JP2009244073A - Ionization type gas sensor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an ionization type gas sensor capable of reducing the radioactivity and obtaining high operation reliability and detecting a detection target gas with high accuracy.
An ionization gas sensor for detecting a gas concentration according to an amount of change in which an ionization current generated by ionization of a gas by radiation decreases due to the presence of gas particles, wherein a radiation source and a collecting electrode are arranged in the chamber. And a detection unit provided with a voltage application unit that provides a potential difference between the collector electrode and the chamber, a concentration calculation unit, and a temperature detection unit. The concentration calculation unit detects from the detection unit. The gas concentration value obtained by comparing the output value with one of at least two calibration curve data recorded at different temperatures and recorded in advance between the temperature values related to the two calibration curve data Based on the output change amount in the temperature region, the gas concentration is calculated by correcting the temperature with a concentration correction amount corresponding to the temperature difference between the measured gas temperature value and the temperature value related to one calibration curve data.
[Selection] Figure 1

Description

  The present invention relates to an ionization type gas sensor.

For example, an ionization gas sensor that uses the principle of an ionization chamber that uses the ionization action of gas by radiation as a sensor for detecting organometallic gases in semiconductor manufacturing processes and smoke particles that indicate the occurrence of fires in offices and factories. Is widely used.
As shown in FIG. 6, a certain type of ionization type gas sensor is affected by the change in the measurement chamber 73 into which the test gas is introduced and the environmental conditions, which are separated from each other by, for example, a plate-like collector electrode 75. A detection unit 71 including a chamber 72 having a compensation chamber 74 for compensation, and a radiation source 76, 77 such as americium 241 disposed in each of the measurement chamber 73 and the compensation chamber 74; A current signal as a detection signal from the electrode 75 (which corresponds to the difference between the ionization current in the measurement chamber 73 and the ionization current in the compensation chamber 74) passes through the wall of the chamber 72 through the insulating member 79 in an airtight manner. the ^- non-inverting output terminal (Vout) of the operational amplifier 83 is connected to the input terminal (Vin ⁺⁾ is the inverting input terminal of one operational amplifier 83 of the collector electrode 75 led to the outside (Vin) It is comprised by the density | concentration calculation part 81 containing the amplifier circuit 82 and CPU85 comprised by what was called a "high input impedance circuit" comprised in the state where the negative feedback was applied continuously (for example, refer patent document 1). . In FIG. 6, 72A is a gas introduction pipe, and 72B is a gas discharge pipe.

In the ionization type gas sensor 70 having such a configuration, the measurement chamber 73 is irradiated with radiation (α rays) emitted from the radiation sources 76 and 77 when a voltage having an appropriate magnitude is applied to the chamber 72 by the voltage applying unit 78. The air in the inner chamber and the compensation chamber 74 is ionized to generate ionization currents. For example, by adding the ionization current flowing in the measurement chamber 73 with the direction of the ionization current flowing in the compensation chamber 74 reversed, The ionization current is canceled and the detected current (output) becomes zero, that is, the equilibrium state is maintained between the measurement chamber 73 and the compensation chamber 74.
Thus, when the test gas introduced into the measurement chamber 73 includes fine particles such as smoke particles and gas particles, radiation is absorbed by the fine particles, and the ionization current in the measurement chamber 73 is changed. Since it decreases and the equilibrium state with the compensation chamber 74 is lost, the concentration of the detection target gas contained in the test gas is detected by detecting the amount of change in the ionization current in the measurement chamber 73.

  In such an ionization type gas sensor 70, for example, reference calibration curve data is acquired in advance using a plurality of standard gases having known concentrations and different concentrations of detection target gases (specific components). In addition, temperature correction for calculating a correction gas concentration value is performed by correcting the calibration curve data so as to conform to actual measurement conditions (for example, temperature conditions).

