WO1992012505A1 - Alarm condition detector such as dangerous concentration detector of noxious gases in a medium - Google Patents

Abstract

The present invention relates to an alarm condition detector such as a dangerous concentration detector of noxious gases in a medium. The invention finds applications in the field of measurment chains coupled to at least one sensor and particularly in the field of security against noxious gases. The invention is particularly characterized in that the detector includes, in addition of the sensor (1), a means (7) for producing at least during the operation of the detector, and also at predetermined moments, a reference value (Vref) representative of the intrinsic condition of the detector, a means (8) for producing at predetermined instants a threshold value (Vthreshold) according to the relationship: Vthreshold = f(Vref, tv), a function both of said reference value (Vref) updated at that instant and of an alarm level (tv), a calculation means (6) for calculating an alarm condition receiving as input the measuring signal (Vmes) and said threshold value (Vthreshold) and producing, as output, said alarm signal.

Description

 ALARM STATE DETECTOR AS A DANGEROUS CONCENTRATION DETECTOR IN HARMFUL GASES IN A MEDIUM.

*, The present invention relates to an alarm state detector i as a dangerous concentration detector e

5 harmful gases in an environment. The invention finds application in the field of measurement chains coupled to at least one sensor and very particularly in the field of safety against harmful gases.

Two main fields of application exist: 10 - industrial installations which produce a multitude of harmful gases, the contents of which are regulated by very strict standards; domestic installations, mainly of heating and cooking, of which the most dangerous harmful gas which they release is carbon monoxide, lethal.

Accidents at work on the one hand, and domestic accidents on the other hand, caused by the emissions of harmful gases are the source of extreme costs. It is a requirement of modern industry to provide solutions which are increasingly suitable for detecting the concentration of harmful gases in an environment.

Hitherto, the detectors have been constructed around a sensor and electronic components, which must be chosen and mounted so as to give maximum satisfaction from the point of view of user safety.

From this point of view, a requirement has emerged alongside the reliability requirements. An alarm state detector which requires special attention for its installation 30 and its maintenance in no way satisfies the safety of users. Up to now, detectors have included consumable elements such as the gas sensor or components whose aging requires a

-_- <_ ι ι_, si-ioπ ι_ιU ucι.cCι.ε ι during v_ι <

35 It is impossible to leave the safety of a user to his good will. In other words, the alarm status detector should be a device that requires no special attention during its operation.

It is a first object of the invention to provide an alarm state detector which does not require any particular maintenance during its lifetime. On the other hand, the technology chosen especially from the point of view of the sensor, requires that the detection chain be adapted to the environment in which the detector is installed. This stems from the fact that the sensor firstly evolves between the time of its manufacture and that of its start-up, depending on the storage and transport conditions in particular. Then, the great diversity of environments in which such a detector is placed leads in most previous embodiments the need that ^: a technician intervenes to carry out the commissioning by manually adjusting adjustment elements such as threshold adjustment potentiometers, etc. .

It is another object of the present invention to provide an alarm state detector which does not require adjustment when it is put into service. In fact, this invention relates to an alarm state detector such as a harmful gas concentration detector in a medium, of the type comprising at least:

a sensor of a magnitude such as the concentration of a gas in a medium, said sensor producing a measurement signal representative of the concentration of at least one gas in the medium under surveillance. The invention is characterized in that the detector further comprises:

- Means for producing at predetermined times a reference value representative of the intrinsic state of the detector;

- Means for producing at predetermined times a threshold value, a function both of said reference value updated at this time and of an alarm level;

- A calculation means U'UÛS cond ion u'â ô iπc receiving as input the measurement signal Vmes and said threshold value and producing as output said alarm signal.

On the other hand, it is possible to detect the concentration of harmful gases in an environment by having a specific sensor for this harmful gas as a sensor in an alarm state detector such as defined in the preamble of claim 1. However, a particular problem arises in the context of 'invention in this guise we want to achieve a very low cost detector. Because, even if security may seem priceless, the quality of a security device is all the more effective as it is widespread. In other words, there is no point in offering high performance security detectors if their cost is prohibitive and does not allow their use. This is particularly the case when the material responsible for harmful emissions is itself very low cost. It is another object of the invention to provide an alarm state detector whose cost is reduced.

