GB2176313A - Apparatus and method for controlling and monitoring mixing of two or more gases - Google Patents

Abstract

An apparatus for controlling and monitoring the mixing of at least two streams of gases comprises a gas flow controller for each stream of gas and a programmable microprocessor for operating the controllers. Control dials for percent gas mixture and total flow rate of gases provide inputs to the microprocessor. The microprocessor in turn adjusts percentage concentration of a gas in the mixed stream of gases by adjusting the flow rate setpoint to the gas flow controllers. The gas flow controller measures the gas flow rate and outputs a signal to the microprocessor which is representative of the actual measure of flow rate. The microprocessor has a display for the calculated percentage mixture of the gases in the mixed gas stream based on the value measured by the flow controller. The operator may then vary the percentage mixture dials until a desired mixture and the total flow rate, as displayed, are achieved. <IMAGE>

Description

SPECIFICATION Apparatus and method for controlling and monitoring mixing of two or more gases This invention relates to an apparatus for monitoring and mixing discrete sources of gas.

The system provides for both manual and electronic control of the flow of the gases to provide a desired mixture.

In the field of anesthesia, the management of oxygen gas flows in various medical gases is crucial, so that it is important to provide the clinician with versatile yet accurate control of oxygen concentrations in the medical gases. Several systems have, therefore, been developed to manage and control oxygen concentrations in the field of anesthesia. A manual system, which has proved to be accurate and reliable in most situations, is that disclosed in United States patent 3,739,799.

This apparatus, commonly sold under the trade mark Quantiflex M.D.M., by Fraser Sweatman Incorporated, provided adjustment on the total flow of anesthetic gas normally consisting of oxygen and nitrous oxide and a control for varying the percentage of oxygen in the anesthetic gas. A rotameter, which is a tube flowmeter, was provided for each gas to visually indicate in the respective rota meter the flow rate of gas. The total flow rate could then be mentally calculated by the summation of the individual flow rates of oxygen and nitrous oxide as shown in the respective rotameter. Such visual display in the rotameters also confirmed the mixture percentage of oxygen set on the controller. The mixture control valve was a thumb wheel which reduced the flow of one of the gases relative to an increase in flow of the other gas.

Systems have been developed which provide for the electronic control of anesthetic gases in analgesia and anesthesia apparatus.

An example is disclosed in United States patent 4,215,409. Digital encoders are used to determine the rotary position of needle valves which control the flow of the anesthesia gases. Electronics are used to calculate the percentage mixture of the gases and digitally display the percent mixture of the gases. The use of needle valves with digital encoders providing for the control in flow of analgesic gases can result in errors in controlling the flows. This can be caused by wear in the needle valves which results in a predetermined rotary position of the valves not producing the predetermined flow rate.

Another type of electronic system for anesthetic gas control is disclosed in United States patent 4,345,612. The apparatus is adapted to provide both electronic and manual control of the medical gases. The system includes microprocessor controlled electronic throttle valves for controlling the flow rate of the oxygen and nitrous oxide gas sources.

The flow rates of the medical gases are predetermined by the provision of a digital keyboard which can be used to enter into the microprocessor the digital values for the flow rates of the gases as desired by the clinician.

The microprocessor then provides a control signal to the electrically controlled throttle valve and, at the same time, indicates on the display the predetermined set value. With a flow rate sensor, the microprocessor continues to vary the opening of the throttle valve until the flow rate sensor indicates that the predetermined flow rate has been achieved in that line. In the event of a power failure, the system may be switched to a manual operation where two fixed throttle valves are used to provide a predetermined flow rate of the oxygen and nitrous oxide gases. This patent discloses a variety of flow rate sensors which may be used, such as the mass flowmeter as disclosed in United States patent 3,938,384.

This system has the apparent draw backs of not providing for variation of the quantities of the medical gases during the manual mode.

Furthermore, the system does not provide a display of the actual measured flow rates as determined by the flow sensors, but instead the flow rates which have been entered into the apparatus on the keyboard by the clinician.

The gas management system, according to this invention may be particularly adapted for use in the analgesia and/or anesthetic field to provide accurate controlled flow rates for oxygen in combination with other medical gases.

The method and apparatus of the invention is also suited for other gas supply control, as may be used in the electronic integratee circuit manufacturing process. The systems provides for electronic control and manual control of the gases.

According to an aspect of this invention, an apparatus for controlling and monitoring the mixing of at least two streams of gases comprises: i) a gas flow controller for each stream of gas comprising a conduit means for delivering a gas to a first two-way valve, said two-way valve having first and second positions to direct a gas to either a corresponding first or second outlet, said first outlet being coupled to an electrical throttling valve, said throttling valve being coupled to an inlet of a tube flowmeter, an outlet of said tube flowmeter being coupled to an electronic mass flowmeter which electronically controls said electric throttling valve, said second outlet of said two-way valve being coupled to a manual throttling valve means for said tube flowmeter, said manual throttling valve means controlling manually flows of gas through said tube flowmeter derived from said second outlet, ii) electronic control means in electronic communication with each of said gas flow controllers, said electronic control means com prising: a) means for outputting a signal representative of a flow rate to each said electronic mass flowmeter, said electronic mass flowmeter establishing a control loop with said electrical throttling valve to adjust and maintain gas flow rate of the corresponding gas at said flow rate, said electronic mass flowmeter having means for outputting to said electronic control means a signal representative of actual gas flow rate of the corresponding gas b) means for converting said actual gas flow rate signals received from said gas flow controllers for visual displaying on a visual display means total flow rate of the mixed gases and percentages of the individual gases in the mixture of gases c) means for imparting a signal for adjusting total flow and a signal for adjusting percentage mixture of the at least two gas streams, d) calculator means for adjusting said signal output means for each said electronic mass flowmeter based on input signals from said means for adjusting total flow and percentage mixture whereby said input signal adjustment means is adjustable until a desired total gas flow rate and desired percentage mixture are achieved as indicated by said visual displays of actual total gas flow rate and actual percentage mixture, and e) means having means for switching said two-way valves for each of said gas flow controllers to either said first position for electronic control mode of gas flow rate or to said second position for manual control mode of gas flow rate, said apparatus when in either said manual control mode or electronic control mode providing visual display of actual gas flow rates on said tube flowmeters and visual display of related total flow rate and percentage gas mixture of said electronic control means on said electronic visual displays.

