US3735138A - Ionization smoke detector - Google Patents

Ionization smoke detector Download PDF

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Publication number
US3735138A
US3735138A US00192827A US3735138DA US3735138A US 3735138 A US3735138 A US 3735138A US 00192827 A US00192827 A US 00192827A US 3735138D A US3735138D A US 3735138DA US 3735138 A US3735138 A US 3735138A
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current
pressure
operating
operating voltage
sensitivity
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US00192827A
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English (en)
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G Rork
A Schlachter
F Simon
R Stryk
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Honeywell Inc
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Honeywell Inc
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    • 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
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
    • H01J41/00—Discharge tubes for measuring pressure of introduced gas or for detecting presence of gas; Discharge tubes for evacuation by diffusion of ions
    • H01J41/02—Discharge tubes for measuring pressure of introduced gas or for detecting presence of gas
    • H01J41/08—Discharge tubes for measuring pressure of introduced gas or for detecting presence of gas with ionisation by means of radioactive substances, e.g. alphatrons

Definitions

  • FIG-4 IS INVENTOR.
  • the ionization type smoke detector consists of a radioactive ionization source, electrodes across which a voltage is applied and ionization current flows and electronic circuitry for measuring the current flow between the electrodes.
  • FIG. 1 is a diagrammatic representation and partial cross-section of an embodiment of an ionization smoke detector of the type described.
  • FIG. 2 is a graphic example of change in current for a change in pressure of the smoke detector with voltage as a parameter.
  • FIG. 3 is a graphic display of change in current for change in ion mobility at fixed voltage.
  • FIGS. 4 and 5 are a graphic display of the Detector Sensitivity to Mobility change as a function of III,.
  • FIG. 6 is a diagrammatic representation of an embodiment having a parallel plate geometry evolved fromone solution of the mathematical relationships explained below.
  • DETAILED DESCRIPTION Ionization type smoke detectors in general, operate on the following principles: Primary particles (high energy electrons called [3 particles, from Nifor example or high energy helium nuclei called 0: particles, from Am for example) are randomly emitted from a radioactive source. These, in turn, collide with air molecules and other vapors-with sufficient energy to ionize them forming positive and negative ion pairs. Each primary particle ionizes a large numberof air molecules in its path. This number for air at ordinary pressure is given to a sufficient degree of accuracy by N E/30, where E is the source energy in electron volts. For example, B particles from Ni have an average energy of about 18,000electron volts. For this energy,
  • the number of primary particles emitted per second from a source is called its activity measured in curies.
  • One millicurie (mC) of Ni produces 3.7 X primaries per second. Therefore, 1 mC of Ni yields 600 (ion pairs/primary) X 3.7 X 10" (Prim/Sec.)
  • This number is the so called generation rate of current in an ionization cell. If these ions are collected at the electrodes at the same rate as they are generated, the current in our Ni example I 22 X 10' (ions prs./sec.) X 1.6 X 10' (coulombs/ion) 35.2 X l0 or 3520 picoamps.
  • I is the saturation current that occurs when the voltage on the cell is raised high enough to collect all the ions produced.
  • the curves of FIG. 3 show current plotted against voltage and the saturation current is clearly apparent from the curves as voltage increases.
  • the radioactive intensity needed for pressure independence in this invention is one capable of producing a rate of ion pair production equivalent to a saturation current in the range of about 450 to about 650 picoamperes.
  • a first optimizing feature of this invention is that the cell is normally operated at a current level (I) below the saturation level (1,). Under this condition of operation, only a fraction of the ion pairs generated are collected. The remainder simply recombine to form neutral molecules again. It may be said that in the steady state, the total number generated per second equals the number collected per second plus the number combining per second.
  • the recombination rate depends among other things on a characteristic of the ion called its mobility which is a measure of the speed that the ion moves in the electric field. The mobility is different for different vapors, as is shown in FIG. 3, and so the rate at which ions recombine differs for different vapors.
  • the mobility is also pressure dependent, that is, the higher the pressure the lower the effective mobility, and since this means larger recombination the collected current is smaller.
  • higher pressures tend to give smaller currents because of the mobility decrease, while higher pressures tend to give larger currents because of the saturation current increase.
  • I I o/ l/ o
  • I saturation current (dL/dP) l" change in saturation current with change in pressure P, desired operating pressure (about 700 mm Hg).
  • AI/l the relative sensitivity
  • Al the ratio of the change in current
  • the attainment of an optimized set of parameters for ionization cells by purely empirical methods is difficult because of the large number of complex physical processes involved in any ionization phenomena.
  • the first step is the determination of the importance of space charge effects on the predicted voltage-current relationship for a variety of simple geometries. We determined that space charge is relevant only in scaling the particular geometry used. We thus derived a relation that depended on recombination and generation rates that was shown to predict I vs. V once a geometry was specified. This result is given by Equation 2.
  • sensitivity we can use Equation 2 to determine sensitivity of a detector as a function of choice of parameters.
  • sensitivity to be either total or fractional change in current for a change in ion mobility, i.e., Al or All] for a given A t/11.. This change in I is illustrated in FIG. 3 which shows V-I curves for ions of two different mobilities.
  • Condition (c) specifies a value for A, the field geometry factor.
  • Equations l (2) and (6) provide a consistent set of three equations which can be satisfied simultaneously by values of the three adjustable parameters A, I and I, so that a high sensitivity consistent with pressure independence is achieved in any chosen design. These equations have solutions over certain physically realizable value ranges of the parameters. If the equations cannot be solved for the values as shown, then one of the specified items of sensitivity or operating voltage must be modified and the equations solved again. This leads to a restricted range of acceptable values for the parameters of sensitivity, operating voltage, saturation current, operating current and geometry.
  • the geometry may take the form of concentric cylinders, parallel plates, or other suitable geometry.
  • a parallel plate geometry such as shown in FIG. 6.
  • Solving equations 1, 2 and 6 provide numbers of I, I, and A which satisfy the equations described for desired sensitivity and pressure independence and in which I, 550 pa. and I 300 pa. for the saturation current and operating current respectively, and the dimensions of the rectangular parallel plates interelectrode spacing, and amount of radioactive material as shown in FIG. 6 satisfy the geometry factor to provide pressure insensitivity without sacrificing values of sensitivity and operating voltage.
  • a single chamber pressure independent ionization type device for detecting products-of-combustion the device being of the type where a unidirectional interelectrode current flows between a pair of electrodes and the occurrence of products-of-combustion causes a current change I, the improved device comprising:
  • geometrical factor includes electrode spacing and electrode size
  • a radioactive source having a radioactive intensity capable of producing a rate of ion pair production equivalent to a saturation current I, of about 450 to about 650 picoamperes,
  • voltage supply means providing an interelectrode operating current I of about 200 to about 500 picoamperes, at an operating voltage V and said geometry factor A, saturation current 1,, operating current 1, operating voltage V, and operating pressure P being interrelated according to the relations 0 (18 /218 m/Mo) thereby causing said operating current to be essentially independent to variations in pressure.
  • a method for designing a pressure independent ionization type products-of-combustion sensing device which, in addition to allowing the specifying of pressure independence, concurrently allows the specifying of arbitrary values of sensitivity and of operating voltage within a restricted range, the method comprising the steps of a. specifying 1) pressure independence, (2) an arbitrary value of sensitivity, (3) and an arbitrary operating voltage;