To this end, the inventors have found that the approach implemented in the prior art which consisted in monitoring the increase in the concentration of a harmful gas in the trace state (sometimes of the order of a few ppm or parts per million) was the cause of an unnecessary complication of the processing circuit and the sensor and therefore the source of a significant additional cost.

It is another object of the invention to provide an alarm state detector of a harmful gas which does not require the costly detection of traces of a specific harmful gas.

To this end, the invention also relates to a relatively majority gas detector in the medium, the measured value then being much greater in this case than in that of a trace gas.

Other advantages and characteristics of the present invention will be better understood with the aid of the description and of the appended fiyureb which are: - Figure 1: a block diagram of the detector of the invention;

- Figures 2a to 2c: curves of the evolution of the measurement voltage and other reference values in the assembly of FIG. 1;

- Figure 3: a second embodiment of the detector. In Figure 1, there is shown a block diagram of an alarm state detector for harmful gases in a medium according to the invention. An energy source 5 such as a battery electrodes with a voltage of 3 volts supplies, or does not supply, on the one hand a sensor 1, and on the other hand all the electronic circuits of the detector. The sensor 1 can be of the electrochemical type, but also of a semiconductor type. It detects the concentration of at least one given gas in a medium.

The output signal from the sensor 1 is supplied to the input of a chain 2, 3 for acquiring gas concentration measurement data mainly comprising an analog-to-digital conversion circuit 2 whose input receives the voltage across the terminals of the sensor 1 in analog form and the output of which produces a digital signal which is transmitted to a circuit 3 producing a measurement signal Vmes representative of the state of the sensor 1.

^fc2i' ^t3i ^{et t}4i Φ¹^- seront décrits plus loin. En particulier, le circuit de conversion analogique numérique est activé sous le séquencement t_^ . L^e circuit de conversion analogigue numérigue peut comporter un échantillonneur bloqueur, suivi d'un convertisseur analogique numérique proprement dit. Les échantillons prélevés à la cadence t-,^ sont convertis en mots binaires sur une longueur par exemple de 8 bits.The detector circuits are clocked by a sequencer 4 which produces noted time slots

^fc 2i ' ^t 3i ^{and t} 4i Φ ¹ ^ - will be described later. In particular, the analog-to-digital conversion circuit is activated under the sequencing t_ ^. ^T analogigue numérigue conversion circuit may include a sample and hold, followed by a digital to analog converter itself. The samples taken at the rate t -, ^ are converted into binary words over a length for example of 8 bits.

In one embodiment, a filtering of the measurements from the sensor is performed. This filtering can be carried out either at the input of the digital conversion circuit by an analog filter made up of resistance-capacitance cells, or on the digital samples themselves by digital filtering. In this latter variant, the filtering is carried out by the circuit for calculating the measurement value Vmes under the form of an average of the values measured over time, the instants of measurement of this average being chosen by a timing t _2j _ supplied by the sequencer 4 or by a predetermined number (for example of 16 samples) of previous values on the date t _2j _.

In this embodiment, the voltage measured at the terminals of the sensor 1 is converted into a succession of N = 16 values V ^ to V _N and the circuit 3 for calculating the measurement value Vmes executes the average: Vmes = (V_ + V ₂ +... + V _N ) / N

Other means or filtering functions are possible such as a weighted average in the laguelle the averaged values V- ^ to V ₁₆ are weighted by predetermined or calculated coefficients C-_ a C _lg .

On the other hand, the detector of the invention also comprises a means 7 for calculating a reference value such as a voltage Vref and a means 8 for calculating a threshold value such as a voltage Vseuil.

