Toxic gas detection

Abstract: Circuit means is disclosed for performing the functions of the self-monitoring and diagnosis of means for the detection of toxic and/or harmful and/or explosive gases, checking the operating condition and efficiency of the various constituent parts, either continuously or at specified intervals, or under manual control.

Description: The present invention refers to a toxic gas detection device according to the preamble of claim 1. Devices of this kind are known from WO90/12315. In this document a circuit is described which tests whether an amperometric electrochemical gas sensor monitoring the concentration of toxic gases in an atmosphere is working properly. The sensor has sensing electrodes, a reference electrode and a counter electrode and in normal operation the potential between the sensing and reference electrodes id fixed by means of an operational amplifier. The sensor is periodically tested to see whether it is working properly by applying a pulse potential between the sensing and reference electrode. If the sensor is working properly, this results in a flow current through the sensor and through the circuit which is detected by an alarm device connected to the potentiometer by a switch during the test periods. If no current is detected by the alarm device the sensor is not working properly and an alarm is given by the device. Another device of the above mentioned kind is disclosed in U.S. 4,219,806. This device being a detector of a significant gas conditions in the atmosphere comprises one or more elements for sensing the conditions. This elements are connected to an alarm channel which sounds a warning alarm or control a corrective device such as a gas valve in the event of an excessive atmospheric condition. The detector includes a threshold sensing the alarm amplifying channel and a trouble sensing electronic valve or stage responsive to a failure of one of the sensing elements both to enable operation of a trouble channel to cause the alarm to indicate the failure and also to inhibit operation of the alarm channel. The trouble channel may also relay power supply failures such as decrease in battery voltage. The object of the present invention consist in providing a toxic gas detector of the above mentioned kind which is able to execute testing routines comprising self-monitoring and diagnosis of the detection devices capable of monitoring the critical parameters for the devices together with the function of critical components therein, indicating any faults or abnormalities by suitable means as well as the functioning of the alarm device as a whole by checking the fact that the components downstream of the sensors are in a perfect operating condition. The invention solves the above mentioned problem by means of the combination of features of the characterising part of claim 1. The subclaims discloses further improvement of the invention. This test procedure is brought into action as described below. An example of the invention will now be described with reference to the accompanying drawings, in which :- Figure 1 shows the detector as a box diagram. Figure 2 shows the power supply fur the detector of Figure 1. Figure 3 shows the microprocessor connections for the detector of Figure 1, Figures 4 and 5 show alternative interfaces required for an electrically compatible connection between the gas sensor, the acoustic indicator and the monitoring circuit in the detector of Figure 1, and Figure 6 shows the buzzer interface for the detector of Figure 1. A monitoring circuit 1 has an alarm LED 2 and a test signal LED 3. Power is supplied by a supply block 4. A gas sensor 6 and an acoustic sensor 7 are connected to the monitoring circuit 1 through an interface 5. A manual test switch 8 is provided; 8' is the corresponding relay. In Figure 2 an electrical mains connection 10 feeds through a continuous transformer 11 to feeds 9 and 9' fitted with a voltage regulator 12. Figure 3 shows the monitoring circuit microprocessor with its electrical signal inputs and outputs for controlling the timing of the operations which mainly affect user unit 14, alarm 15, sensor 16, input 1/ (RS > MAX) or input 18 (RS < MIN), buzzer 19, LEDs 21, 22, test switch 23 and the other circuit components, relay 25, buzzer control 20 and logic input 24 (voltage or current). When under manual or timed control, microprocessor 1 switches off sensors 31 or 31' (logic signal 16 or 16') by means of the circuit illustrated in Figure 4 or Figure 5, according to the type of heating elelment operation. Switching off the sensor for a predetermined period (a few seconds) causes it to cool. Subsequent switching on of the sensor provides an opportunity for examination of the thermal transient (through an analysis of the output voltage "V out" from the measuring side of the sensor). Typically, during the initial seconds of the heating transient "V out" has voltage values in excess of the alarm threshold, and thus logic signal 15 in Figure 3 is activated (ALARM). When this signal is detected by microprocessor 1 a check is made to ensure that the alarm condition has been generated correctly. For testing the acoustic signalling device, a manual or timed command in the form of signal 19 (BUZZER 32) is activated and logic signal 20, which represents the functioning of the acoustic indicator from the electrical point of view is detected and checked for correctness.. The acoustic efficiency of the indicator must be checked by the user, who must recognise a particular acoustic message when the test is activated. Relay 25 is tested by means of a manual or timed control, by activating relay 25' by microprocessor 1 and subsequently checking logic signal 26 generated by an auxiliary relay contact. Obviously in the case of automatic timed command, the acoustic and/or visual signals will be of a different type and quality (for example flashing of the test LED 3) so as not to mislead users, giving rise to an unjustified panic situation. Flashing of the LED when the test has a positive outcome (the equipment is functioning correctly) implicitly provides a check on the functioning of the LED itself. If this were not the case a steady light or no light from the LED in the event of a positive test result would not allow any fault in the signalling device to be detected. Figures 4 and 5 illustrate two possible embodiments of the circuit for measuring the resistance of the heating element. Figure 4 shows an embodiment in which the heating element is operated at constant current. Circuit 27 (consisting of R1, R2, R3, R4, R5, Q1, Q2, Q3, U1 and D1) forms a current generator which can be switched off by means of logic input 16 and which can provide two different values of current which can be selected through logic input 24 (I1/I2). In this case the resistance is measured by detecting the voltage across the heating element (comparators U2 and U3). In Figure 5, the embodiment in which the heating element is operated at constant voltage, circuit 27', consisting of R1, R2, R3, R4, R8, R9, Q1, Q2, Q3 and U1, forms a voltage generator which can be switched off by means of logic input 16' and which can provide two different voltage values which can be selected by means of logic input 24' (V1/V2). In this case the resistance is measured by monitoring the current flowing in the heating. element (by measuring the voltage at terminals 30 of R7).

