US3768313A

US3768313A - Method of determining the risk of explosion of a preferably gaseous medium and apparatus for carrying out this method

Info

Publication number: US3768313A
Application number: US00203180A
Authority: US
Inventors: S Johansson; L Johnsson; K Strid
Original assignee: INDUSTRILABORATORIET AB
Current assignee: INDUSTRILABORATORIET AB
Priority date: 1970-12-02
Filing date: 1971-11-30
Publication date: 1973-10-30
Anticipated expiration: 1990-10-30
Also published as: DE2159359A1; GB1369038A; SE346619B; FR2117514A5

Abstract

The risk of explosion of a preferably gaseous medium is determined by measuring a parameter, which is determinative of the inflammability of the medium, by means of a probe, which comprises an ignition chamber and an ignition initiator in said chamber. A successively varied amount of energy is sequentially supplied to the ignition initiator, until the amount of energy, which is operatively therein, has reached such a level that the medium in the ignition chamber only just ignites. The actuation level, at which the medium in the ignition chamber actuated by the ignition initiator ignites, constitutes a measure of said parameter of the medium and is determined by adequate, preferably electrical means.

Description

United States Patent Johansson et al.

[451 Oct. 30, 1973 [54] METHOD F D R ININ H RISK on 1,744,416 1/1930 Schanly 73/26 EXPLOSION OF A PREFERABLY GASEOUS Egg g; 3; 135 g ompson 3,186,213 6/1965 Donnell t. 73/36 [75] Inventors: Stig R. Johansson, Jonkoping; Lars Primary Examiner-james J Gin g 322 222383 ig o gg l'gzi Attorney-Cushman, Darby & Cushman [73] Assignee: lndustrilaboratorict AB, Jonkoping, Sweden [57] ABSTRACT 22 Filed; No 30 1971 The risk of explosion of a preferably gaseous medium is determined by measuring a parameter, which is de- [21] APPL 203,180 terminative of the inflammability of the medium, by

means of a probe, which comprises an ignition cham- [30] Foreign Application Priority Data her and an ignition initiator in said chamber. A suc- Dec 2 1970 Sweden 7875/70 cessively varied amount of energy is sequentially sup plied to the ignition initiator, until the amount of en- [52] U 8 Cl 73/432 R 73/35 73/26 ergy, which is operatively therein, has reached such a [Sl] Int Cl G01, 25/54 level that the medium in the ignition chamber only [58] Field 36 15 R just ignites. The actuation level, :at which the medium 73/25 in the ignition chamber actuated by the ignition initiator ignites, constitutes a measure of said parameter of [56] References Cited the medium and is determined by adequate, preferably 1 t l A UNITED STATES PATENTS 6 ac means 2,939,312 6/1960 Jacobs et al. 73/36 14 Claims, 14 Drawing Figures PAIENTEIlucran mm 3.768313 SHEET 10F 7 3 L Zb J I 17 2021 --.m' s 27' 28 29 A nu/25 'III mow I rlllr M- k b l:

PAIENTEDncrso 1975 327681313 SHEET 2 OF 7 l PATENTEDUBI 30 I915 3768B 1 3 SHEET 5 [IF 7 METHOD OF- DETERMINING THE RISK OF EXPLOSION OF A PREFERABLY GASEOUS MEDIUM AND APPARATUS FOR CARRYING OUT THIS METHOD This invention relates primarily to a method of deter mining the risk of explosion of a preferably gaseous medium and secondarily to apparatus for carrying out said method.

Accidents caused by gas and dust explosions are frequent. Besides material damages, which may be important, a sudden explosion in a usually but not necessarily gaseous medium usually also entails serious damages to persons; the number of victims of accidents of this type is comparatively great. Precautionary measures are of course essential. A necessary measure in the protective work has hitherto always consisted of the measurement of some physical or chemical property of a suspect or potentially explosive gas mixture or the like. A common method consists of the determination of the chemical composition of the gas, e.g. by means of a hot-wire instrument, after which a comparison with known data of so called explosion limits is made in order to arrive at a conclusion regarding the explosibility of the gas.