JP 2002-365264 A

  However, in the ionization type gas sensor 70 having the above-described configuration, it is an essential constituent requirement that the measurement chamber 73 and the compensation chamber 74 include the radiation sources 76 and 77, respectively, which is sufficient for the management and handling of the radiation sources 76 and 77. It is necessary to pay attention to the above, and it is desired that the radioactivity is reduced.

  Further, as shown in FIG. 7, the calibration curve data in the ionization type gas sensor 70 has temperature dependence, and the relationship between the gas concentration and the output change amount varies depending on the temperature. Therefore, the calibration curve data is simply based on the temperature difference. If the correction coefficient is set to the correction coefficient, the gas concentration value obtained has a problem of low reliability.

  The present invention has been made on the basis of the circumstances as described above, and it has a configuration in which the radioactivity is reduced, and high operational reliability is obtained. An object of the present invention is to provide an ionization type gas sensor that can be detected with high accuracy.

The ionization type gas sensor of the present invention is an ionization type gas sensor that detects the concentration of the detection target gas in accordance with the amount of current change in which the ionization current generated by the ionization action of the gas due to radiation decreases due to the presence of the gas particles of the detection target gas.
A conductive chamber defining a measurement chamber into which a test gas is introduced is provided, a radiation source and a collector electrode are disposed in the chamber, and a voltage that provides a potential difference between the chamber and the collector electrode A detection unit provided with an application unit; a concentration calculation unit that calculates a gas concentration of the detection target gas based on a detection signal from the detection unit; and a temperature detection unit that detects the temperature of the target gas. And
In the concentration calculation unit, calibration curve data indicating a relationship between at least two gas concentration values acquired in advance and different detection temperatures and detection output values obtained in the detection unit is recorded.
The gas concentration value obtained by comparing the detection output value obtained in the detector with one of the two calibration curve data selected from the calibration curve data is the temperature value related to the two selected calibration curve data. Based on the amount of change in output in the temperature region between, the temperature is corrected with a concentration correction amount corresponding to the temperature difference between the measured temperature value of the test gas and the temperature value related to the one calibration curve data, The gas concentration of the detection target gas is calculated.

  In the ionization type gas sensor of the present invention, three or more calibration curve data having different temperatures are recorded, and the temperature value related to each calibration curve data is set with a uniform temperature range. It is preferable that

  In the ionization type gas sensor of the present invention, a cylindrical chamber whose both ends are hermetically sealed is used, a radiation source is disposed on one end wall of the chamber, and a collector electrode made of a wire is provided in the chamber. It is preferable that the structure is arranged so as to extend in the axial direction of the chamber at a position coaxial with the central axis of the chamber.

  According to the ionization type gas sensor of the present invention, basically, since the radiation source disposed in the chamber is one, the radioactivity can be reduced, The gas concentration value obtained by comparing the detection output value obtained in the detection unit with one of the two calibration curve data relating to the temperature region including the measured temperature of the test gas is based on the output change amount in the temperature region. Thus, when the temperature of the test gas changes due to the temperature correction with the concentration correction amount according to the temperature difference between the measured temperature of the test gas and the temperature related to the one calibration curve data, the calibration curve Since the output instruction value can be prevented from abruptly changing due to the re-selection, the concentration of the detection target gas contained in the detection gas can be accurately detected. In particular, even when gas detection in a low concentration region, which has been difficult to measure with high accuracy in the past, can be performed with high accuracy.

FIG. 1 is an explanatory diagram showing an outline of the configuration of an example of an ionization gas sensor according to the present invention, and FIG. 2 is a cross-sectional view showing the configuration of a detection unit of the ionization gas sensor shown in FIG.
The ionization gas sensor (hereinafter simply referred to as “gas sensor”) 10 includes a detection unit 11 that uses the principle of an ionization chamber that uses the ionization action of gas by radiation, and a detection target based on a detection signal from the detection unit 11. And a concentration calculation unit 30 for calculating the concentration of the gas.

