Abstract:
The apparatus includes a mixing chamber for mixing gas from a gas supply to be tested with air to form a gas-air mixture. The mixing chamber includes a gas inlet port through which the gas is received, an air intake port through which the air is received, and an outlet port through which the gas-air mixture leaves the mixing chamber. The apparatus includes a flow control valve that can be connected to the gas supply to enable a user to selectively adjust the flow rate of the gas from the gas supply to the apparatus. The apparatus also includes a mass flow sensor positioned between the flow control valve and the gas inlet port of the mixing chamber for measuring the mass flow rate of the gas flowing into the mixing chamber. A motorized fan is positioned in a conduit connected to outlet port of the mixing chamber for drawing the gas-air mixture out of the mixing chamber to a location where it can be sniffed by the user to detect odor. The apparatus also includes a tachometer for measuring the rotational speed of the motorized fan and a microprocessor controller responsive to signals from the tachometer for controlling the rotational speed of the motorized fan such that it is maintained at a substantially constant speed. The apparatus can be linked to a personal computer to enable data to be exchanged between the personal computer and the apparatus.

Description:
This application is a continuation-in-part of prior application U.S. Ser. No. 09/121,512, filed Jul. 23, 1998, now U.S. Pat. No. 5,996,396. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Technical Field 
     The present invention relates generally to apparatus for determining odor levels in odorized gas streams. 
     2. Description of the Related Art 
     Federal and state regulations require that combustible gases (e.g., natural gas) in transmission lines be odorized to provide a quick and easy means for detecting the presence of gases. Odorization involves adding small amounts of a chemical with a distinctive odor to the gas stream. This odor facilitates leak detection and provides an early warning of potentially unsafe conditions. 
     The odor must be readily detectable by a person with a normal sense of smell when gas is present at a given concentration in air. The concentration is typically specified as a certain percentage of the lower explosive limit (LEL) of the gas. 
     Federal and state odorizing regulations require gas operators to conduct periodic sampling at distribution sites to assure proper odor levels. Portable instruments, sometimes referred to as odorometers, have been devised to determine gas odor levels using olfactory testing. These instruments are available from a number of manufacturers including Bacharach, Inc. and Heath Consultants, Inc. 
     In one known device, gas to be analyzed enters the instrument through a gas intake port. The gas passes through a low pressure regulator and then a flow adjustment valve, which is operated by a user. The gas then enters a sensor chamber, which measures gas flow. A signal from the sensor chamber is processed, converted to a digital signal and fed to a liquid crystal display (LCD). The gas from the sensor chamber is then mixed with air drawn in by an air blower. The air-gas mixture then exits the device through an exhaust port, where it can be sniffed by a user. 
     One test that is commonly performed with such a device is known as a threshold odorant test (TOT). This test is used to determine a threshold gas-air concentration at which odor is barely detectable. In this use, the operator slowly opens the flow adjustment valve, allowing the gas sample to enter the instrument, while at the same time breathing normally with his or her nose placed close to the exhaust port. When odor in the sample is detected, the operator depresses a read button to observe a reading on the LCD corresponding to gas flow. A look-up chart is then used to convert the reading into a percent gas concentration, which is then hand-written somewhere for recordation. 
     Another use of the device is to determine odor characteristics at given gas-air concentrations. In this use, a gas/air mixture at a preset concentration is sniffed by various persons who then categorize the odor level as “absent,” “barely detectable,” “readily detectable,” “strong” or “obnoxious.” 
     A significant problem with these known devices it that they fail to provide accurate and repeatable gas concentration readings. Except for the LCD output, the instruments are substantially analog devices and lack adequate control means to ensure steady operation. For instance, the fans or blowers used to draw air through the devices cannot be sufficiently controlled to ensure constant speed, which is critical in determining gas concentration. 
     Furthermore, the known devices fail to provide adequate mixing of the sample gas and air. Consequently, the gas-air mixtures are not uniformly mixed, resulting in testing inaccuracies. 
     In addition, known devices are not equipped with recording means for storing and/or analyzing test data. Consequently, users must hand-write the data somewhere and use a look-up table to cross-reference data with gas concentration values. This is a potential source for further errors. 
     BRIEF SUMMARY OF THE INVENTION 
     Thus, a primary object of the invention is to provide a device for determining odor levels in gas streams that produces highly accurate and repeatable results. 
     Another object of the invention is to provide a device for determining odor levels that is microprocessor based. 
     A further object of the invention is to provide a device for determining odor levels that thoroughly mixes the gas sample to be tested with air. 
     Yet another object of the invention is to provide a device that automatically records test data and is thereby substantially “paper-free.” 
     A further object of the invention is to provide a device that analyzes test data, obviating the need for look-up tables and data interpretation. 
     A further object of the invention is to provide a device that can download test data stored therein to a personal computer (PC) for analysis, report preparation and storage in a main database. 
     Still another object of the invention is to provide a device that can be linked to a printer to print data stored in the device. 
     Yet another object of the invention is to provide a device that can be uploaded with data such as test location and technician information from a PC. 