The drawback of methods such as the abovementioned one resides therein that they are indirect and therefore, unreliable. Accordingly, it would be better to measure some explosion property, such asthe selfignition temperature (T or the ignition energy (E), but one is then faced with the difficulty of carrying out the rneasurement'in such a way that no explosion occurs at the place in question. Furthermore, the purely experimental difficulties are considerable. Thus, there is, eg no simple and accurate in situ method of measuring the ignition energy, which is that property, which should be measured in this connection, in the first instance.

A primary object is this invention is to eliminate these drawbacks. This object is attained by carrying out the method and designing the apparatus according to the invention in the manner, which is set forth in the characterizing class of claims 1 and 2, respectively. The expression ignition initiator herein means an incandescent or heating body, which usually should be heated to incandescence, a spark gap, over which an electric spark or discharge can be created, or equivalent means, which may be activated to such 'an activating level that it can ignite the medium in the ignition chamber by transferring energy thereto. In the two abovementioned cases the means for activating the ignition initiator comprises means for heating the incandescent or heating body and means for generating at least one electric impulse applied to the spark gap, respectively. In the two instances the activation level is represented by the (surface) temperature of the heating body and the energy contents of. the impulse, respectively. Thanks to the fact that the probe is so designed that an electric spark, which bridges the spark gap, orthe heated body can ignite a representative sample quantity of the medium in the ignition chamber of the probe without any possibility for an explosion to propagate into a surrounding medium, direct measurements in a possibly explosive medium, which may be gaseous, liquid or solid (pulvereous) are thus made possible. The flame generated within the probe is prevented from giving rise to an explosion in the medium surrounding the probe, thanks to the fact that the mesh of the gauze of the probe is so minute that it is certainty smaller than the quenching distance of that or those media, in which the probe is to be utilized.

Further features and advantages of the method and the apparatus according to the invention will become apparent from the following particular description and the annexed drawings, which diagrammatically and as non-limiting examples illustrate some preferred embodiments of the invention and in which:

FIG. 1 is a simplified wiring diagram of a probe and its appurtenant apparatus for determining the risk of explosion in a given medium;

FIG. 2 is a longitudinal section of a first embodiment of a measuring probe having a spark gap;

FIG. 3 is a cross section on line IIIIII in FIG. 2;

FIG. 4 is a longitudinal section of a second embodiment of a probe having a spark gap;

FIG. 5 is a side view corresponding to FIG. 4;

FIG. 6 is a longitudinal section of still another embodiment of a measuring probe having a spark gap;

FIG. 7 is an end view of the probe according to FIG.

" 6 as seen in the direction of the arrows VIIVII;

FIG. 8 is a simplified wiring diagram of a probe having a heating or incandescent body as well as appurtenant equipment for determining the risk of explosion of a given medium; I

FIG. 9 is a longitudinal section of a first embodiment of the probe according to FIG. 8;.

FIG. 10 is a cross section on lineX-X according to FIG. 9;

FIG. 11 illustrates a second embodiment of a detail of the probe according'to FIG. 8;

FIG. 12 is a simplified wiring diagram of a modified probe with its appurtenant equipment;

FIG. 13 is a voltage-time diagram; and

FIG. 14 is a longitudinal section of a further embodiment of the measuring probe.

In the drawings the same or similar details are designated with the same reference characters throughout.

The apparatus illustrated in FIG. 1 comprises an adjustable high-voltage source 1,

coaxial cables

2a, 2b, 3, 4 and .5, the measuring probe, which is generally designated 6', a pressure spark gap, which is interconnected between the cables 2 and 3 4, a terminating resistor 8, a high-ohmic resistor 9, the resistance of which may be of the order of M9, an oscilloscope 10, which is connected between the coaxial cables 4 and 5 through a capacitive voltage divider 11, and an indicator 12, which is in a suitable way, e.g. by .means .of a conductor 38, coupled to a transducer in the probe 6 for indicating whether or not the medium, in which the probe is introduced, has been ignited by thespark traversing the spark gap 13 in the probe 6.

