Abstract:
A method of stabilizing temperature effects on a gas analyzer includes transmitting infrared energy through a plurality of cells in a reference cell drum to a sensor in the gas analyzer. The plurality of cells include at least one cell containing a reference gas and at least one cell containing an air reference. The method also includes interposing a filter in at least one of the plurality of cells in the reference cell drum, wherein energy from the infrared beam is reduced. The method further includes rotating the reference cell drum such that the infrared energy fully passes through each of the plurality of cells for a limited period of time such that the sensor generates a substantial sine wave signal.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates generally to the field of gas analyzers. More particularly, the present invention relates to a technique for optical stabilization of temperature effects on an infrared gas analyzer. 
     Infrared (IR) gas analyzers typically include an infrared energy source and an infrared sensor. Disposed between the infrared energy source and the infrared sensor is a cell containing a gas mixture to be analyzed. A filter is generally carried on a rotatable wheel such that multiple filters can be rotated into position between the infrared energy source and infrared sensor. Infrared energy passes through the sample and is reduced by the presence of any gas that absorbs the infrared energy. Selectively imposing filters in the path of the beam of infrared energy changes the wavelength band of the infrared energy. Typically, each filter passes only radiation at a narrow band corresponding to a characteristic absorption wavelength band of a particular gas of interest. As such, through use of an infrared filter that is selected for each gas to be monitored in the gas sample, only infrared energy that can be absorbed by that gas is allowed to pass through the filter to be detected by the sensor. 
     In most IR gas analyzers, there is the need for some form of calibrating reference for the sensor. Conventional calibrating references include chopped dual beam, rotating filters or any other method that produces a varying optical energy signal on the sensor. FIG. 1 shows a graph  5  depicting a typical electrical output of a sensor in a conventional IR gas analyzer, where the low sensor data points (located at the −1 level) represent the voltage measured by the sensor when the filter wheel or chopper is positioned at a dark time (i.e., when all of the IR energy from the IR source is blocked such that the sensor is dark). All measurements are referenced to this point. There is also a reference gas or reference filter that represents a constant attenuation of the IR source energy. These smaller peaks (represented by the 0.6 levels on graph  5 ), allow for the calibration of the larger peaks that have been (or will be) attenuated by the unknown gas to be measured. By subtracting the dark time from these two peak readings and taking the ratio of the known reference peak to the unknown gas peak, conventional gas analyzers obtain an absolute reading that is representative of the gas amount being measured. 
     FIG. 1 also shows the sensor signal square wave or “flat top” output. This output is typical of a dual path chopped or negative gas filter type gas analyzer. When the chopper or filter is in alignment with the IR source and sensor, the energy on the sensor is maximum and remains there until the chopper or filter goes out of alignment. The impingement of the maximum energy on the sensor during the alignment period causes the flat top on the signal wave form. This square wave in combination with the difference in peak levels with respect to the same level of the dark time means that, in order to maintain the electronic signal without distortion, the buffer amplifier must pass all frequency from DC to the 3  rd , 5 th  and possibly higher (e.g., 7 th ) harmonic of the fundamental. Thus, filtering the sensor noise now requires a balance in distortion and noise. 
     Ideally, in order to use conventional methods for determining gas amounts, the sensor should be linear. In practice, however, the best infrared sensors (in terms of cost versus speed versus sensitivity) are nonlinear. Nonlinear sensors used in gas analyzers are error prone in relative measurements due to the changing temperature of the sampling environment. To reduce the errors caused by the changing temperature and complex data signals, IR type gas analyzers typically use expensive sensor coolers and sampling environment heaters to reduce the effects of sensor data error. 
     Thus, there is a need to reduce gas reading errors in gas analyzers without using expensive heaters and coolers that are slow to come to temperature. Further, there is a need to use non-linear sensors in gas analyzers without the data measurement errors which result from changes in environmental temperature. Even further, there is a need to stabilize measurements which are typically unstable by simplifying the sensor data wave signal. 
     BRIEF SUMMARY OF THE INVENTION 
     One embodiment of the invention relates to a method of stabilizing temperature effects on a gas analyzer. The method includes transmitting infrared energy through a plurality of cells in a reference cell drum to a sensor in the gas analyzer. The plurality of cells include at least one cell containing a reference gas and at least one cell containing an air reference. The method also includes interposing a filter between the bulb and the sensor in at least one of the plurality of cells in the reference cell drum, wherein energy from the infrared beam is reduced. The method further includes rotating the reference cell drum such that the beam of infrared energy fully passes through each of the plurality of cells for a limited period of time such that the sensor generates a substantial sine wave signal. 
