EP0384209B1 - Method for the operation of an ionization smoke detector, and ionization smoke detector - Google Patents

Abstract

This detector exhibits a measuring chamber which can be ionised by a radioactive source, is accessible to the environmental air and has a first electrode connected to a d.c. feed voltage and a measuring electrode, the potential of which changes as a function of the density of the smoke when smoke enters the measuring chamber, and is measured for the purpose of generating a smoke alarm signal when it reaches a predetermined value, the potential of the measuring electrode being measured for at least one further electrical field strength and being compared with at least one second potential value which occurs at the second field strength in accordance with the law of small ion deposition when there are smoke aerosols in the measuring chamber. <IMAGE>

Description

The invention relates to a method for operating an ionization smoke detector according to the preamble of patent claim 1.

It is known to use an open ionization chamber to detect the increasing aerosol content (smoke) in the air. A radioactive element generates an ion current in the ionization chamber, which is reduced by the so-called small ion accumulation effect in the presence of smoke aerosols. Conventional ionization smoke detectors trigger an alarm via the detection line when a predetermined threshold value for the ion current or a potential caused thereby (at the measuring electrode) is exceeded. So-called analog detectors are becoming more and more popular used (DE-AS 22 57 931, DE-OS 29 46 507, EP 0 070 449). Depending on the analog value of the respective measuring chamber current, a corresponding signal is generated for the evaluation device.

A fire alarm system normally consists of a large number of fire alarms that are connected in groups to a fire alarm control panel via power supply and signal lines. The evaluation of the analog signals requires an assigned unique identification signal for each detector and its respective measured value. In order to detect a fire as immediately as possible, it is necessary to issue analog signals in short time intervals. Since a large number of fire detectors are usually connected to a common cable, there is a large accumulation of signals. A very high-quality detector identification word, each consisting of a signal sequence and an identification data word containing the associated analog value, are imperative, as is a high-quality cable network for secure transmission to the control center, which is often far away from the detectors (EP 0 121 048 or EP 0 070 449) . In the control center itself, a relatively high level of complexity is required for the data processing of the many signal sequences (EP 0 067 339).

This effort is made in order to detect changes in the measuring chamber current which are not due to a fire as early as possible and to avoid false alarms (DE-AS 22 57 931 or DE-OS 29 46 507).

Apart from climatic influences such as temperature, pressure etc. as well as signs of aging, in particular the radioactive element, the proper operation of such smoke detectors is impaired by soiling which naturally varies greatly depending on the atmosphere in which the detector is exposed. A distinction is essentially made between two harmful effects based on different types of pollution. If the contamination on the insulation of the component carrying the measuring electrode predominates, leakage currents lead to a reduction in the sensitivity or even to a non-response. In order to detect this state in good time, solutions have already been proposed (DE-PS 20 29 794, EP 0 033 888, DE-OS 30 04 753 or DE-PS 20 04 584).

If, on the other hand, the radioactive element is predominantly contaminated, for example due to dust deposits, there is a reduction in the measuring chamber current due to a reduction in the kinetic energy or the ionization ability of the radioactive radiation; the Ionization smoke detectors become more sensitive to smoke. If the radioactive element continues to become soiled unnoticed, a false alarm will occur if appropriate measures are not taken.

Various solutions have already been proposed for early detection of this highly critical condition of a detector. In the case of conventionally operating threshold detectors, for example, one or more additional prewarning thresholds are provided, which trigger even with a relatively small decrease in chamber current (CH-PS 629 905 or CH-PS 574 532). In order to be able to check the function of the ionization smoke detectors from the control center or to determine the actual sensitivity, or more precisely, the voltage difference to be overcome for an alarm triggering at the measuring electrode, it has also been proposed to increase the voltage at the outer electrode of the measuring chamber continuously or step by step increase (DE-PS 20 19 791, DE-PS 202 764 or DE-PS 20 50 719). DE-OS 21 21 382 also suggests evaluating only changes in the measuring chamber current that extend over longer periods of time in order to distinguish whether smoke or, for example, dirt is the cause of a change in the chamber current. Extremely slow changes in the current are due to the effects of dirt In addition, the latter document also suggests the installation of a radiation detector with which the radioactivity is measured directly in order to be able to immediately detect changes in the ionization power. The installation of auxiliary electrodes is also described in the same document in order to be able to more clearly identify or compensate for an increase in the insulation leakage current.

It is also known from EP 0 121 048 to equip each ionization smoke detector with so-called interference levels. Additional thresholds are formed below the alarm threshold and an overlaid long-term drift is also taken into account. A comparable method has also become known for analog detectors (EP 0 070 449).