The resistance of the resistor 8 is suitably approximately equal to the impedance of the coaxial cables 2 5, which have the same impedance of the order of 50 Q. A resistor 14, which may have a resistance of the order of 0 10 k0, may be connected in serieswith the spark gap 13 in those instances, when it is desirable that the energy developed in the spark shall be particularly they are discharged very rapidly via the pressure spark gap '7, when that ignites. The spark gap 7 may be designed in such a way that it is ignited either on account of the fact that the voltage thereover has exceeded a certain critical level or as a consequence of a highvoltage pulse, which has been applied to an auxiliary electrode (not shown in FIG. 1). In this way two identical pulses will be formed and will propagate from the pulse-generating part, one pulse along the coaxial cable 3, where it is finally completely absorbed by the terminating resistor 8, and the other pulse along the coaxial cable 4, which may be made very long, in the order of 100 m. This pulse may be measured by the oscilloscope 10, when it passes the capacitive voltage divider 11. The pulse continues via the coaxial cable 5, which is terminated by the probe 6, where part of the energy transported by the pulse is dissipated in the spark gap 13 and in thecurrent, limiting resistor 14, if such a resistor is utilized. That part of the energy of the incom: ing pulse, which is not absorbed in the probe, will be conducted back along the coaxial cables 5 and 4 in the form of a reflected pulse, which may be measured by the oscilloscope 10, when it passes the capacitive voltage divider ll. With knowledge of the shape of the incoming and reflected pulses and the resistance of the resistor 14, it is possible to estimate the energy freed in the spark gap 13. Meanwhile the pressure spark gap 7 has had time to become deionized so that it is no longer current conducting. The reflected pulse is therefore conducted directly from the coaxial cable 4 into the coaxial cable 3, where it is finally absorbed in the terminating resistor 8.

If the high-voltage source is suitably calibrated, it might be possible to dispense with the oscilloscope 10 and its capacitive voltage divider 11. g

In the measurement, the object of which is to determine whether a given medium, e.g. a gas mixture, is explosive or not, a pulse is generated having so high an amplitude that a spark with certainty is obtained over the gap 13. If the medium in the probe is then ignited,

. which is indicated by the indicator 12, which may be located at a comparatively great distance from the probe 6, one will know that the medium is explosive. This measuring scheme may be refined thereby that a series of pulses having known, successively increased amplitude, are applied to the spark gap 13 until the spark generated therein by the pulse is sufficiently strong for igniting the medium in the probe. Since the amplitude of the igniting pulse is known, a measure of ignition energy of the medium is also obtained hereby (which in its turn gives information of the composition of the medium).

If it is possible to apply the probe 6 in such a way in a space, which is filled with the medium to be examined, e.g. a tank, a container, or the like that the probe is visible from the outside, so that it is possible to visually determine, whether or not an ignition in the probe has occured, the detector or indicator 12 may be omitted.

In FIGS. 2 and 3 a first embodiment of the probe 6 is diagrammatically illustrated, in which the resistor 14 shown in FIG. 1 is omitted to simplify the drawing. The probe comprises an ignition chamber 20, which is confined by a suitably circular-cylindrical circumferential wall 17, in which four apertures or windows 21 are provided in an angular distance of 90 from each other and two

tight end walls

18, 19. The circumferential wall 17 is tightly surrounded by a fine-mesh gauze 22 having a mesh, which is with certainty smaller than the quenching distance of all media, which may become involved in connection with the probe. The spark gap located. in the ignition chamber is formed by two

electrodes

23a, 23b, the firstmentioned one of which is secured to a set screw 24, which permits adjustment of the electrode distance of the spark gap and with which a locking nut 25 cooperates. The electrode 23b has a metallic head, which is maintained in current-conducting engagement with the inner conductor 30 by being pressed against it by means of a clamping nut 26 of non-conducting material, which is threaded onto the end of the insulator 27 of the coaxial cable 5. The outer conductor 28 of the cable, which is surrounded by a plastic scheath 29, is tightly inserted into and in metallic engagement with that end wall 18 of the conducting housing of the spark chamber, which faces the cable.

