Abstract:
A compensated gas detector incorporates first and second pellistors combined with first and second resistors and an intervening switch. Control circuits can close the switch for normal gas detection. The switch can be opened to carry out diagnostic measurements. A compensation coefficient can be established in order to compensate outs from the pellistors due to mechanical damage thereto.

Description:
FIELD 
       [0001]    The application pertains to gas detectors which incorporate catalytic bead sensors. More particularly, detectors which can compensate outputs from such sensors in response to mechanical damage to the beads, or their mounting structures. 
       BACKGROUND 
       [0002]    Gas detectors which incorporate catalytic bead sensors are widely used to measure combustible gas. Such sensors can be incorporated into a Wheatstone bridge with one sensor providing a sensing function and a second acting as a reference. However, while useful, such circuit configurations are sensitive to mechanical impact. This is especially the case when bead sensors are incorporated into portable gas detectors. When a user drops a detector, the electro/mechanical structure of the Wheatstone bridge can be broken. Outputs from the bridge can be floating even without a gas reaction. This can be a problem when working in an explosive atmosphere. 
         [0003]      FIG. 1  illustrates aspects of electrical connections and mechanical arrangements of known multiple bead sensing units. 
         [0004]    It has been known to use a relatively simple zero compensation technique with bead sensors to make zero gas readings without knowing the reason why the zero gas reading is floating. This process has limited compensation ability and it is undesirable, with explosive gases, to permit zeroing across a wide range. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0005]      FIG. 1  illustrates a prior art sensor configuration; 
           [0006]      FIG. 2  is a diagram of a sensor configuration in accordance herewith; 
           [0007]      FIG. 3  is a flow diagram illustrating a method of operating the sensor configuration of  FIG. 2 ; 
           [0008]      FIG. 4  is a diagram of a gas detector in accordance herewith; and 
           [0009]      FIGS. 5A ,  5 B illustrate aspects of a compensating methodology. 
       
    
    