Claims: A toxic gas detector comprising: a) one or more toxic gas sensors (6, 31, 31') emitting or varying an electric signal depending on the concentration of the toxic gas; b) a control circuit (5) evaluating the electric signals emitted by the at least one sensor (6, 31, 31') and comprising comparator means of the emitted signals with one or more reference signals in order to identify an alarm condition and means for generating and emitting an alarm signal (15) for driving (25) acoustic and or visual alarm warnings (2, 3, 21, 22, 32); c) a testing circuit (1, 5) for testing the operativity of at least the said sensor (6, 31, 31'), having visual and or acoustic warning means (2, 3, 21, 22, 32); d) the testing circuit is formed by a microprocessor (1, 13) programmed to execute continuously or at certain intervals programmed testing routines automatically or under a test command inputted manually (8, 14) and an interface circuit (5) to which the at least one sensor and the microprocessor are connected; e) the interface circuit comprising the control circuit of the at least one sensor and the logical circuit for measuring the electric parameter of the sensor at the receipt of a test command (24) and generating status signal (17, 18) of the sensor and furnishing the said one to the microprocessor for evaluation; f) the interface circuit comprising means for forcing the sensor to enter an alarm condition the output (15) of the control circuit for driving (25) the buzzer (6, 32) being connected to an input of the microprocessor; g) the microprocessor being provided with a programm for evaluating the driving signal (15) emitted by the control circuit during the alarm simulation test; characterised in that h) the sensor (31, 31') comprising heating elements fed by a power supply; i) the microprocessor being programmed in such a way as to switch off sensors (31, 31') (logic signals 16 end 16') by means of circuits (27, 27') and subsequently switch on the sensors thus causing the sensors to emit a thermal transient at the output voltage which exceeds the alarm thershold and which activates the alarm logic signal (15), j) this signal being evaluated by the microprocessor (1) for checking the correct generation of the alarm conditions. A toxic gas detector according to claim 1, characterised in that a) the microprocessor (1, 13) is programmed to execute testing routines of the buzzer (2, 32); b) the interface (5) comprises also the logic circuits for measuring tie electric parameter of the sensor at the receipt. of a test command (19) and generating status signal (20) which is sent to the microprocessor (1, 13) c) the control circuit of the buzzer (2, 32) being a relè (25) which is driven upon receipt of the alarm signal (15) from the control circuit (5) of the at least one sensor (6, 31, 31') by the microprocessor (1, 13) itself. A toxic gas detector according to claim 1 or 2, characterised in that the microprocessor (1, 13) comprises inputs (15, 17, 18, 20, 26) for the logical status signals of the interface (5, 25') connecting to the said microprocessor (1, 13) the at least one sensor (6, 31, 31') the buzzer (2, 32) and the logical circuits and control circuits, and outputs (25', 24, 16, 19) for activating the logical circuits in the interface (5, 25') to execute the testing routine mesurements, A toxic gas detector according to one or more of the preceding claims characterised in that the sensor (6, 31, 31') is provided with an heating element and the logic circuit for executing the status measurements of the continuous test of the sensor (6, 31, 31') comprises measuring means of the electric resistance of said heating element comprising a power supply circuit with an activation/disactivation signal input (16, 16'), a comparator (28), commutator signal inputs (24, 24') for detecting two kinds of current or voltage measurements and two measurements signal outputs (17, 17', 18, 18') each for indicating one of two conditions of the measured resistance being greater or smaller than the reference one.

Inventors: BEGHELLI GIAN PIETRO, BEGHELLI, GIAN PIETRO
Applicants: BEGHELLI SPA, BEGHELLI S.P.A.
Application year: 1992
Main classification: G08B21
IPC codes: G08B21