According to the invention, one of the problems encountered in the prior art is solved by realizing: during the commissioning of the detector of the invention, self-learning of the detector in the environment where it is located;

- During operation of the detector of the invention, self-adaptation of the detector to changes in characteristics, for example of the sensor, changes due in particular to the aging of the components. In this way, the abovementioned disadvantages which are mainly avoided are avoided: the obligation to use calibrated components, the frequent replacement of the sensors, the calibration of these in manufacture, the control of their fishing; eb α réa LiciuâpOi. ELocking and manual adaptation of the measuring chain when installing the detector in the environment where it is installed. To carry out the self-learning of the detector, the means for calculating the reference value is constructed in such a way that this value is calculated during a first step when the sensor is started up in the environment which it must monitor.

For example, when the detector of the invention is equipped with an oxygen detector for monitoring, for example, an oil stove, when it is put into service, it is imperative that the atmosphere of the room is normal, it that is, the oxygen concentration in the room is normal.

To perform the auto-adaptation of the detector, the threshold value is generated by the calculation means 8 from the reference value supplied by the means 7. In general, the threshold value is obtained by a function predetermined recorded in the form of an operator f, such that: Vseuil = f (Vref, tv) where tv is a representative quantity of the sensor 1. This value tv can be predetermined in advance, calibrated to the setting during the manufacture of the detector or even subject to environmental conditions.

Thus, the detector of the invention is constantly updated adapted to external conditions, particularly in the event of aging of the sensor. Note that the invention allows the adaptation of the detector to be executed without external intervention.

The detector of the invention finally comprises a circuit 6 intended to produce an alarm in the form of a signal or message which can take one or more distinct values depending on the alarm cases. The circuit 6 receives on the one hand, the threshold value Vseuil by an output 9 of the circuit 8 and on the other hand, the measurement value Vmes by an input 12 i uv cuuu i. <_ιc x & au i LC .U. tiH-UIt J t-iG .αx ui uc xα. IUO._J UJ- C Vmes. The calculation circuit 6 intended to produce an alarm includes a memory of an alarm condition such as a comparison of the measured value Vmes with the threshold value Vseuil, making it possible to produce, when the condition is verified an alarm signal by an output 11 transmitted to an alarm actuator 10.

The alarm actuator can comprise several elements, alone or in combination, in particular an audible alarm, a visual alarm, a remote transmission of an alarm datum on a line such as a telephone line, and finally can also actuate a trigger to stop the production of harmful vapors guue the detector of the invention can detect. When several elements are provided, the condition recorded in circuit 6 provides as many output states, each element then decoding the alarm message intended for it. In a preferred embodiment, the sequencer 4 also produces sequences t ^ intended for the circuit 8 and t _5j _ intended to produce an alarm. The different signals - ^, t _? ! ' ^t 3i ' ^t 4i ^t 5i can be at different frequencies and at different phases. In FIGS. 2a to 2c, there are shown curves representing the evolution of the voltage measured on a sensor as a function of different states as well as the reference values used in the circuit of the invention. In the embodiment of the invention illustrated with the aid of these figures, it is noted that the measurement signal Vmes is decreasing. However, in the approach of the prior art, the concentration of harmful gases is presumed to increase to generate an alarm situation. One of the merits of the invention is precisely to have reversed this approach which we said above that it forced to use a detector sensitive to traces of harmful products. The sensor used to produce the graphs in FIGS. 2a to 2c, which is the sensor sensor 1 on the detector of FIG. 1, is here a sensor of a majority gas in the medium. In the preferred embodiment of the invention, the majority gas is oxygen and the medium is the ambient atmosphere. The detector of the invention is for example coupled to an oil heating boiler.

It is clear that the characteristics recited above also apply to a detector equipped with a sensor of toxic gas at low concentration, such as carbon monoxide, and the curves and therefore the alarm conditions are therefore reversed. Those skilled in the art will be able to interpret the teachings given by way of illustration. But the invention finds its full force with a majority gas sensor.

In FIGS. 2a to 2c, there is shown on the abscissa or horizontal axis, the times t and on the ordinate, or vertical axis, the voltage measured V _meε across the terminals of the sensor 1. FIGS. 2a and 2b correspond to the operations of two sensors of majority gases in normal atmospheres, and Figure 2c shows an operation of the detector of the invention in an atmosphere in which an alarm situation occurs. In particular, the measurement amplifier or measurement amplification chain makes it possible to obtain measurement voltages according to what is shown in FIGS. 2a to 2C. Thus, the reasoning which is required therein in the following description must be reversed if the amplification is of the inverting type.

