US 3903883 A
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United States Patent [191 Pecina et al.
l l VARIABLE AEROSOL HEATER WITH AUTOMATIC TEMPERATURE CONTROL  Inventors: Richard W. Pecina, Waukegan;
Robert J. Froehlich, Rosemont. both ] Appl. No.1 461,753
( Sept. 9, 1975 1805,53? 4/l974 Burgel ct al, l Zl9/497 182L947 7/l974 Schossow lZX/IM X FOREIGN PATENTS OR APPLICATIONS 285.777 l/l965 Australia l l28/l B Primary E.\'um1'rzm'Richard A. Gaudet ASS/Sit]!!! Exunn'ner lee S. Cohen Allurnen; Agar, or Fi m-Eric P. Schellin 57 ABSTRACT An inhalation therapy device of the type in which a column of water is heated atomized, and combined with a flow of oxygen to form a stream of humidified oxygen which is administered to a patient. An adaptor located at the patient site is connected to the hose carrying the stream. The adaptor includes a thermistor which senses the temperature of the stream immediately prior to the administration to the patient. The thermistor output signal is fed back to a heater control system which controls the amount of heat applied to the column of water in a manner which maintains the temperature constant. The particular temperature being maintained is manually selectable.
5 Claims, 5 Drawing Figures  US. Cl. 128/193; 219/497; 219/502 [5 l] int. Cl. A61M 15/00  Field of Search l28/l93, 194, 192. 212, l28/l B; 219/497 502 [56} References Cited UNITED STATES PATENTS 3.426178 2/l969 Squiers 2l9/SO2 3 434 47l 3/l969 List0n..... l28/l92 3.553.429 ll/l968 Nelson Zl9/497 1634163 l/l972 Shimizu l28/3U3.l8 1659604 5/]972 Melville et al. l28/2l2 1766.914 lU/l973 Jacobs lZil/l94 3.789.853 2/1974 Reinhard l28/l B I 46 l2 f i 4 VARIABLE AEROSOL HEATER WITH AUTOMATIC TEMPERATURE CONTROL BACKGROUND OF THE INVENTION The present invention relates generally to the field of inhalation therapy and more particularly to apparatus which humidifies oxygen before it is adminsistered to a patient.
Inhalation therapy is the medical art of treating a patient with oxygen, or a mixture of air and oxygen, having a high moisture content. This is generally accomplished by atomizing or nebulizing pure water and causing the oxygen to come into contact with it, whereby the oxygen is humidified. A particular system for accomplishing this is disclosed in application Ser. No. 286,692 filed Sept. 6, 1972 by Edward van Amerongen, for Nebulizer, assigned to the assignee of the present application, now U.S. Pat. 3,771 ,72l issued on Novv I3, 1973. Briefly, the aforementioned application shows a system in which a receptacle containing water is adapted as a source of atomized liquid through the agency ofa nebulizer which couples oxygen pressure to the receptacle. A venturi within the nebulizer draws water from the receptacle and directs atomized water and oxygen toward an outlet from which the humidified oxygen flows.
It has been found that the results of the inhalation therapy are improved if the humidified oxygen is warmed before it is administered to the patient. This has generally been accomplished by heating the water in the receptacle before it is atomized. The receptacle is placed in a heater which heats the water by heating the walls of the receptacle. This approach has many disadvantages. For one, it is very inefficient since the useful heat must flow through the walls of the receptacle before reaching the water. This results in bulky equipment and heavy power requirements. A more serious disadvantage is that it is difficult to maintain a desired temperature for the humidified oxygen stream by the method because the heated water flows a relatively long distance, during which its temper-ature can be affected by the environment, before coming into contact with the oxygen.
Many of the problems of the prior art, including those noted above, were solved by an invention disclosed in application Ser. No. 396.782, now US. Pat. No. 3,864,544, for Electric Heating Unit for Liquid", filed Sept. l3, 1973 by Edward van Amerongen, and assigned to the assignce of the present application. Briefly, application Ser. No. 396,782, now US. Pat. No. 3.864544, shows an electric heating unit adapted to be placed between the water receptacle and the nebulizcr. The unit includes an electric heating element which heats a tubular member which conduits a rising column of water from the receptacle to the nebulizer. The heating element is disposed in a bore in a metal block. The block has another bore in which a section ofthc tubular member is disposed. The heating element heats the block which, in turn, heats the tubular membcrv A thermostat is placed close to the block and controls the temperature of the rising column of water by controlling the flow of the electric current to the heating clement.