FIGS. 4 and 5, in which the same reference numerals (possibly with the addition of a, b and so on) as in FIGS. 2 and 3 are utilized for designating the same or like details, illustrate a further development of the probe according to the invention.

As distinguished from the embodiment according to FIGS. 2 3 the circumferential wall of the ignition chamber consists of two

parts

17a, 17b, which are screwed together in FIGS. 4 5. The apertures 21 in the circumferential wall in FIGS. 2 3 have been replaced firstly by three angular rows of smaller apertures 21a in the circumferential wall, secondly by a number of apertures 21b in the left hand end wall 18 of the ignition chamber 20 in the embodiment according to FIGS. 4 5. Inside these openings a fine-mesh gauze 22a or a disk of compressed steelwool or the like corresponding to the gauze 22 has been inserted, the mesh of which is smaller than the quenching distance. The end wall 18 is secured to the outer conductor of the coaxial cable 5 by means of a bushing 31, which is soldered to the outer conductor and has external threads, on which an extension of the end wall 18 is screwed by means of internal threads. To promote the ionization of the gas in the spark gap between the

electrodes

23a and 23b and for accelerating spark break down, an ionization source 32 in the form of a container for a 'y-radiating substance, e.g. cobalt-60, is inserted into the circumferential wall.

According to FIGS. 6 and 7 the probe 6 is supplemented with a blower or fan 33, which promotes the medium circulation through the spark chamber 20. The fan is driven by an encapsulated electric motor 34 and is provided in an fan housing 35, which is joined with the end wall 19 of the spark chamber by means of threads. In this wall apertures 21c are provided, which correspond to the apertures 21b and are covered by a metal-wire arrangement 22b, which'is provided in the spark chamber and corresponds to the gauze or steelwool disk 22a and has a mesh, which is smaller than the quenching distance. Also in the end wall of the fanhousing ventilation openings 36 are provided so that an air stream can pass through the fan housing and the spark chamber.

Thirty-seven designates thermometer pockets, in which temperature-sensitive means, eg thermistors, which form a heat-sensitive transducer, are to be introduced for measuring the temperature difference between the spark chamber 20 and the fan housing 35 for indicating, whether the medium in the spark chamber is ignited or not. For the remote indication of a possible ignition, the measuring means are connected to the indicator 12 (FIG. 1), as is diagrammatically indicated by the conductor 38.

In the embodiment according to FIGS. 8 13 there is utilized as an ignition initiator a heating or incandescent body, which is usually heated to incandescence, instead of the spark gap 13 formed by the

electrodes

23a, 23b. On account hereof, the'apparatus associated with the probe will necessarily differ somewhat from that described above.

The apparatus illustrated in FIG. 8 is comprised of a measuring probe,'which is generally designated 6 and comprises a heating or incandescent body 40 and is connected by a cable, which has four wires in FIG. 8, to apparatus 42, which contains the elements necessary for the heating of the incandescent body 40 and for the measuring or indicating of a possible ignition. A voltage source 43, which may be controlled externally for heating the heating body 40 feeds a Wheatstone bridge consisting of the incandescent or heating body 40 of the measuring probe and the

resistors

44, 45 and 46, of which at least the firstmentioned one is adjustable. The unbalance voltage, which is developed over the bridge on account of the change of the resistance of the incandescent or heating body 40 due to heating is applied to an amplifier 47, which feeds the measuring instrument 48, which is suitably graduated in incandescent or heating body temperature. By adjustment of the adjustable resistor 44 the bridge may be balanced before the measurement commences. Ignition of the medium, in which the measuring probe 6 is introduced, is indicated by a transducer 49, which is comprised in the probe and which by means of the amplifier 50 actuates a suitable indicator, for instance in the form of an incandescent lamp 51, which lights, when the medium is ignited. The transducer is preferably a heat-sensitive emitter or possibly a pressure or light sensor, which is connected to a corresponding indicator. For the transfer of light from the ignition vessel of a light sensitive or photoelextric transducer it is possible to utilize fiber optics.

