GB2032069A - Electrical primer - Google Patents

Abstract

An electrical primer comprises a conductive pyrotechnical priming composition 3 placed in contact with a useful pyrotechnical charge 2 of the primer and two electrodes 6 and 7 in electrical contact with the conductive pyrotechnical composition 3. The resistance of conductive pyrotechnical priming compositions tends to vary with external conditions and time, so that primers employing such compositions tend to be of somewhat unpredictable performance. To alleviate this, a stable electrical resistance 10 is connected in parallel with the conductive pyrotechnical composition 3, between the two electrodes 6 and 7. The invention is applicable to the electrical ignition of pyrotechnical devices. <IMAGE>

Description

SPECIFICATION Electrical primer This invention relates to an electrical primer comprising an electrically conductive pyrotechnical priming composition.

In primers of this type, the conductive pyrotechnical compositions is ignited by applying voltage to two electrodes in electrical contact therewith.

In one embodiment, one of these electrodes is formed by an annular metal disc, the other electrode being formed by a metal body having one flat surface parallel to the disc. The metal disc is separated from the flat surface of the metal body by another annular disc of a dielectric material. The cylindrical cavity defined within the two discs is filled with the conductive pyrotechnical composition.

On one side, the conductive pyrotechnical priming composition is placed in contact with the useful pyrotechnical charge of the primer, which charge may be formed by, for example, mercury fulminate, lead trinitroresorcinate, lead nitride or tetrazene.

The conductive pyrotechnical priming composition may be formed by an explosive selected from the above-mentioned compounds to which a particular conductive material, such as graphite, is added.

Conductive pyrotechnical printing compositions of the above-mentioned type are, in conventional use, attended by disadvantages.

In the first place, it is difficult to obtain perfect homogeneity of the conductive pyrotechnical composition so that, in mass production, considerable differences are observed between the electrical resistances of the various batches of composition.

In addition, a parasitic electrical current, however weak, can cause the primers to be accidentally ignited. Also, the primers are extremely sensitive to electrostatic discharges. In addition, it has been found that the electrical resistance of the conductive pyrotechnical compositions changes as a function of time, producing significant changes in the operating conditions of the primers. Also, experience has shown that a dielectric disc acting as an insulation bridge between the electrodes of a primer can form an electrostatic reservoir capable of creating exceptional operating conditions.

Also, tests have shown that the initiation energy range of known primers is extremely wide. Thus, under certain low voltages, a primer may operate exceptionally with a few microjoules of energy, whereas, other primers are characterised by exceptional misfires on ignition under an energy of the order of one joule.

Most of the disadvantages referred to above may be explained by the fact that the electrical resistance of the conductive pyrotechnical composition varies according to the electrical energy applied thereto. This is due to the fact that the conductive particles (generally of graphite) are unevenly distributed in the pyrotechnical composition of the primers. Under a relatively low voltage, the current flows between these conductive particles over long and winding paths. By contrast, when a primer is subjected to a fairly high voltage, the above-mentioned current paths are short-circuited by micro-arcs so that the primer operates at a dynamic resistance distinctly lower in value than its static resistance under low voltage. Accordingly, it will be appreciated that, under these conditions, the primer may be ignited at a very low energy threshold.

The present invention aims to provide a generally improved primer comprising a pyrotechnical composition.

According to the present invention, there is provided an electrical primer comprising a useful pyrotechnical charge, an electrically conductive pyrotechnical priming composition placed in contact therewith, two electrodes in electrical contact with the conductive pyrotechnical composition, and a stable electrical resistance connected in parallel with the conductive pyrotechnical composition between the two electrodes.

By virtue of the stable electrical resistance connected in parallel with the conductive pyrotechnical composition, the ignition energy is distributed between this parallel resistance and the pyrotechnical composition. In this way, the primer may be rendered more constant in operation than known primers.

In addition, the connection of the stable resistance in parallel with the conductive composition, which may vary widely according to the energy applied, may desensitise the primer to a considerable extent at the level of the misfire thresholds without at the same time significantly influencing the safe operating threshold of the primer.

The invention is particularly applicable to electrical primers in which one of the electrodes is formed by an annular metal disc and the other electrode by a metal body having one flat surface parallel to said disc.

In one such preferred embodiment of the invention, the above-mentioned disc is separated from the flat surface of the metal body by an annular disc of an electrically resistive material which forms the parallel electrical resistance, the cavity formed within the discs being filled with the conductive pyrotechnical composition. Thus, the disc of electrically resistive material replaces the disc of dielectric material iI a known electrical primer.

In one advantageous embodiment of the invention, the parallel electrical resistance has an ohmic value which is substantially from 0.8 to 2 times (or at least in the range 0.1 to 10 times) that of the resistance of the conductive pyrotechnical composition.

The resistance connected in parallel with the conductive pyrotechnical composition may be made of a themoplastic material filled with graphite, so that it is readily possible to obtain a resistance having an ohmic value of the same order as that of the conductive composition.