     DETAILED DESCRIPTION 
       [0010]    While disclosed embodiments can take many different forms, specific embodiments thereof are shown in the drawings and will be described herein in detail with the understanding that the present disclosure is to be considered as an exemplification of the principles thereof as well as the best mode of practicing same, and is not intended to limit the application or claims to the specific embodiment illustrated. 
         [0011]    In one aspect of the present embodiments, resistance values of a reference and an active, gas responsive, bead can be measured independently. The resistance of the active bead can be changed by both gas reactions and external impacts. The resistance of the reference bead is changed by external impacts or environmental reactions, for example poisoning and degradation of a reference bead. 
         [0012]    The consequences of external impacts can be detected by determining if the value of the resistance of the reference bead has changed. By measuring the current which flows through the beads and the voltages across each bead, distorted sensor outputs, due to external impacts, can be detected and compensated. 
         [0013]    In another aspect, detectors which incorporate catalytic bead sensors, and associated reference bead sensors, can in a diagnostic mode, verify the condition of the beads by evaluating reference bead resistance changes due to mechanical impact, or poisoning. If incorporated into a Wheatstone bridge-type of electrical structure, the beads can be switched between an operational state and a diagnostic state to determine bead resistance. 
         [0014]    A present reference bead resistance value can be compared to a prior, pre-stored value to determine if a change has taken place. Where a resistance change indicative of a mechanical or electrical problem is present, the detector can carry out a compensation process. When compensated, the detector can continue to be used even in the presence of explosive gases. 
         [0015]      FIG. 2  illustrates aspects of a compensatable, multi-state, multi-bead sensor/reference unit  10 . Unit  10  incorporates a catalytic bead sensor module  12  which mechanically carries a gas responsive, active bead, pellistor,  12   a  and a reference bead, pellistor,  12   b . A controllable switch SW 1  is provided between active bead  12   a  and two additional fixed resistors R 1 , R 2  which complete a bridge-type structure in combination with beads/resistors  12   a,b  when switch SW 1  is closed. When switch SW 1  is open, the branches  12   a,b  and R 1 , R 2  can operate independently. 
         [0016]    Switch SW 1  is normally closed when unit  10  is measuring gas and is open, off, when checking bead resistance. When SW 1  is open, off, the voltage Vch is divided by the resistance of bead  12   a , the resistance of bead  12   b  and a resistance Rcur to ground. In this state, sensor current I can be calculated using the voltage across Rcur, which can be expected to have a value on the order of one ohm. The resistance value of the reference bead  12   b  and the active bead  12   a  can be calculated by measuring the voltage across the beads and dividing by sensor current I. 
         [0017]      FIG. 3  illustrates aspects of a method  100  of operating a detector  10 - 1 , best seen in  FIG. 4 , discussed below. With SW 1  open, off, initial bead resistance determination can be carried out and the associated values saved, stored, as at  102 . Switch SW 1  can then be closed, turned on, to read the ambient gas concentration as at  104 . 
         [0018]    A determination can be made, as at  106 , as to whether an allowable zero gas concentration range has been exceeded. If not, gas concentration reading continues, as at  104 . Otherwise, as at  108 , switch SW 1  is opened, off, and current bead resistance is compared to one or more previously determined and stored bead resistance values. At  110  a determination is made as to whether reference bead resistance has changed, if not, a gas concentration can be displayed, as at  112 . If a resistance change has been detected, switch SW 1  is closed, on, and a compensation process is carried out. IA zero concentration setting can be displayed as at  116 . 
         [0019]      FIG. 4  illustrates a detector  10 - 1  in accordance herewith. Previously discussed elements have been assigned the same identification numerals as above, and need not be discussed further. 
         [0020]    Detector  10 - 1  can include a portable housing  20 , which might be attachable to clothing of a user. Detector  10 - 1  can be energized by a self-contained battery V as would be understood by those of skill in the art. The housing  20  can also carry control circuits  22 . 
         [0021]    Control circuits  22  can include a user I/O port  22   a , which might include manually operable inputs and one or more outputs, for example a two dimensional numeric gas concentration display  22   a - 1 . Control circuits  22  can also be coupled to an audio/visual alarm indicating output device  22   b , also carried by housing  20 . 
         [0022]    Control circuits  22  can include a switch, such as SW 1 , implemented as a semiconductor switch in turn coupled into the beads  12   a, b  (module  12 ), and resistors R 1 , R 2  as well as Rcur to implement the method  100  discussed previously. Control circuits can, at least in part, be implemented with a programmable processor  22   c , and pre-stored control software  22   d . A data storage unit  22   e  can be used to store resistance values for the beads  12   a,b  measured on an on-going basis to implement the process  100  of  FIG. 3  or variations thereof without limitation. 
         [0023]      FIGS. 5A , B illustrate aspects of measurements and resistance determinations in establishing a compensation parameter, at a particular time, to be used by control circuits  22  in determining compensated concentration values which take into account resistance impact caused changes in the resistance value of reference bead  12   b.    
         [0024]    With reference to  FIG. 5A , gas concentration C, prior to any impact, can be determined by making the following measurements: 
         [0000]    
       
         
           
             
               
                 
                   
                     I 
                     D 
                   
                   = 
                   
                     
                       
                         2 
                          
                         
                             
                         
                          
                         
                           R 
                           b 
                         
                       
                       
                         
                           R 
                           ref 
                         
                         + 
                         
                           R 
                           Det 
                         
                         + 
                         
                           2 
                            
                           
                               
                           
                            
                           
                             R 
                             b 
                           
                         
                       
                     
                      
                     I 
                   
                 
               
               
                 
                   
                     I 
                     R 
                   
                   = 
                   
                     
                       
                         
                           R 
                           ref 
                         
                         + 
                         
                           R 
                           Det 
                         
                       
                       
                         
                           R 
                           ref 
                         
                         + 
                         
                           R 
                           Det 
                         
                         + 
                         
                           2 
                            
                           
                               
                           
                            
                           
                             R 
                             b 
                           
                         
                       
                     
                      
                     I 
                   
                 
               
             
             
               
                 
                   
                     V 
                     
                       R 
                       Det 
                     
                   
                   = 
                   
                     
                       
                         2 
                          
                         
                             
                         
                          
                         
                           R 
                           b 
                         
                          
                         
                           R 
                           Det 
                         
                       
                       
                         
                           R 
                           ref 
                         
                         + 
                         
                           R 
                           Det 
                         
                         + 
                         
                           2 
                            
                           
                               
                           
                            
                           
                             R 
                             b 
                           
                         
                       
                     
                      
                     I 
                   
                 
               
               
                 