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  • Chemical & Material Sciences (AREA)
  • Analytical Chemistry (AREA)
  • Business, Economics & Management (AREA)
  • Emergency Management (AREA)
  • Physics & Mathematics (AREA)
  • General Physics & Mathematics (AREA)
  • Other Investigation Or Analysis Of Materials By Electrical Means (AREA)
US00192827A 1971-10-27 1971-10-27 Ionization smoke detector Expired - Lifetime US3735138A (en)

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Cited By (7)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4051376A (en) * 1975-01-28 1977-09-27 The Radiochemical Centre Ltd. Ionization detectors
US4053776A (en) * 1976-05-25 1977-10-11 The United States Of America As Represented By Thesecretary Of The Interior Sub-micron particle detector
US4704536A (en) * 1983-12-23 1987-11-03 Hochiki Corporation Gas sensor and gas detecting method
US5237281A (en) * 1990-11-13 1993-08-17 Hughes Aircraft Company Ion drag air flow meter
JP2014534422A (ja) * 2011-10-06 2014-12-18 マイクロチップ テクノロジー インコーポレイテッドＭｉｃｒｏｃｈｉｐ Ｔｅｃｈｎｏｌｏｇｙ Ｉｎｃｏｒｐｏｒａｔｅｄ 漏れ電流の存在下でイオン電流を判定するための差動電流測定
US9823280B2 (en) 2011-12-21 2017-11-21 Microchip Technology Incorporated Current sensing with internal ADC capacitor
US12298216B2 (en) 2019-08-02 2025-05-13 Cambridge Enterprise Limited Particle sensor and sensing method

Families Citing this family (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US5189399A (en) * 1989-02-18 1993-02-23 Hartwig Beyersdorf Method of operating an ionization smoke alarm and ionization smoke alarm

Citations (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3448261A (en) * 1965-03-11 1969-06-03 Boris Abel Amiragoff Signal detection and measuring circuit
US3521263A (en) * 1966-02-22 1970-07-21 Cerberus Ag Ionization fire alarm and improved method of detecting smoke and combustion aerosols

Patent Citations (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3448261A (en) * 1965-03-11 1969-06-03 Boris Abel Amiragoff Signal detection and measuring circuit
US3521263A (en) * 1966-02-22 1970-07-21 Cerberus Ag Ionization fire alarm and improved method of detecting smoke and combustion aerosols

Cited By (8)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4051376A (en) * 1975-01-28 1977-09-27 The Radiochemical Centre Ltd. Ionization detectors
US4053776A (en) * 1976-05-25 1977-10-11 The United States Of America As Represented By Thesecretary Of The Interior Sub-micron particle detector
US4704536A (en) * 1983-12-23 1987-11-03 Hochiki Corporation Gas sensor and gas detecting method
US5237281A (en) * 1990-11-13 1993-08-17 Hughes Aircraft Company Ion drag air flow meter
JP2014534422A (ja) * 2011-10-06 2014-12-18 マイクロチップ テクノロジー インコーポレイテッドＭｉｃｒｏｃｈｉｐ Ｔｅｃｈｎｏｌｏｇｙ Ｉｎｃｏｒｐｏｒａｔｅｄ 漏れ電流の存在下でイオン電流を判定するための差動電流測定
US9805572B2 (en) 2011-10-06 2017-10-31 Microchip Technology Incorporated Differential current measurements to determine ion current in the presence of leakage current
US9823280B2 (en) 2011-12-21 2017-11-21 Microchip Technology Incorporated Current sensing with internal ADC capacitor
US12298216B2 (en) 2019-08-02 2025-05-13 Cambridge Enterprise Limited Particle sensor and sensing method

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CH552989A (de) 1974-08-30
DE2242212A1 (de) 1973-05-03

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