According to another aspect of the invention, an apparatus for controlling and monitoring the mixing of at least two streams of gases comprises a gas flow controller for each stream of gas and electronic control means for operating the controller. Each of the gas flow controllers controls flow rate of a stream of gas for mixing with other gases based on input from the electronic control means. Means is provided for inputting to the electronic control means a signal to adjust percentage mixture of a gas in a mixed stream at the at least two gases The electronic control means outputs the signal representative ofa flow rate to the gas flow controller to increase or decrease the gas flow rate. The flow rate signal is determined by the input signal.The gas flow controllers measures gas flow rate and outputs to the electronic controller a signal representative of the actual measure flow rate. The electronic controller has means for converting the measured actual gas flow rates into values representing percentages of any one of the gases in the mixture and representing total gas flow rate.

Means displays percentage gas mixture of each gas and total gas flow rate. The input means provides for variation of the input signal until a desired percentage gas mixture for a gas or total gas flow rate is displayed.

Preferred embodiments of the invention are shown in the drawings wherein: Figure 1 is a schematic representation of the display and control devices of the -apparatus according to this invention; Figure 2 is a flow diagram and controlling devices for the supplies of the medical gases and the mixing thereof; Figure 3 is a block diagram showing the interrelationship of the set point calculator, flow measurement and control units and optical display in electronic digital form for the- flows of the medical gases; Figure 4 is block diagram of the interrelationship of the microprocessor to the various controls and displays of the oxygen management system; Figure 5 is a block diagram of the electronic signal processing by the microprocessor; Figure 6 is a block diagram of the flow measurement and control unit for a corresponding gas line; and Figure 7 is a block diagram of the electronic control for the oxygen analyzer in the patient circuit.

The apparatus, according to this invention which is capable of controlling and monitoring the mixing of two or more gases, can find application in a variety of gas management situations. For example, the apparatus may be used in controlling the mixing of gases used in the field of anesthetic gas delivery. The preferred embodiment of the invention will be described with respect to adapting the apparatus for the delivery of anesthetic gases. However, it is understood that the apparatus may also be used to manage the mixing- of gases which are used in vapor deposition for electronic integrated circuit manufacturing and other areas of commercial vapor deposition processes.

The oxygen management system, according to this preferred embodiment of the invention, provides a complete system for controlling and monitoring oxygen in an anesthetic gas delivery system. It combines the features of a stand-alone oxygen monitor with those Qf a sophisticated gas mixer providing the clinician with versatile control of oxygen concentrations in either the normal electronic mode of control or the manual override mode of control. The gas mixer portion of the oxygen management system allows either nitrous oxide and/or air to be mixed with oxygen in the analgesic/anesthetic procedure. The system is provided with warnings of any deviation from steady state operating conditions and unsafe levels of oxygen in the anesthetic gas mixture as selected by the clinician.

An oxygen analyzer is provided for the fresh gas side of the patient breathing circuit with appropriate program in its microprocessor. It is appreciated that a variety of oxygen sensors are available, such as the galvanic cell type, polarographic type and paramagnetic type. This allows the oxygen management system to monitor and compare patient oxygen concentration against the percent oxygen concentration adjusted by the clinician, An alarm is provided if this amount deviates by more than a certain percentage from that of the delivered concentration. A further alarm is also provided if the oxygen concentration begins to drift. Besides these two alarms, .high and low values for oxygen concentration can also be set by use of two thumb switches.

The patient oxygen analyzer is electrically independent of the micro controller used for the oxygen management system, but is able to transfer readings to the oxygen management system for some of the alarm functions.

The oxygen management system has a communications link that allows information to be output to an external unit for recording purposes or further analysis. With reference to Figure 1, the display panel 10 for the oxygen management system is shown. The total flow of the medical gases may be adjusted by control dial 12 where rotation in the direction of arrow 14 increases the total flow of the gases. Rotation of the dial 12 in the opposite direction decreases the flow of gases. Dial 18 may be rotated in the direction of arrow 20 to increase in the total flow of the gases the percent by volume of oxygen. When the user moves the dial 18 until the desired mixture is read out on display 22, a corresponding amount of oxygen should be in the mixed gas stream as indicated in display 24.The system is also adapted to display percent nitrous oxide in a three component anesthetic gas composition of nitrous oxide, air and oxygen. For the predetermined total flow rate, dial 19 may be rotated in the direction of arrow 21 to increase in the total flow of the gases the percent by volume of nitrous oxide. Display 23 indicates the percent nitrous oxide in the total flow of gases as e result of rotating the dial 19 in either increase or decrease direction.

The oxygen analyzer, with a sensor mounted in the patient circuit, analyzes the percentage by volume of oxygen in the gas stream. The oxygen analyzer may be calibrated at both 100% and 21% oxygen in the patient stream. Calibration at 100% oxygen is accomplished by flowing pure oxygen through the system and pushing calibration button 26.

When pure air is run through the system, calibration button 28 is pushed to provide calibration at 21% oxygen. Between the ranges of 21% and 100% oxygen, all other concentrations may be readily calculated due to the linear relationship for the oxygen analyzer between these percentages. The alarm set points for percent oxygen in the patient circuit may be dialed up on thumb wheels 30 and 32 which are respectively the upper and lower limits for oxygen in the patient circuit. Should the oxygen concentration in the patient circuit approach either of these extremes, an alarm is sounded.

