Electricity: measuring and testing – Magnetic – Fluid material examination
Patent
1990-01-19
1991-01-29
Snow, Walter E.
Electricity: measuring and testing
Magnetic
Fluid material examination
73 2502, G01N 3100, G01N 2774, G01R 3312
Patent
active
049889460
DESCRIPTION:
BRIEF SUMMARY
The invention relates to apparatus for the detection of gases, and in particular to gas testing apparatus utilising the magnetic susceptibility properties of gases for detection thereof, for example, gaseous oxygen.
Faraday showed, in the last century, that all matter was magnetic, that is either attracted or repelled by a magnetic field. This property, magnetic susceptibility is classified as diamagnetism when matter is repelled by a magnetic field and paramagnetism when it is attracted by a magnetic field. Faraday and others later built sensitive balances to measure these forces in gases. Most gases were found to be diamagnetic, however some gases such as oxygen, nitric oxide and nitrogen dioxide were found to be paramagnetic and hence attracted to a magnetic field. Oxygen in particular has a very strong paramagnetic property.
Two main types of magnetic balances were developed: ones using a uniform magnetic field (GOUY) and ones using a non uniform magnetic field based on the original Faraday design e.g. (Selwood). These were and still are very sensitive, bulky and very delicate laboratory instruments that require large electromagnets. The need to measure oxygen in the labs and in industry lead to the emergence of apparatus which utilised the strong paramagnetic property of oxygen to measure its concentration in a gas mixture of mainly diamagnetic gases. Different methods were developed.
Amongst the most succesful was one based on the Faraday gas susceptibility balance. This is basically a very sensitive torsion balance with a body which has a well defined shape and is made from a stable diamagnetic material such as quartz and filled with a diamagnetic gas such as nitrogen. The body is suspended in a strong and nonuniform magnetic field and the area immediately around it is enclosed and defined by a chamber in which the gas can be changed. If the balance is initially balanced with nitrogen in that chamber, and then the chamber is filled with oxygen, the paramagnetic oxygen gas is attracted to the stronger part of the magnetic field and the body rotates. The twist in the suspension is usually detected by an optical lever system.
Haven, who measured the magnetic susceptibility of gases, experimented with different shapes for the test body. His conclusion was that dumb-bell shape was the most suitable. This became the standard shape used by experimenters using the Faraday method. Early patents on such devices are U.S. Pat. No. 2,416,344 Pauling, U.S. Pat. No. 2,666,893 Munday, U.S. Pat. No. 2,744,234 Munday et al, U.S. Pat. No. 2,962,656 Munday.
The Munday cell became the most used paramagnetic oxygen sensor and in its modern form will consist basically of a small dumb-bell test body. Glass spheres (usually about 3 mm diam) filled with pure nitrogen are joined to make the dumb-bell. A single turn coil and a mirror are cemented to the dumbbell and suspended through a very thin strip of platinum iridium alloy in a non uniform magnetic field. The field is generated by strong magnets which are part of a magnetic circuit into which the cell is pushed and held in position.
The magnetic field inside the cell is shaped by the shape of the pole pieces. The dumbbell being diamagnetic, takes up a position away from the most intense part of the magnetic field. The cell is a gas chamber which has a front window so as to allow the movement of the dumb-bell to be monitored, and houses the pole pieces and the test body and is sealed gas tight with epoxy. Electric pins in the back of the cell allow an electric current to pass through the suspension strip, round the single turn feedback coil and out through the bottom half of the suspension strip and lower electric pin. Gas is admitted via gas connectors ending with nozzles. The gas impinges on the inner wall of the cell and the nozzle has to be oriented so that the flow produces a minimum disturbance to the dumb-bell (flow errors).
When the gas surrounding the dumb-bell is diamagnetic, eg nitrogen; the dumb-bell will take a certain position (the null position); if this gas
Holman Danny F.
Kocache Riad M. A.
Servomex (UK) Ltd.
Snow Walter E.
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