Humidity compensated carbon dioxide gas measurement and control system

Abstract: A carbon dioxide gas measurement and control system which comprises a thermal conductivity CO 2 sensor and a humidity sensor which produce separate and independent output signals which are compared in a microprocessor, thereby generating a humidity compensated carbon dioxide gas measurement which is accurate to within ±0.1%.

Description: The present invention relates generally to a method and system for accurately measuring carbon dioxide gas concentration in a substantially saturated or non-saturated gaseous environment, e.g., a cell-culture incubator, regardless of absolute humidity changes. Background of the Invention Measurement and control of the CO 2 concentration in laboratory cell-culture incubators is most commonly accomplished by means of a thermal conductivity detection system. The thermal conductivity cell, or detector, is a differential thermometer, set up as an electronic bridge circuit that is balanced to be equal, with two thermistors in a common block or metallic housing to add thermal stability. (See U.S. Patent No. 3,929,584, ((Mansfield)). This cell is most commonly placed in a working environment that is isolated from the surrounding area, but is not restricted to its placement. In some instances, the detector cell is located in an air flow path external from the working environment, but contiguous with the environment. One thermistor sensor is enclosed in the block/housing and detects chamber temperature only. The other thermistor sensor is exposed to the chamber environment. The measured difference between the two thermistor sensors is the thermal conductivity (density) of the atmosphere, or its ability, when moved at an even rate, to remove the small amount of heat from the exposed sensor. If all other factors remain constant and only the carbon dioxide content is varied, the "TC" cell output (when properly calibrated) will indicate changes in CO 2 concentration. Unfortunately, the TC cell is affected by barometric pressure, temperature, humidity and the velocity of air flow past the sensor cell. These variables are controlled or compensated for with the use of electronic zeroing circuitry to compensate for changes in temperature and relative humidity levels. In monitoring the effects of CO 2 in an atmosphere, absolute humidity must be held constant so any change in thermal conductivity is caused only by a change in the CO 2 concentration. Under the worst circumstances, a change in absolute humidity can cause such a significant change in thermal conductivity that the controller can shift the CO 2 content by as much as 4%. To maintain a stable humidity level in laboratory incubators, a pan of water is placed within the working environment and its temperature allowed to equilibrate. The incubator, working atmosphere, must reach a point of near saturation in order to maintain an absolute humidity level that will not change with ambient conditions. For the laboratory investigator that does not want to operate their incubator in a saturated condition, but does want accurate CO 2 control, the drifting of the thermal conductivity sensor's reference becomes a problem with regard to the accuracy of the CO 2 gas concentration in the incubator. That is, as the absolute humidity changes, so does the reference base of the CO 2 gas sensor. When operating a dry incubator, as opposed to a saturated one, ambient humidity fluctuations will affect CO 2 zero calibration. Since the fluctuations possible in extreme ambient temperature changes have less effect on the total absolute humidity, the CO 2 calibration can be affected as much as 1½% in the worst case which does not represent as severe a problem, but does create an error that could prove critical in the pH level of the cell media being cultured within the incubator working chamber. US 3,848,569 discloses an analogue humidity-compensated carbon dioxide gas detection control system and method in an incubator, utilizing a humidity sensor, a thermal conductivity based carbon dioxide sensor and bridge circuitry including a potentiometer which is arranged to vary its resistance and thereby compensate the carbon dioxide reading for variations in humidity. A control system is provided which initiates a supply of carbon dioxide to the incubator until a selected level is reached. No offsetting of a zero reference point is disclosed. DE-A-3,640,734 disclose a digital temperature and humidity-compensated gas pollution detector utilizing a semiconductor sensor such as a tin dioxide sensor to detect the pollutant and temperature and humidity sensors. The output of the pollution detector is corrected by a microprocessor which utilizes a two-dimensional look-up table of stored humidity and temperature values. No offsetting of a zero reference point is disclosed. The present inventor has developed a unique system for detecting or measuring CO 2 gas concentration in an enclosed environment using a thermal conductivity sensor without the inaccuracies of the prior art systems which are caused by fluctuations in absolute humidity. In one aspect the invention provides a humidity compensated carbon dioxide gas detection and control system for use in a substantially saturated or non-saturated gaseous environment which comprises: a humidity sensor means capable of providing a humidity level output; a thermal conductivity sensor means capable of providing a carbon dioxide concentration output; means for calculating the difference between said humidity level output and a stored humidity level detected when the zero reference of the thermal conductivity sensor means was originally set or calibrated, to generate a humidity-compensated carbon dioxide concentration output, said means