US2537775A

US2537775A - Ion vacuum gauge

Info

Publication number: US2537775A
Application number: US546902A
Authority: US
Inventors: Stephen M Macneille
Original assignee: Individual
Current assignee: Individual
Priority date: 1944-07-27
Filing date: 1944-07-27
Publication date: 1951-01-09
Anticipated expiration: 1968-01-09

Description

Jan. 9, 1951 s. M. M NEILLE ION VACUUM GAUGE Filed July'27, 1944 noun) INVENTOR. JZe ohen m. MzZcA ezZZZe FfIE.B

am/ r- Patented Jan. 9, 1951 ION VACUUM GAUGE Stephen M. MacNeille, Oak Ridge, Tenn., as-

signor, by mesne assignments, to the United States of America as represented by the United States Atomic Energy Commission Application July 27, 1944, Serial No. 546,902

7 Claims. 1

This invention relates to gas pressure measurement and particularly to a method and apparatus for ascertaining the pressure of a gas by noting the relationship between electrons bombarding the gas and the ionization resulting therefrom.

It has long been known that electrons in passing through a gas at low pressure will ionize the gas, and that the number of ions formed is a function of the gas pressure. In the usual arrangement the intensity of the electronic bombardment has been held constant and the pressure ascertained by noting the intensity of the ionization produced thereby.

These devices, designated ionization gauges, comprise a source of electrons, usually a heated filament, and two spaced electrodes, one of which is positive and the other negative with respect to the filament or cathode. The positive electrode. the anode, collects the electron current and the negative electrode, the plate, collects the positive ions. The practice in the past has been to control the filament emission to maintain a constant electron current to the anode and measure the plate current as an indication of the amount of ionization and hence the pressure.

In accordance with the present invention, an ionization gauge is operated with its ion current held constant at a predetermined value by suitable regulation of the electronic discharge which preferably is accomplished by varying the temperature of the filament.

An object of the present invention is the provision of a circuit arrangement which will operate to maintain constant the ion current in an ionization gauge and which will measure the electron current in terms of gas pressure.

Other objects and advantages of the invention will become apparent from the following description when taken in connection with the accompanying drawing in which Fig. l is a. simplified circuit diagram showing the operating principle of the invention; and,

Fig. 2 is a circuit diagram illustrating a preferred form of the invention.

As shown in Fig. 1 an ionization tube II) has a 2 cult constants are such that when a predetermined current, for example one microampere, flows to the plate [5 as indicated by a suitable meter [7, the electron current is a known function of the pressure. A suitable meter l8 responds to this electron current and may be calibrated in pressure units if desired. The source I2 is variable so that when the pressure changes, the temperature of the cathode II can be adjusted to keep the ion current at its predetermined value. Any change in filament or cathode current is reflected in the electron current, and thus the pressure change appears in the reading of the meter It.

It will be appreciated that the design of the control arrangement will be such that at or near the pressures expected to be measured, as, for instance, 10- mm. of mercury, the filament current required will be low enough to insure long life. This aspect is important and involves one of the most advantageous features of the invention. This will be clear if it isremembered that when the electron current is held constant, as heretofore, an increase in pressure requires an increase in filament current whereas, with the constant ion current arrangement of the present invention, an increase in pressure requires less filament current. of course, in the unlikely event that the pressure decreased considerably beyond the expected pressure, the filament current would have to be increased in order to keep the ion current constant. However, unexpected increases in the degree of vacuum seldom occur whereas increases in pressure, especially in large volume systems, occur with such frequency as to be almost expected.

A self regulating arrangement incorporating the invention is shown in Fig. 2 as comprising an ionization gauge tube 20 having its filament 2| connected to be energized from a suitable source of alternating current 22 through a saturable reactor 23 and a transformer 24. As an aid in explaining the circuit arrangement, certain numerical values will be given which have been found to be satisfactory. It will, however, be under-' stood that that circuits with other values may readily be devised. Also connected to the source of supply 22 is a conventional power supply including a full wave rectifier 25 the output of which is filtered and impressed across a voltage divider comprising the

resistances

26, 21, and 28 having associated therewith two voltage regulating tubes 29 and 3|! and a resistance 3|. The filament 2! of the ion tube 20 is connected between

resistances

26 and 21 so that it will be held approximately 15 volts above ground potential. The plate 32 of the tube 20 is connected to ground through a one megohm resistor 33 thereby giving the plate 32 a potential of approximately volts negative with respect to the filament 2|. The grid 34 of the ion tube is connected between the two

voltage regulating tubes

29 and 30 so that the grid 34 is given a potential of approximately 150 volts above ground and about 135 volts positive with respect to the filament 2|. With these voltages, it is evident that the grid 34 is an anode and will collect all the electrons emitted by the filament 2 I while the plate 32 being the most negative electrode will collect the positive ions which may be formed by electron bombardment.