  In the detection unit 11, an electrode structure 20 including a collecting electrode 21 is attached in an airtight manner to an opening on one end side of a cylindrical base 13, and a holder 16 for holding a radiation source 15 in an opening on the other end is airtight. A cylindrical chamber 12 that is mounted and defines a measurement chamber 14 into which a test gas is introduced is provided, and a voltage applying means 18 that provides a potential difference between the collector electrode 21 and the chamber 12. In FIG. 2, 12A is a gas introduction pipe, and 12B is a gas discharge pipe.

  The base 13 constituting the chamber 12 is made of a conductive material such as stainless steel and functions as a counter electrode for the collector electrode 21.

  The electrode structure 20 includes a collector electrode 21 made of a wire made of a hard metal having a small wire diameter, and a cylindrical electrode made of, for example, stainless steel that holds the collector electrode 21 in a state of being inserted therein. A collector electrode holder 22, a guard ring 24 made of, for example, stainless steel, which holds the collector electrode holder 22 via a cylindrical inner insulating member 23A made of, for example, a fluororesin, and a collector electrode holder holding portion of the guard ring 24 Is formed with a cylindrical outer insulating member 23B made of, for example, a fluororesin and inserted in the inside, and in a state in which the electrode structure 20 is mounted on the base body 13, It is positioned coaxially with the central axis of the base body 13 and extends outwardly from the one end face of the guard ring 24 in the axial direction. .

Examples of the material constituting the collector electrode 21 include stainless steel, tungsten, iron, molybdenum, and the like, and even if the surface of these wires is subjected to, for example, Ni-Au plating treatment. Good.
The wire diameter of the collector electrode 21 is preferably, for example, φ1.0 mm or less, and more preferably φ0.4 to φ0.8 mm. When the wire diameter of the collector electrode 21 is excessive, the applied voltage required when performing a predetermined gas measurement is high, while when the wire diameter of the collector electrode 21 is excessively small, self-maintenance is performed. The shape becomes smaller and it is more susceptible to vibrations.

The radiation source 15 is made of, for example, a mass of americium 241 and is held by a recess formed at a central position (position coaxial with the central axis of the chamber 12 and the collector electrode 21) on the inner surface of the holder 16 made of, for example, stainless steel. It has been fixed.
The radiation source 15 has a radioactivity of 10 kBq (kilo becquerel) or less, for example, about 8 kBq (kilo becquerel).

The voltage applying means 18 in the gas sensor 10 is constituted by, for example, a DC power supply, and applies a negative voltage to the base 13 of the chamber 12, for example.
On the other hand, the collector electrode 21 is maintained at, for example, a ground potential state. Therefore, positive ions generated by the ionization action are collected in the chamber 12 and electrons having high mobility are collected in the collector electrode 21. An electric field is formed within 14.

The concentration calculation unit 30 includes, for example, an integration circuit 32 including an operational amplifier OP, a low-pass filter circuit 33, a gain circuit 34, and a CPU 35 having a calculation unit 37 that calculates a gas concentration. In FIG. 1, 38A is an A / D converter, and 38B is a D / A converter.
In the integrating circuit 32, a switch (reset switch) SW for discharging the electric charge charged in the capacitor C is connected in parallel to the capacitor C.
Reference numeral 65 in FIG. 1 is, for example, a thermistor which is a temperature detection means for detecting the ambient temperature of the detection unit 11 (the temperature of the test gas introduced into the detection unit 11).

The CPU 35 in the gas sensor 10 has a recording unit 39, and at least two or more calibration curve data at different temperature values acquired in advance for the gas sensor 10 are set in the recording unit 39.
The temperature values related to these calibration curve data are set at a uniform temperature range, for example, 10 ° C. within the operating temperature range of the product. In this embodiment, for example, 0 ° C., 10 ° C., 20 ° C. Five calibration curve data relating to five temperature values of 30 ° C. and 40 ° C. are set. The temperature value is not particularly limited, and can be set as appropriate depending on, for example, the environmental conditions in which the gas sensor 10 is used.
The calibration curve data is set for each detection target gas such as SiH ₄ gas or TEOS gas.

