Continuous adjusting apparatus for detecting gaseous impurities with a corona discharge

An apparatus for continuously adjusting the corona discharge currrent of a pair of electrodes exposed to an atmosphere of gaseous impurities, particularly halogen. The device consists of a power supply to cause a corona current to flow in a pair of electrodes in series with a summing resistor. Any change in the gaseous impurities which causes a change in the corona current is sampled in the resistor, detected, delayed, amplified and fed back in proper phase to the control element of the power supply to cause the corona current to remain constant. During the finite delay time the change in corona current causes an audible alarm to sound until the corona current is restored to the new level of impurity concentration. The corrective action is continuous for any level of impurity concentration.

BACKGROUND OF THE INVENTION 
The present invention relates generally to apparatus for detecting 
impurities in a gaseous atmosphere with a corona discharge and more 
particularly, to a continuously adjusting corona current source. 
Apparatus for detecting gaseous impurities, such as halogens, in 
atmospheres with a continuous corona discharge are known in the prior art. 
Such devices are known to be sensitive indicators of the presence and 
concentration of gaseous impurities. Of special interest is the fact that 
the corona discharge current level diminishes with increasing 
concentration of halogen gases, since halogen gases possess positive ions, 
which combine with the negative ozone ions within the corona discharge to 
decrease the space charge current flow. The recombinations of ions is 
dramatic in that a large current change occurs for a small concentration 
of gaseous impurities. As a result of this phenomenon, these devices are 
capable of detecting halogen gases in very low concentrations. Likewise, 
the ability to maintain an optimum level of corona current within a 
constantly changing background level of impurities is highly desirable. To 
maintain this optimum corona current flow over a long period of time 
regardless of slowly varying background levels, humidity, air flow, 
sensing tip variations, electronic components drift, contamination and 
human adjustment errors requires a continuously adjusting apparatus. 
The typical prior art devices used some automatic means to establish an 
initial adjustment, independent of manual methods, and then assumed this 
condition to remain stable; or else made timely periodic recalibration 
setups at fixed intervals. 
Since the prior art states that the devices are inherently unstable 
requiring troublesome frequent recalibration, there were many attempts to 
resolve the problem. 
SUMMARY OF THE INVENTION 
The principal object of this invention is to overcome the deficiencies of 
heretofore known methods of halogen gas leak detectors by providing a 
novel and improved continuously adjusting apparatus using a servo feedback 
control technique to cause the corona discharge current to be maintained 
at an optimum sensitive level. 
A more particular objective of the invention is to provide a means of 
continuously adjusting the corona current in either direction to maintain 
optimum corona discharge currents while alerting the operator to any 
corrective change in the corona current which indicates a change in the 
halogen concentration. 
The servo feedback control technique used in this invention samples the 
corona discharge current in a summing resistor placed in series with the 
corona discharge current. The resulting voltage is compared to a reference 
and applied to the high voltage power supply feedback winding to maintain 
an empirically determined optimum corona current. Any subsequent change in 
the corona current generates an error voltage at the summing resistor 
which is applied to the feedback winding of the power supply in proper 
phase to cause the corona current to remain constant. Likewise, the error 
voltage is amplified and applied to threshold detector circuits which 
signal an increase or a decrease in the corona current as a change in the 
gaseous impurity concentration of the atmosphere.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
With reference to FIG. 1A of the simplified schematic, the principal object 
of this invention is explained as follows: A high voltage pulse type 
transformer 2 is connected to a conventional blocking oscillator 1 with a 
feedback control 7. The generated pulses in the transformer secondary are 
rectified and filtered in the network 3 to provide a highly negative 
voltage to the corona generating sensor 4 which in series with the summing 
resistor 5 completes the corona discharge current path to the transformer 
secondary. The level of the corona current is initially established by a 
bias current in the feedback winding of the blocking oscillator 1. This 
bias current is controlled by the feedback control circuit 7 after a time 
delay 6 and comparison of the reference voltage 8 with the voltage at the 
summing resistor 5. After the initial setup, any changes in the corona 
current through the summing resistor 5 are sensed as an error voltage when 
compared to the reference voltage 8. This error voltage is applied through 
the feedback control 7 to the feedback winding of the blocking oscillator 
1 in proper phase to cause a correction of the corona discharge current to 
the original value. The RC time delay 6 applies the correct timing to keep 
the servo loop "critically damped" to prevent hunting or oscillating. 
