GB1592066A

GB1592066A - Gas-measuring and gas-warning apparatus

Info

Publication number: GB1592066A
Application number: GB3496077A
Authority: GB
Original assignee: Auergesellschaft GmbH
Current assignee: Auergesellschaft GmbH
Priority date: 1976-08-19
Filing date: 1977-08-19
Publication date: 1981-07-01
Also published as: DE2637557B1; FR2362391A1; IT1083398B; DE2637557C2

Description

(54) IMPROVEMENTS IN OR RELATING TO GAS-MEASURING AND GAS-WARNING APPARATUS (71) We, AUERGESELLSCHAFT GMBH, a German Body Corporate, of 24 Friedrich Krause-Ufer, 1000 Berlin 65 (West), 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 present invention relates to methods of and apparatus for calibrating gas-measuring and gas-warning apparatus of the kind which operates with semi-conductor detecting elements.

Gas-measuring and gas-warning apparatus is intended to provide a reliable and timely warning of dangerous increases in poisonous or explosible gases and vapours and, when required, to initiate counter measures, and for this reason it requires alarm thresholds to be calibrated and set.

The calibration is generally performed by feeding to the gas-measuring apparatus a gas mixture containing the component to be measured. In many cases the so-called flowthrough method is used to do this. According to this method, the gases, i.e. the component to be measured and air, are mixed in a specific ratio corresponding to the concentration to be calibrated for. The mixture so obtained is fed into the gas-measuring apparatus and used to calibrate it.

In apparatus in which semi-conductor gas detecting elements are used, problems are often encountered in setting the calibration values, particularly when the gas-detecting elements operate in bridge circuits. If the full gas-concentration to be calibrated for is fed to the gas-detecting element then there is a large and relatively fast rise in the signal caused by the presence of the component to be measured and a correspondingly large deflection at the measuring instrument. This signal, however, becomes considerably smaller after a relatively short period if the applied concentration remains the same and it often fails to assume any definite stable value. Interrupting the infeed of gas and feeding it in again after scavenging with fresh air results in a lower indication, particularly in the initial range.This behaviour by semiconductor gas-detecting elements makes it difficult to arrive at a reliable calibration.

Accordingly the present invention consists in a semiconductor gas measuring or warning arrangement in combination with apparatus comprising a first duct one end of which is a gas inlet for connection to a source of gas to be calibrated or measured and the other end of which opens into an expansion chamber, and a second duct one end of which is an outlet for supplying the gas to the inlet of the semiconductor gas measuring or warning arrangement and the other end of which also opens into said expansion chamber, said expansion chamber being of greater cross-sectional area than said ducts.

The invention also consists in a method employing the combination set forth above.

In order that the invention may be more clearly understood, reference will now be made to the accompanying drawings which show one particular embodiment thereof by way of example and in which: Figure 1 is a diagrammatic view of the flow path of gas in apparatus constructed in accordance with the present invention for supplying gas to a gas measuring arrangement, Figure 2 is an axial section through a part of the apparatus of Figure 1, Figure 3a shows signal waveforms obtained from a gas measuring arrangement where gas has been supplied directly thereto, and Figure 3b shows signal waveforms from the gas measuring arrangement when supplied with gas through the device shown in Figures I and 2.

Referring now to the drawings the appa ratus shown in Figure 1 comprises an inlet I to which gas for calibration or measurement is fed. The gas, after passing through a fine mesh permeable filter 2, is fed by a gas pump 3 through a through-put regulator 4 to a delay device 5 in the form of an expansion chamber. After passing through the device 5 the gas is supplied to the measuring inlet of a gas measuring arrangement 12. Having passed through the device 5 the gas reaches a double magnetic valve 8, and can be supplied to an outlet 10 through port 8/2 of the valve 8. The outlet 10 is arranged to supply the gas which has passed through the apparatus to an inlet 13 of a semiconductor gas measuring apparatus 12.

When the apparatus is being used to calibrate the gas measuring arrangement 12, a known concentration of gas is supplied to the inlet 1. The valves 6, 7 and 8/1 are closed so that the gas mixture flows through the filter 2, the regulator 4 into the device 5. In the device 5 the flow of gas is delayed so that there is a slow build up of gas concentration at outlet 10. This avoids a sudden, unwanted rise or excursion of the output signal of the measuring arrangement 12 that would occur if the gas had first not passed through the device 5.

This can best be appreciated by reference to Figures 3a and 3b of the drawings. Figure 3a shows output signals from the measuring arrangement 12 when the gas mixture is applied directly to its input 13. From these graphs it can be seen that the output signal of the measuring arrangement undergoes a considerable excursion which is in excess of the true value which should be caused by the particular gas concentration. Figure 3b shows the output signals when the same concentration has been supplied to input 13 via the apparatus just described. From this Figure it can be seen that his excursion has been suppressed by the delay caused by the expansion chamber.