                   
                     V 
                     
                       R 
                       b 
                     
                   
                   = 
                   
                     
                       
                         
                           ( 
                           
                             
                               R 
                               ref 
                             
                             + 
                             
                               R 
                               Det 
                             
                           
                           ) 
                         
                          
                         
                           R 
                           b 
                         
                       
                       
                         
                           R 
                           ref 
                         
                         + 
                         
                           R 
                           Det 
                         
                         + 
                         
                           2 
                            
                           
                               
                           
                            
                           
                             R 
                             b 
                           
                         
                       
                     
                      
                     I 
                   
                 
               
             
           
         
       
     
         [0025]    After having made the above measurements, gas concentration can be established by: 
         [0000]    
       
         
           
             C 
             = 
             
               
                 
                   V 
                   Rb 
                 
                 - 
                 
                   V 
                   RDet 
                 
               
               = 
               
                 
                   
                     
                       ( 
                       
                         
                           R 
                           ref 
                         
                         - 
                         
                           R 
                           Det 
                         
                       
                       ) 
                     
                      
                     
                       R 
                       b 
                     
                   
                   
                     
                       R 
                       ref 
                     
                     + 
                     
                       R 
                       Det 
                     
                     + 
                     
                       2 
                        
                       
                           
                       
                        
                       
                         R 
                         b 
                       
                     
                   
                 
                  
                 I 
               
             
           
         
       
     
         [0026]    With reference to  FIG. 5B , a measured value of concentration C′, after impact can be determined as follows: 
         [0000]    
       
         
           
             
               I 
               D 
               ′ 
             
             = 
             
               
                 
                   2 
                    
                   
                       
                   
                    
                   
                     R 
                     b 
                   
                 
                 
                   
                     R 
                     ref 
                   
                   + 
                   
                     R 
                     Det 
                   
                   + 
                   
                     2 
                      
                     
                         
                     
                      
                     
                       R 
                       b 
                     
                   
                   + 
                   
                     Δ 
                      
                     
                         
                     
                      
                     
                       R 
                       ref 
                     
                   
                   + 
                   
                     Δ 
                      
                     
                         
                     
                      
                     
                       R 
                       Det 
                     
                   
                 
               
                
               I 
             
           
         
       
       
         
           
             
               I 
               R 
               ′ 
             
             = 
             
               
                 
                   
                     R 
                     ref 
                   
                   + 
                   
                     R 
                     Det 
                   
                   + 
                   
                     Δ 
                      
                     
                         
                     
                      
                     
                       R 
                       ref 
                     
                   
                   + 
                   
                     Δ 
                      
                     
                         
                     
                      
                     
                       R 
                       Det 
                     
                   
                 
                 
                   
                     R 
                     ref 
                   
                   + 
                   
                     R 
                     Det 
                   
                   + 
                   
                     2 
                      
                     
                         
                     
                      
                     
                       R 
                       b 
                     
                   
                   + 
                   
                     Δ 
                      
                     
                         
                     
                      
                     
                       R 
                       ref 
                     
                   
                   + 
                   
                     Δ 
                      
                     
                         
                     
                      
                     
                       R 
                       Det 
                     
                   
                 
               
                
               I 
             
           
         
       
       
         
           
             
               V 
               
                 R 
                 Det 
               
               ′ 
             
             = 
             
               
                 
                   
                     2 
                      
                     
                         
                     
                      
                     
                       R 
                       b 
                     
                      
                     
                       R 
                       Det 
                     
                   
                   + 
                   
                     2 
                      
                     
                         
                     
                      
                     
                       R 
                       b 
                     
                      
                     Δ 
                      
                     
                         
                     
                      
                     
                       R 
                       Det 
                     
                   
                 
                 
                   
                     R 
                     ref 
                   
                   + 
                   
                     R 
                     Det 
                   
                   + 
                   
                     2 
                      
                     
                         
                     
                      
                     
                       R 
                       b 
                     
                   
                   + 
                   
                     Δ 
                      
                     
                         
                     
                      
                     
                       R 
                       ref 
                     
                   
                   + 
                   
                     Δ 
                      
                     
                         
                     
                      
                     
                       R 
                       Det 
                     
                   
                 
               
                
               I 
             
           
         
       
       
         
           
             
               V 
               
                 R 
                 b 
               
               ′ 
             
             = 
             
               
                 