The mass flow meters measure the flow rates of oxygen and the other selected gases of air and/or N20 to provide a read out on display 34 for the actual total gas flow rate.

Assuming that a change in the total flow rate is desired, the dial 12 is rotated in the appropriate direction until a new measured or computed value for total flow rate is displayed on display 34. This is the actual total flow rate measured. Assuming there is no change required in the desired percent of oxygen and N20 if present in the gas stream, the microprocessor adjusts the flow of oxygen and the nitrous oxide such that their proportions, once the desired total flow is achieved, is at a ratio equal to the prior ratio at steady state.

Alternatively, the total flow may remain the same but the percent of oxygen and/or nitrous oxide in the mixture can be varied by rotating dials 18 and 19. Rotation of either dial 18 or 19 varies the flow rate of oxygen or nitrous oxide. The percent oxygen of display 22 and percent nitrous oxide of display 23 are the actual percent oxygen and nitrous oxide based on the measured flow rates of the respective gases.

In view of the mass flow meters measuring directly the actual flow rates of oxygen as shown in display 36 and of the other gases, nitrous oxide in display 38 and air in display 39, the user can see the actual amounts of oxygen and the other gases passing through the system. At the base of the display, a switch 40 may be moved from the "off" position at 42 to select in combination with oxygen, one of air at position 44, nitrous oxide at position 46 or combination of nitrous oxide and air at 47.Thus the user has before him a digital read out of the total flow of the anesthetic gas, percent mixture of oxygen and/or nitrous oxide in the gas, confirmation by the oxygen analyzer of the actual amount of oxygen, percentage wise, in the patient's circuit and a digital read out of the amounts in litres per minute of the oxygen, nitrous oxide and/or air flowing through the management system to be mixed with other materials. Furthermore, by the use of tube flowmeters in combination with the mass flow meters, the operator is provided with a back-up visual reading on the tube flowmeters of the actual flow rates in a manner to be discussed.

The oxygen management system may be operated in the normal mode in which electronic control dominates. The normal mode is selected by push button 52. Alternatively, the system may be operated in the manual or override mode as selected by push button 54.

Usually it is desired to commence operation of the system in the override or manual mode.

Thus on start up, the override button cari be actuated or upon turning the unit on, the system automatically commences in the override or manual mode. In the manual mode, the flows of oxygen and the other gases may be selected respectively by dial 56 for oxygen, dial 58-for air and dial 60 for nitrous oxide.

For example, assuming that oxygen, nitrous oxide and air are desired, the dial 56 may be adjusted until the float ball is at the level 62 which is a reading of approximately 1.9 litres per minute. The nitrous oxide dial 60 may be adjusted until the float ball indicates a flow rate at level 64 which is approximately 3.9 litres per minute. Similarly, dial 58 may be adjusted until the float ball indicates a flow rate at 65 which is 2.0 liter per minute.

As will be discussed with respect to Figures 2 and 3, the oxygen management system has mass flow meters which measure the flow rate of the respective streams. Therefore, providing there is power in either the main source of power or in the battery back-up (optional), displays 36, 38 and 39 will digitally indicate the respective flows confirming the 2. 1 litres per minute of oxygen and the 4.9 litres per minutes of nitrous oxide and 2.0 liters per minute of air. The oxygen management system provides both digital read-out and flowmeter read-outs of the actual flow rates of nitrous oxide, oxygen and air, and can also provide read out of the total flow rate and percent mixture of oxygen when in the manual mode.

The remaining items on display are self-explanatory. Several types of alarm lights are used. The oxygen warning alarm light 68 illuminates when measured oxygen concentration is less than 30%. The low limit alarm for oxygen concentration causes display 24 to flash when measured oxygen concentration is below the low alarm limit set by dials 32. The high limit alarm results in the display 24 flashing when measured oxygen concentration is above the high alarm limit set by dial 30. A drift alarm may also be provided to alert when measured oxygen concentration has changed by more than a predetermined percentage, such as 3%, since last change of control settings when the unit is operated in steady state.

Additional alarms include the total flow failure light 70 which is illuminated when total flow rate decreases without a corresponding user input. Both the lamp 70 is lit and the total flow display 34 commences to flash.

Flow fault for oxygen, nitrous oxide and air can be indicated by additional lamps and when there is a decrease in total flow of oxygen, or the nitrous oxide or air flow, which may be caused by insufficient supply of oxygen, nitrous oxide or air, the display flashes to indicate insufficient pressure of the respective gas to maintain the total flow required.

"Battery In Use" lamp 74 becomes lit when the system is running from battery power.

"Low Battery" lamp 76 becomes lit when battery level is below a predetermined value.

In accordance with this oxygen management system, the user has all the essential digital displays of total flow, oxygen percentage and flows of oxygen and the selected nitrous oxide and/or air. Furthermore, the corresponding flow rates are indicated by flowmeter tubes 80 for oxygen, 82 for air and 84 for nitrous oxide.

Turning to Figure 2, the tubing system and manual and electronic control devices for the flow of gases is shown. The oxygen management system includes an air~inlet 86, nitrous oxide inlet 88, and an oxygen inlet 90. The switch 40 is moved to either position 44, 46 or 47 to open the corresponding rotary valves 92, 94 and or 102 to allow the selected gas to pass through the gas selector valving 96 and the corresponding tubing 98, 100 and 104. For either selection of air or nitrous oxide, the two-position valve 102 allows oxygen to flow into tubing 104 for the selection of either the air or nitrous oxide. In the respective tubing 98, 100 or 104, the corresponding pilot valve 106, 108 and 110 is provided.

