GB1581236A - Ionisation detector chamber - Google Patents

Ionisation detector chamber Download PDF

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Publication number
GB1581236A
GB1581236A GB2766876A GB2766876A GB1581236A GB 1581236 A GB1581236 A GB 1581236A GB 2766876 A GB2766876 A GB 2766876A GB 2766876 A GB2766876 A GB 2766876A GB 1581236 A GB1581236 A GB 1581236A
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GB
United Kingdom
Prior art keywords
electrode
detector
chamber
measuring electrode
measuring
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Expired
Application number
GB2766876A
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Chloride Group Ltd
Original Assignee
Chloride Group Ltd
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Chloride Group Ltd filed Critical Chloride Group Ltd
Priority to GB2766876A priority Critical patent/GB1581236A/en
Priority to IL52357A priority patent/IL52357A/en
Priority to FR7719956A priority patent/FR2393303A1/en
Priority to IE1360/77A priority patent/IE45520B1/en
Priority to JP7725177A priority patent/JPS5333600A/en
Priority to DE19772729599 priority patent/DE2729599A1/en
Priority to CA281,806A priority patent/CA1091826A/en
Priority to CH815377A priority patent/CH616768A5/fr
Priority to US06/058,613 priority patent/US4328424A/en
Publication of GB1581236A publication Critical patent/GB1581236A/en
Expired legal-status Critical Current

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Classifications

    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N27/00—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means
    • G01N27/62—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating the ionisation of gases, e.g. aerosols; by investigating electric discharges, e.g. emission of cathode
    • G01N27/64—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating the ionisation of gases, e.g. aerosols; by investigating electric discharges, e.g. emission of cathode using wave or particle radiation to ionise a gas, e.g. in an ionisation chamber
    • G01N27/66—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating the ionisation of gases, e.g. aerosols; by investigating electric discharges, e.g. emission of cathode using wave or particle radiation to ionise a gas, e.g. in an ionisation chamber and measuring current or voltage
    • G—PHYSICS
    • G08—SIGNALLING
    • G08B—SIGNALLING OR CALLING SYSTEMS; ORDER TELEGRAPHS; ALARM SYSTEMS
    • G08B17/00—Fire alarms; Alarms responsive to explosion
    • G08B17/10—Actuation by presence of smoke or gases, e.g. automatic alarm devices for analysing flowing fluid materials by the use of optical means
    • G08B17/11—Actuation by presence of smoke or gases, e.g. automatic alarm devices for analysing flowing fluid materials by the use of optical means using an ionisation chamber for detecting smoke or gas
    • G08B17/113—Constructional details

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  • Chemical & Material Sciences (AREA)
  • Physics & Mathematics (AREA)
  • General Physics & Mathematics (AREA)
  • Health & Medical Sciences (AREA)
  • Analytical Chemistry (AREA)
  • Biochemistry (AREA)
  • Life Sciences & Earth Sciences (AREA)
  • Electrochemistry (AREA)
  • Chemical Kinetics & Catalysis (AREA)
  • General Health & Medical Sciences (AREA)
  • Toxicology (AREA)
  • Immunology (AREA)
  • Pathology (AREA)
  • Business, Economics & Management (AREA)
  • Emergency Management (AREA)
  • Fire-Detection Mechanisms (AREA)