                   
                     ( 
                     
                       
                         R 
                         ref 
                       
                       + 
                       
                         R 
                         Det 
                       
                       + 
                       
                         Δ 
                          
                         
                             
                         
                          
                         
                           R 
                           ref 
                         
                       
                       + 
                       
                         Δ 
                          
                         
                             
                         
                          
                         
                           R 
                           Det 
                         
                       
                     
                     ) 
                   
                    
                   
                     R 
                     b 
                   
                 
                 
                   
                     R 
                     ref 
                   
                   + 
                   
                     R 
                     Det 
                   
                   + 
                   
                     2 
                      
                     
                         
                     
                      
                     
                       R 
                       b 
                     
                   
                   + 
                   
                     Δ 
                      
                     
                         
                     
                      
                     
                       R 
                       ref 
                     
                   
                   + 
                   
                     Δ 
                      
                     
                         
                     
                      
                     
                       R 
                       Det 
                     
                   
                 
               
                
               I 
             
           
         
       
     
         [0027]    Having made the above measurements, concentration after impact, C′, can be established by: 
         [0000]    
       
         
           
             
               C 
               ′ 
             
             = 
             
               
                 
                   V 
                   Rb 
                   ′ 
                 
                 - 
                 
                   V 
                   RDet 
                   ′ 
                 
               
               = 
               
                 
                   
                     
                       ( 
                       
                         
                           R 
                           ref 
                         
                         + 
                         
                           Δ 
                            
                           
                               
                           
                            
                           
                             R 
                             ref 
                           
                         
                         - 
                         
                           R 
                           Det 
                         
                         - 
                         
                           Δ 
                            
                           
                               
                           
                            
                           
                             R 
                             Det 
                           
                         
                       
                       ) 
                     
                      
                     
                       R 
                       b 
                     
                   
                   
                     
                       R 
                       ref 
                     
                     + 
                     
                       R 
                       Det 
                     
                     + 
                     
                       2 
                        
                       
                           
                       
                        
                       
                         R 
                         b 
                       
                     
                     + 
                     
                       Δ 
                        
                       
                           
                       
                        
                       
                         R 
                         ref 
                       
                     
                     + 
                     
                       Δ 
                        
                       
                           
                       
                        
                       
                         R 
                         Det 
                       
                     
                   
                 
                  
                 I 
               
             
           
         
       
     
         [0028]    A compensation parameter can be determined by: 
         [0000]    
       
         
           
             Compensation 
             = 
             
               
                 
                   
                     
                       
                         
                           ( 
                           
                             
                               R 
                               ref 
                             
                             + 
                             
                               R 
                               Det 
                             
                             + 
                             
                               2 
                                
                               
                                   
                               
                                
                               
                                 R 
                                 b 
                               
                             
                           
                           ) 
                         
                          
                         
                           
                             R 
                             b 
                           
                            
                           
                             ( 
                             
                               
                                 Δ 
                                  
                                 
                                     
                                 
                                  
                                 
                                   R 
                                   ref 
                                 
                               
                               - 
                               
                                 Δ 
                                  
                                 
                                     
                                 
                                  
                                 
                                   R 
                                   Det 
                                 
                               
                             
                             ) 
                           
                         
                       
                       - 
                     
                   
                 
                 
                   
                     
                       
                         ( 
                         
                           
                             R 
                             ref 
                           
                           - 
                           
                             R 
                             Det 
                           
                         
                         ) 
                       
                        
                       
                         
                           R 
                           b 
                         
                          
                         
                           ( 
                           
                             
                               Δ 
                                
                               
                                   
                               
                                
                               
                                 R 
                                 ref 
                               
                             
                             + 
                             
                               Δ 
                                
                               
                                   
                               
                                
                               
                                 R 
                                 Det 
                               
                             
                           
                           ) 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   
                     
                       R 
                       ref 
                     
                     + 
                     
                       R 
                       Det 
                     
                     + 
                     
                       2 
                        
                       
                           
                       
                        
                       
                         R 
                         b 
                       
                     
                   
                   ) 
                 
                  
                 
                   ( 
                   
                     
                       R 
                       ref 
                     
                     + 
                     
                       R 
                       Det 
                     
                     + 
                     
                       2 
                        
                       
                           
                       
                        
                       
                         R 
                         b 
                       
                     
                     + 
                     
                       Δ 
                        
                       
                           