While the electric heating unit of van Amerongcn has proven to be a large advance in the art of inhalation therapy, there are many problems remaining. It has been found by medical practitioners that it is desirable,
in terms of achieving the most effective therapy, to have a particular temperature for the humidified oxygen stream when it is administered to the patient. In most cases, the temperature of the humidified oxygen stream should be the same as the physiological temperature of the patient, generally 98.6". In a few instances, such as in pediatrics, the temperature of the stream should be lower, such as 92. Due to the lack of accuracy of prior art devices and the many variables involved in the inhalation therapy environment, no prior art device has proven capable of providing a stream of humidified oxygen which has a constant and controllable temperature when being administered to the patient. Since the stream is heated by heating the water before it is nebulized and combined with oxygen, such variables as the length of the hose connecting the nebulizer to the patient, the room temperature, the presence or absence of, and degree of, air movement over the hose, variable pressure of the oxygen supply, and the variability of the heater itself, all influence the temperature of the stream at the patient site.
SUMMARY OF THE INVENTION It is the general purpose of the present invention to provide a device for inhalation therapy which permits the temperature of the humidified oxygen stream at the patient site to be accurately and automatically con trolledv To attain this, the invention provides a unique heater control circuit which controls the heating of the water in accordance with the temperature of the humidified oxygen stream at the patient site. A temperature sensing adaptor is connected between the hose carrying the stream of humidified oxygen and the device used to administer the humidified oxygen to the patient. An electrical signal representative of the sensed temperature is fed back from the adaptor to the heater control circuit and used to automatically maintain the temperature constant. The particular temperature being maintained is variable and can be selected at the will of the operator. Another temperature sensor senses the temperature of the heating element and will override the stream temperature sensor if the heating element becomes hot enough to cause vapor lock or otherwise overheats.
Therefore, it is an object of the present invention to provide an inhalation therapy device which maintains the temperature of the humidified oxygen stream at the patient site constant.
It is another object of the present invention to provide an inhalation therapy device which delivers a stream of humidified oxygen to the patient site at a controllably variable temperature.
It is a further object of the present invention to provide an inhalation therapy device which automatically compensates for environmental variables to deliver a stream of humidified oxygen to the patient site at a constant temperature.
BRIEF DESCRIPTION OF THE DRAWINGS Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying draw ings, in which like reference numerals designate like parts throughout the figures thereof, and wherein:
FIGv I shows 'tili overall view of a preferred embodiment of the lfwbl ltion',
FIG. 2 shows the preferred embodiment cut away along line 22 of FIG. 1;
FIG. 3 is a schematic diagram of the sensor and control circuitry of the invention;
FIG. 4 is a schematic diagram of the power supply circitry of the invention;
FIG. 5 is a schematic diagram of the heater power circuitry of the invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIGS. 1 and 2. the heating and control unit is designated by the numeral 10. The heating portion 12 of the unit is connected between nebulizer l6 and water receptacle l8. As is more fully disclosed in the aforementioned van Amerongen application. a source of oxygen under pressure is connected to the nebulizer l6 and flows through it into hose 20. The oxygen flow creates a pressure drop in the nebulizer which causes a column of water to be drawn from receptacle 18 through tubular member 38. An electric heating element 42 is located in a horizontal bore in metal block 40. A portion of tubular member 38 is disposed in a vertical bore in block 40. As the rising column of water passes through tubular member 38, it is heated by heat passing from heater element 42 through block 40 to tubular member 38. A power cord 34 hav ing a ground prong 33 is provided to couple electric power from a conventional source of electrical energy.
The control portion of unit 10 is generally designated by numeral 14, and includes a printed circuit board 48 upon which electrical circuit elements are mounted in the conventional manner. Also mounted on the board 48 are three neon tubes 50, 52 and 54. When in a completely assembled condition, these tubes are disposed behind semi-transparent areas 56, 58 and 60, rcspcctively, on the face of the unit, these areas having indicia thereon to indicate circuit conditions. The particular function of these tubes, as well as the circuit components, will be fully described below. Projecting from the face of the unit is temperature control knob 32. Disposed in heating block 40 is a temperature sensing device 44, such as a thermistor. The device 44 provides an electrical output signal on wire 45 which is indicative of the temperature of the block 40. Also disposed in heat conducting relationship with block 40 is thermal fuse 46. Fuse 46 is connected in series with heater element 42 to disconnect current from the heater element in case of overheating. The fuse also protects the unit in case of a circuit malfunction. since the fuse opcrates directly from sensed heat and does not rely on control circuit operation.