Hereinafter, the gas detection operation by the gas sensor 10 will be described.
The gas sensor 10 is used in a gas detection system together with a pyrolyzer and a pump. The configuration of the gas detection system will be described with reference to FIG. 3. A pyrolyzer 41 that generates particulate oxide by heating the test gas to about 800 to 900 ° C. is provided in the chamber 12 of the gas sensor 10. The gas introduced into the chamber 12 of the gas sensor 10 is connected to the provided gas introduction pipe 12 </ b> A, and the gas introduction means including, for example, the suction pump 48 is provided through the gas flow rate adjustment means including the gas flow rate adjustment valve 47 and the buffer 46. It is connected to the discharge pipe 12B. 3, 42 is a gas sensor, 43 is a data logger, 44 is a particle removal filter, 45 is a flow meter, 46 is a buffer, and 49 is an exhaust duct.

  In this gas detection system 40, the test gas is introduced into the measurement chamber 14 of the chamber 12 at a gas flow rate adjusted to an appropriate size, and controlled to an appropriate size by the voltage application means 18, for example. By applying a negative (−) voltage to the substrate 13 constituting the chamber 12, the air in the measurement chamber 14 is ionized by the action of radiation (α rays) from the radiation source 15, thereby generating electrons and negative ions. When ions are attracted to the collector electrode 21 that functions as an anode, an ionization current flows between the wall of the chamber 12 that functions as a cathode and the collector electrode 21, and an input current signal corresponding to the magnitude of the ionization current is an integration circuit. 32, and the output signal is input to the CPU 35 via the low-pass filter circuit 33 and the gain circuit 34, and based on the output signal. The concentration of the detection target gas is calculated, but when gas particles relating to the detection target gas are included in the detection gas, the radiation (α rays) from the radiation source 15 is absorbed by the gas particles. Accordingly, the ionization current is reduced, and the concentration of the detection target gas is calculated according to the amount of change (the degree of reduction) of the ionization current.

In the above description, when calculating the concentration of the detection target gas, the detection output value V obtained by the detection unit 11 is converted into a temperature region (t1 <) where the actually measured temperature value (T) obtained from the thermistor (temperature detection means) 65 is included. Based on the output change rate α in the temperature region, the gas concentration value (z1, z2) obtained by contrasting with one of the two calibration curves according to T <t2) is measured. And a temperature correction with a concentration correction amount (R) corresponding to a temperature difference (Δt) between a temperature value (t1 or t2) related to one of the calibration curves, and a corrected gas concentration value is obtained. Here, the density correction amount (R) is obtained by (Expression 1) R = α × Δt = {(z2−z1) / (t2−t1)} × Δt, Δt = T−t1 or Δt = t2−T. Is done.
Hereinafter, the temperature correction method according to the present invention will be described with specific numerical examples.

The gas detection system shown in FIG. 3 is configured using the gas sensor 10 having the following specifications, the applied voltage to the chamber is set to −5 V, and the silane gas (SiH ₄ gas) as the test gas is 0.3 liter / min. As shown in FIG. 4, the temperature of the silane gas is 0 ° C., 20 ° C. and 40 ° C. When a calibration curve serving as a reference in this case has been acquired in advance, a similar gas particle detection test was performed on silane gas at 30 ° C. (actually detected value detected by the temperature detecting means). When the voltage is 25 V, as shown in FIG. 5, the gas concentration value obtained by comparing the sensor output value with a reference calibration curve of 20 ° C. is, for example, 5. Since the gas concentration value obtained by comparing with the standard calibration curve of ppm and 40 ° C. is, for example, 7.5 ppm, the gas concentration value corresponding to the detection output value of the sensor in silane gas at 30 ° C. is 5.6 to It is assumed that it is within the concentration range of 7.5 ppm.
Therefore, based on the output change rate in the temperature range of 20 to 40 ° C., the concentration correction amount (R == R) calculated according to the above equation 1 according to the temperature difference between the measured temperature value of the silane gas and the temperature related to the reference calibration curve. By correcting the temperature in 0.95), the gas concentration value corresponding to the sensor output value of 2.25 V in the silane gas at 30 ° C. is calculated to be 6.55 ppm.

