GB1575767A - Apparatus for the unambiguous indication of the proportion of combustibles in a gaseous sample - Google Patents

Description

(54) APPARATUS FOR THE UNAMBIGUOUS INDICATION OF THE PROPORTION OF COMBUSTIBLES IN A GASEOUS SAMPLE (71) We, DRÄGERWERK AKTIEN GESELLSCHAFT, a German Company, of Moislinger Allee 53-55, 2400 Liibeck, Federal Republic of Germany, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- The invention relates to an apparatus for the unambiguous indication of the proportion of combustibles in a gaseous sample.

Gas analysis apparatus which operates on the basis of determining the "heat tone", that is to say measuring the heat released when the combustibles in a gaseous sample are burned, requires in the sample an oxygen content sufficient for complete combustion. With relatively high concentrations of combustible gas, the oxygen is not sufficient. Therefore an ambiguous measurement result may occur, in that the indication can point both towards a low concentration and also towards a high concentration. For methane for example the situation is that according to the combustion equation CH4+202CO2+2H20+193.5 Kcal, the CH4 content can be correctly determined quantitatively up to 9.5%. With higher content values it is approximately proportional to half the oxygen content.An indication of 1% CH4 could also mean a concentration of 90% and an indication of 2% CH4 could mean a concentration of 80% etc.

This kind of indication ambiguity excludes the use of apparatus operating with determining only the heat tone. An unambiguous result can be achieved with measuring a further property.

Federal German Patent No. 1698549 discloses an apparatus for indicating the proportion of combustibles in a gaseous sample which operates on the "heat tone" principle and comprises an additional arrangement for measuring thermal conductivity in order to eliminate indication ambiguity which is possible with relatively high concentrations of combustible gas. The apparatus includes a Wheatstone bridge circuit in which in two adjacent branches a heat tone resistance wire and a compensation resistance wire are arranged in each case. In the two other branches of the bridge circuit arrangement there are situated in each case a thermal conductivity resistance wire and a matching compensation resistance wire. An indicating instrument is situated in the bridge diagonal.

The two first-mentioned resistance wires and the last-mentioned thermal conductivity resistance wire are arranged in a common measuring chamber through which the gas to be measured flows. The compensation resistance wire for thermal conductivity is situated in a closed chamber which is filled with the gas which is to be examined, but without its combustible components. By means of different temperatures at the resistance wires, a heat tone action is brought about by combustion on the other hand only thermal conduction on the other hand.An unambiguous result is achieved by designing the thermal conductivity resistance wire so that in the normal measuring range of the apparatus from e.g. 0 to 5% CH4 only negligible influence is exerted on the indication, but in the case of high methane contents of up to 100% the relatively low heat tone value is overtaken, which necessarily means that the instrument pointer undergoes a deflection beyond the measuring range. Unambiguous indication of the proportion of the combustibles is ensured with this apparatus only if they have a thermal conductivity value deviating from the non-combustible components of the sample. An equal value could be given for example in the case of admixing methane and butane.Because of the deflection of the pointer of the indicating instrument beyond the measuring range, further pursuing of the actual concentration relationships is not possible.

From Federal German Patent No.

2036371, an apparatus is also known with an arrangement for inhibiting the falling-off of the indication caused by incomplete oxidation when there are high contents of combustible gas. The measuring principle is again based on the measuring of the heat tone which occurs at oxidation. For that purpose the apparatus comprises a Wheatstone bridge with a measuring resistance conductor subjected to. the sample and with a compensation resistance conductor which is connected in series and is also exposed to the sample. In the air-gas mixture which is also to be measured, there is also arranged a resistance conductor whose resistance changes in dependence on the thermal conduction properties.

Connected to this conductor is an electrical switch which is acted upon by a current or voltage change proportional to the resistance change, as a control variable, and which, above the range of complete oxidation, switches-on a current, this current directly or indirectly holding a measuring instrument at full deflection.

Thus, with this apparatus with additional thermal conductivity arrangement again it is attempted to arrive at an unambiguous indication. Here again a disadvantage is the unreliability already mentioned hereinbefore in connection with the other apparatus in the case of equal thermal conductivity values of the combustible components and the other components.

Moreover, after the full deflection of the measuring instrument, it is no longer possible to pursue further the actual concentration relationships. Furthermore, the oxygen content cannot be ascertained.

According to the present invention, there is provided apparatus for the unambiguous indication of the proportion of combustibles in a gaseous sample, comprising receiving means for receiving said sample- for combustion in said means, first measuring means for measuring heat of combustion produced when said sample is combusted in said receiving means, second measuring means for measuring oxygen content in said receiving means, and an indicating instrument connected and common to both of the first and second measuring means and arranged to indicate said heat of combustion and to indicate when said oxygen content has fallen below a desired value.