An adaptor 22 having a temperature sensing device 26 is connected at the patient site between the end of hose and the coupler, hoses, etc., 24 of the device which administers the humidified stream of oxygen to the patient. The temperature sensing device 26 is of the type, such as a thermistor. which provides an electrical signal indicative of the temperature being sensed. The exact structure of the adaptor 22 is not critical, being primarily a matter of design based on such factors as the relative sizes and shapes of the hoses and connectors. The important considerations are that the adaptor be located at the patient site and that the temperature sensing device 26 be disposed so as to sense the temperature of the stream of humidified oxygen. In this fashion. an electrical signal representing the temperature of the stream of humidified oxygen at the patient site is transmitted via conductor 28, plug 30 and socket 36, to the control circuitry in portion 14 of unit 10.
The electrical circuitry aspect of the invention will now be described. Referring to FIG. 4, the power supply circuit receives A.C. power line input on terminals H and N. The A.C. power is applied across the primary windings of transformer 402 and across neon indicator tube 50. To permit the unit to be used with either l 15 VAC or 220 VAC power, the primary windings can be interconnected in two ways. If I I5 VAC is applied, terminal A is connected to terminal C and terminal B is connected to terminal D. Connected in this manner, winding A-B is in parallel with winding C-D. lf 220 VAC is applied, terminal B is connected to terminal C. Connected in this manner, winding A-B is in series with winding C-D. In either mode of operation, the proper secondary voltage results and is applied to bridge rectifier circuit 404. The bridge rectifier output is filtered by filter capacitor 406 to provide operating voltage Vcc. A regulating circuit comprised of the parallel combination of Zener diode 410 and capacitor 412 in series with resistor 408 provides regulated voltage Vreg of an amplitude less than that of Vcc.
Referring now to the sensor and control circuitry of FIG. 3, the previously mentioned hose thermistor 26, located at the patient site, is connected to the control circuitry by plug 30 and socket 36. The socket 36 has contacts 35 and 37 which contact the signal carrying conductor of plug 30. The ground conductor of plug 30 contacts the ground conductor of socket 36. As an examination of the diagram will show, when plug 30 is inserted into socket 36, thermistor 26 is connected to socket 36, terminal 35 and resistor 76 is disconnected from terminal 35. The function of resistor 76 is to provide a low resistance to control circuitry 14 which will cause a shut-down of the heater power if plug 30 is removed from control unit l0. Socket terminal 35 is connected in series with trim resistor 72, resistor 74, and temperature control variable resistor 31 across Vreg. The setting of variable resistor 31 is varied by previ ously mentioned knob 32. The junction of a resistor 31 and thermistor 26 is connected to input terminal of differential amplifier 66. Resistors 62 and 64 provide a voltage divider across Vreg, the junction of resistors 62 and 64 being connected to terminal 68 of amplifier 66. A feedback path from the output of amplifier 66 to input terminal 68 is provided by resistor 78 and capacitor 80. The output of amplifier 66 is coupled by resistor 82 to terminal 84 of differential amplifier 86. The other input terminal 88 receives a ramp voltage from a ramp generator circuit designated generally by the numeral 91. The ramp generator includes a programmable UJT 94 with its gate electrode connected to the junction of programming resistors 90 and 92. The cathode of 94 is connected to ground through series diodes 102 and 104. The anode of 94 is connected to terminal 88 of amplifier 86 and coupled to Vreg through resistor 98. A capacitor 96 is connected across the anode-cathode circuit of 94. A resistor 100 connects the anode of diode 102 to Vreg. The output of differential amplifier 86 drives light emitting diode 108. which is connected to Vcc through resistor 106.
Vreg is also applied to a series combination of resistor and capacitor 112. For reasons to be discussed later, resistor H0 has a very high value of resistance and capacitor 1 12 has a high value of capacitance. The
junction of resistor and capacitor 112 is connected via resistor 114 to input terminal 116 of differential amplifier 118. The other input terminal 124 is connected to the junction of voltage dividing resistors and 122. Resistor 126 provides a feedback path from the output terminal of amplifier 118 and input terminal 116. The output of amplifier 118 is coupled by resistor 128 to the base of transistor 130. A light emitting diode 132 is connected in parallel with transistor 130 and in series with resistor X. The series parallel combination resistor X, transistor 130 and light emitting diode 132 is connected across Vcc.