Description

(54) IONISATION DETECTOR CHAMBER (71) We, CHLORIDE GROUP LIMITED, a Company registered under the Laws of England, of 52, Grosvenor Gardens, London SW1W OAU, do hereby declare this invention for which we pray that a patent may be granted to us, and the method by which it is to be performed to be described in and by the following statement: This invention relates to a detector including an ionisation chamber for detecting the presence of smoke so as to be capable of giving a signal for giving an alarm that a fire is imminent in a building.
An object of the invention is to provide a simply constructed small detector which is nevertheless very sensitive to the presence of smoke.
According to the present invention, a smoke detector including an ionisation chamber has a rod-shaped first electrode, a second electrode, and a measuring electrode, means for ionising gas in the chamber including the space between the three electrodes, and a shielding sleeve closely surrounding the first electrode except for an exposed conducting area disposed adjacent the measuring electrode and nearer to the measuring electrode than the second electrode is.
The disposition of the measuring electrode in relation to the first electrode concentrates to the electric field within the part of the chamber adjacent to the first electrode, and ensures that the presence of smoke has a much greater effect on the ionisation current flowing between the measuring electrode and the second electrode than on the ionisation current flowing between the first electrode and the measuring electrode.
The shielding sleeve may be of insulating material, and is preferably in contact with the sides of the first electrode, except for a short length of the sides of the inner end. The first electrode can be positioned concentrically within the second electrode constituting the ionisation chamber wall, and that effectively divides the space within the chamber into two areas, a central area between the measuring electrode and the exposed area of the first electrode, and a larger surrounding area between the measuring electrode and the wall of the ionisation chamber.
The ionisation means is conveniently a piece of radio-active material for example a foil or radium or americium and that can be positioned anywhere in the ionisation chamber although a convenient position is on the measuring electrode centrally opposite the exposed part of the first electrode.
In one preferred construction the chamber has an insulating base carrying the measuring electrode which in turn carries a foil of americium; the ionisation chamber consists of an upstanding wall around the base and an end wall closing the chamber, all those walls being perforated while the first electrode extends through the end wall of the ionisation chamber in the form of a cylindrical conducting rod which is a tight fit within an insulating sleeve.
In that preferred construction, the sensitivity of the detector may not be accurately reproducable from sample to sample if the dielectric constant of the insulating material varies from sample to sample. The critical region of the detector is the region of high electric field between the first electrode and the measuring electrode and by terminating the insulation short of the end of the elongate first electrode so that the last part of the length of its sides is not covered by insulation the insulation is removed from this region of high electric field and in consequence variations in the dielectric constant are not reflected in substantial variations in the sensitivity of the detector. The detector is also made more sensitive as the surface area of the exposed end of the elongate first electrode is larger.This modifies the electric field pattern in the region between the first electrode and the measuring electrode, so giving increased sensitivity.
In general, the shielding sleeve must extend reasonably near to the end of the elongate electrode to prevent the lines of the electric field going directly from the positive electrode to a surrounding negative electrode rather than by way of the measuring electrode.
The invention may be carried into practice in various ways, and one embodiment will now be described by way of example with reference to the accompanying drawings of which: FIGURE 1 is a sectional elevation of an ionisation type smoke detector, FIGURE 2 is an exploded view of components of the detector; and FIGURE 3 is a circuit diagram of the detector.
The detector includes an ionisation chamber consisting of the space between an insulating base 14 and a perforated wall 13 upstanding from the sides of the base 14 with an enclosed top. A measuring electrode 15 is mounted in a central recess in the base 14 and centrally mounted on the exposed face of the measuring electrode 15 is an elongate foil 16 of americium.
Closely spaced centrally from the foil 16, is the exposed end of a cylindrical electrode 11 which is a close fit within an insulating sleeve 12 which serves to mount the electrode 11 in, and insulate it from, the top wall of the chamber 13.
The insulation 12 extends down the larger part of the length of the sides of the elongate cylindrical rod electrode 11 but terminates a little short of the exposed end of that electrode. This means that the insulation is removed from the region of high intensity field between the electrodes 11 and 15 so that the sensitivity of the detector does not depend to any substantial degree on the dielectric constant of the material of the insulating sleeve 12. The sleeve terminates sufficiently near the end of the electrode 1 to prevent the lines of electric field passing directly from the first electrode to the second electrode rather than by way of the measuring electrode 15.
In order to prevent excessive leakage currents from measuring electrode 15 to the second electrode 13 over the surface of the insulating base 14 a conducting guard ring could be fitted in an annular groove in the upper surface of the base surrounding the disc 15 and within the perforated wall 13.
The surface of the base 14 between the measuring electrode 15 and the perforated wall 13 constituting the negative electrode is corrugated at 21 to increase the length of the creep path over insulation between those electrodes.
The perforated wall 13 is part of a sub assembly constituting the outer wall of the ionisation chamber, and comprising three components. The perforated wall 13 is a simple mesh cup with a top having a central aperture for the insulator 12, and a frustoconical side wall, and that fits within an inner moulding 22 having four radially directed retaining ribs 23, extending from a rim 24 to a central disc 25, and then there is an outer moulding 26 fitting around the apertured part of the inner moulding 23 and having four equally circumferentially spaced slots in its frusto-conical side wall, as indicated at 27, which are opposite the ribs 23. The arrangement allows ambient air free access to the ionisation chamber within the wall 13, while yet there is no direct passage for air to enter without having to deviate around the ribs.