     Another embodiment of the invention relates to a gas analyzer including a source which emits infrared energy, a sensor which detects infrared energy emitted from the source, a reference cell drum interposed between the source and the sensor, and at least one filter. The reference cell drum includes a plurality of cells through which the infrared energy passes from the source to the sensor. The plurality of cells includes at least one cell containing a reference gas and at least one cell containing an air reference. The reference cell drum is configured to rotate such that the infrared energy fully passes through each of the plurality of cells for a limited period of time such that the sensor generates a substantial sine wave signal. The at least one filter is located within the at least one cell containing an air reference. The at least one filter provides broad spectrum attenuation to reduce the infrared energy passing through the at least one cell containing an air reference. 
     Another embodiment of the invention relates to a gas analyzer which stabilizes temperature effects without using heaters and coolers. The gas analyzer includes means for transmitting infrared energy through a plurality of cells in a reference cell drum to a sensor. The cells include at least one cell containing a reference gas and at least one cell containing an air reference. The gas analyzer also includes means for filtering infrared energy passing through the at least one cell in the reference cell drum containing an air reference and means for rotating the reference cell drum such that the infrared energy fully passes through each of the plurality of cells for a limited period of time such that the sensor generates a substantial sine wave signal. 
     Other principle features and advantages of the present invention will become apparent to those skilled in the art upon review of the following drawings, the detailed description, and the appended claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will become more fully understood from the following detailed description, taken in conjunction with the accompanying drawings, wherein like reference numerals denote like elements, in which: 
     FIG. 1 is a graph illustrating sensor energy to voltage transforms in a sensor of a conventional gas analyzer; 
     FIG. 2 is a perspective, exploded view of a portion of a gas analyzer including a reference cell drum, an infrared source, and an infrared sensor; 
     FIG. 3 is a bottom view of the portion of the gas analyzer of FIG. 2 which depicts a series of optical transmittance path rings superimposed on the reference cell drum; 
     FIG. 4 is a graph illustrating sensor energy to voltage transforms of the sensor of the gas analyzer of FIG. 2; 
     FIG. 5 is a graph of a transfer function of a theoretically perfect sensor and an actual sensor; and 
     FIG. 6 is a graph of sensor error versus temperature for the transfer function of FIG.  5 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to the drawings and first to FIG. 2, a portion  10  of a gas analyzer includes a reference cell drum  12 , a reflector body  14 , a bulb  16 , and a sensor  18 . Reference cell drum  12  includes cells  20 ,  22 ,  24 , and  26 . Cells  20  and  24  contain a reference gas. Cells  22  and  26  contain air and include a filter  28 . In a preferred embodiment, filter  28  is a neutral density optical attenuator. 
     Reflector body  14  is a cylindrical component housing bulb  16  and having one open end. The interior of reflector body  14  includes tapered walls which are reflective to aid in directing light from bulb  16  out the open end of reflector body  14 . Bulb  16  is an infrared energy source, such as, a 5v flashlight bulb which emits a beam of infrared energy. Sensor  18  is any of a variety of gas analyzer sensors and is positioned on one side of reference cell drum  12  opposite reflector body  14 . As depicted in FIG. 2, reference cell  24  is currently located between reflector body  14  and sensor  18 . However, in operation, reference cell drum  12  is rotatable such that cells  20 ,  22 ,  24 , and  26  pass one after the other between reflector body  14  and sensor  18  as drum  12  rotates. Infrared energy from bulb  16  passes through each of cells  20 ,  22 ,  24 , and  26  as reference cell drum  12  rotates. The full spectrum (that the sensor filters can pass) of the infrared source energy will pass through reference cells  20  and  24  and impinge on sensor  18  when cells  20  and  24  are aligned with sensor  18 . 
     FIG. 3 illustrates a bottom view of portion  10  including five (5) circles  30  which are not part of portion  10  but depict an exemplary path of optical sensor  18  relative to reference cell drum  12  as the drum rotates. Circles  30  also illustrate the path that the infrared beam from bulb  16  will travel across reference cell drum  12 . One circle at the top of FIG. 3 is completely aligned or “full on” with cell  20 , which contains a reference gas, the next circle in the clockwise direction shows the sensor only halfway aligned “half on” cell  20 , and the next circle in the clockwise direction is just out of line with or “full off” cell  20 . This third circle, where the sensor  18  is completely out of line with any of cells  20 ,  22 ,  24 , and  26 , represents the “dark time” of the sensor. The cycle repeats in reverse to the last circle that is back to complete alignment or “full on” with cell  22 , which contains air and filter  28 . As the drum rotates at a constant speed, the “full on” time and the “full off” time does not last long. As such, the time that the full infrared beam is detected by sensor  18  is limited, thereby removing the flat tops of the detected signal wave and generating a near sine wave. Ideally, a perfect sine wave would be preferred but, for ease of manufacture and lower cost, an offset beam through a circle (as shown in FIG. 3) is used to simplify the sensor data wave signal. 