During the time delay required for correction of the error current, the 
error voltage across the summing resistor 5 is compared to the reference 
voltage 8 in a fast and sensitive 2X amplifier 9 to initiate the alarm 
circuits. 
With reference to FIG. 1B of the block diagram, the output of the 2X 
amplifier 9 is routed to the current decrease detector 10 and to the 
current increase detector 12. A decrease in the discharge current 
activates the alarm frequency select gate 11 which triggers the dual 
frequency timer 14 to jump from a fixed low frequency to a high fixed 
audio frequency which causes the audio output 15 to sound an alarm. An 
increase in the discharge current activates the timer disable select gate 
13 which disables the dual frequency timer 14 stopping the low frequency 
output to the audio output 15. Thus, under static conditions a low 
frequency tick is heard at the audio output 15, an increase in discharge 
current stops the tick, and a decrease in discharge current causes a high 
frequency squeal at the audio output 15. Since sudden and significant 
changes in the discharge current are caused by the application of gaseous 
impurities to the sensor 4, the audio output 15 is a measure of the 
presence and direction of a gaseous leak. 
The following are references on the block diagram, FIG. 1B, which do not 
appear in the above description of the preferred embodiment: Blocks 1, 2, 
3, 5, 6, 7, 8 and resistors R28 and R29. 
DETAILED DESCRIPTION OF THE ELECTRONIC CIRCUITRY 
Referring to FIG. 2 in detail: 
A high negative DC voltage required to establish a corona in the sensor tip 
is generated by a controlled blocking oscillator Q1 and the pulse forming 
transformer T1. The high voltage pulses at pin 2 of T1 are applied to R2, 
which limits peak current through the rectifiers CR1, CR2, and CR3 where 
the pulses are rectified to pass only a negative current to a PI type 
filter composed of C1, R27 and C2 to provide the negative current through 
R1 to the corona generating sensor tip. The shell of the sensor is 
connected to the common grounded bus. Pin 1 of T1 and the positive end of 
the PI filter capacitors C1 and C2 are connected to the summing resistor 
R5 to cause the current through the sensor to pass through R5. This causes 
R5 to develop a positive voltage with respect to ground proportional to 
the current through the sensor. Capacitor C3 maintains a constant voltage 
across R5 for small variations in the corona currents. 
The voltage across the summing resistor R5 is applied to two circuits. The 
first is the control circuit for the blocking oscillator. The second is 
the detector circuit to indicate the presence of gaseous impurities in the 
atmosphere. In the first control circuit the voltage developed across the 
summing resistor R5 is applied through a large value resistor R6 to charge 
capacitor C9. This RC combination causes a delay time before a change in 
the voltage across R5 is applied to the control amplifier U3A. The voltage 
across C9 is applied to the non-inverting input of the operational 
amplifier U3A pin 5. The operational amplifier U3A has a gain of one and 
is used to isolate the voltage on C9 from the control circuit of 
operational amplifier U3B. Operational amplifier U3B having a gain of 10, 
inverts any changes in the delayed voltage from the summing resistor R5 
and U3A, compares this voltage to a stable reference supplied by zener 
regulator CR4 and voltage divider resistors R16 and R17 applied to pin 3 
of U3B, and applies this voltage through resistor R18 to the control 
winding pin 5 of transformer T1. This voltage is the feedback voltage, 
which is passed through the feedback winding of T1 pins 5 and 6 through 
current limiting resistor R4 and R3 to the base input of the power supply 
transistor Q1. This voltage controls the output level of the blocking 
oscillator and as such forms a closed loop feedback system to maintain the 
corona current through resistor R5 and the sensor at a stable constant 
level. Likewise any changes in the current through resistor R5 is inverted 
and applied through this feedback circuit to cause correction of the 
corona current to the initial preset value referenced to the stable 
reference voltage. Herein is the principal embodiment of this invention. 
In normal operation, feedback control systems must contain time delay 
elements which limit the speed of operation from the time an error voltage 
is detected until the time that the correction is applied. This delay time 
is just sufficient to permit the operation of an alarm circuit, which 
signals that an error voltage has been generated in the corona current. To 
obtain the voltage necessary for the alarm circuits, resistor R14 is 
connected to the summing resistor R5 and thence to the non-inverting 
input, pin 5 of the operational amplifier U2A which provides isolation 
from resistor R5 and amplifies the error voltage two times for proper 
operation of the alarm circuits. 