Upstream of valves 106 and 108 are flow restrictor failsafe valves 112 and 114. Only when oxygen pressure is present in line 104 are the pressure control valves 112 and 114 opened to allow the flow of nitrous oxide and/or air as controlled by the solenoid selector valve. The oxygen flowing through valves 112 and 114 is returned to pilot valve 110 via line 116.

The mode select, which powers the electrical lead 118 to the solenoids 119, is controlled by pressing either the normal mode switch 52 or the override mode switch 54. In the normal mode, the valve 119 is open thereby allowing pressurized oxygen in branch line 121 to pressurize pilot valves 106, 108 and 110 to move same to direct flow of the respective gases through lines 120, 122 and 124. In the override mode, the valve 119 is closed so that-the pilot valves 106, 108 and 110 are in a second position which directs the flow of respective gas through lines 172, 174 and 176 In each of lines 120, 122 and 124 are corresponding electric throttling valves or electric flow control valves 126, 128 and 130. Each of these valves are in communication with flowmeter tubes 82, 84 and 80 respectively via lines 138, 140 and 142. Flowmeter tube 82 communicates with the electronic flowmeter and controller 144 via line 146. Flowmeter tube 84 communicates with electronic flowmeter 145 via line 148. Flowmeter tube 84 communicates with electronic flowmeter 150 via line 152. Upon selecting air and/or nitrous oxide, the corre sponding flowmeter 82 or 84 indicates a flow therethrough. With respect to display of Figure 1, both nitrous oxide and air have been selected thus providing a reading in flowmeters 82 and 84. The flowmeter and controllers 144 and 145 measure the flow rates of air and/or nitrous oxide and provide analog electric signals in lines 154 and 156. Correspondingly, the oxygen flow through flowmeter 80 is measured by electronic flowmeter 150 to provide an analog signal in line 157 representative of the measured flow rate.The gases from the flowmeters 144, 145 and 150 through lines 158, 159 and 160 are mixed at T-junction 162. The mixed gas passes through a pressure relief valve to an outlet which leads to the standard type of vapourizer normally used in anesthetic procedures.

The flowmeter and controller 144 includes a comparator system with feedback in line 169 to the corresponding electric flow control valve 126 to adjust the flow rate of the desired gas until the analog signal output in line 154 matches the analog signal set point 168.

Similarly with flowmeters 145 and 150, feedback is provided in lines 171 and 173 to the electric flow control valve 128 and 130 until the analog output in line 156 and 157 matches the analog signal set point input 170 and 175. The set point signals 168, 170 and 175 are determined by the microprocessor in accordance with the change in the corresponding controls 12, 18 or 19 for total flow and percent mixture of oxygen and percent mixture of nitrous oxide.

In the override or manual mode as selected by button 54, the pilot valves 106, 108 and 110 are positioned to bypass the corresponding electric flow control valves 126, 128 and 130 via lines 172, 174 and 176. Each flowmeter is mounted in a corresponding support block 178, 180 and 182. For flowmeter 82, needle valve 184 is provided in block 178.

Needle valve 186 is provided in block 180 and in block 182, needle valve 188 is provided. The needle valves may be adjusted when in the manual override mode to provide the desired flow rate of oxygen and the selected air and/or nitrous oxide gas.

It is preferred that on start-up the oxygen management system be in the override or manual mode. As shown in Figure 2, a flow bypass 190 is provided, such that when the system is started up in the override mode, oxygen flowing through line 176 flows through the bypass into line 142 and up through flowmeter 80. This bypasses the needle valve 188 so that regardless of the position in which the needle valve is provide, a minimum flow rate of oxygen of, for example, 2 or 3 litres per minute, as determined by the flow bypass, is achieved to ensure that oxygen is always present in the system. A flow restrictor may be incorporated in line 100 for the source of nitrous oxide to limit flow of N20 to an upper value.This limit on the flow rate of nitrous oxide, in combination with the minimum flow rate of oxygen determined by the bypass 190, ensures a minimum 30% mixture of oxygen in the total flow. The combined use of the nitrous oxide flow restrictor 113 and the bypass 190 provides this added safety margin in the overall operation of the apparatus.

With reference to Figure 3, the flowmeter and controller 150 for the oxygen line 152 is shown. Correspondingly, the flowmeter controls 144 and 145 for the air and nitrous oxide lines 146 or 148 are shown. A set point calculator 192, which may be part of the microprocessor for controlling the oxygen management system, provides analog outputs in lines 168, 170 and 175 to the flow measurement and control units 144, 145 and 150.

The flow measurement and control unit 150 adjusts the flow of oxygen in line 160 to a level determined by the set point calculator 192. Similarly, the flow measurement and control units 144 and 145 provide gas flows of air and nitrous oxide in lines 158 and 159 in accordance with the analog signal in lines 168 and 170.

The set point calculator, which may be part of the microprocessor, determines the rate of change and direction from signals in lines 168, 170 and 1 75 in accordance with inputs from the total flow control dial 12, the percent oxygen dial 18 and the percent nitrous oxide dial 19. The air/nitrous oxide select switch 40 provides input via line 119. The signal for the total flow control dial position is input via line 202. The signal from the mixture control dials 18 and 19 are input to the set point calculator via lines 204 and 205. Neither the total flow control nor the mixture control provide for absolute setting of the dials to predetermine a desired flow rate. Instead, the dials can be moved by the operator until the desired actual flow, as read out on the displays is realized.The advantage of the system is that the operator, in obtaining the necessary information from the displays, knows exactly what is happening with the system and is assured that the necessary flows are being provided because the display only provides readouts with respect to what is actually flowing out of the system. The set point calculator 192, therefore, does not in any way serve to preset the flow measurement and control units 144, 145 and 150. Instead, the operator, by continuing to rotate the total flow control and percent mixture controls, achieves the desired settings as displayed on the displays 22 and 23 and the total flow display 34. In this way, the flow rate of the respective gases is adjusted by the clinician. The digital read out provides the clinician with the actual flow rates of the gases rather than what the unit may be attempting to achieve.This provides assurance to the operator that he is adjusting the system to the desired flow rates, where the operator continues to adjust the dials until the total flow is desired and the percent mix ture of oxygen is obtained.