The gas flow path in the apparatus also includes a passage which by-passes the device 5 and which includes a magnetic valve 6 and an adjustable constrictor or throat 9.

This by-pass passage also leads to the valve 8 and can be converted to outlet 10 by port 8/1 of valve 8.

Finally the output end of device 5 is connected via a passage to an exhaust outlet Il, flow of gas through this passage being controlled by a magnetic valve 7.

The volume of the chamber in the devices is greater than that of the measuring cell in the measuring apparatus and the crosssections of the tubes 15 greater than the cross-section of the ducts in the remainder of then apparatus.

After each calibration measurement has been made the apparatus is scavenged with fresh air, that is air without the gaseous component which is to be measured. For this purpose, the magnetic valves 6 and 7 and output 8/1 of the magnetic valve 8 are opened and fresh air is pumped through the apparatus, i.e. through device 5, the open magnetic valve 7, and the exhaust gas outlet 11, and through the by-pass passage 14 which includes the magnetic valve 6, constrictor 9 and the open outlet 8/1 of magnetic valve 8. This by-pass passage or circuit allows the scavenging period for the gasmeasuring arrangement 12 to be set by means of the adjustable constrictor 9. This scavenging procedure is carried out after each calibration point has been set on measuring arrangement 12.

Fig. 2 shows in greater detail one embodiment of the device 5. As previously described the device 5 is essentially an expansion chamber and gas mixture is fed into or out of the chamber as the case may be by an inlet and an outlet in the form of screwed connections or plug-in pipe connections. These connections are attached to tubes 15 which are located within the chamber and have sintered filter discs 16 fixed over their open ends. The chamber is filled with packing in the form of glass wool 17, and the discs 16 prevent the glass wool 17 from being dragged to the inlet 13 of the measuring arrangement 12.The packing 17 apart from glass wool, may also consist of small balls or rings of glass or ceramic material or particles of a similar shape, and is intended to prevent the setting up of a preferred direction of flow within the chamber of the device 5, and consequently a steep rise in gas concentration at the input of the measuring arrangement. It will also be seen from the Figure 2 that gas passing from the outlet of one pipe 15 through the chamber and into the inlet of the other pipe has to flow in a direction which is essentially opposite to the flow of the gas through the remainder of the apparatus to the gas measuring arrangement.

It will be appreciated that the apparatus described equally finds application in the measurement of unknown gas concentration by preventing too sudden a rise in the output signal of the semiconductor gas measuring arrangements.

WHAT WE CLAIM IS:- 1. A semiconductor gas measuring or warning arrangement in combination with apparatus comprising a first duct one end of which is a gas inlet for connection to a source of gas to be calibrated or measured and the other end of which opens into an expansion chamber and a second duct one end of which is an outlet for supplying the gas to the inlet of the semiconductor gas measuring or warning arrangement and the other end of which also opens into said expansion chamber, said expansion chamber being of greater