To assist in understanding the invention and to show how it may be carried out, an embodiment thereof will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 shows an electrical primer in section, for use in the electrical ignition of a pyrotechnical device; and Figure 2 is a diagram illustrating operation of the electrical primer.

Referring to Figure 1, the primer comprises an outer substantially cylindrical metal casing 1 accommodating a useful pyrotechnical charge 2 of the primer and and an electrically conductive pyrotechnical composition 3. The useful pyrotechnical charge 2 is separated from the outer metal casing 1 by a metal cup 4 of which the base is provided with a central opening 5. The useful pyrotechnical charge 2 is covered by a metal cap 2a which compresses the charge 2 towards the conductive pyrotechnical composition 3. The conductive composition 3 is in contact with the useful charge 2 through the central opening of the cup 4.

The conductive composition 3 is placed in electrical contact with two electrodes 6 and 7. The electrode 6 is formed by a metal body having one flat surface 8 in contact with the conductive composition 3. The electrode 7 is an annular metal disc arranged below the metal cup 4, parallel to the flat surface 8. The disc 7 and the metal body 6 are electrically insulated with respect to the outer metal casing 1 by an insulating cup 9 which surrounds the disc 7 and the body 6.

In addition, the flat surface 8 of the body 6 is separated from the disc 7 by another annular disc 1 0 substantially identical in its dimensions with the disc 7. The disc 10 has a predetermined electrical resistance which is stable in respect of the energy applied thereto. The cylindrical cavity defined by the - apertures 7a and 1 Oa of the discs 7 and 10 is filled with the conductive pyrotechnical charge 3.

The disc 10 forms an electrical resistance connected in parallel with the resistance formed by the conductive composition 3.

The resistance of this disc 10 preferably amounts to substantially between 0.8 and 2 times the natural resistance of the conductive pyrotechnical composition 3.

The resistance 10 may be made of a thermoplastic material or of rubber rendered conductive by particles of graphite.

The proportion by weight of graphite is preferably from 3 to 10%, based on the total weight of the disc 1 0. During production, the resistive disc 10 is compressed under a pressure of the order of 1000 bars. Under these conditions, the disc 10 has a resistivity of from 3000 to 2000 ohms.cm, depending on its graphite content.

In order to obtain good electrical contact between the resistive disc 10 and the electrodes formed by the disc 7 and the body 6, the resistive disc 10 is compressed between these two electrodes 6 and 7 under the effect of the pressure exerted by the useful charge 2.

The operation of the electrical primer will now be described with reference to Figure 2.

Referring to Figure 2, it can be seen that the disc 10 arranged between the electrodes 6 and 7 is equivalent to a resistance Rp connected in parallel with the resistance Rs formed by the conductive pyrotechnical composition 3. Under these conditions, when the primer is ignited, the ignition energy is distributed between the composition 3 and its parallel resistance formed by the disc 10. Since the electrical resistance of the conductive composition 3 varies widely according to the energy applied to the electrodes 6 and 7, the connection of a fixed resistance in parallel with the resistance of this composition 3 enables the primer to be considerably desensitised at the level of the misfire threshold without at the same time significantly influencing the safe operating threshold of the primer.

The resuit may be illustrated by the following calculations: Where a voltage U = 1 (one) is applied between the electrodes 6 and 7 of the primer in the static mode, the total energy W consumed in the primer is equal to (1/Rs = 1/Rp)t in which Rs is the static resistance of the conductive composition 3, Rp is the resistance of the disc 10 and t the time factor.

The ratio WR5/W of the energy consumed in the conductive composition 3 to the total energy W is equal to: (1/Rs)t (1/Rs + 1/Rp)t Since Rs is generally of the same order of magnitude as Rp, it will be assumed for the purposes of simplification that Rs = Rp. Accordingly, WRS/W is equal to 1/2.

In the dynamic mode, the dynamic resistance Rd of the conductive composition 3 is equal to1Rs, where kis capable of varying from 2 to 10, over the range of electrical energies with which the present embodiment is concerned.

In this case: WRd/W= kt(1 = k) Where k = 2 WRd/W=2/3 Where k = 5 WRd/W=S/6 Wherek=10 WRd/W= 10/11 The above calculations show that, in the static mode, the conductive composition 3 only dissipates half the energy applied whereas it dissipates a larger fraction thereof when the dynamic resistance Rd becomes very different from the static resistance Rs.

The difference between the so-called safe voltage and the so-called operating voltage considerably increases the difference in the energy consumed in the primer at the corresponding threshold.

In practice, the so-called operating voltage is equal to between about 2 and 3 times the safe voltage. The corresponding voltage step considerably promotes the intake of current into a resistance of dynamic behaviour which, then, is considerably weaker. In this case, the probability of operation increases for a given energy, which is reflected in low initiation energies of the primer whereas, at low voltage, this probability is greatly reduced in the illustrated primer, which requires, relatively, an enormous amount of energy.