Owing to the invention, it is possible to ensure an unambiguous measurement result in gas analysis apparatus even when measuring high concentration of combustible constituents. Equal thermal conductivity values of the combustible components with the remainder of the gas sample, such as prevent obtaining an unambiguous result in known gas analysis apparatus, are of no significance. The fact that the foreign gases, that is to say the combustible gases also, displace the oxygen means that there is an interdependence, so that measuring the oxygen content makes it possible to draw conclusions regarding the foreign gas content.

In a preferred embodiment, the apparatus includes a measuring chamber which receives the gaseous sample and in which there are arranged connected in series with each other a measuring resistance wire effecting the combustion of the combustibles of the sample by heating and also a compensation resistance wire which is heated to a lesser extent, the point of connection thereof forming one corner of a Wheatstone bridge circuit, and the bridge signal being applied together with the signal from an oxygen measuring circuit including an oxygen sensor in the measuring chamber, and also an adjustable threshold value switch, to a summating amplifier to which an indicating instrument is connected.The selection of the oxygen concentration at which the threshold value switch responds, ensures that the heat tone has not already gone back to a relatively low indication value because of a lack of oxygen present.

The output voltage of the threshold value switch is added to the bridge signal. Thus the indication of the indicating instrument is kept up.

Advantageously, the threshold value switch after responding and together with the bridge signal, switches the indicating instrument to full deflection. With this advantageous solution it becomes clear at once that the concentration of combustibles has reached the maximum desired critical value in accordance with the preset oxygen concentration, and has possibly already exceeded it.

With a further feature according to which the oxygen sensor is connectable directly with the indicating instrument through a switch and an amplifier, it is possible in a simple manner to observe the increase in combustible gases from the drop in the oxygen concentration. As regards the question of staying in such an atmosphere, it is of great importance to the persons concerned to know the level of breathability with regard to the oxygen content.

In order to use the advantages of fully developed and established measuring instruments, there is used an oxygen sensor which operates on the per se known electrochemical principle.

In order that the invention may be clearly understood and readily carried into effect, reference will now be made, by way of example, to the accompanying diagrammatic drawing, which shows apparatus for the unambiguous indication of the proportion of combustibles in a gaseous sample.

Referring to the drawing, the apparatus for measuring and indicating unambiguously the proportion of combustibles in the sample operates on the heat tone principle. Accordingly, a resistance wire arranged in a branch of a Wheatstone bridge as a detector element 1 is heated to such an extent that combustible gases and vapours from the sample burn with the oxygen which is present. The heat tone which is produced is the measure of the quantity of combustibles. It is indicated in a suitable way by an instrument 5. In order to eliminate the thermal conductivity of the sample which would disturb the measurement result and the effect of the ambient temperature on the measurement value, there is arranged in the other branch of the Wheatstone bridge as a compensation element 2 a resistance wire. A potentiometer 3 completes the Wheatstone bridge.The deflector element 1 and the compensation element 2 are arranged in the measuring chamber 11 into which the sample is introduced. The heat tone signal as a bridge signal goes to a summating amplifier 4. Also situated in the measuring chamber 11 and subjected to the action of the sample, is an electrochemical oxygen sensor 6. Its signal goes to the summating amplifier 4 in an oxygen measuring circuit by way of an adjustable threshold value switch 10. A potentiometer 7 is used for adjusting the oxygen sensor 6. The indicating instrument 5 is connected to the summating amplifier 4.

The sample introduced into the measuring chamber 11 impinges on the detector element 1, the compensation element 2 and the oxygen sensor 6. Because of the combustion of the combustible components, with the same compensation of the thermal conduction value, the bridge signal is produced which goes to the summating amplifier 4 and then the indicating instrument 5. In parallel therewith, the oxygen, sensor 6 measures the oxygen content of the gas sample. After dropping to the value set in the threshold value switch 10, with which complete combustion is still achieved, the oxygen signal is given additionally to the summating amplifier 4.

The joint signal at all events has the result that the indicating instrument 5 goes to full deflection.

By way of a switch 8 and an amplifier 9 the oxygen sensor 6 can be connected directly to the indicating instrument 5, cutting out the signal from the summating amplifier 4. In this way the oxygen concentration in the sample can be followed up directly, so that it is possible to make a decision regarding its breathability.

WHAT WE CLAIM IS: 1. Apparatus for the unambiguous indication of the proportion of combustibles in a gaseous sample, comprising receiving means for receiving said sample for combustion in said means, first measuring means for measuring heat of combustion produced when said sample is combusted in said receiving means, second measuring means for measuring oxygen content in said receiving means, and an indicating instrument connected and common to both of the first and second measuring means and arranged to indicate said heat of combustion and to indicate when said oxygen content has fallen below a desired value.

2. Apparatus as claimed in claim 1, wherein the higher the measured value of the heat of combustion the higher the reading of said indicating instrument.