In FIG. 2a, two constant levels V _maχ and V _{min have been represented} . The values V _maχ and v _min are representative values of sensor malfunction and can be determined at the factory during the manufacture of the sensor. Thus, a capricorn of a given type intended for a specific application will be characterized by a pair ( ^v _max r ^v min ^ * ^At the initial instant, t = 0, corresponding to a powering up of the detector of the invention, the value rήinimuiû VΓÛXÛ, stored at a first address of a random access memory, is transferred to another address as the initial value of another parameter: the reference value _re f. The value of reference is therefore initially loaded at the minimum value. This operation is a preferred mode of self-learning as defined above.

During a period at the end of which the time t = 1 is reached, the detector of the invention is polarized and charged so as to not take account of the transient states during the first power-up. The details of this power-up operation will be described later.

The average value of the voltages measured at the sensor terminals is then calculated at time t = 1 and is worth

Vmesd ⁾ '

At each detection instant t = 1, 2, 3, ..., from r = 1, the reference voltage Vref is compared to the measured value Vmes. The test carried out by circuit 6 firstly has a condition: ^v ref < ^v mes ⁽ D

If condition (1) is satisfied, V _re £ is loaded at the value V _mes on the measurement date. Otherwise,

Your _re remains unchanged. In FIG. 2b, the behavior of another type of sensor has been shown in which the nominal voltage of the sensor increases from its activation in a normal atmosphere up to a value Vc at an instant t = i, then then decrease. In this case, the detector of the invention records an increase in the reference voltage since the condition (1): _re; c <V _mes is verified and that the circuit for calculating the reference voltage then fixes: ^v ref ^{= v} mes' Therefore, the reference voltage Vref is adapted to the evolution of the detector and therefore the detector of the invention is self-adaptive. In fact, this self-adaptation characteristic is described in the case of the evolutions of uapLeui. dib, it actually applies to other changes caused by variations in the other components. It is thus possible to choose low cost components. The rest of the operation according to FIG. 2b is strictly identical to that explained in FIG. 2a.

In FIG. 2c, the evolution over time has been shown for a sensor fitted to the detector of the invention in the event that an accident occurs during combustion.

Over time, the voltage across the sensor should follow the indicated curve V _{norιr [} .

Each time the detector is interrogated, circuit 6 calculates beforehand: ^v threshold = ^v ref ^"tv < ⁴ > which is a special case of the above-mentioned relation:

Vseuil = f (Vref, tv) where the operator f is here a subtractor, since the sensor is a majority gas sensor. In particular, if the detector worked with a noxious gas sensor, the operator would, in a preferred embodiment, be an adder.

The magnitude tv is recorded in a given area of the detector's memory. Then, circuit 6 tests two other conditions (2) and

(3): ^v min < ^v mes < ^v max < ² > ^v threshold <mes < ³ >

The above condition test (2) is carried out in such a way as to prevent a faulty sensor from disturbing the operation or reducing the safety performance of the assembly.

In particular, the value _m ^ _n is reached when the voltage of the battery which polarizes the entire assembly is too low. This situation occurs in particular when the detector of the invention is contained in a box containing electric batteries (two standard 1.5 Volt batteries).

On the other hand, the sensors used. Without the invention are sensors whose nominal voltage, that is to say the voltage in a reference medium in which the concentration of majority gases is normal, decreases with over time. In this case, of course, the detector of the invention must trigger from a certain threshold, in particular when the measured voltage, even without the indication of a too low oxygen concentration reaches a too low value.