Alternatively, some other kind of radiation sensitive or ionization sensitive transducer and a corresponding indicator may also be utilized.

Measurement is carried out in the following way: With the voltage source 43 set to supply so low a voltage that the heating of the incandescent body caused thereby can be neglected, the bridge is balanced by means of the resistor 44. After that, the voltage supplied by the voltage source 43 is increased until the lamp 5 indicates that ingition has occured; The ignition temperature T is then indicated on instrument 48.

FIGS. 9 and 10 illustrate, more explicitly an embodiment of the measuring probe 6 shown in FIG. 8. This probe comprises an ignition chamber 20a, which is defined by a suitable circular-cylindrical circumferential wall 170, in which four apertures or windows 21d (FIG. 10) are provided at an angular distance of 90, and two

tight end walls

18a, 19a. The circumferential wall 17c is tightly surrounded by a gauze 22c, which covers the apertures 21d and the mesh of which is with certainty smaller than the quenching distance for the media, in which the probe isto be utilized. The incandescent or heating body 40, which has the shape of a strip of resistor material, is by means of set screws 54 secured into brackets 55, which are mounted in an insulator 56. This also carries the transducer 49. The cable 41 is introduced through a bushing 57, which is secured to the end wall 18a and has internal threads, and is by soldering or by other means connected to the transducer 49 and the incandescent body brackets 55. Sealing between the cable 41 and the probe casing is obtained by means of rubber packing 59, which is surrounded by two washers 58 and is pressed against the cable 41 by a bushing 60 having external threads.

If the probe 6 can be introduced in a space filled with a medium, the explosiveness of which is to be investigated, in such a way that the, probe is visible from the outside so that it is possible to see, when an ignition in the probe occurs, it is possible to omit the. transducer 49, the amplifier 50 and the indicator 51. In a modified. embodiment of the apparatus according to FIG. 8, which is illustrated in FIG. 11, the incandescent, or heating body consists of two metal strips 40a, 4012, which are welded together at 40c to form a thermo electric'couple. A probe, which is; provided with such an incandescent or heating body is utilized in assosiation with the apparatus 42a illustrated in FIG. 12. An alternating current generator 62 supplies voltage, e.g. of the frequency 1,000 Hz, to the incandescent or

heating body

40a, 40b, 40c through

capacitors

63, 64 and the cable 41a so that the incandescent or heating body is heated. The thermo-electromotive force arising in the incandescent or heating. body as a consequence of its heating'is extracted from the cable 41a through blocking

inductors

65,66, which block the alternating current, and is applied to the amplifier 470, which feeds the measuring instrument 48a.

In FIG. 12 there is furthermore shown a .device, which permits the measurement to be carried out automatically. A function generator 67 generates a rampfunction voltage according to FIG. 13, which is repeated with a suitable (adjustable) time interval. Electronic circuit 68 gates this voltage for actuating the alternating current generator 62 in such a way that the power given off thereby increases simultaneously as said voltage. At the start of the measurement the gate 68 is opened, the power given off by the generator 62 then causes the temperature of the incandescent or

heating body

40a, 40b, 40c to increase in dependence of the increasing voltage. 7

When ignition occurs in the probe 6, the transducer 49a brings about closingof the gate 68 through the amplifier 50a, so that the alternating current generator 62 is rendered inoperative. Suitably, the apparatus is designed in such a way that the reading of the instrument 48a, which indicates the self-ingition temperature T", remains until the next measuring period is commenced.