[Gas sensor specifications]
Chamber (12): Material: Stainless steel, outer diameter: φ20 mm, inner diameter: φ15 mm, length: 55 mm,
Collector electrode (21): Material; Tungsten with Ni—Au plating on the surface, Wire diameter: φ0.3 mm, Position in the measurement chamber; Position coaxial with the central axis of the chamber,
Radiation source (15): material; americium 241, radioactivity; 8 kBq (kilo becquerel),

  Thus, according to the gas sensor 10, an input current signal is obtained as a detection signal from the detection unit 11, and this input current signal is integrated by the integration circuit 32 including the operational amplifier OP to thereby detect the concentration of the detection target gas. Can be calculated. Specifically, the output Vout is calculated based on the following equation (2), and the concentration of the detection target gas corresponding to the output Vout obtained thereby is calculated based on the calibration curve data acquired in advance.

  Accordingly, the chamber structure can be configured as having only the measurement chamber 14 and having the single radiation source 15, so that the gas concentration is detected based on the difference in ionization current between the measurement chamber and the compensation chamber. The radiation can be reduced while having the same operational reliability as the above.

  Moreover, the gas concentration value obtained by comparing the detection output value obtained in the detection unit 11 with one of the two calibration curve data relating to the temperature region including the measured temperature of the test gas is the temperature region. Based on the amount of change in output, the temperature of the test gas is changed by the temperature correction with the concentration correction amount corresponding to the temperature difference between the measured temperature of the test gas and the temperature related to the one calibration curve data. In this case, since a rapid change in the output instruction value due to the reselection of the calibration curve can be prevented, the concentration of the detection target gas contained in the test gas can be accurately detected. In particular, even when gas detection in a low concentration region, which has been difficult to measure with high accuracy in the past, can be performed with high accuracy.

  Furthermore, the detection unit 11 includes a cylindrical chamber 12 that defines a measurement chamber 14 in which both ends are hermetically sealed and into which gas to be detected is introduced, and is provided at a central position on one end wall of the chamber 12. A configuration in which a radiation source 15 is disposed, and a collecting electrode 21 made of a wire is positioned coaxially with the central axis of the chamber 12 so as to penetrate the other end wall in an airtight manner and be led out to the outside. As a result, the intensity of the electric field formed in the measurement chamber 14 can be uniformly formed in the axial direction by applying a voltage of an appropriate magnitude to the chamber 12, so that It is possible to reliably detect the ionizing current generated by the ionizing action.

  Furthermore, since the collector electrode 21 is maintained at the ground potential state and a negative voltage is applied to the chamber 12, electrons with high mobility are collected on the collector electrode 21, which is a positive electrode. Efficiency can be obtained.

As mentioned above, although embodiment of this invention was described, this invention is not limited to said embodiment, A various change can be added.
For example, in the present invention, voltage application means may be provided so that a predetermined potential difference is obtained between the collector electrode and the chamber. For example, a voltage is applied to both the collector electrode and the chamber. Or a configuration in which a positive voltage is applied to the collector electrode.
Further, the voltage application means for the chamber can be configured by a variable voltage power source. In such a configuration, the magnitude of the voltage applied to the chamber is based on the type and / or concentration of the detection target gas. By setting, the intensity of the electric field formed in the measurement chamber is adjusted, for example, sensitivity adjustment according to the type of the detection target gas can be performed, and the detection target gas can be detected with high accuracy.
Furthermore, the number of calibration curve data (temperature) to be acquired in advance is not particularly limited, but is preferably 3 to 5 in practice.
Furthermore, it is not necessary that the collector electrode itself is led out of the chamber. For example, an external lead for power supply is connected to the base end portion of the collector electrode, and the external lead is led out of the chamber. It may be configured.
Furthermore, the gas sensor of the present invention can also be applied to a smoke detector for sensing carbon monoxide, hydrogen sulfide, hydrocarbons, carbon dioxide, methane, butane and the like.