     As reference cell drum  12  turns from a gas reference (e.g., cell  20 ) to an air reference (e.g., cell  22 ), the energy on sensor  18  remains near constant from one peak to the next. This constant energy is accomplished by the use of a neutral density filter, filter  28 , on the air reference cell (e.g., cell  22 ). Filter  28  acts as a broad spectrum attenuator, reducing total energy. If the reference gas and neutral density filter are matched, the end result is that the peaks of the signal sensed by sensor  18  will all have the same amplitude, as illustrated by a graph  45  in FIG. 4. A comparison between the signal peaks of graph  45  versus the signal peaks of graph  5  shows that the flat tops of the peaks have been substantially eliminated. A perfect match cannot be accomplished over the range of sample gas to be measured, but getting close makes removal of the remaining error a smaller and more straight forward correction. 
     To illustrate the operability of portion  10  in optical stabilization of temperature effects on the IR gas analyzer, a linear (linear_y i :=·i 0.96) and nonlinear (Nonlinear_y i :=i·0.99) function for a sensor transfer function are compared. Using the linear function as a measurement of perfection, the following will show the errors that can be caused by temperature changes. FIG. 5 illustrates a graph  55  of the linear and non-linear functions. Notably, there is little difference in the two slopes. Nonetheless, this small amount of imperfection can cause a significant error due to changing temperature. Thus, even the best sensor can be improved by the optical method of the preferred embodiment. 
     Using these two sensor transform functions (linear and nonlinear) it can be seen that for a linear transform function, with a ten (10) unit change in temperature, it can be compared with the same temperature change at any other temperature offset over the full range of temperatures and there is no difference in the two signals. In other words, a ten (10) unit change in temperature is ten (10) units, regardless of the temperature offset. Thus, for example, regardless of whether the temperature changes from 20 to 30 units, or from 380 to 390 units, the OK_RATIO difference (defined below) is still zero because the temperature change (i.e., 10 units) is the same in both cases.                  OK   —          Ratio   x       :=           linear   —          y   x       -       linear   —          y     X   +   10                 linear   —          y     X   +   offset         -       linear   —          y     X   +   10   +   offset                     (   1   )                     OK   —          Ratio   1       -       OK   —          Ratio   200         =   0           (   2   )                                
     If, however, the nonlinear transfer function is used, the difference over the temperature range is 1.8%. In other words, a ten (10) unit change in temperature can look like a 10.18 unit change in temperature at a different device temperature.                Ratio   x     =           Nonlinear   —          y   x       -     Nonlinear     y     x   +   10                 Nonlinear   —          y     X   +   offset         -       Nonlinear   —          y     X   +   10   +   offset                     (   3   )                   Ratio   1     -     Ratio   200       =     1.827   ·   %             (   4   )                                
     Now, if the method disclosed herein for using an attenuator (e.g., filter  28  ) in the optical path is used, this will reduce the energy to the sensor&#39;s larger peak data and reduce the 1.8% error down to near zero.                  FIX   —          Ratio   x       :=           Nonlinear   —          y     x   +   attenuate         -       Nonlinear   —          y     x   +   10   +   attenuate                 Nonlinear   —          y     x   +   offset         -       Nonlinear   —          y     x   +   10   +   offset                     (   5   )                     FIX   —          Ratio   1       -       FIX   —          Ratio   200         =       -   0.02332     ·   %             (   6   )                                
     FIG. 6 illustrates a graph  65  showing the full temperature range errors for the equations above. A line  70  represents Ratio x , a line  74  represents OK_Ratio x , and a line  78  represents FIX_Ratio x . Most of the 1.8% error (shown by line  70 ) could have been calibrated out of the temperature effects using a heater at a high temperature where the error slop is less. However, the warm-up time would be very long to get to that temperature. 
     Line  74 , at the level  1  amount, shows ideal response over the full temperature range and line  78  (almost on top of line  74  ) is the sensor reference gas reading with correction as described herein. Although the correction is not perfect, it is much closer to ideal response then line  70 . Advantageously, portion  10  provides reduced gas reading errors over temperature by reducing the sensor&#39;s dynamic range requirement. Further, there is reduced sensor noise output by reducing sensor output signal frequency response requirements. The method and apparatus used herein may be used, for example, in a CO 2  gas analyzer. In a prototype of such an analyzer, the method according to the preferred embodiment has been tested and performance exceeds the performance of a commercial unit with heaters by a factor of three (3) in stability without needing warm-up to meet the commercial unit specifications. As one of skill in the art would understand, the disclosed method may also be used in analyzers for other gases, such as an anesthetic gas analyzer. 
     While the embodiments illustrated in the FIGURES and described above are presently preferred, it should be understood that these embodiments are offered by way of example only. Other embodiments may include, for example, different types and variations of gas analyzers. The invention is not limited to a particular embodiment but extends to various modifications, combinations, and permutations that nevertheless fall within the scope and spirit of the appended claims.