to flow through said chamber, and means for applying a magnetic field
REFERENCES:
patent: 2416344 (1947-02-01), Pauling
patent: 2962656 (1960-11-01), Munday
patent: 3026472 (1962-03-01), Greene et al.
patent: 3292421 (1966-12-01), Meyer
patent: 3612991 (1971-10-01), Greene
Holman Danny F.
Kocache Riad M. A.
Edmonds Warren S.
Servomex, Ltd.
Strecker Gerard R.
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