US 2504772 A
Description (OCR text may contain errors)
April 18, 1950 H TE 2,504,772
I ELECTRICAL DETECTOR OF VAPORS AND THE LIKE Filed July 8, 1947 l1 ca 5'" RELAY l6 4. i W 3 I m y Ia Inventor:
William C.Whi te,
y His Attorney- Patented Apr. 18, 1950 ELECTRICAL DETECTOR OF VAPORS LIKE ANDTHE William C. White, Schenectady, N. Y., assignor to General Electric Com New York pany, a corporation of Application Jub 8, 1947, Serial No. 759,641
11 Claims- -(Cl. 175183) This invention relates to improvements in the electrical methods and apparatus for detecting certain finely divided atmospheric substances described and claimed in a copending application Serial No. 754,657, filed June 14, 1947, by Chester W. Rice, and assigned to the assignee of the present invention. It has for its general object the provision of a method and apparatus having improved quantitative sensitivity.
The features of the invention desired to be protected are pointed out in the appended claims. The invention itself together with its further objects and advantages will be best understood by reference to the following specification taken in connection with the appended drawing which represents one embodiment of the invention in a convenient industrially or otherwise useful vapor detector designed to detect the presence of finely divided substances and vaporsin any suitable inert atmosphere acting as a, carrier of the substance.
In the aforesaid application of Chester W. Rice, there are described and claimed methods and apparatus for detecting certain finely divided substances such as certain gases, vapors, smokes or other particles of matter, in the atmosphere or elsewhere, by passing a sample of an atmosphere, which may be suspected of containing a concentration of the substance to be detected, into an electrical discharge device under conditions which permit the substance to induce positive ion formation at a positively charged heated electrode, the positive ions so formed being collected by a negatively charged electrode to produce an indicating current which increases with the concentration of the substance. Devices of this type have been found to be qualioccurs by ionization of the substance itself when it comes into contact with the heated electrode.
For others, such as the halogens and their compounds, the substance appears to cause ion formation only in the presence of what may be called sensitizing materials, such as the alkali metals or their compounds, and it. appears in this case that the sensitizing materials themselves are actually ionized to afford the desired positive ion current.
Now I have found that the quantitative sensitivity (i. e. the rate of ion formation) of devices of this nature may vary with variations in the rate of air flow in the space between the electrodes. If that rate is too rapid, apparently only a limited number of vapor particles have opportunity to strike the hot positively charged electrode and induce ion formation. Therefore, the sensitivity of the device may decrease as the rate of air flow is increased. On the other hand, while sensitivity may be higher at low rates, if the air flow has too low a rate, response of the device to vapor concentration changes will be. slow because of the time delay in the passage of the vapor through the device. Moreover prolonged or excessive exposure of the electrodes to detectable vapors which may occur at low air flow rates has a deleterious effect on sensitivity. The speed at which sensitivity recovers from the latter effect after cessation of the prolonged or excessive exposure is slow but may be hastened by further fiow of relatively pure air. An optimum rate of air fiow which provides the best compromise between these conditions may be selected for any given set of operating conditions.
The foregoing indicates the following as one desirable and useful mode of operation of the device when, for example, it is used to detect the presence of a toxic or other vapor in a room or other space. In order initially to take full advantage of the greater sensitivity characterizing low air velocities, the device may be permitted initially to circulate the air through its interelectrode space only by virtue its own heat convection currents created by its hot electrodes or by the aid of relatively weak blower action. When an initial concentration of vapor is indicated by an increase in current in the device, the rate of flow may be increased by suitable means, for example, the increase of anode current may be caused to operate a suitable amplifier and relay, which may in turn activate a blower. The blower in turn, will greatly increase the air flow through the device and thus tend to cause the detector to recover quickly from any decrease in the sensitivity resulting from any prolonged or excessive exposure occurring because of the initial relatively higher vapor concentration in the interelectrode space. Moreover, with increased air flow velocity, the time of passage through the detector will be small for which reason it will respond more quickly to changes in the degree of concentration of the substance detected. If the positive ion current subsequently be reduced either because the vapor concentra- Referring now to the drawing it will be noted that an electronic discharge device I of the type disclosed in the aforementioned Rice application is indicated schematically. It may, for example, comprise an envelope 2, a heated positively charged electrode 3, a negative electrode 4, an air inlet orifice and an outlet orifice G. The heater wire 1 of the electrode 3 may be energized by any suitable means such as a full line voltage heater or by the secondary 8 of a transformer 9 suitably connected to the heater wire and in turn energized by any suitable source of power imposed upon its primary coil l0. Any suitable source of voltage for establishing a potential difierence between the electrodes 3 and 4 may be employed, for example, the battery II. For the purpose of giving an indication of the magnitude of the current flowing between the electrodes, there may be provided a microammeter or simila current detecting instrument l2. The current range of microammeter I! will preferably be chosen so as to distinguish between any minute currents which might flow between the anode and the cathode in the absence of the substance to be detected and the greater currents which will flow when the presence of the substance gives rise to ions creating the detection current.