It has also become known from EP 0 067 339 to use changes in the measuring chamber quiescent current caused by fluctuating ambient conditions as a criterion for whether the detector is in the correct operating state at all.

All of the methods that have become known so far have no path that allows a sufficiently certain distinction to be made as to whether dirt deposits on the radioactive element or Floating smoke aerosols are the cause of a reduction in the measuring chamber current. The response of a detector to so-called prewarning thresholds requires a person-to-be check to see whether a fire is emerging, ie a responsible operator triggers an extensive alarm organization. In the large number of cases, contamination is the cause of the triggering of the pre-warning threshold, but there is a risk that attention will be lowered or at least a great deal of uncertainty will be raised. Fire detectors that suppress an alarm in the event of a relatively slow change in the measuring chamber current harbor the risk that they will slowly detect smoldering fires very late or not at all. A very strong short-term contamination or, for example, condensation on the radioactive emitters cannot be distinguished with the aid of these methods from a current change in the measuring chamber caused by a rapid rise in smoke.

In principle, these shortcomings also have the known analog systems. Even with a comparatively high technical effort, only a few of the errors that actually occur can be caused by contamination to be faked, to be recognized. In most known solutions relating to most analog detectors, either contamination or aging is assumed in the event of a very slow change in the measuring chamber current values, or a less meaningful evaluation of the fluctuations in the measuring chamber current occurring in normal operation is carried out.

The invention has for its object to provide a method for operating an ionization smoke detector, with which it can be reliably detected whether the change in the measuring chamber current is caused on the one hand by the entry of smoke aerosols or on the other hand by pollution or other impairment of the radioactive source.

This object is achieved according to the invention by the features of the characterizing part of patent claim 1.

In the method according to the invention, use is made of the knowledge that the measuring chamber current assumes different values when the field strength changes, depending on whether a current reduction is due, for example, to contamination and thus partial coverage of the radioactive element or the entry of smoke aerosols. Regardless of the degree of contamination, there will be a measuring chamber behave differently when the voltage drop changes due to the increase or decrease in the applied supply voltage than if there are smoke aerosols floating in the measuring chamber. According to the attachment law by Schweitler (DE-AS 12 59 277), the relative change in the ion concentration depends on the residence time of the ions in a volume element under consideration. However, the ion residence time depends on the electric field strength. In other words, with increasing field strength in the ionization chamber, the relative change in the measuring chamber current is always smaller with the same smoke density. With the same smoke density, lower field strengths (for example of a few V / cm) result in a greater percentage reduction in the chamber current compared to the reduction in higher field strengths. The reason is the ability to attach to aerosols, which decreases with increasing field strength.

On the basis of the knowledge described above, a large number of configurations of the method according to the invention are possible. It can be used both in an arrangement consisting of one or two ionization chambers and in a system operating with threshold values or analog values. A relatively simple embodiment of the invention can work as follows.

In the case of an unsaturated ionization chamber, which operates in a field strength range of a few volts / cm, which is favorable for the accumulation of ions on smoke aerosols, a defined change in the field strength is carried out when a predetermined change in the measuring chamber current is achieved. If smoke aerosols are the cause of the triggering of the change in field strength, the new (changed) chamber flow corresponding to the Accumulation Act will set in. If, for example, the field strength has been increased significantly, it is no longer optimal for the ion attachment, and a correspondingly lower value for the chamber current will result. If, on the other hand, a deposit of dirt or a film of moisture on the radioactive preparation is the cause of the change in chamber current, there will be a substantially greater change in the ionization current if the conditions are otherwise the same if the field strength increases. Based on the evaluation of the chamber current values occurring with the different field strengths, a decision can be made as to whether a fire alarm should be triggered or whether, for example, the relevant fire detector only needs cleaning. A false alarm due to dirty or condensed radioactive emitters can be avoided by the invention.

The method according to the invention also allows a change in the field strength at certain time intervals in order to be able to detect even a small amount of contamination and, if necessary, to initiate a corresponding correction of the sensitivity to smoke. The method can be used both in analog ionization smoke detectors and in those that operate as threshold detectors. In this way, the field strength switchover and the evaluation can only be triggered after one or more changes of the chamber current of different strengths have been reached. Depending on the severity of the detected contamination, either a correction of the alarm threshold in the case of low contamination or a maintenance request from a certain degree of contamination can then be triggered or the failure of the fire detector can also be signaled in the case of heavy contamination. With the aid of the method according to the invention, smoke densities of different strengths can also be detected in order to trigger corresponding pre-warning and alarm messages. However, the detection of smoke densities of different strengths is also known in the prior art.