The output of amplifier 118 is also coupled by resistor 134 to the base of transistor 136, the collector of which is coupled by resistor 138 to input terminal 140 of differential amplifier 142. Resistors I44 and 146 form a voltage divider across Vreg, and their junction is also connected to terminal 140. Also connected in series across Vreg are trim resistor 150, resistor Y and previously mentioned block thermistor 44. The junction of resistor Y and block thermistor 44 is connected to input terminal 148. The output of amplifier 142 is coupled by resistor 152 to the base of transistor 154, the collector of which is connected to previously mentioned terminal 84 of amplifier 86.
FIG. 5 shows the heater power and ground detector circuitry. ln the ground detector circuit, the grounding prong 33 of the power line plug is coupled to the collector of photo-transistor 133 by resistor I58 and diode 156. The emitter of photo-transistor 133 is connected to power conductor H. The collector of phototransistor 133 is connected to the input of Darlington amplifier 160. Resistor 159 shunts the photo-transistor. Ground indicator neon tube 54 is connected in series with the Darlington amplifier. Diode 162 couples the neon tube 54 and amplifier to power line conductor N. A resistor 164 couples the cathode of diode 162 to power-line conductor H.
In the heater power circuit, heater element 42 and thermal fuse 46 are connected in series with a bidirectional gate-controlled semiconductor device 166 across the power line conductors. A triac is the preferred device. The gate electrode of triac 166 is coupled by a silicon bilateral switch 168 to one plate of capacitor 170. One input to full wave bridge rectifier 172 is connected to the junction of 168 and 170. A photo-transistor I09, shunted by resistor 171, is connected in series with the rectifier output. Capacitors 174 and 180, and resistors 176 and 178, provide filtering. Neon tube 52 is connected in parallel with heater element 42.
The heater power circuit operation will now be described. The current through heater element 42 is controlled by conductive state of triac 166. Triac 166 is controlled by bridge rectifier 172 in the following manner. The triac fires when the charge on capacitor builds up sufficiently to break down semiconductor switch 168 and supply gate current to the triac. Capacitor 170 is charged by the current drawn by rectifier circuit 172. The rectifier current passes through the parallel combination of resistor I71 and photo-transistor 109. When photo-transistor 109 is nonconductive. the relatively high resistance value limits the current drawn by the rectifier circuit to a value insufficient to charge capacitor 170 to the bilateral switch breakdown voltage during a half cycle of the power line voltage. The presence of resistor 17] tends to limit voltage excursions across the photo-transistor. The bridge rectifier connected to the photo-transistor in the manner shown in the drawing permits a bilateral device (triac 166) to be controlled during both polarities of the input waveform by a unilateral device (photo-transistor 109). In order for capacitor 170 to charge sufficiently to cause breakdown of switch 168 and conduction of triac 166, the photo-transistor 109 must be in a conductive state so that the rectifier circuit can draw the necessary cur rent. Thus, by controlling the conductive state of photo-transistor 109, control of the triac 166, and hence control of the current through heater element 42, is achieved. Indicator light 52, being in parallel with the heater element, lights only when the heater element is receiving current and generating heat.
The primary purpose of the control and sensor circuitry of FIG. 3 is to control the conductive state of photo-tranistor 109 as a function of the temperature of the humidified oxygen stream at the patient site. However, the circuitry of FIG. 3 performs three other functions as well. These functions are: to sterilize the waterhandling tubular member in the heating unit; to provide, along with the ground detector circuitry of PK]. 5, a temporary indication as to whether the unit is properly grounded; and, to limit the maximum temperature of the heating block. How the circuit accomplishes these functions is most easily understood by describing first how the circuit operates to accomplish its primary purpose, and then how this operation is modified to achieve the other functions.
Referring now to FIG. 3, the signal from hose thermistor 26, indicative of the temperature of the humidified oxygen stream at the patient site, is applied to terminal 70 of differential amplifier 66. lt has been found in practice that the amount of gain of amplifier 66 is very important for obtaining superior system operating results and the optimum gain was established by experimentation. The output of amplifier 66 is applied to ter minal 84 of amplifier 86. The other terminal receives a ramp voltage from ramp generator 91. The ramp voltage rises from approximately L8 volts (provided by diodes 102 and 104) to approximately 5.4 volts. The circuits are so designed that the signal on terminal 84 will be somewhere within that range so that at some point during the rising ramp, at zero difference occurs. The location of this point determines the waveform of the signal on the output terminal of amplifier 86 which is applied to light emitting diode 108. The light emitted from diode 108 strikes photo-transistor 109 to control the current through the heater element as previously described.