Electrical circuit components are contained in a chamber 29 below the base 14, and within a lower wall 31, and electrical connections 32 respectively from the electrodes 11 and 13 are led down around the sub-assembly 24, 26, into the chamber 29. A connection from the measuring electrode 15 extends directly into the chamber 29, as indicated at 33, and if there is a guard ring, as described above, the connection from that also would extend through the base 14.
There is a surrounding casing 34 having slots for entry of the air, and a lamp 35 can be seen from a central hole in the top of the cover 34 for giving a visual indication of an alarm. That lamp is connected to the circuit components in the chamber 29.
The circuit diagram is shown in Figure 3, and it can be seen the electrodes 11 and 13 are connected respectively to the drain and source electrode of a field effect transistor 17, while the gate electrode of the F.E.T. is connected to the measuring electrode 15.
The circuit is conventional, and operates so that as the voltage on the measuring electrode 15 arise, the drain to source current in the field effect transistor 17 increases, so that the voltage across a resistor R1 also increases and this voltage also appears on the base of transistor TR3 which converts the voltage to a low impedance signal capable of triggering a transistor TR4 which is a programmable unijunction transistor (PUT). TR4 will only trigger, firing the output thyristor CSR1, if the input at A on TR4 exceeds the threshold voltage determined by the setting of a potentiometer RV1. When CSR1 is fied, the current through the detector increases and is usually limited by the circuit in the fire alarm control panel. This constitutes the alarm signal and a local indication is given of the switching of the detector to the alarm condition by the illumination of the lamp 35 which is a light emitting diode D7.
Thus an alarm is given in dependence on the voltage of the measuring electrode 15, and the operation of the detector is such that ionisation current flows between the elec trodes under the influence of the electric field in the ionisation chamber, and the voltage of the measuring electrode 15 is an intermediate voltage determined by the relationship of the impedance between the electrodes 11 and 15, to that between the electrodes 15 and 13.
Under normal conditions, that is with no smoke present in the chamber, the ratio of the impedance between the electrodes 11 and 15 to the impedance between the electrodes 11 and 15 to the impedance between the electrodes 15 and 13 will be set by the electric field concentrations in the two parts of the chamber. These concentrations are in turn determined by the sizes, shapes and spacing of the electrodes. Thus the chamber can be considered to constitute an inner part between the positive electrode 11 and the measuring electrode 15 and a surrounding outer part between the measuring electrode 15 and the negative electrode 13.
The relationship of the measuring electrode 15 to the positive electrode 11 serves to concentrate the electric field at that region because substantially all the lines of electric field flow to the restricted exposed and of the electrode 11 rather than through the insulation to the sides of that electrode Thus when smoke appears within the ionisation chamber, ions attach themselves to smoke particles and lose their mobility. The effect is much more pronounced in the larger region between the measuring electrode 15 and the negative electrode 13, than between the positive electrode 11 and the measuring electrode 15, and the effect is for the impedance of the outer part of the chamber to increase so that the voltage on the measuring electrode 15 increases to the threshold value if the smoke concentration is sufficiently great.
This gives good sensitivity for getting a response from the field effect transistor from quite a small ionisation chamber and the construction is simple because a single piece of radioactive material suffices for both parts of the chamber.
In addition there is only one high impedance leakage path that could affect the dtector's sensitivity, i.e. over the insulator 14 between electrodes 15 and 13. If this is corrugated at at 21 or protected with a guard ring as described above, or both the detector will be more immune to contamination by duct and dirt than are more conventional detectors employing two or more high impedance paths.
WHAT WE CLAIM IS: 1. A smoke detector including an ionisation chamber having a rod-shaped first electrode, a second electrode, and a measuring electrode, means for ionising gas in the chamber including the space between the three electrode, and a shielding sleeve closely surrounding the first electrode except for an exposed conducting area disposed adjacent the measuring electrode and nearer to the measuring electrode than the second electrode is.
2. A detector as claimed in Claim 1 in which the sleeve is an insulating sleeve.
3. A detector as claimed in Claim 2 in which the sleeve is in contact with the sides of the first electrode except for a short length of the sides at the inner end.
4. A detector as claimed in any preceding claim in which the first electrode is on the same side of the measuring electrode as the second electrode.
5. A detector as claimed in any of the preceding claims in which there is a central area between the measuring electrode and the exposed area of the first electrode, and a larger surrounding area between the measuring electrode, and the second electrode.
6. A detector as claimed in any of the preceding claims in which the ionisation means comprises a piece of radio-active material disposed within the chamber.
7. A detector as claimed in Claim 6 in which the radio active material is mounted on the measuring electrode centrally opposite the exposed part of the first electrode.
8. A detector as claimed in any of the preceding claims in which the chamber is defined between an insulating base carrying the measuring electrode and an upstanding perforated wall carried by the base constituting the second electrode.
9. A detector as claimed in Claim 8 in which the insulating base is shaped to establish a long creep path over insulation between the measuring electrode and the second electrode.
10. A detector as claimed in any of the preceding claims including a guard conductor between the measuring electrode and the second electrode, and connected to reduce the leakage current from the measuring electrode to the second electrode.
11. A detector as claimed in Claim 9 and Claim 10 in which the guard conductor is carried in the insulating base between the measuring electrode and the second electrode.
12. A detector as claimed in any preceding claim in which the second electrode is constituted by a perforated wall of the ionisation chamber allowing free entry to ambient air.
13. A detector constructed and arranged substantially as herein specicially described with reference to the accompanying drawings.
**WARNING** end of DESC field may overlap start of CLMS **.