If one assumes a field strength that is favorable for smoke accumulation, an increase in the field strength in the presence of smoke, as described, will be proportionate Set a smaller change in the ion chamber current than if dirt deposits on the radioactive element are the cause of the original chamber current change being achieved. However, if the field strength is reduced under the same initial conditions, smoke leads to a greater change in the chamber flow than a deposit of dirt on the measuring chamber emitter.

To carry out the method according to the invention, it is necessary that the characteristic curve in the measuring chamber (chamber current in relation to the chamber voltage) is (at least point) known. In order to determine the change in the potential with at least one further field strength, reference can be made, for example, to a potential value that the measuring chamber has when new. The reference values can be derived directly, for example, by measuring the new ionization chamber or from its data.

If the characteristic curve is correspondingly favorable, the measurement of the potential with only a second field strength may be sufficient to make a statement as to whether the measured potential change is due to the presence of smoke aerosols or due to dirt deposits on the radioactive material Radiator is conditional. The potentials are preferably measured on the measuring electrode for at least one field strength above and at least one field strength below the first field strength (operating field strength) in order to be able to carry out a reliable evaluation. As already mentioned, the test of an ionization smoke detector for contamination can be initiated, for example, when a decrease in chamber current and thus an increase in potential has taken place. Alternatively, the test can be carried out according to a fixed time program, which is particularly advantageous if, as in analog systems, the data is not evaluated in the individual smoke detectors, but in a control center.

According to the invention, one possibility of carrying out a measurement at a different field strength is to assign a switching device to the test circuit which changes the field strength in the measuring chamber by applying different supply voltages. As an alternative to this, it can be provided that at least two different field strength ranges are continuously formed in the measuring chamber due to a specific structure. For this purpose, an embodiment of the invention provides that the measuring chamber contains at least two pairs of electrodes that are connected to different ones Voltages are connected and the measuring electrodes of both pairs of electrodes are connected to the test circuits. Alternatively, the measuring chamber can have at least two separate measuring electrodes connected to the test circuit and a common counter electrode. The counter electrode has two electrode sections assigned to the measuring electrodes, the distances between which are different from the assigned measuring electrodes. When a predetermined voltage difference compared to the normal state is reached in the chamber area working in the smaller field strength range or the measuring electrode assigned to it, a voltage difference corresponding to the field strength can also be determined in the area working with the higher voltage when exposed to smoke aerosols. If, on the other hand, a deposit of dirt on the radioactive element is the cause of the change in potential in one chamber area, a change in voltage will accordingly clearly occur in the other area. Any deviations in the latter construction depend primarily on the design of the transition areas of the measuring chamber, in particular the field strength acting there.

Fig. 1

zeigt ein Strom-Spannungs-Diagramm eines Ionisationsrauchmelders für verschiedene Bedingungen.

Fig. 2

zeigt ein ähnliches Diagramm wie Fig. 1 mit zusätzlichen Kennlinien.

Fig. 3

zeigt ein ähnliches Diagramm wie die Figuren 1 und 2, jedoch bei Verwendung eines Ohm'schen Widerstands als Referenz für die Meßkammer.

Fig. 4

zeigt im Schnitt eine Ionisationskammeranordnung nach der Erfindung mit unterschiedlichen Feldstärkebereichen.

Fig.5

zeigt eine andere Ionisationskammeranordnung mit unterschiedlichen Feldstärkebereichen.

Fig. 6

zeigt ein Blockschaltbild für den Betrieb eines Ionisationsrauchmelders nach der Erfindung.

Fig. 7

zeigt detailliert den Funktionsablauf für die Steuer- und Auswertelogik des Blockschaltbilds nach Fig. 6.

The invention is described below with reference to drawings.

Fig. 1

shows a current-voltage diagram of an ionization smoke detector for various conditions.

Fig. 2

shows a diagram similar to FIG. 1 with additional characteristics.

Fig. 3

shows a diagram similar to Figures 1 and 2, but using an ohmic resistor as a reference for the measuring chamber.

Fig. 4

shows in section an ionization chamber arrangement according to the invention with different field strength ranges.

Fig. 5

shows another ionization chamber arrangement with different field strength ranges.

Fig. 6

shows a block diagram for the operation of an ionization smoke detector according to the invention.

Fig. 7

6 shows in detail the functional sequence for the control and evaluation logic of the block diagram according to FIG. 6.

Before going into the details shown in the drawings, it should be assumed that each of the features described may be of importance for the invention, either alone or in conjunction with features of the description.