                       
                        
                       
                         R 
                         ref 
                       
                     
                     + 
                     
                       Δ 
                        
                       
                           
                       
                        
                       
                         R 
                         Det 
                       
                     
                   
                   ) 
                 
               
             
           
         
       
     
         [0029]    Compensated concentration C then corresponds to: 
         [0000]    
       
         
           
             C 
             = 
             
               
                 C 
                 ′ 
               
               - 
               
                 
                   
                     
                       
                         
                           
                             
                               ( 
                               
                                 
                                   R 
                                   ref 
                                 
                                 + 
                                 
                                   R 
                                   Det 
                                 
                                 + 
                                 
                                   2 
                                    
                                   
                                       
                                   
                                    
                                   
                                     R 
                                     b 
                                   
                                 
                               
                               ) 
                             
                              
                             
                               
                                 R 
                                 b 
                               
                                
                               
                                 ( 
                                 
                                   
                                     Δ 
                                      
                                     
                                         
                                     
                                      
                                     
                                       R 
                                       ref 
                                     
                                   
                                   - 
                                   
                                     Δ 
                                      
                                     
                                         
                                     
                                      
                                     
                                       R 
                                       Det 
                                     
                                   
                                 
                                 ) 
                               
                             
                           
                           - 
                         
                       
                     
                     
                       
                         
                           
                             ( 
                             
                               
                                 R 
                                 ref 
                               
                               - 
                               
                                 R 
                                 Det 
                               
                             
                             ) 
                           
                            
                           
                             
                               R 
                               b 
                             
                              
                             
                               ( 
                               
                                 
                                   Δ 
                                    
                                   
                                       
                                   
                                    
                                   
                                     R 
                                     ref 
                                   
                                 
                                 + 
                                 
                                   Δ 
                                    
                                   
                                       
                                   
                                    
                                   
                                     R 
                                     Det 
                                   
                                 
                               
                               ) 
                             
                           
                         
                       
                     
                   
                   
                     
                       ( 
                       
                         
                           R 
                           ref 
                         
                         + 
                         
                           R 
                           Det 
                         
                         + 
                         
                           2 
                            
                           
                               
                           
                            
                           
                             R 
                             b 
                           
                         
                       
                       ) 
                     
                      
                     
                       ( 
                       
                         
                           R 
                           ref 
                         
                         + 
                         
                           R 
                           Det 
                         
                         + 
                         
                           2 
                            
                           
                               
                           
                            
                           
                             R 
                             b 
                           
                         
                         + 
                         
                           Δ 
                            
                           
                               
                           
                            
                           
                             R 
                             ref 
                           
                         
                         + 
                         
                           Δ 
                            
                           
                               
                           
                            
                           
                             R 
                             Det 
                           
                         
                       
                       ) 
                     
                   
                 
                  
                 I 
               
             
           
         
       
     
         [0030]    The following illustrate exemplary values of parameters when the unit  10  is in different operational states. In Example 1, initial values of catalytic bead resistance are measured with SW 1  open, off, in a diagnostic state: 
         [0000]    
       
         
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 Vdet (V) 
                 1.3661 
               
               
                   
                 Vdet + Vref (V) 
                 2.7339 
               
               
                   
                 Vref (V) 
                 1.3678 
               
               
                   
                 Vrsense (V) 
                 0.2 
               
               
                   
                 Rref (ohm) 
                 6.839 
               
               
                   
                 Rdet (ohm) 
                 6.8305 
               
               
                   
                 Output/1% LEL (V) 
                 0.000925 
               
               
                   
                 Rb (ohm) 
                 2000 
               
               
                   
                 Δrdet (ohm) 
                 0 
               
               
                   
                 Δrref (ohm) 
                 0 
               
               
                   
                 Zero Compensation Factor 
                 −0.9157893 
               
               
                   
                 Gas reading (% LEL) 
                 0 
               
               
                   
                   
               
             
          
         
       
     
       Example 1 
       [0031]    The initial values Rref, and Rdet, both in ohms, can be stored in storage unit  22   e.    
         [0032]    In Example 2, gas concentration can be measured without an alteration of resistance value of the reference bead  12   b . The measured values of Rref and Rdet are compared with previously stored values thereof. If Rref is substantially the same, unchanged, then gas concentration C can be determined, with SW 1  closed, on, in the measuring mode. 
         [0000]    
       
