EP0126036B1 - Inflammable gas detection apparatus with predetermined intervention - Google Patents

Abstract

The detector element (R) is a spiral of fine Pt alloy wire in a bridge network comprising two fixed resistors (R1,R2) and two variable resistors (P1,P2) one (P1) of which is first set in the absence of gas to give the experimentally optimum voltage across the detector (R). The other variable resistor (P2) is then adjusted to set the cathode of a first diode (DS1) below its threshold voltage for a predetermined concn., with two further diodes (DS2,DS3) holding two transistors (TR1,TR2) cut off so that a relay (RL) is de-energised. Gas concn. above the preset level then provokes a voltage variation for rapid switching of the transistors and relay.

Description

The present invention relates to a flammable gas detector device with predetermined intervention.

According to the European invention patent No. 82830111.9 (EP-A-65497), it is known that a flammable gas detector device comprises a detector element in the form of a narrow spiral made of a thin wire of platinum alloy, inserted in a resistance bridge circuit by which the variation in resistance of the detector as a function of the density of the gas which surrounds it determines, in the bridge circuit, an imbalance which makes it possible to obtain a control signal from a device d alarm and / or intervention.

Numerous tests carried out have shown that the detector element used has a sensitivity varying according to variations in the operating electric voltage: the value of this voltage being related to the type of gas with which the detector is present. More precisely, it appeared that the supply voltage of maximum sensitivity, and therefore optimal response, of said detector is 2 V DC for methane gas at a concentration of 2% and 1.6 V DC for propane gas. butane at a concentration of 0.8% and that the variation of the voltage measured at the ends of the detector thus supplied is 0.2 V while passing, in the vicinity of the detector, from the condition of absence of gas to that of presence of gas according to the aforementioned percentage.

The main objective of the present invention is to enable a flammable gas detector element of the spiral type made of platinum alloy wire inserted in a resistance bridge circuit, to operate under conditions of maximum sensitivity.

This has been achieved in accordance with the present invention by adopting the idea of regulating in advance the voltage at the ends of the detector element, inserted in an electrical bridge circuit, to the maximum sensitivity value in relation to the type of gas that one seeks to detect and thus to use the signals supplied by said circuit in the presence of gas to bring the cathode of a diode to its threshold value and, simultaneously, make conductive two diodes and excite a relay signaling and / or control.

The advantages obtained by virtue of the present invention essentially consist in that the apparatus. gives an optimal response for each type of gas to be detected; that said minimum value in percentage of gas to be detected is adjustable at will with speed and precision, that the device is very compact and of minimal weight.

The invention will be explained in more detail below, with the aid of a drawing which represents only one possible embodiment. The single figure shows the electrical diagram of a flammable gas detector device with predetermined intervention according to the invention.

• - un circuit à pont, formé de deux résistances fixes (R1-R2) et de deux résistances variables (P1-P2) apte à fournir aux extrémités d'un détecteur (R) de type à spirale en fil d'alliage de platine, inséré dans une branche dudit circuit, une tension préétablie de sensibilité maximale;
• - un circuit électronique, composé d'une première diode (DS1), de deux autres diodes (DS2-DS3) en série, de deux transistors (TR1-TR2) montés en cascade, de quatre résistances de polarisation (R3-R4-R5-R6), d'un relais (RL) ou d'un dispositif similaire, dont l'excitation permet de mettre en activité un ou plusieurs appareils de signalisation et/ou de controle: toutes ces composantes étant montées selon le circuit de la FIG. 1;
• -- une section en circuit d'alimentation en courant continu, stabilisée par des circuits connus, et simplement indiqués dans le dessin par (V+).

Reduced to its essential structure and with reference to the attached drawing, an apparatus according to the invention comprises:

• - a bridge circuit, formed of two fixed resistors (R1-R2) and two variable resistors (P1-P2) capable of supplying the ends of a detector (R) of the spiral type made of platinum alloy wire, inserted into a branch of said circuit, a preset voltage of maximum sensitivity;
• - an electronic circuit, composed of a first diode (DS1), two other diodes (DS2-DS3) in series, two transistors (TR1-TR2) connected in cascade, four polarization resistors (R3-R4-R5 -R6), a relay (RL) or a similar device, the excitation of which makes it possible to activate one or more signaling and / or control devices: all these components being mounted according to the FIG circuit . 1;
• - a section in DC power supply circuit, stabilized by known circuits, and simply indicated in the drawing by (V +).

The operation is as follows. In rest condition, that is to say there is no gas to be detected in the vicinity of the detector (R), care is taken initially, by acting on the potentiometer (P1), to bring the voltage to the ends of the detector (R) at its value, determined experimentally, of maximum sensitivity with respect to the type of gas which it is desired to detect; then, by acting on the potentiometer (P2), care is taken to fix the cathode (A) of the diode (DS1) at a potential below its threshold potential for a predetermined quantity in relation to the type and concentration of gas that we want to detect so that, by the presence of the diodes (DS2-DS3), the base of the transistor (TR1) ends up being inversely polarized so that said transistor (TR1) and the transistor (TR2) are cut and that the relay (RL) is not energized.

In the presence of gas in the vicinity of the detector (R) in the expected concentration, a variation in voltage is manifested at the ends of the detector (R) which is reflected on the point (A) of the circuit by carrying the cathode of the diode (DS1) at its threshold value: consequently, the transistor (TR1) switches quickly by making the transistor (TR2) conductive and thus causing the relay (RL) to be energized. This operating condition is maintained throughout the time that the detector (R) detects a percentage of the expected gas greater than the preset safety rate.

Example of implementation

Its indicated below the values of the essential components of the device used when considering a concrete form of implementation:

Claims (1)

1. Inflammable gas detection apparatus comprising a resistance bridge circuit constituted of two fixed resistances (R1, R2), an ajustable resistance (P2), and an arm of which is contituted by a gas detection element (R) formed by a tight spiral made of a faint platinous alloy wire, and an electronic circuit, connected to the resistance bridge circuit, for activating one or more signalling and/or control systems, characterized in that the electric resistance bridge circuit comprises a second ajustable resistance (P1) for adjusting the maximal sensitivity voltage of the detection element (R) as a function of the type and the concentration of the gas to detect, and in that the electronic circuit comprises a first diode (DS1) which cathode (A) is connected to said first ajustable resistance (P2) adapted to adjust the threshold potential of this first diode (DS1) as a function of a value that correspond to said maximal sensitivity voltage of the detection e)ement (R), two serial diodes (DS2, DS3) for reverse biasing of the base of a first transistor (TR1) so- long that the potential of said cathode (A) remains lower than said threshold value «in absence» of gas to detect, a second transistor (TR2) connected to the first one, for exciting a relay (RL) for activating said signalling and/or control system when, «in presence» of detected gas, the potential of said cathode is carried to said threshold value, driving into conduction both transistors (TR1, TR2).

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