Referring in detail to FIG. 3: 
The alarm detection and power circuits of FIG. 3 operate as follows: 
The error voltage from the operational amplifier U2A of FIG. 2 which has 
been amplified two times is applied to two separate alarm circuits; 
namely, operational comparator U2B and a dual transistor comparator Q5 & 
Q6. The error voltage applied to the inverting input, pin 2 of operational 
comparator U2B is compared with an adjustable level of the reference 
voltage. This alarm adjust level is set just below the fixed stable 
operating level of the summing voltage across resistor R5. Any increase in 
the summing voltage, which is a measure of an increase in corona current 
or likewise an increase in the gaseous impurity concentration will cause 
the output of the comparator U2B to go to a high voltage level. The output 
of comparator U2B drives the base of transistor Q2 into conduction which 
provides drive current to a timer circuit U1, which changes the time 
constant R8, R7, C6 of the timer U1 and causes a high frequency output at 
pin 3 of timer U1. The high frequency output of U1 is applied through 
audio drive transistor Q3 to loudspeaker LS1. 
After the initial setup of said summing amplifier feedback circuit the 
output of the timer circuit U1 pin 3 is a short pulse caused by the time 
constant of resistors R7, R8, & R9 and capacitor C6 through the normally 
conducting transistor Q4 from the main supply voltage. Transistor Q4 is 
held in conduction by the base drive supplied from the collector resistor 
of voltage comparators Q5 & Q6, which compare the summing error voltage 
from resistor R5 with a mute adjust voltage preset to a level just above 
the fixed stable summing voltage across resistor R5. Any decrease in the 
summing voltage which constitutes a significant decrease in the 
corresponding corona discharge current and likewise a decrease in the 
gaseous impurity concentration of the atmosphere will cause the comparator 
Q5 & Q6 to cut off drive current to transistor Q4 which will prevent drive 
into the timing circuit U1 and mute the output of the loudspeaker. 
Whenever any changes occur in the corona discharge current caused either by 
some changes in the components of the detecting apparatus or by a 
detection of a change in the gaseous impurity concentration of the 
atmosphere as detected by the disposed sensor, the error voltage generated 
at the summing resistor R5 starts to correct for this change in an effort 
to return the system to a stable operating condition. This correction is 
continually in operation even during the time that the error is detected 
and an alarm signal indicates that a change is occurring. The finite time 
between returning the system to a stable condition is used to indicate 
that a change has occurred in the discharge current. The signals provided 
by the alarm circuit are the steady tick audio output during stable 
operation, the high frequency squeal whenever an increase in the gaseous 
impurities is detected, and a muting of any audio output signal whenever a 
decrease in the gaseous impurities is detected. Between these three unique 
sounds the operator of the detector will be able to detect any significant 
changes in the gaseous impurity of the atmosphere and to locate leaks in 
halogen gaseous systems. 
In the operation of the impurity detector of the present invention, 
normally, the detector is used in an environment which contains some 
background amounts of the impurities to be detected. Herein lies one of 
the problems associated with previous detectors; that is calibration and 
recalibration of the most sensitive operating condition of the instrument. 
When the detector of the present invention is first turned on, maximum 
corona discharge current is caused to flow until the atmosphere to which 
the sensor is exposed is sensed and the corona discharge current reaches a 
fixed balanced condition. Now any increase or decrease from the stable 
condition causes an error voltage in a closed loop servo system. This 
error voltage is detected and used to signal the increased or decreased 
level of the corona current which represents a change in the gaseous 
impurity level of the atmosphere. The error voltage is applied as a 
continuously correcting voltage to the corona current high voltage source 
to 11 maintain the level at a fixed stable and sensitive level. 
The above description should be considered exemplary and of the preferred 
embodiment only. Other modifications of the invention will occur to those 
who make and use the invention. It is desired to include within the scope 
of the present invention all such inventions which come within the scope 
and meaning of the appended claims. 
The following are references on schematics which do not appear in the above 
detailed descriptions: Resistors; R10, R11, R12, R13, R15, R19, R20, R21, 
R22, R23, R24, R25, R26, R28, R29, R30. Capacitors; C4, C5, C7, C8, C10, 
C11, C12, C13, C14, C15, C16. Miscellaneous; Sensor, Swi, DS1, B1, B2, B3, 
B4.