It is explanatory drawing which shows the outline of a structure in an example of the ionization type gas sensor of this invention. It is sectional drawing which shows the structure of the detection part of the ionization type gas sensor shown in FIG. It is explanatory drawing which shows the outline of a structure in an example of the gas detection system using the ionization type gas sensor shown in FIG. 1 and FIG. It is explanatory drawing which shows an example of the calibration curve data in the ionization type gas sensor of this invention. It is explanatory drawing for demonstrating the calculation method of the density | concentration of the detection target gas in the ionization type gas sensor of this invention. It is explanatory drawing which shows the outline of a structure in an example of the conventional ionization type gas sensor. Is an explanatory diagram showing an example of the output characteristics of the SiH ₄ gas in the ionization gas sensor.

Explanation of symbols

10 Ionization type gas sensor (gas sensor)
DESCRIPTION OF SYMBOLS 11 Detection part 12 Chamber 12A Gas introduction pipe 12B Gas discharge pipe 13 Base | substrate 14 Measurement room 15 Radiation source 16 Holder 18 Voltage application means 20 Electrode structure 21 Current collection electrode 22 Current collection electrode holder 23A Inner insulation member 23B Outer insulation member 24 Guard ring 30 Density calculation unit 32 Integration circuit 33 Low-pass filter circuit 34 Gain circuit 35 CPU
37 arithmetic unit 38A A / D converter 38B D / A converter 39 recording unit 40 gas detection system 41 pyrolyzer 42 gas sensor 43 data logger 44 particle removal filter 45 flow meter 46 buffer 47 gas flow rate adjustment valve 48 suction pump 49 exhaust duct DESCRIPTION OF SYMBOLS 65 Thermistor 70 Ionization type gas sensor 71 Detection part 72 Chamber 72A Gas introduction pipe 72B Gas discharge pipe 73 Measurement room 74 Compensation room 75 Collector electrode 76,77 Radiation source 78 Voltage application means 79 Insulating member 81 Concentration calculation part 82 Amplifying circuit 83 Operational amplifier 85 CPU
OP operational amplifier C capacitor SW switch (reset switch)

Claims (3)

In an ionization type gas sensor that detects the concentration of the detection target gas in accordance with the amount of change in the current, which is reduced by the presence of gas particles of the detection target gas, due to the ionization action of the gas by radiation,
A conductive chamber defining a measurement chamber into which a test gas is introduced is provided, a radiation source and a collector electrode are disposed in the chamber, and a voltage that provides a potential difference between the chamber and the collector electrode A detection unit provided with an application unit; a concentration calculation unit that calculates a gas concentration of the detection target gas based on a detection signal from the detection unit; and a temperature detection unit that detects the temperature of the target gas. And
In the concentration calculation unit, calibration curve data indicating a relationship between at least two gas concentration values acquired in advance and different detection temperatures and detection output values obtained in the detection unit is recorded.
The gas concentration value obtained by comparing the detection output value obtained in the detector with one of the two calibration curve data selected from the calibration curve data is the temperature value related to the two selected calibration curve data. Based on the amount of change in output in the temperature region between, the temperature is corrected with a concentration correction amount corresponding to the temperature difference between the measured temperature value of the test gas and the temperature value related to the one calibration curve data, An ionization type gas sensor, wherein a gas concentration of a detection target gas is calculated.   3. Three or more calibration curve data having different temperatures are recorded, and the temperature value of each calibration curve data is set with a uniform temperature range. Ionization type gas sensor.   The chamber has a cylindrical shape whose both ends are hermetically sealed, a radiation source is provided on one end wall of the chamber, and a collecting electrode made of a wire is placed at a position coaxial with the central axis of the chamber. The ionization type gas sensor according to claim 1, wherein the ionization type gas sensor is arranged so as to extend in an axial direction of the gas sensor.

Images