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

Claims

**WARNING** start of CLMS field may overlap end of DESC **. ratus shown in Figure 1 comprises an inlet I to which gas for calibration or measurement is fed. The gas, after passing through a fine mesh permeable filter 2, is fed by a gas pump 3 through a through-put regulator 4 to a delay device 5 in the form of an expansion chamber. After passing through the device 5 the gas is supplied to the measuring inlet of a gas measuring arrangement 12. Having passed through the device 5 the gas reaches a double magnetic valve 8, and can be supplied to an outlet 10 through port 8/2 of the valve 8. The outlet 10 is arranged to supply the gas which has passed through the apparatus to an inlet 13 of a semiconductor gas measuring apparatus 12. When the apparatus is being used to calibrate the gas measuring arrangement 12, a known concentration of gas is supplied to the inlet 1. The valves 6, 7 and 8/1 are closed so that the gas mixture flows through the filter 2, the regulator 4 into the device 5. In the device 5 the flow of gas is delayed so that there is a slow build up of gas concentration at outlet 10. This avoids a sudden, unwanted rise or excursion of the output signal of the measuring arrangement 12 that would occur if the gas had first not passed through the device 5. This can best be appreciated by reference to Figures 3a and 3b of the drawings. Figure 3a shows output signals from the measuring arrangement 12 when the gas mixture is applied directly to its input 13. From these graphs it can be seen that the output signal of the measuring arrangement undergoes a considerable excursion which is in excess of the true value which should be caused by the particular gas concentration. Figure 3b shows the output signals when the same concentration has been supplied to input 13 via the apparatus just described. From this Figure it can be seen that his excursion has been suppressed by the delay caused by the expansion chamber. The gas flow path in the apparatus also includes a passage which by-passes the device 5 and which includes a magnetic valve 6 and an adjustable constrictor or throat 9. This by-pass passage also leads to the valve 8 and can be converted to outlet 10 by port 8/1 of valve 8. Finally the output end of device 5 is connected via a passage to an exhaust outlet Il, flow of gas through this passage being controlled by a magnetic valve 7. The volume of the chamber in the devices is greater than that of the measuring cell in the measuring apparatus and the crosssections of the tubes 15 greater than the cross-section of the ducts in the remainder of then apparatus. After each calibration measurement has been made the apparatus is scavenged with fresh air, that is air without the gaseous component which is to be measured. For this purpose, the magnetic valves 6 and 7 and output 8/1 of the magnetic valve 8 are opened and fresh air is pumped through the apparatus, i.e. through device 5, the open magnetic valve 7, and the exhaust gas outlet 11, and through the by-pass passage 14 which includes the magnetic valve 6, constrictor 9 and the open outlet 8/1 of magnetic valve 8. This by-pass passage or circuit allows the scavenging period for the gasmeasuring arrangement 12 to be set by means of the adjustable constrictor 9. This scavenging procedure is carried out after each calibration point has been set on measuring arrangement 12. Fig. 2 shows in greater detail one embodiment of the device 5. As previously described the device 5 is essentially an expansion chamber and gas mixture is fed into or out of the chamber as the case may be by an inlet and an outlet in the form of screwed connections or plug-in pipe connections. These connections are attached to tubes 15 which are located within the chamber and have sintered filter discs 16 fixed over their open ends. The chamber is filled with packing in the form of glass wool 17, and the discs 16 prevent the glass wool 17 from being dragged to the inlet 13 of the measuring arrangement 12.The packing 17 apart from glass wool, may also consist of small balls or rings of glass or ceramic material or particles of a similar shape, and is intended to prevent the setting up of a preferred direction of flow within the chamber of the device 5, and consequently a steep rise in gas concentration at the input of the measuring arrangement. It will also be seen from the Figure 2 that gas passing from the outlet of one pipe 15 through the chamber and into the inlet of the other pipe has to flow in a direction which is essentially opposite to the flow of the gas through the remainder of the apparatus to the gas measuring arrangement. It will be appreciated that the apparatus described equally finds application in the measurement of unknown gas concentration by preventing too sudden a rise in the output signal of the semiconductor gas measuring arrangements. WHAT WE CLAIM IS:-

1. A semiconductor gas measuring or warning arrangement in combination with apparatus comprising a first duct one end of which is a gas inlet for connection to a source of gas to be calibrated or measured and the other end of which opens into an expansion chamber and a second duct one end of which is an outlet for supplying the gas to the inlet of the semiconductor gas measuring or warning arrangement and the other end of which also opens into said expansion chamber, said expansion chamber being of greater

cross-sectional area than said ducts.

2. A combination as claimed in claim 1, wherein the ends of the ducts in the expansion chamber are so disposed that in use the gas in the expansion chamber tends to flow in a direction opposite to the direction of flow of gas through the first and second ducts as the gas travels to the measuring or warning arrangement.

3. A combination as claimed in claim 2, wherein the ends of the ducts in said expansion chamber have filters permeable to said gas, the chamber containing packing material for restricting the rate of gas flow through the chamber.

4. A combination as claimed in claim 3, wherein the packing comprises either glass balls, glass rings or glass wool.

5. A combination as claimed in any one of claims 1 to 4, and including a passage bypassing said chamber.

6. A combination as claimed in claim 5, wherein said by-pass passage includes valve means for preventing flow of gas through said passage.

7. A combination as claimed in claim 6, wherein said by-pass passage and the output of said chamber are connected to valve means operative to couple both the output of said chamber and said by-pass passage to the gas outlet of said apparatus.

8. A combination as claimed in any one of claims 5 to 7, wherein said by-pass passage includes an adjustable flow regulator.

9. A combination as claimed in any one of claims 1 to 8, wherein the output of said chamber is also connectable to an exhaust outlet via a passage, said passage including further valve means for opening or closing said passage.

10. A combination for calibrating semiconductor gas measuring or warning arrangements or for monitoring gas concentrations with semiconductor gas measuring or warning arrangements substantially as hereinbefore described with reference to the accompanying drawings.

I 1. A method of calibrating semiconductor gas measuring or warning arrangements or a method of monitoring gas concentrations with semiconductor gas measuring or warning arrangements utilising a combination as claimed in any one of the preceding claims and comprising supplying gas to the inlet of the first duct of the apparatus and coupling the outlet of the second duct to the measuring inlet of the semiconductor measuring or warning arrangements.