Claims (13)

**WARNING** start of CLMS field may overlap end of DESC **. trodes under the influence of the electric field in the ionisation chamber, and the voltage of the measuring electrode 15 is an intermediate voltage determined by the relationship of the impedance between the electrodes 11 and 15, to that between the electrodes 15 and 13. Under normal conditions, that is with no smoke present in the chamber, the ratio of the impedance between the electrodes 11 and 15 to the impedance between the electrodes 11 and 15 to the impedance between the electrodes 15 and 13 will be set by the electric field concentrations in the two parts of the chamber. These concentrations are in turn determined by the sizes, shapes and spacing of the electrodes. Thus the chamber can be considered to constitute an inner part between the positive electrode 11 and the measuring electrode 15 and a surrounding outer part between the measuring electrode 15 and the negative electrode 13. The relationship of the measuring electrode 15 to the positive electrode 11 serves to concentrate the electric field at that region because substantially all the lines of electric field flow to the restricted exposed and of the electrode 11 rather than through the insulation to the sides of that electrode Thus when smoke appears within the ionisation chamber, ions attach themselves to smoke particles and lose their mobility. The effect is much more pronounced in the larger region between the measuring electrode 15 and the negative electrode 13, than between the positive electrode 11 and the measuring electrode 15, and the effect is for the impedance of the outer part of the chamber to increase so that the voltage on the measuring electrode 15 increases to the threshold value if the smoke concentration is sufficiently great. This gives good sensitivity for getting a response from the field effect transistor from quite a small ionisation chamber and the construction is simple because a single piece of radioactive material suffices for both parts of the chamber. In addition there is only one high impedance leakage path that could affect the dtector's sensitivity, i.e. over the insulator 14 between electrodes 15 and 13. If this is corrugated at at 21 or protected with a guard ring as described above, or both the detector will be more immune to contamination by duct and dirt than are more conventional detectors employing two or more high impedance paths. WHAT WE CLAIM IS:
1. A smoke detector including an ionisation chamber having a rod-shaped first electrode, a second electrode, and a measuring electrode, means for ionising gas in the chamber including the space between the three electrode, and a shielding sleeve closely surrounding the first electrode except for an exposed conducting area disposed adjacent the measuring electrode and nearer to the measuring electrode than the second electrode is.
2. A detector as claimed in Claim 1 in which the sleeve is an insulating sleeve.
3. A detector as claimed in Claim 2 in which the sleeve is in contact with the sides of the first electrode except for a short length of the sides at the inner end.
4. A detector as claimed in any preceding claim in which the first electrode is on the same side of the measuring electrode as the second electrode.
5. A detector as claimed in any of the preceding claims in which there is a central area between the measuring electrode and the exposed area of the first electrode, and a larger surrounding area between the measuring electrode, and the second electrode.
6. A detector as claimed in any of the preceding claims in which the ionisation means comprises a piece of radio-active material disposed within the chamber.
7. A detector as claimed in Claim 6 in which the radio active material is mounted on the measuring electrode centrally opposite the exposed part of the first electrode.
8. A detector as claimed in any of the preceding claims in which the chamber is defined between an insulating base carrying the measuring electrode and an upstanding perforated wall carried by the base constituting the second electrode.
9. A detector as claimed in Claim 8 in which the insulating base is shaped to establish a long creep path over insulation between the measuring electrode and the second electrode.
10. A detector as claimed in any of the preceding claims including a guard conductor between the measuring electrode and the second electrode, and connected to reduce the leakage current from the measuring electrode to the second electrode.
11. A detector as claimed in Claim 9 and Claim 10 in which the guard conductor is carried in the insulating base between the measuring electrode and the second electrode.
12. A detector as claimed in any preceding claim in which the second electrode is constituted by a perforated wall of the ionisation chamber allowing free entry to ambient air.
13. A detector constructed and arranged substantially as herein specicially described with reference to the accompanying drawings.
GB2766876A 1976-07-02 1976-07-02 Ionisation detector chamber Expired GB1581236A (en)