1 shows characteristic curves of a chamber arrangement of an ionization smoke detector in which an ionization measuring chamber which is freely accessible to the ambient air and a closed ionization reference chamber, which each have a radioactive element, are connected in series. The chamber voltage UK is plotted on the abscissa and the chamber current IK is plotted on the ordinate. The characteristic lines with a solid line represent the course of the characteristic curve of the measuring chamber when new MK (new) and in the presence of smoke MK (smoke) of a predetermined constant density. The dash-dotted characteristic curves RK show the characteristic curve of the reference chamber. The dashed characteristic curve MK (dirty) represents a characteristic curve if the radioactive element in the measuring chamber is significantly contaminated.

Starting from a normal voltage U _N across both chambers, a voltage potential corresponding to intersection point C is established at the common measuring electrode. If a potential shift is detected on the measuring electrode during operation, for example by the voltage difference X, an intersection point D is reached. According to the invention, the chamber voltage is now changed to build up a different field strength, for example by switching down to the voltage value U _P1 . When the measuring chamber is new, the working point A would be set at the measuring electrode. However, if smoke was the cause of the potential change X, the reduced characteristic MK (smoke) applies. If the chamber voltage is reduced, the potential B at the measuring electrode will set. The potential difference between A and B is a₁. If, on the other hand, dirt on the radioactive element was the cause of the potential change X, the measuring chamber characteristic MK (dirty) comes into play, and there is now an intersection point K, ie only the potential difference b 1 is reached.

If, on the other hand, a switchover to a higher chamber voltage U _{P2 occurs} after the voltage difference X has occurred, the potential L would set at the measuring electrode when new. If, on the other hand, there is smoke in the chamber, the intersection M results, ie the potential difference a₂. This can now be evaluated for smoke detection. However, if the lamp is dirty, the cause of the potential change X at the nominal voltage would have been the point of intersection N at the higher test voltage. This large potential difference b₂ would be available for reliable contamination detection. The very large potential differences result from the fact that the characteristic curves at the higher chamber voltage are largely in the saturation range.

However, when the chamber voltage is reduced to smaller evaluable potential differences compared to the nominal voltage, it can already be discriminated whether smoke or dirt was the cause of the lowering of the potential at the nominal voltage. When the chamber voltage is increased, there are not only higher potential differences under the characteristic curves and intersection curves selected here, but also significant differences with regard to the cause of the chamber current decrease or potential change. It can also be seen that at low chamber voltage the ratio of the potential differences a 1 to b 1 is greater than 1. In contrast, at a test voltage higher than the normal voltage, the ratio of the potential differences a₂ to b₂ is less than 1. If an average normal chamber voltage is assumed, dirt deposits with a lower test voltage have less of an effect than smoke. At a higher one Test voltage, on the other hand, dirt has a much stronger effect than smoke in the measuring chamber. As already mentioned, there are large potential differences in the case of saturation ratios of the chambers, in particular with the test voltage U _P2 shown, which enable an accurate evaluation of the prevailing smoke density or a clear discrimination as to whether dirt is present on the radiator.

The diagram of FIG. 2 is largely the same as that of FIG. 1, but it shows a more detailed evaluation option using the method according to the invention. The solid lines MK (new) and MK (smoke) as well as the dashed line MK (dirty) correspond to those in FIG. 1. An additional characteristic characterizes the measuring chamber at MK (little smoke) with a given, identical smoke density during the measurement. An additional characteristic curve MK (little dirt) characterizes the measuring chamber with less dirt deposition on the radioactive element. The course of the reference chamber characteristics is identical to that of FIG. 1.

If there is relatively little smoke in the measuring chamber, the potential difference y results at the measuring electrode. This can be a reason to switch to the higher test voltage U _P2 . Is smoke the cause of the potential change? y has been, the measuring electrode voltage will shift from the intersection point L to the intersection point P, which causes a change in potential d at the measuring electrode. If, on the other hand, the accumulation of dirt was the cause, the measuring chamber characteristic curve follows the described course MK (little dirt). Starting from the point of intersection at U _N reached after the occurrence of the potential difference y, the potential shifts from the measuring electrode at the test voltage U _P2 to the point of intersection R. The potential difference from the points L to R now reaches the larger value f instead of the difference d when exposed to smoke due to dirt. The potential difference d can serve as a prewarning for low smoke, and if the potential difference f occurs, this can be interpreted as an indication that the ionization detector needs to be cleaned.