comprising a microprocessor responsive to said difference to offset a CO 2 gas control system zero reference point corresponding to said stored humidity level, said microprocessor using a look-up table comprising specific zero reference point offset values. The system may optionally also include a means for controlling the flow of carbon dioxide gas into the environment in accordance with the humidity compensated carbon dioxide gas concentration output. It is also desirable to have a means for measuring the temperature of the environment in order to adjust the humidity level output accordingly. In another aspect the invention provides a process for measuring and controlling carbon dioxide gas in a substantially saturated or non-saturated environment which comprises: measuring the humidity level of said environment; measuring the carbon dioxide concentration of said environment; calculating the difference between said humidity level and a stored humidity level detected when the zero reference of the thermal conductivity sensor means was originally set or calibrated and in response to said difference generating, in a microprocessor including a look-up table, a humidity-compensated carbon dioxide concentration output by offsetting a CO 2 gas control system zero reference point corresponding to said stored humidity level, said look-up table comprising specific values used to offset said zero reference point. Preferably, the flow of the carbon dioxide gas into the environment can be regulated in accordance with the humidity compensated carbon dioxide gas concentration measurement. The present invention removes the inaccuracies due to the effect of changes in absolute humidity levels on the zero calibration point of the thermal conductivity sensor which is used to measure and control the CO 2 content within any controlled atmosphere (e.g., a cell-culture CO 2 incubator). This is accomplished in one embodiment by having the output of the thermal conductivity detector multiplexed with the output of an absolute humidity detector via firmware in a microprocessor control system. The output from the microprocessor controller is a humidity-corrected signal that will maintain a stable reference output with respect to constantly changing absolute humidity levels in the controlled atmosphere. By constantly correcting this output due to changes in absolute humidity levels, accuracy and errors were reduced from a high of 4% for conventional systems to less than 0.2% for preferred embodiments of the present invention. These errors are primarily caused by manually or automatically zeroing the thermal conductivity detection system in a potentially unstable environment. This zeroing is typically conducted by the product user or non-compensating type control system. The present invention is also capable of specifically measuring and controlling the CO 2 content in an enclosed (e.g., a laboratory CO 2 incubator) by means of a thermal conductivity detector, where the relative humidity level may be allowed to equilibrate to that of surrounding ambient conditions as well as conditions that may be increased to virtual saturation. The two factors that contribute to inaccuracies in a thermal conductivity gas control system, i.e. relative humidity and dry-bulb temperature, are taken into consideration in the measurement system of the present system of the present invention to arrive at the absolute humidity compensated carbon dioxide gas concentration. The present invention also provides many additional advantages which shall become apparent as described below. A preferred embodiment of the invention is described below by way of example only with reference to Figures 1 to 4 of the accompanying drawings, wherein like parts have been given like numbers, and: Fig. 1 is a schematic representation of an incubator having humidity, CO 2 and temperature sensors disposed therein according to the present invention; Fig. 2 is a block diagram of the humidity compensated CO 2 gas measurement and control system in accordance with the present invention; Fig. 3 is a circuitry diagram of the humidity compensated CO 2 measurement and control system in accordance with the present invention; and Fig. 4 is a flow chart of the process steps used to compensate CO 2 gas measurements for changes in absolute humidity. A thermal conductivity CO 2 sensor measurement output signal is multiplexed with an absolute humidity sensor measurement output signal via a microprocessor. The sensors are disposed within an enclosed container (i.e., a cell culture incubator) such that they are within the path of the circulating CO 2 gas. Both the CO 2 and absolute humidity detection devices are connected, via a shielded interconnecting cable, to their respective electronic interface circuit board. The electronically conditioned signals generated from both the CO 2 and absolute humidity sensors are then independently passed on to a microprocessor for firmware multiplexing. As changes are detected by the absolute humidity sensor, adjustments are electronically made on a continuous basis to the zero reference base of the thermal conductivity gas control circuit. This compensation system maintains an accurate gas control system that is unaffected by changes in absolute humidity levels. Fig. 1 shows a cell-culture incubator 11 comprising multiple perforated shelves 7 disposed between wall ducts 8. The outer surface of wall ducts 8 and the inner surface of incubator housing 20 form a passageway 9 therebetween, wherein gas contained within housing 11 is circulated in the direction of the arrows such that gas moves from the humidity water pan 10 up towards blower wheel 5 attached to blower motor 4 and returned to the bottom of incubator 11 via passageways 9. As the gas is dispelled from