3. Apparatus as claimed in claim 1 or 2, wherein said measuring means is connected to said indicating instrument by way of an adjustable threshold value switch which responds when said oxygen content has fallen below a desired value.

4. Apparatus as claimed in any preceding claim, wherein said first measuring means comprises a heated measuring resistance conductor of an electrical bridge including a compensation resistance conductor which is heated to a lesser temperature.

5. Apparatus as claimed in claim 4 as appended to claim 3, and further comprising a summating amplifier to which the signal from the bridge and the signal from the switch are applied.

6. Apparatus as claimed in claim 3 or 5, as appended to claim 2, wherein the responding of said switch produces a full reading of said indicating instrument.

7. Apparatus as claimed in claim 5, or claim 6 as appended to claim 5, and further comprising a second switch by way of which said second measuring means can be

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (9)

**WARNING** start of CLMS field may overlap end of DESC **. example, to the accompanying diagrammatic drawing, which shows apparatus for the unambiguous indication of the proportion of combustibles in a gaseous sample. Referring to the drawing, the apparatus for measuring and indicating unambiguously the proportion of combustibles in the sample operates on the heat tone principle. Accordingly, a resistance wire arranged in a branch of a Wheatstone bridge as a detector element 1 is heated to such an extent that combustible gases and vapours from the sample burn with the oxygen which is present. The heat tone which is produced is the measure of the quantity of combustibles. It is indicated in a suitable way by an instrument 5. In order to eliminate the thermal conductivity of the sample which would disturb the measurement result and the effect of the ambient temperature on the measurement value, there is arranged in the other branch of the Wheatstone bridge as a compensation element 2 a resistance wire. A potentiometer 3 completes the Wheatstone bridge.The deflector element 1 and the compensation element 2 are arranged in the measuring chamber 11 into which the sample is introduced. The heat tone signal as a bridge signal goes to a summating amplifier 4. Also situated in the measuring chamber 11 and subjected to the action of the sample, is an electrochemical oxygen sensor 6. Its signal goes to the summating amplifier 4 in an oxygen measuring circuit by way of an adjustable threshold value switch 10. A potentiometer 7 is used for adjusting the oxygen sensor 6. The indicating instrument 5 is connected to the summating amplifier 4. The sample introduced into the measuring chamber 11 impinges on the detector element 1, the compensation element 2 and the oxygen sensor 6. Because of the combustion of the combustible components, with the same compensation of the thermal conduction value, the bridge signal is produced which goes to the summating amplifier 4 and then the indicating instrument 5. In parallel therewith, the oxygen, sensor 6 measures the oxygen content of the gas sample. After dropping to the value set in the threshold value switch 10, with which complete combustion is still achieved, the oxygen signal is given additionally to the summating amplifier 4. The joint signal at all events has the result that the indicating instrument 5 goes to full deflection. By way of a switch 8 and an amplifier 9 the oxygen sensor 6 can be connected directly to the indicating instrument 5, cutting out the signal from the summating amplifier 4. In this way the oxygen concentration in the sample can be followed up directly, so that it is possible to make a decision regarding its breathability. WHAT WE CLAIM IS:

1. Apparatus for the unambiguous indication of the proportion of combustibles in a gaseous sample, comprising receiving means for receiving said sample for combustion in said means, first measuring means for measuring heat of combustion produced when said sample is combusted in said receiving means, second measuring means for measuring oxygen content in said receiving means, and an indicating instrument connected and common to both of the first and second measuring means and arranged to indicate said heat of combustion and to indicate when said oxygen content has fallen below a desired value.

2. Apparatus as claimed in claim 1, wherein the higher the measured value of the heat of combustion the higher the reading of said indicating instrument.

3. Apparatus as claimed in claim 1 or 2, wherein said measuring means is connected to said indicating instrument by way of an adjustable threshold value switch which responds when said oxygen content has fallen below a desired value.

4. Apparatus as claimed in any preceding claim, wherein said first measuring means comprises a heated measuring resistance conductor of an electrical bridge including a compensation resistance conductor which is heated to a lesser temperature.

5. Apparatus as claimed in claim 4 as appended to claim 3, and further comprising a summating amplifier to which the signal from the bridge and the signal from the switch are applied.

6. Apparatus as claimed in claim 3 or 5, as appended to claim 2, wherein the responding of said switch produces a full reading of said indicating instrument.

7. Apparatus as claimed in claim 5, or claim 6 as appended to claim 5, and further comprising a second switch by way of which said second measuring means can be

connected to said indicating instrument other than via said summating amplifier.

8. Apparatus as claimed in any preceding claim, wherein said second measuring means operates electrochemically.

9. Apparatus for the unambiguous indication of the proportion of combustibles in a gaseous sample, substantially as hereinbefore described with reference to the accompanying drawing.