After evaluation, the chamber voltage can be switched back to its nominal value U _N. However, if the potential difference at the measuring electrode becomes larger during operation and reaches the value x, for example, then a switchover to the higher test voltage U _P2 takes place again. Now, as already described in connection with Fig. 1, in the presence of smoke, the potential difference a₂ for an alarm evaluation or the potential difference b₂ for the deposition of dirt will set. b₂ indicates heavy soiling of the detector and, if the degree of soiling is very high, can be used as an indication of a smoke detector that is no longer fully functional.

The diagram in FIG. 3 is based on a chamber arrangement in which the ionization reference chamber is replaced by an ohmic resistor. The straight line of resistance passing through point U _N intersects the new measuring chamber line at point U. If a potential difference z is reached by changing the chamber current, a switchover to the low chamber voltage U _{P1 takes place} . In the event of smoke, the intersection point P with the characteristic curve MK (smoke) results. The potential difference m 1 is reached. In the event of contamination, the measuring chamber characteristic curve takes the dashed MK (dirty) curve. The straight line at U _P1 intersects the dashed curve of the measuring chamber at point Q. The potential difference now assumes the value r 1. When switching to a higher test voltage U _P2 , the point of intersection T and the potential difference m₂ result in the action of smoke. With dirt, however, the measuring electrode potential shifts to the intersection S, and the potential difference r₂ is measured. Opposite the in Fig. 1 obtained potential differences, the differences determined in the arrangement of FIG. 2 are smaller, but here too the ratio m 1 to r 1 is greater than 1 (test voltage U _P1 ). The corresponding ratio m₂ to r₂ is less than 1 at the test voltage U _P2. A clear evaluation of whether dirt or smoke has caused the change in the chamber current can thus be carried out.

In order to make a very detailed and reliable decision about the smoke density and the degree of pollution, it can also make sense to switch to a reference chamber (characteristic curve RK). Now the intersection points L, M and N and, accordingly, the potential differences a₂ and b₂ would offer themselves for a very precise evaluation with the same chamber ratios.

A targeted characteristic curve can also be set with the aid of a combination of resistors and, if applicable, a reference chamber, in order to obtain potential differences with which either smoke or dirt is preferably evaluated.

As already mentioned, the evaluation of whether there is smoke in the detector or whether it is contaminated can be carried out in the Detectors themselves or at a central location. If the evaluation is carried out at a central point, it can be advantageous to also change the chamber voltage for a change in the electric field strength from a central point, for example by changing the supply voltage line by line. However, if you choose a version in which the ionization chambers and the circuit design are housed in a common housing, it is expedient to carry out the test procedure with each detector depending on its respective measuring chamber state. In order to be able to automatically carry out a test only for a certain smoke detector working on the same detection line, ie voltage supply line, and to leave the other detectors in the monitoring state, a voltage switchover or field strength change is expediently carried out only in the detector to be tested in each case. It goes without saying that the electronic switching circuit of the detector contains the necessary switching options and the necessary evaluation and signal modules.

The method described above has the advantage that it can be carried out with conventionally designed ionization chambers. On the other hand, it depends in a lot For example, to report a rapidly developing fire in a short time is to be preferred to the arrangement described below.

FIG. 4 shows an ionization chamber arrangement 10 which consists of a measuring chamber 11 and a reference chamber 12. The reference chamber 12 has a reference chamber electrode 13, and the measuring chamber 11 has an outer measuring chamber electrode 14. Both chambers 11, 12 have an outer measuring electrode 15 in common and an inner measuring electrode 16, which are separated from one another by suitable insulation 17. Radioactive emitters are arranged on both sides of the inner measuring electrode, the arrows in the chambers 11 and 12 indicating the range of the radioactive rays. The electrodes 13, 15 and 16 are flat. The outer electrode 14, on the other hand, has a stepped cup-shaped design with a central section 18 and a section 19 running around it in an annular manner, which sections are connected to one another by a substantially axial annular wall section 20. As a result, the central measuring electrode 16 acts largely with the central section 18 of the outer electrode 14 together and the outer measuring electrode 15 substantially with the outer annular portion 19 of the outer electrode 14. Thus there are two regions of different field strengths in the measuring chamber 11, the transition field strength regions not being included. For example, a supply voltage of 12 volts is applied to the outer electrode 14 and the reference chamber electrode 13. As explained, the field strength in the central region is lower than in the outer region, since the outer electrode 14 or the section 19 is at a smaller distance from the outer measuring electrode 15 than the middle section 18 from the inner measuring electrode 16. Is now in the chamber arrangement Fig. 4, the deposition of dirt on the radioactive emitter of the measuring chamber 11 cause of a change in the inner measuring electrode 16, which is operated in the region of the lower field strength, so a different potential will set at the outer measuring electrode 15 working in the region of the higher field strength. If FIG. 1 is used analogously for comparison, and if the potential on the inner electrode 16 had shifted from the working point C to D, then the potential L on the outer electrode 15 shifts to the point N. This clarifies the The example used in the method has not taken into account the compensating currents flowing through the potential difference between the measuring electrodes. If, on the other hand, smoke is the cause of the potential reduction, the electrodes 15 In contrast, 16 changed values, since the attachment of ions to smoke aerosols is more favorable in the area of smaller field strength than in the area of larger field strength. The relationships shown in Figures 1 to 3 can be applied accordingly.