If the oxygen concentration decreases, the measured voltage shown by the solid line curve V _mes decreases faster than V _norm . When condition (3): threshold ^< "mes becomes false, circuit 8 producing an alarm message is excited. Conversely, if the sensor is a noxious gas sensor, the monitored concentration increases in an alarm situation and the condition (3) would then be:

Vmes <Vseuil. In one embodiment, to mask false alarms, especially if the phenomenon is slowly evolving, a delay is imposed on the circuit before it sends the alarm message to the actuators. In particular, in the case where the detector is connected to an actuator of an audible alarm as a first degree of alarm, then is connected to a switch blogging the operation of the apparatus producing harmful gases, as the phenomenon having provoked the drop in concentration may disappear, it is not necessary to trigger even such a first level of alarm. A timing circuit produced by a time counter is therefore interposed between the output of the alarm message generator and the alarm actuators whose delay is predetermined according to the application. As indicated above, the above reasoning concerns a chain of direct amplification. If the amplifier is an inverter, the conditions and functions become: ^v mes < ^v ref ^(i? > ^V min <Vs < ^v max < ² '> ^v mes < ^v threshold < ³ '> ^v threshold = ^v ref ^{+ tv} < ⁴ '> In a preferred embodiment represented in FIG. 3, the detector of the invention mainly comprises an electro-chemical sensor 50 specifying oxygen, such as the KEXX sensor from the company Japan Storage, connected via a predetermined gain amplifier 51 at the input port 52 of the analog converter 53 of a micro-controller M50725 from the company Mitsubishi, micro-controller referenced 54 in the drawing and of which at least one output port 55 is connected to an actuator of alarm like a buzzer 56. The detector is produced on a card printed in a box, not shown in the drawing, open to ambient air to control its concentration and is powered by two standard batteries 57 and 58 of 1, 5 volts. It is a merit of the invention to allow the use of a low voltage source just sufficient to polarize the components of the detector. This allows the use of inexpensive power sources.

A program is loaded into an internal memory 59 of the microcontroller 54 which manages the various functions executed by the microcontroller and which follow the operations described above with the aid of FIG. 1.

In order to allow reliable use during the lifespan of the sensor which is approximately ten years, it is necessary to provide the detector with a means of energy saving.

In a particular embodiment, to reduce consumption, due to the fact that the variations in concentration of majority gas are slower than those in harmful gases, it is possible to provide periods of deactivation of the supply alternating with periods of detection .

In a preferred embodiment, the deactivation period lasts 150 seconds, during which the

- J- t »v ^ u.≈iiv_.≈ uc x iiu x yc υ υ α.ι_, i vαu i. xc ιι ι.ι υ- υυιn.-. uxo-u_. mj -τ can be slowed down, or the latter placed in the standby state if it includes a HALT mode. During this period, the power supply Val of the batteries is disconnected above 57 and 58 of any circuit internal to the detector and which consumes electricity. In particular, the program executed in the microcontroller places an output 61 for controlling the power supply, in particular of the amplifier, but also of a reference voltage generator 62 intended for a corresponding input 63 of the internal analog-digital converter circuit 53 of the microcontroller 54 in the inactive state. In this state, the voltage Vr and the voltage V fall back to ground. The reference voltage generator comprises a Zener diode which is charged up to the voltage Val by means of a resistor Rr and whose supply Val is supplied by an inverting amplifier Ar. The reference voltage Vr is supplied by the output Vr. The voltage V, which is to the nearest loss, exactly equal to the supply voltage Val, is however brought to ground level during the controlled disconnection periods when the output 61 of the microcontroller 54 is at the high level, by a inverting amplifier As whose output is referenced V.

In a following period, for example of the same duration, the microcontroller reactivates the circuits, such as the circuit 51 or the converter circuit 53 by its input 63 and their supply and detection is resumed. The power control output then returns to the active level. As the amplifiers Ar and As are inverting amplifiers in the embodiment, the selective supply is effective when the output 61 is low. The state of this output is switched by the execution of the software stored in memory 59.

In one embodiment, the microcontroller is equipped with an output 65 which always emits a succession of pulses intended to enable an alarm 66 to be triggered if the mi io-conLiôlêui fails. This so-called watchdog output 65 discharges a capacitor C, the output of which is transmitted to an adaptation amplifier AO. Amplifier output AO adapter is connected to a safety actuator 66 such as for example the electromagnet to cut off the operation of the monitored heater, and a fault flag is placed on suitable outputs 67 and 70 of the microcontroller.