The optical coupling provides electrical isolation between the control and sensor circuitry and the heater power circuitry. A commercially available optical coupling device which contains both the light emitter and photo-transistor within a sealed compartment is used.
The maximum temperature which the heating block is allowed to reach is determined by the block termistor circuit. Block thermistor 44 provides a signal representative of block temperature. This signal is applied to terminal 148 of amplifier 142. The other terminal 140 receives a reference potential from the junction of resistors 144 and 146. Trim resistor 150 is adjusted so that, when the block temperature is below the prescribed maximum the output signal from amplifier 142 biases transistor 154 to be nonconductive. When the block temperature rises to the prescribed maximum, the output signal from amplifier 142 causes transistor 154 to become conductive. When conductive, transistor I54 clamps terminals 84 of amplifier 86 close to ground. the result being that the output of amplifier 86 approaches Vcc. whereby the voltage across diode 108 is insufficient to cause light emission. Thus. phototransistor ceases to conduct and current through the heater element is cut off. When the block tempera ture lowers. transistor 154 goes out of conduction and the hose thermistor again controls the current through the heating element.
It is advantageous t) sterilize the tubular member of the heating unit when the unit is being turned on to begin operation. The circuit which accomplishes this includes amplifier 118, transistor 136. and associated circuitry. When the unit is connected to the power source. a large capacitor 112 begins to charge through high value resistor 110. The junction of resistor 110 and capacitor 112 is coupled to input terminal 116 of differential amplifier 118. The other input terminal 124 receives a steady reference potential from the junction of resistors 120 and 122 connected across Vreg. Amplifier 118 is so biased that. with the initial conditions described above. its output signal causes transistor 136 to conduct. The conduction of transistor 136 causes resistor 138 to be effectively in parallel with resistor 146 since one end of resistor 138 is clamped close to ground through the transistor. This changes the reference potential on terminal 140 of amplifier 142 so that the heating block reaches a much higher temperature before the signal from block thermistor 44 causes the output of amplifier 142 to bring transistor 154 into conduction. When the capacitor [12 has charged to a preselected voltagc. the output signal from amplifier 118 turns transistor 136 off. whereby the reference point connected to terminal 140 of amplifier 142 changes to a value determined solely by Vreg, resistor 144 and resistor 146, and the block thermistor circuit operates as previously described.
It is advantageous for the ground indicator lamp 54 to indicate proper grounding only for a short period of time after the unit is connected for operation. Referring to FIG. 5, indicator lamp S4 is connected in series with a Darlington amplifier 160 between rectifier 162 and conductor H of the power line. When prong 33 is connected to earth ground, a small amount of current will flow from resistor 158 through diode 156 and resistor 159 to conductor H. This current will cause the Darlington amplifier to conduct thereby causing neon tube 54 to light. The conduction path is from conductor N through diode 162. If earth ground is not present at 33, no current will flow and the Darlington amplifier will not conduct.
The previously described output signal from ampli fier 118 is coupled to the base of transistor 130. causing the transistor to conduct. Since light emitting diode 132 is shunted by transistor 130, the diode does not emit light when the transistor is conducting. Light emitting diode 132 is optically coupled to photo-transistor 133 so that, during the sterilizing period. phototransistor I33 receives no light and is. in effect, an open switch. If prong 33 is grounded at this time. the circuit will operate as described above and indicator lamp 54 will light. If prong 33 is not grounded, lamp 54 remains off.
When the sterilizing period is over. the signal from amplifier 1 18 causes transistor to become noncon ductive. The voltage appearing across diode 132 now causes the diode to emit light. This light is received by optically coupled photo-transistor 133, causing it to become conductive. The conductive phototransistor shunts resistor 159 and effectively clamps the input terminal of Darlington amplifier 160 to its emitter termi nal 16], whereby the amplifier becomes nonconductive anti indicator lamp 54 becomes dark. As is evident. the ground detector circuit is used for indication purposes only and does not modify the operation of the other circuits in any way.
What has been described is an inhalation therapy unit which automatically maintains the temperature of the humidified oxygen stream at the patient site constant. The present invention thus solves many of the problems previously experienced in this field of technology. The temperature being maintained is variable to suit the needs of different patients. Obviously. many modifications and variations of the present invention are possible in light of the above technique. It is to be. therefore, understood that. within the scope of the appended claims the invention may be practiced otherwise than as specifically described.