Such a chamber arrangement has the advantage that time delays after switching to one or more different field strengths due to the respective transient processes can be avoided.

The chamber arrangement 25 shown in FIG. 5 is essentially the same as that of FIG. 4. A measuring chamber 26 and a reference chamber 27 are kept radially at a distance by an outer measuring electrode 28 and an inner measuring electrode 29, which are separated from one another by insulation 30 . The inner measuring electrode 29 has a radioactive emitter on both sides, the arrows shown reflecting the range of the radiation. The reference chamber 27 has a reference chamber electrode 31, and the measuring chamber 26 has an outer electrode which is formed by an inner partial electrode 32 and an outer partial electrode 33, which are insulated from one another by an annular insulation 34. The inner partial electrode 32 is also the same as the measuring electrodes 28, 29 and the reference chamber electrode 31. A part of the outer partial electrode 33 is also flat, followed by a cylindrical section which closes the chamber 26. A different voltage is now applied to the central partial electrode 32 than to the outer partial electrode 33, which results in two areas of different field strength in the measuring chamber 26 - again the transition areas are not included. The middle measuring electrode 29 is essentially assigned to the middle partial electrode 32, while the annular outer measuring electrode 28 is assigned to the annular partial electrode 33.

Applied to the example of FIG. 1, the supply voltage can be U _N and the other U _P2 . When a predetermined electrical potential shift compared to the normal state is reached in the chamber area operating in the smaller field strength range or the measuring electrode assigned to it, an electrical potential shift corresponding to the field strength can also be determined in the area working with the higher voltage U _P2 when exposed to smoke aerosols. If, on the other hand, a deposit of dirt on the radioactive element is the cause of the change in potential in one chamber area, a change in voltage will accordingly clearly occur in the other area.

In the representations of FIGS. 4 and 5, it was assumed that the inner and outer measuring electrodes are at the same electrical potential when they are new at the normal operating voltage. This can be achieved by appropriate geometric dimensioning of the measuring chamber areas operated with different field strengths, e.g. through the choice of coordinated measuring electrode surfaces, chamber volumes and also through the number of ion pairs formed in each case by the radioactive radiation in the two measuring chamber subareas. If different potentials occur at the two measuring electrodes during operation due to the effects of smoke or dirt, the electrical field image changes accordingly. In particular in the area around the electrical insulation between partial measuring electrodes, the flow of equalizing currents is promoted. These compensating currents lead to a reduction in the potential differences and must be taken into account when determining the measuring thresholds.

6 schematically shows a conventional ionization chamber arrangement 40, consisting of a measuring chamber 41 and a reference chamber 42 connected in series therewith, the common inner electrode or measuring electrode 43 being a radioactive radiator on both sides wearing. The chamber arrangement 40 is normally connected to the normal operating voltage U _N (block 45) or a test voltage U _P (block 46a) via a switch 44. A comparator 47 is connected to the measuring electrode 43 via an electronic circuit 46, which preferably contains a field effect transistor. Four threshold levels are provided in the comparator 47, namely alarm threshold value 48, dirt threshold value 49, prewarning threshold value 50 and test threshold value 51. A control and evaluation logic 52 is connected to the output of the comparator 47, from which an output to a prewarning signal stage 53 for smoke, one to one Pollution signal level 54 and one goes to an alarm signal level 55.

The circuit shown works as follows. During the normal operating voltage U _N , only low field strengths of a few volts / cm are effective for the ion transport in the chambers 41 and 42. The potential arising at the measuring electrode 43 is fed to the comparator 47. If the potential reaches the test threshold 51, for example potential W in FIG. 2, the control and evaluation logic is actuated accordingly. This is used to actuate switch 44 and to switch to a higher test voltage U _P2 (46a). This occurs during the test period at the higher voltage or the higher electric field strength a potential R, the comparator responds with its dirt threshold value, and a contamination signal is triggered in stage 54 via the control and evaluation logic. If, however, this potential is not reached, but rather potential P, the prewarning threshold 50 is reached via comparator 47 and, with the aid of the control and evaluation logic 52, a prewarning signal is emitted via stage 53, which indicates that the smoke density is low. The control and evaluation logic of the detector 40 is left in this state in order to immediately trigger an alarm in the event of a further increase in smoke after the alarm threshold 48 has been reached (alarm signal level 55). If, however, the alarm threshold is not reached within a predetermined time or the potential P is undershot again (in the direction of normal value L), the detector is switched back to its normal monitoring state with the supply voltage U _N. However, if the test threshold potential W is reached again, a new test cycle is triggered.