From each flow measurement and control unit, an output is provided. For control unit 144, its analog signal output is via line 154 to flow signal monitor 208. Correspondingly, the control units 145 and 150 have outputs via lines 156 and 157 in analog signal form to the flow signal monitor unit. The flow signal monitor unit converts the analog signals into digital signals and in combination with the sta tus information from the set point calculator input in address 212, the optical display unit 214 is addressed via line 216. The optical display unit consists of the digital displays 22, 23, 34, 36, 38 and 39. The percent oxygen indicated in display 24 by the oxygen analyzer 218 having an oxygen sensor 220 is input to the optical display by address222 to in turn display the percent oxygen on display 24.

With reference to Figure 4, the microproces sor, as located on computer board 224, is interfaced with the various electronic controls and displays. The computer board is normally powered by a 115 volt normal input at 226 which is stepped down by transformer 228 to a regulated power supply 234. Various refer ence voltages are provided by the power sup ply 234 for operation of the electronic control.

The air/nitrous select switch 40 has input via lines 119 to the microprocessor 224. The to tal flow- control dial, a dual channel rotary dial encoder, 12 has input via lines 202 to the microprocessor. The inputs are in the form of digital signals indicating the rate at which the dial is turned and the direction indicating either an increase or decrease in the flow rates. The percent mixture dials 18 and 19 also have digital inputs to the microprocessor via lines 204 indicating the rate at which the dial is turned and the direction in which the dial is turned. The microprocessor is programmed to analyze the inputs from the total flow dial 12, the oxygen percent mixture dial 18 and nitrous oxide percent mixture dial 19 to provide outputs to the mass flpw controllers 144, 145 and 150 which incorporate the flow control valves 1-26, 128 and 130.The set point calculator in the microprocessor, as per Figure 3, has analog signal outputs via lines 168, 170 and 175 to the mass flow controller for the respective gases.

The microprocessor provides an output 241 which enables a switch to turn on the relay 240 which controls the valves 106, 108 and 110 for either the normal or override mode of operation as selected by switches 52 and 54 in the display panel. The microprocessor is programmed to start up in the override or manual mode. The system may be activated by moving the gas selector switch 40 from the "off" position 42 to the desired gas mixture. In this state, the relay 240 is in its first position where the valves 106, 108 and 110 direct the gas flows through the tube flowmeters. After the microprocessor has done an integrity check, in accordance with the software program, on all functions and everything has checked out as being operative, power is applied to line 241.At this stage, if the clinician is ready for the "normal" mode of control, switch 52 may be closed to actuate relay 240, which in turn closes contact 242. Actuation of relay 240 in turn actuates valve 119 via line 118, which in turn moves all valves 106, 108 and 110 to their first positions, so that the respective electronic flow controllers 144, 145 and 150 take over in the manner described with respect to Figure 3. At any time, the clinician can switch back to the override mode by pushing switch 54 which opens the contact 242 and thereby deactuates the relay 240-to move the valves to their second positions.The software -of the microprocessor may be adapted so that on sensing a malfunction somewhere else in the system, which would indicate that it can no longer electronically control the gas flows, the microprocessor can remove the power from line 241 which in turn deactuates the relay 240 to move the valves to their first position. In this manner, only the clinician can select the "normal" mode when the system is checked out.

However, either the microprocessor or the clinician can switch back to the "override" mode to provide another aspect in the safe operation of the system.

The microprocessor communicates with the lamps indicating the warnings as discussed with respect to Figure 1 via line 244to selectively illuminate the lamps 68 and 70 in the manner discussed with respect to Figure 1.

Similarly, the microprocessor communicates with the display panel to provide various readouts on the displays via line 248. In the event of sensing a condition that requires actuation of the audible alarm250, the microprocessor selectively actuates the alarm via line 252.

The audible alarm maybe turned off for a predetermined period of time by pressing the audio "off" switch 67 as shown in Figure 1. A separate system failure alarm 253 may be provided separate of alarm 250.

The oxygen monitor unit 218 communicates with the microprocessor via line 254 and line 256 of group- 222. The oxygen sensor 220 is connected to the oxygen monitor via line 258.

The oxygen monitor includes its own microprocessor, to be discussed with respect to Figure 7, to provide data to the microprocessor 224 via line 256. The percent oxygen is shown on display 24.

The microprocessor 224 is also capable of communicating with external peripheral systems via lines 260 and 262. For example, the information being displayed may also be re corded and/or printed.

Figure 5 illustrates the internal components of the microcomputer board 224. The system includes a microprocessor 264 with a random access memory 268 and an EPROM memory 270. An analog to digital converter 272 converts all incoming analog signals to digital signals. The incoming analog signals include the analog signal output from the flow measurement and control units 144, 145 and 150 of Figure 3 which provide the relevant information concerning the oxygen flow rate in line 157 and the air and nitrous oxide flow rates in line 154 and 156. Inputs to the microprocessor from the total flow control dial 12 and the percent mixture control dials 18 and 19 are provided in digital signal input via lines 202, 204 and 205 into digital input port 276.