blower wheel 5 it passes over humidity sensor 1, thermal conductivity sensor 2 and temperature sensor 3. The humidity level and CO 2 sensors interact with a microprocessor in accordance with the block diagram shown in Fig. 2. The humidity sensor means 30 is capable of accurately measuring the environmental atmosphere to detect the presence of humidity that is temperature compensated for stability and accuracy over the entire operating range of the system. As such incubator 11 having a heat generating means will form an environmentally controlled chamber useful for research of cell studies in the laboratory requiring specific control of surrounding temperature and pH levels. Pressurized carbon dioxide is externally supplied and connected to incubator 11 and its flow controlled by means of a solenoid valve 40 disposed between CO 2 supply means 42 and incubator 11. The level of carbon dioxide in the controlled atmosphere chamber of incubator 11 is typically maintained at a constant level, impervious to changes in temperature and humidity levels, by microprocessor 34 which continuously monitors these parameters and maintains the desired CO 2 concentration level by proportionally opening and closing solenoid valve 40 allowing pressurized CO 2 supplied via storage tank 42 to enter controlled atmosphere chamber. The thermal conductivity cell, or detector, 2 is a differential thermometer, set up as an electronic bridge circuit that is balanced to be equal, with two thermistors in a common block or metallic housing to add thermal stability. That is, the thermal conductivity of the CO 2 disposed within chamber is continuously measured by a thermistor that is part of a Wheatstone bridge circuit that also provides a linear voltage output for use by microprocessor 34. The humidity system's sensor 2 utilizes a monolithic integrated circuit detector 30 coupled with a thin film platinum RTD sensor 50 for temperature measurement and compensation within the humidity detection system for stability over the entire incubator operating humidity range. The monolithic integrated circuit used for humidity measurement is a capacitance sensor, when incorporated in a complimentary metal oxide semiconductor (CMOS) circuit, such that it converts the signal into a linear voltage output for use by microprocessor control system 34. These two signals are multiplexed together within microprocessor 34 using firmware. Microprocessor 34 then compares the present humidity level detected by humidity sensor means 30 with the stored humidity level 36 detected when the zero reference of the CO 2 sensor was originally set or calibrated. A compensation value is then established, similar to an offset, by comparing values in look-up table 38. Look-up table 38 comprises differential data that represents specific values to offset the CO 2 gas control system zero reference point. The difference between the two humidity measurements is then referenced to the actual value of the CO 2 measured by thermal conductivity sensor means 32. The measured CO 2 concentration is then adjusted according to this differential and the output is passed to control panel display 44 for user reference. Once the actual CO 2 concentration is detected and re-calibrated, microprocessor 34 signals CO 2 gas control means 41 to either open or close solenoid valve 40 to allow pressurized CO 2 to enter chamber in order to maintain a desired CO 2 concentration level therein. Fig. 4 is a flow chart of the process steps discussed immediately above. Fig. 3 shows the circuitry used in accordance with the present invention, wherein the humidity level is input into the electronic circuit system from the combination humidity/temperature sensor, not shown, located within the working environment. The humidity/temperature sensor is connected to the electronic circuit system via humidity sensor input 6. The humidity level input is conditioned by operational amplifier U1A whose output then goes directly to the humidity circuitry on the microprocessor circuit board via output 1 which is then passed on to the humidity parameter display of the incubator. Simultaneously, the conditioned output from operational amplifier U1A is passed on to the multiplexing circuitry. The humidity/temperature sensor is also connected to the electronic circuit system via temperature sensor input 2. The temperature level input is combined with the conditioned humidity level input via operational amplifier U3B, wherein a correction factor is applied to the combined signals which is then converted to an absolute humidity value. The carbon dioxide sensor is connected to the electronic circuit system via carbon dioxide sensor input 4. The signal received from input 4 is conditioned by operational amplifier U2A. This conditioned output signal goes directly to the auxiliary thermal-conductivity monitoring circuitry on the microprocessor circuit board via output 3. Simultaneously, the conditioned output signal is also passed on to the multiplexing circuitry. That is, the conditioned carbon dioxide output signal from U2D is sent to the multiplexing circuitry where it is multiplexed with the zero corrected, absolute humidity output from U3B (i.e., U3B adds correction for varying absolute humidity levels from zero referenced Rh level) and then goes directly to the monitoring circuitry on the microprocessor circuit board via output 5. The carbon dioxide and humidity levels are compared, the absolute humidity output is verified with the absolute humidity level detected when the carbon dioxide gas system zero was referenced, and the carbon dioxide gas system internal zero reference is adjusted (i.e., compensated) if there has been any absolute humidity level changes. Any carbon dioxide set point error is corrected by the addition