The functional sequence of the control and evaluation logic 52 is shown in more detail in FIG. 7. When the test threshold 51 (FIG. 6) is reached, a memory 60 is set and a control signal is sent to the switch for switching the voltage (line 61). To only after the by switching the voltage caused to settle transients a further evaluation of the measuring electrode potential, a delay element T _v1 comes into action, which is connected to the dirt threshold 49 via the line 62. If, after the delay time has elapsed, the signal corresponding to the contamination (potential R in FIG. 2) is present, a signal from the memory 60 corresponding to the higher voltage U _{P2 is} also present at the gate G1 as the second voltage, so that the signal indicates the contamination Output 64 driven and a contamination signal (stage 54; see also FIG. 6) triggered. If the threshold (dirt; potential R in FIG. 2) has not been reached after the delay time has elapsed, a gate G2 receives a negated signal. Furthermore, a signal from memory 60 characterizing the higher operating voltage is also present at gate G2. The gate G2 triggers a delay element T _v2 , the time constant of which is greater than that of the delay element T _v1 . After the time of T _v2 , the observation time is started by a timer T _v3 . If the alarm threshold at the higher test voltage is reached within the observation time, the conditions of a gate G3 are fulfilled. The alarm output 65 is triggered and thus the alarm signal is triggered (stage 55; see also FIG. 6). However, if the alarm threshold is not reached during the observation period, this is the case potential P due to low smoke density, the conditions for a gate G4 are met, and the prewarning output 63 is activated and a prewarning signal is triggered (stage 53; see also FIG. 6). If there is then no further potential shift caused by smoke rise during the observation time, the delay stage T _{v3 sends} a signal to a timing element M _v . This timer bridges the transient processes that occur when the supply voltage is reset to the monitoring state. At the same time, the memory 60 is reset. The detector works again under normal conditions. However, if the test threshold 51 is reached again, a new test cycle takes place. It goes without saying that finer graded threshold values acting in the same way can be used in an extended test.

To carry out the method, it is not necessary for each ionization fire detector to be individually assigned complete control, evaluation and signal electronics as described above. At least some of the electronics can be located in the monitoring center in order to proceed according to the method either in a predetermined order or after reaching predetermined changes in the chamber flow Evaluation with the respective detector to be checked can be interconnected via lines.

Claims (18)

1. A method for the operation of a smoke detection system having a chamber open to ambient atmosphere, said chamber containing a radioactive source for ionization of said chamber, said chamber having a first electrode (18; 32) connected to a DC supply voltage (45), and a measuring electrode (16; 29;43) whereby said DC supply voltage effects a corresponding first field strength generating an ionization current, with the distribution of said field strength depending upon the characteristics of said radioactive source and the atmosphere in said measuring chamber (11; 26; 41), the potential at said measuring electrode (16; 29; 43) or the current between said electrodes (16; 32 or 16; 29; 43), respectively, changing upon the entrance of smoke into said measuring chamber (11; 26) in dependence of the density of the smoke, said changing being measured in order to generate a smoke alarm signal if a first predetermined value is reached, at least a further field strength being generated in said measuring chamber (11; 26; 41) for test purposes, with the potential value measured at said further field strength being an indication of the contamination in said measuring chamber (11; 26; 41), characterized by the following method steps:

   a) said further field strength is selected such that in case of smoke on the one side according to the agglomeration of the law of ions and in case of a contamination of said radioactive source on the other side a significantly different field strength distribution or a significantly different ionization current, respectively, results, and at least a further measuring of said potential at said measuring electrode or of said ionization current is carried out,

   b) the potential value or current value, respectively, yielded by said further measuring (potential at point M or N; B or K; P or R; Q or P; S or T) is compared with known measuring values referred to said measuring chamber in a smokeless state or to an actual potential or current value, respectively, measured or calculated, respectively, for the smokeless or approximately smokeless state,