The probe according to FIG. 14 contains as its incandescent or heating body a thin-walled tube 40d of resistor material, which corresponds to the incandescent or heating wire 40 in FIGS. 9 and 10 and the incandescent or

heating body

40a, 40b, 40c in FIG. 11 and which is directly heated by current from a pair of

conductors

78, 79 comprised in the cable. in the illustrated embodiment, and is secured in an insulator 70, which is joined to the lower end wall 18c of the probe 6. As wasthe case in the embodiment according to FIG. 6, the circumferential wall of the ignition chamber 20b in FIG. 14 consists of two parts 17d, 172, which are joinedby threads and which together with the end wall of the probe are contempated to constitute the return lead for the current through the tube 40d. As isthe embodiment according to FIG. 9, the cable 41b is secured in the end wall 180, which is provided with an abutment, by means of a rubber packing 59b, which is surrounded by two washers 58b and is compressed by means of a tightening screw 60b. The heating or incandescent body 40d, which has the shape of a thin-walled tube, may consist e.g. of tantalum and may have a length and an outer diameter, which suitably exceed about 50 mm and about 10 mm, respectively. The distance between the outer circumferential surface of the tube 40d and the inner surface of the circumferential wall 17d, l7e of the probe housing should suitably exceed about mm. The openings 21d in the circumferential wall 17c in FIGS. 9 It) have been replaced, firstly by a number of apertures Zle in the lower end wall 18c of the ignition chamber 20b, secondly by a number of apertures 21f in the upper end wall 19c of the ignition chamber 20b in the embodiment according to FIG. 14. Inside each one of these sets of openings there is provided a fine-

mesh gauze

22c and 22d, respectively, or a disk of compressed steelwool or the like having a mesh, which is smaller than the quenching distance, the gauzes and the disk corresponding to the

gauze

22a and 22b, respectively. A fan 71 promotes the medium flow through the ignition chamber 20b. The fan is driven by an encapsulated electric motor 72 and is provided in a fan housing 73, which is joined to the upper end wall 19c by means of threads. In the end wall 7 I of the fan housing 73 ventilation apertures 75 are provided so that an air stream can pass through both the fan housing 73 and the ignition chamber 20b.

Thermometer pockets 76 include temperaturesensing means. e.g. thermistors, which form a heatsensitive transducer and are introduced for measurement of the temperature difference between the ignition chamber 20b and the fan housing 73, whereby ignition of the medium in the ignition chamber is indicated. For remote indication of ignition, the measuring means are connected to an indicator, which corresponds to the indicator 51 in FIG. 8 as is diagrammatically indicated by the dash lines38a, 38b in FIG. 14.

The embodiments described above and illustrated in the drawings are, of course, to be regarded merely as non-limiting examples and can as to their details be modified in several ways within the scope of the invention concept. Thus, the indicator may suitably be combined with or replaced by a preferably acoustical and- /or optical alarm. Furthermore, new embodiments, which are also within the scope of the invention, may be created by appropriate combination of details taken from different embodiments described above.

What we claim is:

l. A method of determining the risk of explosion of a preferably gaseous medium by means of a probe, which comprises an ignition chamber, which is provided with apertures, a fine-mesh gauze covering said apertures having a mesh smaller than the quenching distance of said medium and comprising the step of measuring a parameter, which is determinative of the inflammability of the medium, particularly the ignition energy or the ignition temperature, including supplying a successively varied amount of energy to an explosion initiator, which is arranged in said ignition chamber, until the amount of energy, which is operative in the explosion initiator, has reached such a level that the medium in the ignition chamber only just ignites, and measuring the activation level, at which the medium in the ignition chamber actuated by the explosion initiator ignites, said activation level constituting a measure of said parameter of the medium.