A digital to analog converter 274 is provided to output to the flow measurement controls 144, 145 and 150 the necessary analog signal to cause a change in the flow rates of the medical gases. The analog signals are output to the flow measurement controllers via lines 168, 170 and 175. A digital input port 276 is provided to receive inputs from the various remaining control switches of the display panel discussed with respect to Figure 1 and the mass flow controller from information to be discussed with respect to Figure 6. An optical display driver 278 is provided for driving the optical displays 280 as discussed with respect to Figure 1. A digital output port 282 provides the necessary control signals for the control relays in switching from override to normal mode, illuminating the lamps for the various alarms, etc.The microprocessor 264 communicates with these various segments of the computer board via the address buss 284.

These components in turn communicate with the microprocessor via data buss 286.

With reference to Figure 6, the flow measurement and control unit is shown in more detail. It is understood that the arrangement of Figure 6 is applicable to the flow measured ment and control unit 150 for the oxygen, 144 for the air and 145 for nitrous oxide as per Figure 3. Taking for example the oxygen flow measurement and control unit, it consists of the electric flow control valve 130 as per Figure 2 and the mass flow sensor 150. The gas flows through the electric flow control valve via line 124 and into the mass flow sensor via line 152 with the intermediate flow through the tube flowmeter 80 as shown in Figure 2. The output of the flowmeter 150 is via line 160 to be mixed with the other metered gas. The flow measurement and control unit includes a comparator system 288.The analog signal is provided via line 175 from the set point calculator 192 of the microprocessor. The analog signal in line 175 is of a voltage level representative of the extent to which the total flow control and mixture control dials 12 and 18 have been rotated. The flow sensor 150 provides a signal in line 290 to amplifier 292 which provides an analog output in line 157 which is representative of the actual flow rate reading. As shown in Figure 3, this signal is passed to the flow signal monitor 208 for reading on display 36. The amplifier 292 includes a read-only memory 294 which includes a calibration curve relating signal 290 to the actual flow rate of the gas through line 152. The microprocessor 264 communicates with the read-only memory of each flow measurement and control unit via data link 296 to digital input 276.Upon activation of the unit, the microprocessor addresses the ROM 294 of each flow measurement and control unit to place in its memory the calibration curve so as to provide a digital read out in respective display 36, 38 or 39 and in display 34 of the true actual flow rates as measured by the flow sensors. Thus the amplifier 292 in outputting an analog signal via line 157 provides input to the microprocessor by the analog to digital converter 272. This digital signal is then compared to the calibration curve to provide the exact flow in litres per minute of the gas through line 152. In this manner, the calibration for each flow measurement and control unit is automatically provided without the need to calibrate each control unit after it is installed in the system. This provides a very significant advantage.The flow measurement and control units may be replaced as modules in the systems with their own calibration curves already included. This avoids extended down time of the system for recalibration and also avoids the possibility of replacement of the flowmeter with one calibrated for a gas other than the gas to be handled. In the read-only memory for each flow measurement and control unit, various identification data may be included. Most importantly, data identifying the gas for which the unit has been calibrated. This avoids the problem that, during maintenance and disassembly of the unit and reassembly, should one or more of the flowmeters be mixed up, the apparatus can determined this and sound an alarm to indicate that the mass flowmeters have been improperly reassembled.For example, it may be determined that an oxygen mass flowmeter has been assembled in the line which includes nitrous oxide and conversely.

The amplifier 292 also provides internal output in line 298 which is fed to the comparator device 288. This produces an error signal in output 300 of the comparator which is amplified in amplifier 302 to provide a signal in line 304 to the flow control valve 130. Depending upon the potential and sign of the signal, the valve is either slightly open or closed to change the flow rate, which is again sensed by the flow sensor 150, resulting in further feedback to the comparator 288 with continued adjustment until the error signal in line 300 is less than a predetermined level.

The oxygen analyzer 218 is shown in more detail in Figure 7. The analyzer includes a microprocessor 306 which has a serial interface with microprocessor of computer board 264 via interface 254, 256. The digital display 24 for the oxygen analyzer may be either controlled by microprocessor chip 306, as shown in Figure 7, or through the central microprocessor chip of the computer board 224 as -shown in Figure 4. The microprocessor 306 includes a system program in a read-only memory 308 which communicates via the address buss line 310 and data buss line 312.

Thumb wheel switches 30 and 32 of Figure 1 have inputs to the microprocessor 306 via lines 312. The warning lamps and audible alarms and switches are controlled by lines 314. The oxygen analyzer is powered from the normal 115 volt source in line 316 which is reduced in power supply 318. A battery power back up is provided at 320 in the event of power failure.

The oxygen sensor 220 has its output amplified at 222 to the analog to digital converter 322 which provides input via line 324 to the microprocessor 306. In accordance with the system program, an output is provided in lines 326 to the display 24 indicating the actual amount of oxygen sensed by the oxygen sensor 220 in the patient circuit. By separating the oxygen analyzer from the main oxygen management system, there is less opportunity for failure or mistake both in indicating the percent oxygen in the patient circuit and delivering the required oxygen concentration.

The microprocessor 264 includes its own software program in EPROM 270 which controls the many functions of the system including reacting to inputs from the total flow control and the percent oxygen control, analyzing inputs of the measured oxygen and other gas flow and displaying the results. Since the system is based on reacting to a control dial change rather than attempting to set the flow rates at a predetermined level, the software is designed to read the oxygen, nitrous oxide and air flow rates and correspondingly display these flow rates on displays 36, 38 and 39.

The software calculates the percent oxygen and total flow rates which are in turn read out on displays 22 and 34. The system will, in reacting to either an input of total flow dial change or percent oxygen dial change, maintain the other one or more variables at the steady state level. For example, should the clinician adjust dial 12 to either increase or decrease the total flow, then an adjustment is made to the corresponding flow controllers.

For an increase in total flow, the oxygen, nitrous oxide and air set points are increased by the set point calculator 192 to open the flow valves in proportion to their current ratio.