Priority Applications (9)

Application Number Priority Date Filing Date Title
GB2766876A GB1581236A (en) 1976-07-02 1976-07-02 Ionisation detector chamber
IL52357A IL52357A (en) 1976-07-02 1977-06-21 Ionisation detector chamber
FR7719956A FR2393303A1 (en) 1976-07-02 1977-06-29 IONIZATION CHAMBER DETECTOR
IE1360/77A IE45520B1 (en) 1976-07-02 1977-06-30 Inisation detector chamber
JP7725177A JPS5333600A (en) 1976-07-02 1977-06-30 Ionizing detector
DE19772729599 DE2729599A1 (en) 1976-07-02 1977-06-30 DETECTOR WITH IONIZATION CHAMBER
CA281,806A CA1091826A (en) 1976-07-02 1977-06-30 Ionisation detector chamber
CH815377A CH616768A5 (en) 1976-07-02 1977-07-01
US06/058,613 US4328424A (en) 1976-07-02 1979-07-18 Ionization detector chamber

Applications Claiming Priority (1)

Application Number Priority Date Filing Date Title
GB2766876A GB1581236A (en) 1976-07-02 1976-07-02 Ionisation detector chamber

Publications (1)

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GB1581236A true GB1581236A (en) 1980-12-10

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Application Number Title Priority Date Filing Date
GB2766876A Expired GB1581236A (en) 1976-07-02 1976-07-02 Ionisation detector chamber

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GB (1) GB1581236A (en)

Cited By (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
GB2117560A (en) * 1982-03-09 1983-10-12 Emile Hugon Capacitive smoke detector
GB2182198A (en) * 1985-10-31 1987-05-07 Gent Ltd Ionisation detection chamber
GB2202369A (en) * 1987-02-06 1988-09-21 Kernforschungsz Karlsruhe Radioactive-gas detection apparatus comprising an ionization chamber

Cited By (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
GB2117560A (en) * 1982-03-09 1983-10-12 Emile Hugon Capacitive smoke detector
GB2182198A (en) * 1985-10-31 1987-05-07 Gent Ltd Ionisation detection chamber
GB2182198B (en) * 1985-10-31 1991-05-15 Gent Ltd Ionisation detection chamber
GB2202369A (en) * 1987-02-06 1988-09-21 Kernforschungsz Karlsruhe Radioactive-gas detection apparatus comprising an ionization chamber

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Legal Events

Date Code Title Description
PS Patent sealed
732 Registration of transactions, instruments or events in the register (sect. 32/1977)
PCNP Patent ceased through non-payment of renewal fee
PE20 Patent expired after termination of 20 years

Effective date: 19970629