Measuring and testing – Gas analysis – By thermal property
Patent
1993-03-24
1994-12-06
Williams, Hezron E.
Measuring and testing
Gas analysis
By thermal property
324204, G01N 2774
Patent
active
053699807
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The present invention relates to a method and apparatus for the determination of the proportion of a paramagnetic gas such as oxygen in a mixture of gases which are not all paramagnetic.
BACKGROUND OF THE INVENTION
Various methods have been proposed in the past for detecting gases in gaseous mixtures. Faraday showed in the last century that all matter is magnetic. Matter strongly attracted to a magnetic field was termed paramagnetic and matter repulsed by a magnetic field was termed diamagnetic. Few gases have strong paramagnetic properties, amongst those with the strongest are oxygen O.sub.2, nitrogen dioxide NO.sub.2 and nitrous oxide NO.
This property exhibited by only a few gases has been used in the past for the determination of the proportion of a paramagnetic gas, usually oxygen, in a gas mixture using generally four methods:
1. Magnetic Wind. This method suffers from effects such as thermal conductivity of background gases, low sensitivity, long response time and is not used much nowadays.
2. Differential Pressure. This method requires a reference gas and in some forms requires an alternating magnetic field. It can be made to have a somewhat fast response, but the smallness of the differential pressure created requires very sensitive flow sensors.
3. Magnetic Auto-balance. A torsion balance with automatic feed back control via an optical system. The most popular method, but suffers mainly from slow response time, and gas flow errors.
4. Relative Change in Permeability. Many different approaches to this method have been attempted without a great deal of success due to the extreme smallness of the resulting change in permeability.
Thus there remains a need for a method and apparatus for the determination of a proportion of paramagnetic gas in a mixture of gases which is relatively simple to use and has a quick response to changes in the gas mixture being measured.
SUMMARY OF THE INVENTION
The present invention provides a method of determining the proportion of a paramagnetic gas in a mixture of gases comprising: magnetic field; oscillation whereby to determine the proportion of a paramagnetic gas in the mixture of gases surrounding said resiliently mounted member.
It is well established that the magnetic force F acting on an element, a sphere for example but not necessarily, of a volume magnetic susceptibility K.sub.o, a volume V and a mass M placed in a magnetic field of strength H and a gradient dH/dx is given by: ##EQU1## where K.sub.s is the net volume magnetic susceptibility of the gas surrounding the element. It will be appreciated that a variation in the partial pressure of a paramagnetic gas in the gas surrounding the element will cause K.sub.s to vary. It therefore follows that the force F on the element given by equation (1) will vary according to the partial pressure of the paramagnetic gas surrounding the element.
If the element is caused to oscillate in the magnetic field, the parameters of the oscillation, such as the damping or the frequency of oscillation are affected by the force F given by equation (1) which in turn is affected by the partial pressure of paramagnetic gas surrounding the element. This is the basis of the present invention.
BRIEF DESCRIPTION OF THE DRAWINGS
In order that the present invention be more fully understood preferred embodiments will be described by way of example with reference to the accompanying drawings, in which:
FIG. 1 illustrates basic apparatus operating according to the present invention;
FIG. 2 illustrates a block diagram of one preferred form of control circuitry for use with the apparatus of FIG. 1; and
FIG. 3 is a schematic diagram of a flow cell incorporating the apparatus of FIG. 1.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Suppose an element is suspended within a magnetic field by an elastic member such that there is only one main mode of oscillation and that is in a direction perpendicular to the magnetic field. If the elastic constant of the mounting member is S, then the equation of motion of the oscillati
REFERENCES:
patent: 3672388 (1972-06-01), Ringwoll
patent: 3742344 (1973-06-01), Hummel
patent: 4563894 (1986-01-01), Karrer
Brock Michael J.
Servomex (UK) Ltd.
Williams Hezron E.
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