METHOD AND DEVICE FOR MEASURING THE SMOKE CONCENTRATION IN AIR

Abstract: Disclosed is a method and a device for measuring the smoke concentration in air. In a smoke chamber (5) are located two electrodes (2, 3), connected to a voltage source for forming a circuit in which a DC current flows. A radioactive radiation source (1) to ionize the air molecules is located in the vicinity of the smoke chamber, so that the number of ionized air molecules which are formed is dependent upon the radiation intensity of the ion source (1) and the concentration of smoke particles in the smoke chamber (5). The charging voltage will further imply that a cloud of high ion concentration is built up close to the surface of the electrodes. The ion cloud will be discharged capacitively upon a plurality of short voltage pulses (12) applied to the electrodes (2, 3) to thereby result in current pulses (13) substantially greater than the DC current flowing through the chamber (5).

Description: Method and device for measuring the smoke concentration in air The present invention concerns a method and a device for measuring the smoke concentration in air, of the kind which is stated in the introductory part of claim 1 and 5. More specifically the invention concerns detection of smoke particles in air which is ionized by a radioactive source. The number of ions is reduced when smoke particles are present, and this can be measured using various means, preferably by locating electrodes in the vicinity of the radioactive source for forming a smoke chamber. Measuring the reduction of the ionization current implies some problemes, as small currents are involved. It is desirable to keep the radiation from the radioactive source as low as possible. If the strength of the radiation is less than 1 micro curie, the current will be 10-100 pA. In a previously known method, a reference resistor is used to create a voltage divider together with the smoke chamber. The reference resistor can either be a special resistor or another ionized chamber which is not exposed to smoke from the environment. There are certain problems connected with the use of reference resistors having these values. The reference resist r should be in the magnitude of 10 11-1012 ohm, and it has proved to be difficult to provide highly resistiv materials having sufficient stability. This is generally associated with difficulties measuring such small currents under varying atmoshperic conditions. Besides, it is necessary that the input impedance in the amplifier is substantially greater than the reference resistor and the resistance in the smoke chamber, to avoid leakage current in the amplifier influencing the voltage division. Even if MOSFET or special FET transistors are used, one will always face the problem of creating sufficient impe¬ dance between the input terminal of the transistor, and the other terminals under high air moisture. This particularly applies if the components and the substrate or circuit board to which these are connected are dirty. A further principle to measure the impedance of the chamber, is to connect the smoke chamber to a reference capacitor and an amplifier to create an oscillating circuit. The circuit will oscillate with a frequency varying with the resistor in the smoke chamber. Normally it is easier to manufacture a capacitor with sufficient stability than a resistor with high resistivity, but this solution sets up the same restrictions to the input impedance of the amplifier as the above mentioned solutions. It is therefore an object of the present invention to provide a method and a device for measuring the smoke concentration in air, without the disadvantages which are present in the prior art. The object of the invention is achieved with a method and device having features as stated in the characterizing part of patent claim 1 and 5. Additional features will be clear from the dependent claims. The invention will now be described by means of a preferred embodiment, and with reference to accompanied drawings, wherein, Fig. 1 is a circuit diagram which states the principles for the kind of measurement upon which the invention is based, Fig. 2 is a circuit diagram of a first embcdiment from the prior art. Fig. 3 is a circuit diagram of a second embodiment from the prior art, Fig. 4 is a circuit diagram of a third embodiment from the prior art. Fig. 5 is a graphic diagram of the response after DC voltage is connected to the smoke chamber, Fig. 6 is a graphic diagram of the response after voltage pulses has been connected to the smoke chamber, Fig. 7 is a circuit diagram of a first embodiment according to the present invention, and Fig. 8 is a circuit diagram of a second embodiment according to the present invention. By first referring to Fig. 1, a radioactive source 1 ionizes the air gap between two electrodes 2, 3. If a DC current source 4 is connected to the electrodes 2, 3, a current I will flow in the circuit. This current is determined by the number of ions formed in the air gap per unit time. This will among other things be dependent upon the intensity of the radiation from the radioactive source. Current I is not a linear function of the voltage U as it is for resitors. It will, however, increase slightly with increasing volatage due to the recombination of ions being reduced. If smoke particles come between the electrodes 2, 3, the rate of ionization will be reduced, that is the number of ions which is generated per unit time will be less. This implies that current I will be reduced. The two electrodes 2, 3 and the air gap represent an impedance where the stationary impedance is a function of the concentration of smoke particles. In the following, the electrodes 2, 3 and the air gap is designated a smoke chamber 5. As it is a desire to use weak radioactive sources, most likely with a radiation less than 1 micro curie (35 kBq), current I will be only 10-100 picoampere (pA). In Fig. 2 and 3 are shown two methods for measuring the currents in question. In Fig. 2, the smoke chamber 5 is conncected in series with am electrical reference resistor 6 having a value R f , so that a voltage divider is formed where the measurement voltage U can be expressed as U is amplified by an amplifier 7 and fed into an alarm circuit (not shown). Since R. is in the order of magntitude 10 11-1012 ohm, resi.stor R 1 P . fl 6 f should also have a value which is in the same range. The reference resistor can be reference chamber similar to the smoke chamber but a chamber which is closed so that smoke will not enter. A corresponding method is achieved by putting a middle electrode 8 in a smoke chamber 5, as shown in Fig. 3. The chamber 5 is made up from two impedances connected in series, where the smoke particles will give the greatest variation in current I in the part of the chamber 5 which i farthest away from the radioactive source. In Fig. 4 there is shown an additional known principle for measuring the impedance of the chamber. The smoke chamber 5 is connected to capacitor 9 having a value C and an amplifier 7, so that an oscillating circuit is forme The oscillating circuit will oscillate with a frequency (f) following from the equation f = V C ref The oscillating frequency will therefore vary with R J-_. and can be measured and provide a signal to an alarm circuit (not shown). The present invention takes its basis in a dynamic impedance which can be observed when a DC voltage is connected to the chamber, which voltage has short voltage pulses superimposed. If the chamber is connected to a DC voltage 10, the current 11 will follow the graph shown in the Fig. 5, i.e. after a short inclination it will stabilize on a constant value. The constant current 11 is determined by the number of ions, that is the number of free charges which form in chamber 5. This number is proportional to the intensity of the radioactive radiation source, and to a certain degree dependent upon the magnitude of the DC voltage 10, as a higher voltage will reduce the number of recombinations. Furthermore, it shows that a connection of short voltag pulses 12, superimposed on the DC voltage 10, results in current pulses 13 which are substantially greater than the stable (constant) DC curent 11 shown in Fig. 6. This is explained by a high ion concentration building up close to the surface of the electrodes. This is discharged by a capasitive displacement current at rapid increase of the voltage. We see that the current immediately after the voltage pulse will be reduced to close to zero, and afterwards be rebuild, that is it must be rebuild a new ion concentration close the the surface of the electrode. The mean value of the current in a chamber changes very little due to the current pulses. This means that the current pulses have a magnitude which is determined by the length of the voltage pulses and the distance between the pulses: Impulse = 1/2*1mean*period time/pulse time The ratio between pulse length and pulse distance can e.g. be between 1/1000 and 1/100. It has shown to be particularly practical to have a ratio between pulse length and pulse distance at about 1/500, something which leads to the current pulses 13 being almost 300 greater than the constant DC current. If smoke particles come into the smoke chamber, the number of produced ions will be reduced, and the time to rebuild the ion concentration will increase. The fact that the current pulse, which is set free by the capasative discharging of the ions, is almost 300 times greater than the constant current, which flows through the chamber at connected DC current, will highly reduce the restrictions of the input impedance of the amplifier 7. In Fig. 7 there is shown an embodiment according to present invention. A pulse generator 14 of per see known type will produce voltage pulses 12 of desired length and in desired intervals. The voltage pulses are connected to the electrodes 2, 3 and this will result in the formation of current pulses 13. The amplifier 7 is, in this example, realized by using a simple bipolar npn-transistor. Accordingly, Fig. 8 shows an example of an embodiment with a bipolar pnp- ransistor. The good dynamics of the bipolar transistors make these a very good amplifying element for the current pulses 13.