What is claimed is:
1. In combination with an inhalation therapy device of the type wherein a column of water is drawn from a receptacle through an electric heating unit. whereby the water is heated. and into an atomizing device. whereby the heated water is atomized. and wherein the heated water is combined with a flow of oxygen to prt vide a stream of humidified oxygen and wherein said stream ofhumidified oxygen is delivered through an extended hose to the site of a patient. and wherein the stream of humidified oxygen is thereat administered to said patient. the improvement which comprises:
means for sensing the temperature of the stream of humidified oxygen at the said patient site immediately prior to its administration to the said patient and for generating an electrical signal representa tive of said temperature; and.
control means responsive to said signal for controlling the amount of heat generated by said electric heating unit to maintain the stream at a preselected temperature;
wherein said control means comprises:
gate controlled bilateral switch means for controlling the flow of electric current through said electric heating unit;
means for amplifying said electrical signal; and.
means connected to the gate electrode of said bilat eral switch means for controlling the conductive state of said swtich means in response to said amplified electrical signal;
means for sensing the temperature of said electric heating unit and generating an output signal indicative thereof; and,
means for overriding said amplified electrical signal and causing said bilateral switch means to be in its non'conductive state when said output signal indicates a preselected maximum temperature.
2. In combination with an inhalation therapy device of the type wherein a column of water is drawn from a receptacle through an electric heating unit. whereby the water is heated. and into an atomizing device. whereby the heated water is atomized. and wherein the heated atomized water is combined with a flow of oxygen to provide a stream of humidified oxygen which is administered to a patient, the improvement which comprises:
means for sensing the temperature of the stream of humidified oxygen immediately prior to its administration to the patient and for generating an electrical signal representative of said temperature; and
control means responsive to said signal for controlling the amount of heat generated by said electric heating unit to maintain the stream at a preselected temperature said control means comprising gate controlled bilateral switch means for controlling the flow of electric current through said electric heating unit;
means for amplifying said electrical signal;
means connected to the gate electrode of said bilateral switch means for controlling the conductive state of said swtich means in response to said amplified electrical signal;
means for sensing the temperature of said electric heating unit and generating an output signal indicative thereof;
means for overriding said amplified electrical signal and causing said bilateral switch means to be in its non-conductive state when said output signal indicates a preselected maximum temperature; and
circuit means for temporarily modifying the operation of said overriding means such that the bilateral switch means is not caused to become nonconductive until said output signal indicates another preselected maximum temperature which is higher than said first-mentioned preselected maximum temperature 3. The combination of claim 2. wherein said control means includes means for visually indicating if the de vice is properly grounded.
4. The combination of claim 3, wherein said ground indicating means includes:
an indicator lamp;
an amplifier in series with said lamp;
circuit means for biasing said amplifier into conduc tion when the device is connected to ground;
a phototranistor connected to said amplifier so as to cause said amplifier to become non-conductive when said photo-transistor is conductive;
a light emitting diode optically coupled to said photo- 10 transistor; and,
switch means responsive to the activation of said circuit operation modifying means for energizing said light emitting diode only when said circuit opera tion modifying means is energized.
5. In combination with an inhalation therapy device of the type wherein a column of water is drawn from a receptacle through an electric heating unit, whereby the water is heated and into an atomizing device, whereby the heated water is atomized, and wherein the heated atomized water is combined with a flow of oxygen to provide a stream of humidified oxygen which is administered to a patient the improvement which comprises:
means for sensing the temperature of the stream of humidified oxygen immediately prior to its administration to the patient and for generating an electrical signal representative of said temperature; and,
control means responsive to said signal for controlling the amount of heat generated by said electrical heating unit to maintain the stream at a preselected temperature, said control means comprising:
gate controlled bilateral switch means for controlling the flow of electric current thorugh said electric heating unit;
means for amplifying said electrical signal; and
means connected to the gate electrode of said hilat cral switch means for Controlling the conductive state of said switch means in response to said amplified electrical signal said means for controlling the conductive state of said gate controlled bilateral switch means comprising:
capacitor means for providing triggering current to the gate electrode of said bilateral switch means;
a diode bridge rectifier circuit having one input lead connected to said capacitor means so that said capacitor means is charged by the rectifier current;
a photo-transistor connected in series with said bridge rectifier output; and
a light emitting diode optically coupled to said photo transistor for controlling the conductive state of said photo-tranistor in response to said amplified electrical signal.