Should it be desired to decrease the total flow, then the oxygen, nitrous oxide and air set points are determined by the set point calculator in proportion to their current ratio to result in a decrease in the flows, yet maintain the percent oxygen as determined by the current ratio of the steady state.

On the other hand should it be desired to change the percent oxygen, if the amount of oxygen- is to- be increased, then the oxygen set point is increased and the nitrous oxide and air set points are decreased such that the sum of the set points is at the same level as that during prior steady state operation so that the total flow does not change. In the event that it is desired to decrease the percent oxygen, then the set point for the oxygen is decreased and the set point for the nitrous oxide and air are increased such that the sum of the set points remains the same during steady state operation to maintain the current flow.

In the event that flow of any one of the gases unexpectantly decreases, the microprocessor is adapted in a manner discussed with respect to Figure 4, to switch to the "override" mode. The necessary alarms are sounded to alert the clinician who can immediately respond in the manual mode to correct the problem. At no time is the microprocessor relied upon to attempt to correct the problem.

When the oxygen pressure falls sufficiently, the nitrous oxide orair supply will be shut off completely by the oxygen fajlsafe pressure control valve 114, as shown in Figure 2, which can also be provided in line 98 of the air supply.

The software for the oxygen management system consists of a set of tasks each with its own specific functions to perform. Each task is essentially self-contained, with intertask communications handled exclusively by a set of common memory variables and flags. In general, any task can read these variables and flags, but only one task can write to them.

Task control is handled by a small interrupt driven executive that maintains a software count-down timer for each task. When that timer reaches zero, it stops and task is is considered ready to run. Negative timer values indicate that the task is asleep and positive values give the number of system counts until the task will be ready to run. Tasks may reschedule themselves or other tasks by storing a new value in the appropriate timer, or they can turn tasks off by writing a negative value into that timer.

Tasks timers are polled in a round robin manner by a scheduler routine. The scheduler is passed control of the system each time a task terminates itself. It scans for any task timer value equal to zero and give up control to the associated entry point if found. A task must explicitly return control to the scheduler when it is done (or ready to halt for a while) as the scheduleris not a true context switching task control system.

The tasks which are carried out on a routine basis may be categorized as follows: OMS TASKS ADCONV-reads and controls the analog to digital convertor hardware 272; gets input from flowmeters and valves; stores the result in a buffer for use by the FLOW task.

BINPUT--reads status of digital inputs 276 and stores result in memory flags (e.g. gas selector switch).

BOUT-reads memory flags and writes appropriate data to digital output ports 282.

BUZZER-check status of alarm buzzer flags and start sound generator chip 250 if necessary to produce sounds.

DACONV-takes required setpoint output values for D/A converter and writes them to the chip 274.

DISPLA V-performs necessary conversion to display current flow and percentage values on front panel digital displays 280 and then writes those values to the display driver 278.

FLASHER-turns displays and lamps on and off if a memory flag says they are in alarm.

FLOW-averages the current raw A/D flow rate values and then scales them according to the parameters read in from the PROMs 294; calculates resulting percentages and total flow value.

MONITOR--determines the current system mode and gas selection; also checks for alarm conditions and sets alarm flags if found.

PARSER-uses the micro's onboard interface 260 and 262 to provide a user interface for test, debug and data logging purposes.

SETPOINT--monitors up/down counters associated with each front panel rotary controls 12, 18 and 19; adjusts flow setpoints 168, 170 and 1 75 accordingly when a change is detected.

TRACK--compares current flows measured by FLOW to desired ones set by SETPOINT; sets alarm bits if they differ for too long; also compares oxygen analyzer reading to delivered percentage.

It is understood that these tasks may be implemented by many available forms of software, the programming of which is readily understood by those skilied in the art, such software being entered and stored in the microprocessor memory as discussed with respect to Figure 5.

In the event that the oxygen sensor detects oxygen concentrations in the patient circuit beyond the defined limits, or that the sensor reading has changed without there being a corresponding change in the control settings, an audible alarm will sound and the appropriate visual alarm will be illuminated. The system, however, does not take any further action in attempting to adjust the oxygen concentration. It is left up to the operator to respond to the alarm and make the necessary adjustments in the system. This avoids any possibility of error in the system attempting to adjust aotomatically for decrease of oxygen in the patient circuit and reacting in a manner which could provide hazardous consequences for the patient.

The system for managing oxygen in anesthesia thereby allows the clinician to adjust the controls to obtain the desired total flows and percent oxygen requirements knowing full well that the readouts both digitally and in the flow meters are the actual flow rates. The total flow rate may remain constant as percent oxygen is adjusted and vice versa. With the use of the electronic flow measurement systems with onboard calibration, flow rate measurement accuracy is greatly improved over conventional glass flow tubes. However, the glass flow tubes remain to provide the clinician with a back up, physical indication of the flow rates.

During normal operation of the system, the clinician is prevented from accidentally selecting a hypoxic mixture. Facility is provided for outputs of the information determined by the system to be output to an automated reporting system to provide a complete case record.

For each use of the system, the provision of a manual override system is beneficial where manual adjustments are provided for the selected gas flows. Furthermore, the system may be programmed to commence operation in the manual override mode which ensures that a minimum flow of oxygen is obtained, which may be set by the flow bypass device 190 of Figure 2 at approximately two or three litres per minute. During initial start-up, the program for the microprocessor may be designed to check all systems to ensure that they are operable and not switch over to the normal mode until the system integrity check is completed at which time should the normal switch 52 be activated, the electronic control system is actuated.

Although preferred embodiments of the invention have been described herein in detail, it will be understood by those skilled in the art that variations may be made thereto without departing from the spirit of the invention or the scope of the appended claims.