     These and other objects are accomplished by utilizing a microprocessor-based odorometer for correlating odor levels in gas streams with gas in air concentrations in accordance with the invention. The device includes a mixing chamber for mixing gas from a gas supply to be tested with air to form a gas-air mixture. The mixing chamber includes a gas inlet port through which the gas is received, an air intake port through which the air is received, and an outlet port through which the gas-air mixture exits the mixing chamber. A static mixing element is located at the air intake port to cause turbulence in incoming air flow to promote thorough mixing of the air and gas. The device includes a flow control valve that is connected to the gas supply to enable a user to selectively adjust the flow rate of the gas entering the device. A mass flow sensor is positioned between the flow control valve and the gas inlet port of the mixing chamber for accurately measuring the mass flow rate of the gas flowing into the mixing chamber. A motorized fan is positioned in a hose connected to outlet port of the mixing chamber for drawing the gas-air mixture out of the mixing chamber and moving it to a sniff chamber, where it can be sniffed by the user to detect odor. 
     The device is equipped with a microprocessor controller including a tachometer for determining the rotational speed of the motorized fan. The microprocessor controller is responsive to signals from a fan speed sensor for controlling the rotational speed of the motorized fan such that it is maintained at a substantially constant speed. Consequently, there is substantially constant air flow into the device. 
     A rechargeable lead-acid battery is provided for powering the instrument. 
     The device includes memory for storing test data. It is equipped with a keypad and an LCD display for user interface. 
     The device displays test results in actual air/gas percentages, eliminating the need for look-up charts. 
     The device records complete test results electronically. It can also record any comments by technicians relating to a given test such as, e.g., information on weather conditions at a test site. In addition, test records are stamped with date/time information as well as information on device calibration. 
     The apparatus can be linked to a PC to enable test and device data to be exchanged between the PC and the apparatus. A software package is provided for the PC to generate reports, graphs, and data archives from data read from the device. The device thus eliminates the need to manually write test reports. In addition, critical test data recorded by the device cannot be altered using the device. This feature substantially reduces the risk of test data being falsified by technicians. 
     Test data downloaded from the device to a PC can also be exported to third-party developed applications for further processing. 
     In addition, the software package allows user, test location and calibration information to be easily uploaded from the PC to the device. This feature eliminates the need for technicians to prepare forms and perform repetitive data entry. 
     The device also includes a user authentication feature. Each user is required to enter an identification and corresponding password before the device can be used. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more complete understanding of the present invention and the advantages thereof, reference should be made to the following Detailed Description taken in connection with the accompanying drawings, in which: 
     FIG. 1 is an exploded perspective view of a device for determining odor levels in accordance with the invention; 
     FIG. 2 is an exploded perspective view of the main unit assembly shown in the FIG. 1; 
     FIG. 3 is an exploded perspective view of the mounting panel assembly of the main unit assembly; 
     FIG. 4 is an exploded perspective view of the fan sub-assembly of the main unit assembly; 
     FIG. 5 is an exploded perspective view of the fan printed circuit board assembly of the fan sub-assembly; 
     FIG. 6 is a schematic diagram illustrating gas and air flow through the device; 
     FIG. 7 is a schematic block diagram illustrating microprocessor control of the device; 
     FIG. 8 is a high-level state diagram illustrating operation of the device; 
     FIG. 9 is a high-level reporter state diagram illustrating operation of PC software for analyzing test data from and for programming the device; 
     FIG. 10 is an exemplary screen shot generated by the PC software of a test log; 
     FIG. 11 is an exemplary screen shot generated by the PC software illustrating exporting of data to external applications; 
     FIG. 12 is an exemplary screen shot generated by the PC software illustrating data filtering; 
     FIG. 13 is an exemplary screen shot generated by the PC software of a graph of test readings at a given test location; 
     FIG. 14 is an exemplary screen shot generated by the PC software showing details of a given test conducted using the device; 
     FIG. 15 is an exemplary screen shot generated by the PC software illustrating pre-loading of user and test location data on the device; 
     FIG. 16 is an exemplary screen shot generated by the PC software showing details of a qualification test taken by a given user; and 
     FIG. 17 is an exemplary screen shot generated by the PC software of a graph of the results of multiple qualification tests taken by a given user. 
     FIG. 18 is a table illustrating a Relative Air Density Comparison for given elevations at given temperatures. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1 is an exploded view of an odorometer device or instrument  10  for correlating odor levels in gas streams with gas in air concentrations in accordance with the invention. The device  10  includes a portable outer housing or case  12 . The case  12  is equipped with a main unit  14  and a battery  16  for powering the unit. The battery  16  is preferably a rechargeable lead-acid battery. A charger port  18  is provided in the case  12  so that the battery  16  can be charged in situ. Use of a battery facilitates portability of the device, though it should be noted that the device can be configured to be AC powered if desired. 
     An inlet hose  20  is optionally included to facilitate coupling of the instrument with a gas supply or source (not shown in FIG. 1) to be tested. A variety of odorized gases can be tested including, e.g., natural gas and liquefied petroleum gas such as propane and butane. 