   c) from the difference or the relation of said further and said known measuring values (a1, b1; a2, b2; x, y; d, f; m1, r1, m2, r2) the existence of smoke in said measuring chamber (11; 26; 41) or the contamination of said radioactive source, respectively, is derived.
2. The method of claim 1, wherein in case of an elevated field strength and a relatively large change of said potential or said current, respectively, (b2; f; r2) a contamination is assumed while in case of a relatively small change of said potential or said current, respectively, (a2; d; m2) the existence of smoke in said measuring chamber (11; 26; 41) is assumed.
3. The method of claim 1 or 2, wherein in case of a reduced field strength and a relatively large change of said potential or said current, respectively (a1; m1), the existence of smoke in said measuring chamber is assumed while in case of a relatively small change of said potential or said current, respectively, (b1; r1) a contamination in said measuring chamber (11; 26; 41) is assumed.
4. The method of one of the claims 1 to 3, wherein the determination of said potential or the change of said potential, respectively, takes place at said second field strength when said potential for said first field strength reaches a predetermined value.
5. The method of one of the claims 1 to 3, wherein the determination of said potential or the change of said potential, respectively, for said second field strength takes place in accordance with a predetermined time program.
6. The method of one of the claims 1 to 3, wherein said measuring of said potential at said measuring electrode (15; 28; 43) at at least a second field strength is initiated latest when said measured potential at said first field strength reaches an alarm threshold value.
7. The method of one of the claims 1 to 6, wherein said potentials at said measuring electrode (15; 28; 43) are measured for at least one field strength above and at least one field strength below said first field strength (operational field strength).
8. An ionization detection smoke system comprising a measuring chamber (11; 26; 41) adapted to be ionized by a radioactive source, said measuring chamber including a first electrode (18; 32) connected to supply DC voltage (45) and a measuring electrode (16; 29; 43), a first field strength generating an ionization current existing in said measuring chamber, the amount thereof depending upon the characteristics of the radioactive source and the atmosphere in said measuring chamber, with said potential at said measuring electrode (16; 29; 43) or said ionisation current between said electrodes (18; 32; 16; 29; 43) changing upon entry of smoke into said measuring chamber in dependence of the density of the smoke and being measured by a test circuit in order to generate a smoke alarm signal if said potential or said current, respectively, reaches a first predetermined value, a second field strength in said measuring chamber (11; 26; 41) between said electrodes (19; 33; 15; 28; 43) being generated to carry out the method of one of the claims 1 to 7, characterized in that said test circuit transmits said potential values or the changes of said potential values or said current or said current changes, respectively, between said electrodes (18, 16; 19, 15; 32, 29; 33, 28; 43) for at least said first and said second field strength to an evaluation circuit (47; 52) wherein predetermined threshold values for said potential or said current values for said first and said second field strength are stored representing the smoke or the contamination state, respectively, for a comparison with said potential or said potential change or said measured current or said current change, respectively, and an alarm signal circuit (54) is provided generating a contamination signal if said threshold for the contamination state is reached or exceeded.
9. The system of claim 8, wherein at least a contamination (49), a pre-alarm (50) and an alarm threshold (48) are stored.
10. The system of claim 8 or 9, wherein said test circuit is activated a predetermined time duration upon arrival of said pre-alarm threshold (50), however, is deactivated if said alarm threshold value (48) is not reached after said predetermined time duration or if said common potential or said current at said second field strength, respectively, goes below a further lower predetermined potential or current value.
11. The system of one of the claims 8 to 10, wherein said test circuit is included inside the smoke detector.
12. The system of one of the claims 8 to 10, wherein a central separate test circuit is provided connected to individual smoke detectors (40).
13. The system of one of the claims 8 to 12, wherein said test circuit includes a switch means (44) which changes said field strength in said measuring chamber (41) by supply of different DC voltages (U_N, U_P).
14. The system of one of the claims 8 to 13, wherein said test circuit is operated continuously or intermittently.
15. The system of one of the claims 1 to 14, wherein in order to change the sensitivity said test circuit changes said predetermined alarm threshold (48) in dependence of deviations of said measured potentials or said measured currents, respectively, from predetermined initial values.
16. The system of one of the claims 8 to 15, wherein said system is made ineffective if said potential or said current value measured upon contamination approximates the alarm threshold (48)
17. The system of claim 8, wherein said measuring chamber (26) includes at least two pairs of electrodes (32, 29; 33, 28) connected to different voltages and said measuring electrodes (28; 29) of both of said pairs of electrodes (32, 29, 33, 28) are connected to said test circuit.
18. The system of claim 8, wherein said measuring chamber 11 includes at least two separate measuring electrodes (15, 16) connected to said test circuit, said measuring chamber (11) including a common counterelectrode (14) having two electrode portions (18, 19) associated with said measuring electrodes (15, 18), said portions having different distances from said associated measuring electrodes (15, 16).

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