2. Apparatus for determining the risk of explosion of a preferably gaseous medium by determining its imflammability, comprising in combination a probe, which comprises an ignition chamber, which is provided with apertures a fine-mesh gauze covering said apertures having a mesh smaller than the quenching distance of said medium, an explosion initiator which is arranged in said ignition chamber, means connected to said probe for supplying a successively varied amount of energy to the explosion initiator, until the amount of energy, which is operative in the explosion initiator, has reached such a level that the medium in the ignition chamber only just ignites, and electrical means associated with said probe for measuring the activation level, at which the medium in the ignition chamber actuated by the explosion initiator ignites, said activation level constituting a measure of the inflammability of the medium.

3. Apparatus according to claim 2, characterized in that the apparatus is designed for the measurement of the ignition energy of the medium, said ignition initiator being comprised of a spark gap and said means for supplying energy to the ignition initiator being comprised of means for generating at least one electric impulse adapted to be applied to the spark gap, said impulse generating means being connectable to the ignition initiator by means of coaxial cables.

4. Apparatus according to claim 3, characterized in that the means for measuring the ignition energy is an oscilloscope for measuring the energy contents of the impulse, said oscilloscope being connected to one of the members of the group formed by said impulse generator and said spark gap.

5. Apparatus according to claim 2, characterized in that the apparatus is designed for the measurement of the ignition temperature of the medium, and in that said ignition initiator is a heating body and the means for supplying energy to the ignition initiator is comprised of means for heating said heating body.

6. Apparatus according to claim 5, characterized in that said heating body is an incandescent body.

7. Apparatus according to claim 5, characterized in that said heating body is electrically heated.

8. Apparatus according to claim 7, characterized in that said heating body constitutes one arm of a Wheatstone bridge having an adjustable voltage source for heating the heating body connected across one of its diagonals and said means for measuring the activation level connected across its other diagonal, said measuring means being comprised of an instrument for measuring the unbalance voltage of the bridge, which is due to the. alteration of the resistance of the heating body on acount of its heating through the energy supply.

9. Apparatus according to claim 7, characterized in that said heating body is a thermo-electric couple.

10. Apparatus according to claim 9, characterized by the provision of a high frequency voltage source, which is connected to said heating body through reactive means, such as a pair of capacitors, for heating said body, and a voltmeter, which is connected to said thermo-electric couple for determining the voltage thereover, which occurs at the heating thereof, said voltmeter constituting said means for measuring said actuation level.

one of the inputs of said gate circuit for supplying a repetitive ramp function voltage thereto.

13. Apparatus according to claim 12, characterized in that the ramp function voltage is of the sawtooth type.

14. Apparatus according to claim 12, characterized in that the ramp function voltage has an adjustable repetition frequency.

Claims

1. A method of determining the risk of explosion of a preferably gaseous medium by means of a probe, which comprises an ignition chamber, which is provided with apertures, a fine-mesh gauze covering said apertures having a mesh smaller than the quenching distance of said medium and comprising the step of measuring a parameter, which is determinative of the inflammability of the medium, particularly the ignition energy or the ignition temperature, including supplying a successively varied amount of energy to an explosion initiator, which is arranged in said ignition chamber, until the amount of energy, which is operative in the explosion initiator, has reached such a level that the medium in the ignition chamber only just ignites, and measuring the activation level, at which the medium in the ignition chamber actuated by the explosion initiator ignites, said activation level constituting a measure of said parameter of the medium.

8. Apparatus according to claim 7, characterized in that said heating body constitutes one arm of a Wheatstone bridge having an adjustable voltage source for heating the heating body connected across one of its diagonals and said means for measuring the activation level connected across its other diagonal, said measuring means being comprised of an instrument for measuring the unbalance voltage of the bridge, which is due to the alteration of the resistance of the heating body on acount of its heating through the energy supply.

11. Apparatus according to claim 10, characterized in that said voltmeter is graduated in units of temperature.

12. Apparatus according to claom 7, characterized by the provision of an adjustable voltage source for heating said heating body, a gate circuit for governing the output voltage of said voltage source and having two inputs, a transducer comprised in the probe and connected to one of the inputs of said gate circuit, and a function generator, which is connected to the other one of the inputs of said gate circuit for supplying a repetitive ramp function voltage thereto.