The plate or ion current in flowing to ground through the resistor 33 is adapted to give 9. voltage drop of one volt and this drop is applied to the grid 35 of a triode 35, which may be type 6SF5, the plate 38 of which is connected through a resistance 31 to a 300 volt supply. The potential of plate 38 is applied through a resistance 39 to the control grid 40 of a second tube 4|, which may be type 6SH7, the cathode 42 of which is held at 150 volts by proper connection to the voltage supply. The plate 43 of the tube 4| is connected through a D. C. winding 44 0f the saturable reactor 23 to the 450 volt side of the power supply. As stated above, the constants of the particular circuit being described are such that regulation should take place with the ion current collected by plate 32 at a value of one microampere. This current in flowing through the resistance 33 applies a potential of one volt to the grid 35 of the tube 36. In order to check the value of this current, the grid 35 is adapted to be disconnected from this voltage and connected to a regulated potential of one volt by means of a push button switch 45 as shown. If pressing the switch 45 does not alter the meter reading (to be later described), it is established that the known predetermined plate current is flowing. This is the plate current which in accordance with the invention is held constant. A milliammeter 46 is connected in the anode circuit of the ion tube 20 for measuring the magnitude of the electron current necessary to maintain constant the plate current flowing through the resistance 33. The electron current flowing through the meter 46 is a function of the gas pressure in the ion tube 20 and accordingly the reading of the meter 46 is readily convertible into a pressure reading. In order that this meter 46 may be used for difierent ranges, a. series of shunts 41 are made selectively available by means of a switch 48.

It will be noted that the tube 4| has in its screen grid lead a switch 49 which when in its normal position as shown connects this screen grid to a voltage intermediate the voltages of the cathode 42 and the plate 43 of the tube 4|. It has been found that when the filament 2| of the gauge tube 20 ages considerably, there is a tendency to oscillation which sometimes causes sparking across the D. C. winding 44 of the reactor 23. When this occurs, the switch 49 can be thrown to the right to connect the tube 4| as a triode. This reduces somewhat the sensitivity of the circuit but it will still function.

The regulatin circuit just described operates in the following manner: As long as the pressure of the gas in the ion tube 20 remains constant and the plate current flowing through the resistance 33 remains constant, the meter 46 gives a reading indicating this pressure. If the pressure within the tube 20 starts to increase more ions will be formed and the current through the resistance 33 will tend to increase. This will drive the grid 35 of the tube 36 more positive thereby increasing the current flow through its plate 38 which thus becomes less positive thereby rendering the grid 40 of the tube 4| more negative which decreases the current to its plate 43. This decrease in plate current through the tube 4| decreases the current through the D. C. winding 44 of the saturable reactor 23 thereby increasing its impedance and the alternating current drop across it. This decreases the voltage to the transformer 24; less voltage is applied to the filament 2| of the ion tube 20; the filament 2| cools slightly and fewer electrons are emitted; less ionization occurs within the ion tube 20 and the ion current remains constant. Although the ion current remained constant, the electron current became less and the reading of the meter 46 correspondingly changed to indicate the new pressure condition existing within the ion tube 20.

It is to be understood that the invention is not restricted to the specific embodiments shown and described, since these were chosen for the purpose of illustrating the invention, and many modifications will readily suggest themselves to those skilled in the art. Other circuit arrangements may be employed for practising the invention, the scope of which ispointed out in the appended claims.

I claim:

1. The method of detecting variations in pressure of a gas at pressures less than one millimeter of mercury which comprises passing an electron current through the gas to form positive ions, collecting said ions as a current, varying the intensity of the electron current to keep the ion current constant at a predetermined value, and measuring the variations in the electron current.

2. The method of measuring gas pressures less than a millimeter of mercury, which comprises ionizing a portion of the gas at a predetermined rate which is independent of the pressure and measuring the magnitude of the ionizing agent required to produce ionization at said predetermined rate.

3. In an ionization pressure gauge, an ionization tube comprising a filament, an anode and a plate immersed in a gas the pressure of which is to be measured, voltage means for rendering the filament positive with respect to the plate and negative with respect the anode, means for supplying heating current to the filament to cause it to emit electrons, whereby electrons move to the anode and ions are formed in the gas and move to the plate to form a current and means responsive to this ion current for varying the heating current to the filament whenever the ion current differs from a predetermined value.

4. In an ionization vacuum gauge comprising ionizing means, an ion collector and an electron collector immersed in a gas the pressure of which is to be measured, means for regulating the ion current comprising means for deriving a potential from the ion current flow and means responsive to said potential controlling the electron current to maintain said potential at a predetermined value.

5. In an ionization pressure gauge comprising an ionization tube having an emitter and an ion collector immersed in a gas the pressure of which is to be measured, in combination, means for regulating the emitter comprising means responsive to ionization in the tube, means comprising a saturable reactor for controlling the power supplied to the emitter and said ionization responsive means being connected to control the saturable reactor whereby to maintain the rate of ionization in the tube at a predetermined value by regulating the emitter.

6. In an ionization pressure gauge comprising ionizing means and an electron collector immersed in the gas the pressure of which is to be measured, in combination, means responsive to ionization connected to regulate the ionizing means and accordingly the magnitude of the electron current in the collector whereby to maintain the ionization at a predetermined rate. means for producing a control impulse equal to that of ionization at the predetermined rate and means for rendering said regulating means momentarily responsive to said impulse producing means to determine whether or not ionization is taking place at the predetermined rate by observing whether or not a change occurs in the magnitude of the electron current.

7. In an ionization pressure gauge comprising ionizing means immersed in a gas the pressure of which is to be measured, in combination, means controlling the ionizing means to maintain the ionization at a predetermined rate comprising means for deriving a voltage proportional to the rate of ionization, a voltage source having a value equal to the voltage derived from said predetermined rate of ionization and means for substituting the separate voltage source for the derived voltagein the. controlling means to check the accuracy of the regulation.

S'I'EPHEN M. MACNEILLE.

REFERENCES CITED The following references are of record in the I file of this patent:

UNITED STATES PATENTS Graw-Hill, 1917, pages 155-157.