Thus, the overall electrical resistance formed by the intrinsic resistance Rs of the composition 3 snd the fixed resistance Rp of the disc 10 connected in parallel therewith is more precise than in a conventional primer. In addition, the association of two resistances of given precision ensures reliable compensation rather than dispersion of static errors.

On the other hand, the electrical resistance of the illustrated primer changes far less with time than in known primers. This is because the connection of a fixed resistance in parallel with the resistance of the conductive composition 3 reduces the resistance deviation in relative terms.

The respective characteristics and performances of a conventional primer and a primer which is as illustrated and differs therefrom in that, instead of an electrically insulating disc, it comprises a resistive disc 10 between the electrodes 6 and 7, are shown by way of comparative example in the following Table::

Conventional primer Primer as illustrated Diameter 8 mm 8 mm Nature of the disc arranged between cellulose acetate graphite-filled the electrode rubber Thickness of this disc 0.10 mm 0.20 mm Nature of the igniting lead trinitroresor charge ci nate, potassium chlorate, barium idem nitrate, calcium silicide, graphite Mean resistance 70 ohms 48 ohms Standard deviation from mean resistance 35% 25% Development of the resistance after 13 to 20% 3 to 5% ageing for 1 month Safe inoperative voltage 3 to 4 volts 6 to 8 volts Operating energy at 10 volts (safety voltage) - for 99% misfires 5 to 10 microjoules 2000 to 5000 micro Joules - for 99% operation 200 to 1000 micro- 50,000 to 400,000 joules microjoules Operating' energy at 18 volts (operating voltage) - for 99% misfires 5 to 10 microjoules 40 to 100 microjoules - for 99% operation 160 to 400 micro- 300 to 1000 micro joules joules Operating time of the primer under normal 80 microseconds 80 microseconds ignition energy Comparison of the numerical data set out in the above table shows that the primer as illustrated is comparable in its pyrotechnical performance with the conventional primer. By contrast, the primer as illustrated has a resistance whcih is slightly more constant and distinctly more stable as a function of time.

In addition, the threshold voltage for complete misfiring passes on average from 3.5 to approximately 7 volts. At a voltage of 10 volts (at the safety threshold) the primer as illustrated is about 1000 times less sensitive than the conventional primer, which difference is considerable. However, at a voltage of 18 volts, i.e. at the operating threshold of the primer, the primer as illustrated is only about two times less sensitive than the conventional primer, which is negligible in practice.

Accordingly, the primer as illustrated is considerably safer in operation then the conventional primer.

The invention is by no means limited to the example which has just been described and to which numerous modifications may be made without departing from the scope of the invention, as defined in the appendant claims.

Thus, the invention is applicable both to primers operating at a low voltage (for example, 10 to 20 volts) and to primers operating at medium voltage (for example, 200 volts).

The illustrated primer is also applicable to detonators which have to withstand considerable accelerations.

In addition, the shape of the electrodes 6 and 7 and the shape of the resistance 10 connected in parallel with the conductive pyrotechnical composition 3 may differ from the shapes shown in the drawings.

The resistance 10 may be made of any suitable material having a resistivity comparable with that of the conductive composition 3, providing this resistance is sufficiently stable as a function of the energy applied thereto.

Claims (9)

1. An electrical primer comprising a useful pyrotechnical charge, an electrically conductive pyrotechnical priming composition placed in contact therewith, two electrodes in electrical contact with the conductive pyrotechnical composition, and a stable electrical resistance connected in parallel with the conductive pyrotechnical composition between the two electrodes.

2. An electrical primer as claimed in claim 1, wherein one of the electrodes is formed by an annular metal disc and the other electrode is formed by a metal body having one flat surface parallel to said disc, wherein the disc is separated from the flat surface of the metal body by an annular disc of an electrically resistive material forming said parallel resistance, the cylindrical cavity formed within said two discs being filled with the conductive pyrotechnical composition.

3. An electrical primer as claimed in claim 1 or2, wherein said parallel electrical resistance has an ohmic value which is substantially from 0.8 to 2 times that of the static resistance of the conductive pyrotechnical composition.

4. An electrical primer as claimed in any of claims 1 to 3, wherein the parallel electrical resistance comprises a thermoplastic material filled with graphite.

5. An electrical primer as claimed in any of claims 1 to 3, wherein the parallel resistance comprises rubber containing from 3 to 10% of graphite.

6. An electrical primer as claimed in claim 4 or 5, wherein said resistance has a resistivity af between 300 and 2000 ohms. cm.

7. An electrical primer substantially as hereinbefore described with reference to the accompanying drawing.

8. An electrical primer as claimed in claim 7 and having properties as recited in the Table herein.

9. A pyrotechnical device provided with an electrical primer as claimed in any preceding claim.