For the. purpose of causing forced air flow through the device when desired, any suitable means maybe provided such as a blower l3 having a cowling aligned with the outlet orifice 6 and having for example, a fan It driven by a motor l5, energized by any suitable source of voltage l6. Normally, blower l3 will be either unenergized or energized at a relatively slow speed whereby the rate of air flow through the device I will be relatively small for initial detections of vapor. Under those circumstances, the sensitivity of the device will be at or near its maximum value. After an initial amount of the substance to be detected has entered the device, the rate of air flow may be caused to be stepped up in order that the device will respond more quickly to changes in the concentration of the substance to be detected in the atmosphere and in order to minimize slow sensitivity recovery and possible loss of sensitivity due to long continued presence of the substance. As one suitable means for increasing the air fiow in this manner, I have shown an amplifier and relay I'l connected across and actuated by the voltage of. a resistor l8 in series with the circuit of the electrodes. Amplifier I! may be arranged to energize a solenoid l9 which upon energization causes a switch 20 to close the energizing circuit for'the blower I3. It will be apparentwith this arrangement when an initial concentration of vapor is introduced into the devic I by the air stream or by convection currents, the blower circuit will be automatically energized and the air flow increased to produce the aforementioned eifects.
It may be desirable to introduce by well known means a slight time delay in the opening of switch 1 20 in order that the blower may remain in action long enough to cause the device to be well flushed with air after a drop in current sufficient to deactivate the relay'. This will remove any residual amounts of the substance which might give rise to currents interfering with normal operation of the device and cause erratic action.
While I have shown and described a particular embodiment of my invention, it will be obvious to those skilled in the art that various changes and modifications may be made without departing from my invention in its broader aspects and I, therefore, aim in the appended claims to cover all such changes andvmodiflcations as fall within the true spirit and scope of my invention.
What I claim as new and desire to secure by Letters Patent of the United States is:
1. The method ofoperating a detector for a finely divided atmospheric substance of the type comprising an electrical discharge device including a pair of electrodes, means for causing a flow of an atmosphere contaimng said substance to pass between said electrodes, means for imposing an electric potential difference between said'electrodes, means for heating the more positive one of said electrodes and means for detecting changes in the current between said electrodes caused by the formation of positive ions within said device due to the presence of said substance, which method comprises initially restricting said fiow to a relatively slow rate whereby to obtain opti-- mum sensitivity of said detector to an initial introduction of said substance thereinto and substantially increasing said rate of fiow after initial detection of said substance.
2. The method as in claim 1 in which said substance is of a class having an ionization potential less than the lectron work function of the more positive of said electrodes.
3. The method as in claim 1 wherein said substance is of the class consisting of the alkali metals and compounds thereof.
4. The method as in claim 1 in which said substance is of the class consisting of the halogen elements and compounds thereof.
5. A detector for a finely divided atmospheric substance comprising an electrical discharge device including a pair of electrodes, means for causing a flow of an atmosphere containing said substance to pass between said electrodes, means for imposing an electric potential difference between said electrodes, means for heating the more positive one of said electrodes and means for detecting changes in the current between said electrodes caused by the formation of positive ions in said device due to the presence of said substance, and means controlling said first mentioned means for initially restricting said fiow to a relatively slow rate. whereby to obtain optimum sensitivity of said detector to an initial introduction of said substance thereinto and means controlling said first mentioned means for substantially increasing said rate of flow after initial detection of said substance.
6. A detector as in claim 5 in which said substance is of a class having an ionization potential less than the electron work function of the more positive of said electrodes.
'7. A detector as in claim 5 wherein said" substance is of the class consisting of the alkali metals and compounds thereof.
8. A detector as in claim 5 in which said substance is of the class consisting of the halogen elements and compounds thereof.
9. A detector as in claim 5 wherein said two last mentioned means comprise a relay responsive to the fiow of current in said device and adapted to energize said first mentioned means upon initial flow of current in said device.
causing a flow of atmosphere containing said substances to pass between said electrodes, means for imposing an electrical potential difierence between said electrodes, means for heating the more positive of said electrodes, and means for detecting the presence of said substance by the current caused to flow between said electrodes by the formation of positive ions within said device and means responsive to the changes in current flow in said device adapted to energize said first mentioned means for increasing said flow after initial detection of said substance.
11. The method of operatin a detector of finely divided substances in an atmosphere which detector has high sensitivity at low rates of flow of said atmosphere through said detector but is readily subject to contamination at low rates of 3 initial introduction of said substance thereinto and substantially increasing said rate of flow after initial detection of said substance.
WILLIAM C. WHITE.
REFERENCES CITED The following references are of record in the file of this patent:
UNITED STATES PATENTS Name Date Y Goddard June 9, 1931 Salzberg et al Nov. 16. 1943 Number