Claims (11)

1. An apparatus for controlling and monitoring the mixing of at least two streams of gases, said apparatus having: i) a gas flow controller for each stream of gas comprising a conduit means for delivering a gas to a first two-way valve, said two-way valve having first and second positions to direct a gas to either a corresponding first or second outlet, said first outlet being coupled to an electrical throttling valve, said throttling valve being coupled to an inlet of a tube flowmeter, an outlet of said tube flowmeter being coupled to an electronic mass flowmeter which electronically controls said electric throttling valve, said second outlet of said two-way valve being coupled to a manual throttling valve means for said tube flowmeter, said manual throttling valve means controlling manually flows of gas through said tube flowmeter derived from said second outlet, ii) electronic control means in electronic communication with each of said gas flow controllers, said electronic control means comprising: a) means for outputting a signal representative of a flow rate to each said electronic mass flowmeter, said electronic mass flowmeter establishing an analog loop with said electrical throttling valve to adjust and maintain gas flow rate of the corresponding gas at said flow rate, said electronic mass flowmeter having means for outputting to said electronic control means a signal representative of actual gas flow rate -of the corresponding gas b) means for converting said actual gas flow rate signals received from said gas flow controllers for visual displaying on a visual display means total flow rate of the mixed gases and percentages of the individual gases in the mixture of gases c) means for imparting a signal for adjusting total flow and a signal for adjusting percentage mixture of the at least two gas streams, d) calculator means for adjusting said signal output means to each said electronic mass flowmeter based on input signals from said means for adjusting total flow and percentage mixture whereby said input signal adjustment means is adjustable until a desired total gas flow- rate and desired percentage mixture are achieved as indicated by said visual displays of actual total gas flow rate and actual percentage mixture, e) means having means for switching said two-way valves for each of said gas flow controllers to either said first position for electronic control mode of gas flow- rate or to said second position for manual control mode of gas flow rate, said apparatus when in either said manual control mode or electronic control mode providing visual display of actual gas flow rates on said tube flowmeters and visual display of related total flow rate and percentage gas mixture of said electronic control means on said electronic visual displays.

2. An apparatus of claim 1, wherein means is provided in association with said manual throttling valve means for bypassing said manual throttling valve means when said two-way valve means is in said second position, said bypass means providing a predetermined minimum flow rate of gas through said tube flowmeter regardless of manual throttling valve means setting.

3. An apparatus of claim 1, wherein means is provided in association with said manual throttling valve means for restricting flow to a maximum flow through said manual throttling valve means when said two-way valve means is in said second position, said means for restricting flow providing a predetermined maximum flow rate of a gas through said tube flowmeter regardless of increased manual setting on said manual throttling valve means.

4. An apparatus of claim 1, wherein said converter means additionally converts said actual gas flow rate signals for displaying on said visual display means actual gas flow rates for each of the gas streams.

5. An apparatus of claim 1, wherein one of said gas flow controllers is adapted for controlling flow of oxygen gas, means for analyzing percent oxygen concentration in a mixed stream of gases, said electronic control means having means for displaying a value for analyzed percent concentration of oxygen determined by said oxygen analyzer means, said display means for calculated percent concentration of oxygen gas and said display means for analyzed percent concentration providing visual confirmation that said electronic -control means is functioning properly, said electronic control means having means for comparing said analyzed value for percentage oxygen concentration against said calculated value for oxygen concentration, said comparator means actuating an audible alarm when said com- pared values differ by a predetermined level.

6. An apparatus of claim 1, wherein each of said means for adjusting total flow and for adjusting percentage mixture comprises a manually rotatable dial which generates said signal based on direction of rotation of said dial and rate of rotation of s,aid dial.

7. An apparatus of claim i, wherein three gas flow controllers are provided to control flows of oxygen, nitrous oxide and air for purposes of anesthesia, said conduit means of each of said gas flow controllers having an inlet, a valving system for said three inlets to control admission of oxygen with air and/or nitrous oxide, said valving means generating a signal to indicate which of said gases are being mixed and transmitting said gas selection signal to said electronic control means to enable said electronic control means to control and monitor mixing of gases through the respective gas flow controllers.

8. An apparatus of claim -7, wherein a fail safe valve is provided in each of said conduits delivering air and nitrous oxide gases, said fail safe valves having means for closing said valves automatically in response to a lack of oxygen pressure in said conduit delivering oxygen gas.

9. An apparatus of claim 7, wherein each of said two-way valves is shifted from said second position to said first position by pressurized oxygen gas in a branch conduit coupled to each of said two way valves, an electrically actuated valve in said oxygen branch conduit which is coupled to said conduit for delivering oxygen, said electronic control means signaling said electrically actuated valve to open when said electronic control mode is selected.

10. An apparatus of claim 9, wherein said two-way valve has means for automatically returning it to said second position when pressurized oxygen or electrical power supply is removed to revert control to said manual control mode necessitating thereby reset of said electronic control means to resume said electronic control mode.

11.An apparatus for controlling and monitoring the mixing of at least two streams of gases, said apparatus comprising a gas flow controller for each stream of gas and electronic control means for operating said controller, each of said gas flow controllers controlling flow rate of a stream of gas for mixing with other gases based on input from said electronic control means, means for inputting to said electronic control means a signal to adjust percentage mixture of a gas in a mixed stream of the at least two gases, said electronic control means outputting a signal representative of a flow rate to said gas flow controller to increase or decrease gas flow rates, said flow rate signal being determined by said input signal, said gas flow controller measuring gas flow rate and outputting to said electronic controller a signal representative of said actual measured flow rate, said electronic controller having means for converting said measured actual gas flow rates into values representing percentage of any one of the gases in the mixture and representing total gas flow rate, means for displaying percentage gas mixture of each gas and total gas flow rate, said input means providing for variation of said input signal until a desired percentage gas mixture for a gas or total gas flow rate is displayed.