Claims: Claims 1. A method for measuring the smoke concentration in air, wherein two electrodes (2,3) located in a smoke chamber (5) are connected to a voltage source for forming a circuit in which a DC current flows, where a radioactive radiation source (1) to ionize the air molecules is located in the vicinity of the smoke chamber, so that the number of ionized air molecules which is formed is dependent upon the radiatio intensity of the ion source (1) and the concentration of smoke particles in the smoke chamber (5), and the charging voltage will further imply that a cloud of high ion concent¬ ration is built up close to the surface of the electrodes, c h a r a c t e r i z e d in that the ion cloud is discharged capacitively upon a plurality of short voltage pulses (12) being applied to the electrodes (2,3) which results in current pulses (13) substantially greater than the DC current flowing in the circuit. 2. Method according to claim 1, c h a r a c t e r i z e d in the voltage pulses (12) having a duration in the range of 0,05 mS - 1,0 mS, preferably 0,2 mS. 3. Method according to claim 1-2, c h a r a c t e r i z e d in the ratio between the pulse length and the pulse distance" of the voltage pulses being in the range of 1/1000 - 1/100, preferable about 1/500. 4. Method according to claim 1-3, c h a r a c t e r i z e d in that the current pulse (13) is amplified using a bipolar transistor (7). 5. Device for measurement of the smoke concentration in air, comprising a smoke chamber (5) having two electrodes (2,3) and a radioactive source for radiation (1), to which ionize the air molecules, whereby the number of ionized air molecules formed is a function of the intensity of the radiation from the ion source (1) and the concentration of smoke particles in the smoke chamber (5), further is provided a voltage source which is connected to the electrodes (2,3) forming a circuit in which a DC current flows, c h a r a c t e r i z e d in that in the circuit of the electrodes (2,3) there is connected a pulse generator (14) to apply one or more voltage pulses (12) to the electrodes (2,3) to create current pulses (13) substantially greater than DC current flowing in the circuit. 6. Device according to claim 5, c h a r a c t e r i z e d in that a bipolar transistor (7) is connected in the circuit with electrodes (2,3) to amplify the current pulses (13).

Inventors: RENNEMO BJOERN, RENNEMO, BJOERN
Applicants: RENNEMO BJOERN, RENNEMO, BJOERN
Application year: 1992
Main classification: G01N27
IPC codes: G08B17