         
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 Vdet (V) 
                 1.3851 
               
               
                   
                 Vdet + Vref (V) 
                 2.7529 
               
               
                   
                 Vref (V) 
                 1.3678 
               
               
                   
                 Vrsense (V) 
                 0.2 
               
               
                   
                 Rref (ohm) 
                 6.839 
               
               
                   
                 Rdet (ohm) 
                 6.9255 
               
               
                   
                 Output/1% LEL (V) 
                 0.000925 
               
               
                   
                 Rb (ohm) 
                 2000 
               
               
                   
                 Δrdet (ohm) 
                 0.095 
               
               
                   
                 Δrref (ohm) 
                 0 
               
               
                   
                 Raw Gas value 
                 9.3192825 
               
               
                   
                 Gas reading (% LEL) 
                 10.235072 
               
               
                   
                   
               
             
          
         
       
     
       Example 2 
       [0033]    In Example 3, changes to the value of the resistance of reference bead  12   b  are measured and the compensation parameter is determined. In this regard, with SW 1  open, off, in the diagnostic mode, the measured values of Rref and Rdet are compared with pre-stored values. If Rref has changed, a value of the change in Reref is determined. The new Rref value is then stored in the storage unit  22   e . 
         [0000]    
       
         
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 Vdet (V) 
                 1.3661 
               
               
                   
                 Vdet + Vref (V) 
                 2.7471 
               
               
                   
                 Vref (V) 
                 1.381 
               
               
                   
                 Vrsense (V) 
                 0.2 
               
               
                   
                 Rref (ohm) 
                 6.905 
               
               
                   
                 Rdet (ohm) 
                 6.8305 
               
               
                   
                 Output/1% LEL (V) 
                 0.000925 
               
               
                   
                 Rb (ohm) 
                 2000 
               
               
                   
                 Δrdet (ohm) 
                 0 
               
               
                   
                 Δrref (ohm) 
                 0.066 
               
               
                   
                 Zero Compensation Factor 
                 −8.0264921 
               
               
                   
                 Gas reading (% LEL) 
                 0 
               
               
                   
                   
               
             
          
         
       
     
       Example 3 
       [0034]    In Example 4, gas concentration is measured with reference to an updated, changed value of resistance of the reference bead  12   b . In this regard, the measured values of Rref, Rdet with SW 1  closed, on, in the measuring mode are compared with the latest values or data stored in unit  22   e . If the value of Rref has not changed, gas concentration C can be determined with the following results. 
         [0000]    
       
         
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 Vdet (V) 
                 1.3783 
               
               
                   
                 Vdet + Vref (V) 
                 2.7593 
               
               
                   
                 Vref (V) 
                 1.381 
               
               
                   
                 Vrsense (V) 
                 0.2 
               
               
                   
                 Rref (ohm) 
                 6.905 
               
               
                   
                 Rdet (ohm) 
                 6.8915 
               
               
                   
                 Output/1% LEL (V) 
                 0.000925 
               
               
                   
                 Rb (ohm) 
                 2000 
               
               
                   
                 Δrdet (ohm) 
                 0.095 
               
               
                   
                 Δrref (ohm) 
                 0.066 
               
               
                   
                 Raw Gas value 
                 2.2085798 
               
               
                   
                 Gas reading (% LEL) 
                 10.235072 
               
               
                   
                   
               
             
          
         
       
     
       Example 4 
       [0035]    In summary, changes in the values of the resistance of the reference bead  12   b  of a gas detector can be determined. In a diagnostic mode, the detector can measure and then establish catalytic bead resistance values. Abnormal gas concentration values, caused by reference bead resistance value changes due to mechanical impact, poisoning or reference bead degradation can be compensated so that the user can continue to carry and rely on output indicia from the detector. 
         [0036]    From the foregoing, it will be observed that numerous variations and modifications may be effected without departing from the spirit and scope of the invention. It is to be understood that no limitation with respect to the specific apparatus illustrated herein is intended or should be inferred. It is, of course, intended to cover by the appended claims all such modifications as fall within the scope of the claims. Further, logic flows depicted in the figures do not require the particular order shown, or sequential order, to achieve desirable results. Other steps may be provided, or steps may be eliminated, from the described flows, and other components may be add to, or removed from the described embodiments.