The so-called watchdog output 65 is connected to a trigger circuit 69 which comprises a pair of diodes D1 and D2. The diodes are mounted head to tail between output 65 and the supply line Val and their common point is transmitted to the input of an inverting amplifier AO whose output is protected by a diode D3 connected in reverse in series.

The output of the diode D3 is connected to a common point between the input of an inverting amplifier Al and an output 70 of the microcontroller 54. The output of the amplifier Al is connected to the input of an inverting amplifier A2 which attacks the base of a transistor Tr connected between the supply Val and a first terminal of relay 66. The other terminal of relay 66 is connected to ground and the coil of relay 66 is protected by a diode D.

When a fault appears in the detector, the output 70 is placed at the low level. Capacitor C discharges and the output of amplifier A0 goes low. Therefore, the relay 66 is triggered and the device on which the detector of Figure 3 is mounted can be put out of service by means of this relay.

In addition, the microcontroller program, in one of the situations evoked during the discussion of the previous figures, can also, as a second alarm severity level trigger the general cut-off, for example if the alarm output on the buzzer 56 is maintained beyond a time delay of for example five minutes.

In general, the status indicator lines are connected to a display and / or transmission means of status messages indicating to the user the reason for the failure or the status of the detector. Indeed, the detector of the invention can be connected to a remote alarm center. The chosen link can be RS232 or peri-computer with connection to the switched telephone network like the Minitel network in France.

These state indicator outputs 67 are connected in a preferred example to light-emitting diodes associated with status messages locally indicating to the user the reason for the failure or the state of the detector. In one embodiment, used in particular when the detector of the invention charges a circuit drawing a lot of current, especially when it is started, the terminal 75 for resetting the initial state (RESET) of the microcontroller is connected to the positive supply line Val by a resistance - capacitance load circuit 70 so as not to subject the microcontroller to large voltage variations on such occasions. Such a circuit 70 includes a capacitor 71 connected between ground and the input terminal 75. The input terminal is connected to the voltage Val by a resistor 72 in parallel with a diode 73.

Other arrangements can be used in the present invention, in particular by implementing the conventional circuits making it possible to test the detector when it is produced around such a microcontroller.

On the other hand, the exemplary embodiments have been given by way of illustration.

