This invention relates in general to gas detection devices and in particular to a new and useful device for determining the concentration of substances with paramagnetic properies, particularly oxygen.
The invention particularly concerns a device for the determination of the concentration of substances with paramagnetic properties, particularly oxygen, in substance mixtures consisting of a revolving, subdivided cell assembly, in which a first series of cells is filled with gas having variable oxygen contents and a second series of cells contains a control gas with a constant oxygen content, and an assembly of coils in which the magnetic induction produced by the parmagnetic substance is converted into an electrical signal.
A similar device is known from German OS No. 19 24 228, in which is described a first magnet system, consisting of permanent magnet, pole shows and magnetic air gap, and a second, identical magnet system which extends over a revolving chamber system. The chamber system is constructed with a centrifugal force fan wheel of circular disc shape and open chambers, through which the gas to be measured flows alternating and with closed chambers with a control gas being passed between the pole shoes of both permanent magnets. The chamber sequence is arranged to have a chamber with a control gas between the pole shoes of the first permanent magnet when a chamber with a test gas is located between the pole shoes of the second permanent magnet. The magnetic induction produced by the permanent magnets is changed into an electrical signal by measuring coils, which, at constant rpm of the measuring chamber assembly, is proportional to the partial oxygen pressure of the gas to be measured. This measuring voltage is further electrically processed and indicated on an indicator.
The known measuring device requires, however, a mechanical design of the pole shoes of the permanent magnet which guarantees a constant geometric arrangement under operating conditions. This can be obtained only at unjustifiable expense. For example, a change in the air gap between the poles of the permanent magnet by approximately 10.sup.-8 mm has an effect on the measuring signal given in the coil assembly similar to a change in the oxygen content in the test gas by approximately 1% by volume. Thus, if the oxygen concentration is to be measured with the aid of such a known assembly with an accuracy greater than 1% by volume of oxygen as is usual in practice, then the gaps between the poles of the permanent magnets must be produced with an accuracy greater than 10.sup.-8 mm and kept at this distance since changes in the gaps, e.g. due to mechanical vibrations result in disturbing microphonic effects. Such exacting demands on the mechanical processing and stability makes the applicability of the known assembly in actual practice doubtful.
The gap between the pole shoes of the magnets is no longer important when, for example, the magnet pole and coil assembly is located on only one side of the revolving cell assembly. But in this case there is the disadvantage that the magnetic field does not completely penetrate the measuring chamber and the control chamber.
It is known from U.S. Pat. No. 3,076,929 and German OS No. 31 45 542 that there are problems in arranging the mechancial chamber and control chamber firmly in a housing and surrounding both chambers with one pair of coils each. The former coil is used to produce a magnetic field and the latter coil, for the transformation of the change in its magnetic induction into an electrical signal. The coils are connected either by a transformer connection (U.S. Pat. No. 3,076,929) or by a bridge connection (German OS No. 31 45 542), and the measuring signal as produced by them are indicated in an evaluating unit.
In these known coil assemblies the test gas and the control gas serve as coil shells or jackets. The self-induction of the coil is thereby changed in proportion to the concentration of paramagnetic components in the gas mixture. This change in the induction causes an unbalance in the measuring circuit the size of which can be used to determine the concentration of paramagnetic substances.
Such a measuring device does react with sensitivity to changes in the concentration of paramagnetic substances in the test gas, but additional efforts are required to prevent variations in the expansions of the coil carriers due to variations in ambient temperatures, or at least to equalize these. A change in the geometric expansions of the coils due to temperature fluctuations is reflected in a change of their self-induction, which is on the same order of magnitude as the change in the self-induction due to the change in the concentration of paramagnetic substance in the measuring chamber.