of carbon dioxide gas. This is performed by providing a voltage output that opens a solenoid valve that allows carbon dioxide to enter the working environment in metered amounts. This type of control and compensation system is accurate to within ±0.1% of the actual concentration measured. The total error created by temperature and humidity changes can subsequently be held to approximately ±0.1% as well as relating to the resolution capability of the output display EXAMPLE 1 For example, an incubator operating at 37°C. with an ambient relative humidity level of 35% (absolute humidity is equal to 14.6 g moisture/kg of dry air and (102 grains of moisture per pound of dry air)), and with the CO 2 detector system zero calibrated under these conditions, maintains the CO 2 gas level at 5% of the environment. Water is now placed in the incubator to raise its humidity level to - saturation. After stabilization, the temperature remains at 37°C. and the CO 2 concentration is based on the displayed value. The relative humidity in the incubator should now be approximately 98% (absolute humidity is equal to 44 g moisture/kg of dry air (308 grains of moisture per pound of dry air)). A verification of the actual CO 2 concentration reveals that it is now approximately 9% of the environment. This example shows what can happen if an incubator is CO 2 zero calibrated dry and, thereafter, water is added without re-zeroing. That is, if an incubator is zeroed the actual CO 2 content differs by almost 4% from that shown on the display. EXAMPLE 2 An incubator utilizing a humidity compensated CO 2 gas measurement and control system according to the present invention was heated to 37°C. with an ambient relative humidity level of 35% (absolute humidity equal to 14.6 g moisture/kg of dry air 102 grains. of moisture per lb. of dry air), and with the CO 2 detection system referenced under these conditions. The CO 2 gas level is maintained at 5% of the environment. Water is now placed in the incubator to raise its humidity level to saturation. After stabilization, the temperature remains at 37°C. and the relative humidity in the incubator should now be 98% (absolute humidity equal to 44 g moisture/kg of dry air (308 grains of moisture per lb. of dry air)). The CO 2 concentration is based on the displayed value which is zero corrected (humidity compensated) for the relative humidity change from the original incubator setup humidity level. A verification of the actual CO 2 concentration reveals that the level has remained at 5% of the incubator's environment. While I have shown and described several embodiments in accordance with my invention, it is to be clearly understood that the same are susceptible to numerous changes apparent to one skilled in the art. Therefore, I do not wish to be limited to the details shown and described but intend to show all changes and modifications which come within the scope of the appended claims.

Claims: A humidity compensated carbon dioxide gas detection and control system for use in a substantially saturated or non-saturated gaseous environment which comprises: a humidity sensor means (1) capable of providing a humidity level output; a thermal conductivity sensor means (2) capable of providing a carbon dioxide concentration output; means for calculating the difference between said humidity level output and a stored humidity level detected when the zero reference of the thermal conductivity sensor means was originally set or calibrated to generate a humidity-compensated carbon dioxide concentration output, said means comprising a microprocessor (34) responsive to said difference to offset a CO 2 gas control system zero reference point (36) corresponding to said stored humidity level, said microprocessor using a look-up table comprising specific zero reference point offset values. The system according to claim 1, further comprising a means (41) for controlling the flow of carbon dioxide gas into said environment in accordance with said humidity compensated carbon dioxide gas concentration output. The system according to claim 1 or 2, further comprising means (3) for measuring the temperature of said environment. The system according to claim 3, further comprising means for adjusting said humidity level output in accordance with said temperature. A process for measuring and controlling carbon dioxide gas in a substantially saturated or non-saturated environment which comprises: measuring the humidity level of said environment; measuring the carbon dioxide concentration of said environment; calculating the difference between said humidity level and a stored humidity level detected when the zero reference of the thermal conductivity sensor means was originally set or calibrated, and in response to said difference, generating in a microprocessor (34) including a look-up table (38), a humidity-compensated carbon dioxide concentration output by offsetting a CO 2 gas control system zero reference point (36) corresponding to said stored humidity level, said look-up table comprising specific values used to offset said zero reference point (36). The process according to claim 5, further comprising the step of controlling the flow of said carbon dioxide gas into said environment in accordance with said humidity compensated carbon dioxide gas concentration measurement. The process according to claim 5 or claim 6, further comprising the step of measuring the temperature of said environment. The process according to claim 7, further comprising a means for adjusting said humidity level output in accordance with said temperature.

Inventors: BUTTS CHARLES G, BUTTS, CHARLES G.
Applicants: GEN SIGNAL CORP, GENERAL SIGNAL CORPORATION
Application year: 1995
Main classification: G01N25
IPC codes: G01N25, C12M3, C12M1