Claims

CLAIMS 1. Alarm state detector as a detector of the concentration of harmful gases in a medium, of the type comprising at least: - a sensor (1) of a size such as the concentration of a gas in a medium, said sensor producing a measurement signal representative of the concentration of at least one gas in the medium under surveillance, characterized in that the detector also comprises: - means (7) for producing at least when the detector, then at predetermined times, a reference value (Vref) representative of the intrinsic state of the detector; means (8) for producing at predetermined times a threshold value (Vseuil), according to the relation: Vseuil = f (Vref, tv) function both of said reference value (Vref) updated at this time and of an alarm level (tv); - A means (6) of calculating an alarm condition receiving as input the measurement signal (Vmes) and said threshold value (Vseuil) and producing as output said alarm signal. 2. Detector according to claim 1, characterized in that it comprises a chain (2, 3) for the acquisition of gas concentration measurement data mainly comprising an analog digital conversion circuit (2) and a circuit (3 ) developing a measurement signal (Vmes) representative of the state of sensor 1. 3. Detector according to claim 2, characterized in that it also comprises a sequencer (4) which produces time slots (t -. ^, T ₂ t _β i and t ^), in particular for activating the analog conversion circuit numeric under the seqUéiCêiûeiiL (i _j ). 4. Detector according to claim 3, characterized in that the analog-digital conversion circuit comprises a blocking sampler, followed by a converter analog digital whose samples taken at the rate (t- ^) are converted into binary words over a length for example of 8 bits supplied to a circuit (3) for calculating the measurement signal (Vmes) proper in digital form. 5. Detector according to one of the preceding claims, characterized in that it comprises a means for filtering the measurements (V- _j _ to V ₁₆ ) coming from the sensor (1) comprising an analog filter made up of resistance cells- capacity at the input of the analog-to-digital conversion circuit and / or a digital-type filter executing a weighted average or not of the values measured over time (V-, at ^ g), the instants of measurement of this average being chosen by a timing (t ₂ _) provided by the sequencer (4) or by a predetermined number of values previous to the date t ₂ ^. 6. Detector according to one of the preceding claims, characterized in that it includes a memory of a pair of values (V _maχ ) and (V _mj _ _n ) which characterize a determined application of the detector, in that at a initial instant (t = 0) as a power-up of the detector, the minimum value (Vmin), respectively maximum (Vmax), is transferred to an address containing the reference value (V _re f). 7. Detector according to one of the preceding claims, characterized in that the sensor (1) is a specific sensor of at least one major gas for detecting an alarm situation, in that the means (8) for calculating the threshold voltage (Vseuil) performs the operation: Vseuil = Vref - tv said tv value can be predetermined in advance, calibrated to the setting during the manufacture of the detector or even subject to environmental conditions, such as a quantity cci ci Relative to id fleeing from the sensor êL of the detector application, and being recorded in a given zone of the memory of the detector. 8. Detector according to claim 7, characterized in that that the circuit (7) for calculating the reference value, in order to update said reference value, compares the last reference value (Vref) stored with the measurement value (Vmes) calculated during the operating period in progress according to the relation: ^v ref < ^v mes ^ in that, if condition (1) is satisfied, _ref is loaded at the said measurement value (V _mes ) calculated during the current operating period, otherwise, the said reference value ( ^v _re f) remains unchanged so as to carry out the auto-adaptation of the detector. 9. Detector according to claim 1, characterized in that the circuit (6) for activating an alarm is activated by evaluation of the relationships: ^v min < ^v mes < ^v max < ² > ^v threshold < ^v mes < ³ > so that if conditions (2) or (3) are not satisfied, it will emit an alarm signal. 10. Detector according to claim 9, characterized in that, to mask false alarms, a delay circuit is interposed between the circuit intended to emit an alarm message and the alarm actuators auxguels are intended for these messages. 11. Detector according to one of the preceding claims, characterized in that it mainly comprises a specific electro-chemical sensor αe oxygen connected via a predetermined gain amplifier to the input port of the analog converter d '' a microcontroller, at least one output port of which is connected to an alarm actuator such as a buzzer, and in that the detector is produced on a printed card in a box open to ambient air to control the concentration of it and in that it is powered by standard batteries. 12. Detector according to one of the preceding claims, characterized in that it includes a memory in which is recorded a program which manages the various functions performed by the microcontroller. 13. Detector according to claim 12, characterized in that it comprises an energy saving means so as to allow reliable use during the lifetime of the sensor. 14. Detector according to claim 13, characterized in that the energy saving means consists of a succession of periods of deactivation of the power supply alternating with detection periods. 15. Detector according to claim 14, characterized in that during the deactivation period the frequency of the clock activating the microcontroller is slowed down, or the latter placed in standby state if it includes a HALT mode, and period during Laguelle disconnects the power supply of the above batteries from any consumer circuit, and / or in that the program executed in the microcontroller places a power control output, in particular of the amplifier, but also of a generator. reference voltage intended for the internal analog-digital converter circuit of the microcontroller in the inactive state, and in that in a following period, for example of the same duration, the microcontroller reactivates the circuits and their supply and the detection is resumed, the power control output then goes back to the active level. 16. Detector according to claim 11, characterized in that the microcontroller is equipped with an output (65) which permanently emits a succession of pulses intended to enable an alarm to be triggered if the microcontroller fails, the so-called watchdog output, discharging a capacitor (C) whose output is transmitted to an adaptation amplifier (AO), and in this respect that the output of the adaptation amplifier is connected to a safety actuator (66) for example a solenoid for cutting off the operation of the monitored heating appliance, and in that a fault indicator is placed on outputs suitable for the microcontroller, in that these status indicator outputs are connected to a means of visualization and / or transmission of status messages indicating to the user the reason for the failure or the status of the detector . 17. Detector according to claim 16, characterized in that a reset terminal (RESET) of the microcontroller is connected to the positive supply line by a resistance - capacitance load circuit so as not to subject the microcontroller to large variations in voltage during the periods of detection.