     FIG. 2 is an exploded view showing the main unit assembly  14  in greater detail. The main unit assembly  14  includes a mounting panel assembly  22 , a main printed circuit board (PCB) assembly  24 , a fan sub-assembly  26 , and a regulator assembly  28 . 
     The main PCB assembly  24  includes a processor board  30  containing the microprocessor circuitry for controlling and operating the instrument as will be described further below. The main PCB assembly  24  also includes a 4×20 character LCD display  32 . 
     The regulator assembly  28  comprises a low flow regulator  34  for regulating the pressure of gas from the gas supply. It also includes a flow control valve  36 , preferably a precision needle valve, that can be adjusted by a user to control the flow rate of gas from the gas supply. 
     FIG. 3 illustrates the mounting panel assembly  22  in greater detail. The mounting panel assembly  22  comprises a base panel  38  and an overlay panel  40 . The overlay panel  40  includes a membrane keyboard or keypad  42  having a 7×7 matrix. A connector cable  44  from the keypad  42  connects to the main PCB assembly  24 . The base and overlay panels  38 ,  40  include a window  46  through which the LCD display  32  can be viewed. It also includes an air intake port  48 , a gas inlet port  50 , an opening  52  for receiving an adjustment knob of the flow control valve  36 , an opening  54  for receiving a data port connector  56 , and a gas outlet port  58 . 
     The air intake port  48  leads to a mixing chamber  60  directly therebeneath. A screen  62  and a mixing element  64 , preferably comprising a static (i.e., non-rotating) fan, are positioned at the air intake port  48 . It should be noted that other types of mixing elements can alternatively be used, e.g., a series of baffles. The mixing chamber  60  includes a gas entry port  66  through which gas enters the mixing chamber at a location slightly below the fan element  64 . A polyethylene elbow tube  68  supplies gas to the gas entry port  66 . 
     A nose piece  70 , which forms a “sniffing chamber” at which gas is sniffed by a user, is connected to the outlet port  58 . 
     FIG. 4 is a detailed exploded view of the fan sub-assembly  26  (of FIG.  2 ). The fan sub-assembly  26  comprises a fan PCB assembly  72 , which is connected on opposite sides by first and second conduits or hoses  74 ,  76  and first and second tubes  78 ,  80 . 
     The hoses  74 ,  76  are preferably flexible polypropylene hoses, each having a length of four inches and a one-inch inner diameter. The inlet end  82  of the first hose  74  connects directly with the mixing chamber  60  to receive flow therefrom. The outlet end  84  of the second hose  76  connects directly with the gas outlet port  58  for exhausting gas from the device  10 . 
     The tubes  78 ,  80  are preferably of quarter-inch outer diameter, each having an eight-inch length. The inlet end  86  of the first tube  78  is connected to the outlet of the flow control valve  36 . The outlet end  88  of the second tube  80  is connected to the elbow tube  68  leading to the mixing chamber  60 . 
     The fan PCB assembly  72  (of FIG. 4) is shown in greater detail in FIG.  5 . The fan PCB assembly  72  includes a printed circuit board  90 , on which a mass flow sensor  92  and a motorized fan  94  are connected. The mass flow sensor  92  is connected on opposite sides to the tubes  78 ,  80  to measure flow of gas therethrough. The mass flow sensor  92  transmits signals indicative of the gas flow rate through circuitry on the PCB  90  to the microprocessor controller on the main PCB assembly  24 . 
     Also mounted on the PCB  90  is a fan speed sensor  96 , preferably an infrared device. The sensor  96  includes an infrared emitter/receiver pair located on opposite sides of the fan  94 . The emitter sends an infrared beam to the receiver. The beam is interrupted each time a fan blade crosses it as the fan  94  rotates. A pulsed signal indicative of the fan rotation speed is thereby transmitted by the sensor  96  to a tachometer  97  in the microprocessor controller. A closed loop control circuit comprising the infrared sensor/tachometer and a digitally controlled drive circuit are used to keep the fan at a constant speed. The main controller can adjust the fan speed as necessary by outputting a signal to a digital potentiometer in the main PCB  24  to adjust the drive voltage to the fan  94 . 
     Referring now to FIG. 6, the path of air and gas flow through the device  10  will now be described. Gas from a gas supply  98  flows through tubing leading to the regulator  34 . The gas flows through the regulator  34  and thereafter through the flow control valve  36  (when it is open). Thereafter, gas flows through the inlet end  86  of first tube  78  and then through the mass flow sensor  92 , which determines its flow rate. Thereafter the gas flows through the second tube  80  which is connected to the elbow tube  68  leading to the mixing chamber  60 , where the gas is mixed with air. 
     Air enters the instrument  10  through the air inlet port  48  shown in FIG.  3 . It flows through the screen  12  and past the stationary fan element  64  into the mixing chamber  60 . The fan element  64  is designed to swirl incoming air thereby creating turbulence to promote mixing with incoming gas. The air and gas are mixed in the mixing chamber  60  to form a mixed gas/air mixture, which flows into the inlet end  82  of the first hose  74 . The air/gas mixture is drawn through the hose  74  by the motorized fan  94  and forced through the second hose  76 , which is connected to the outlet port  58 . The gas/air mixture flows through the outlet port and out of the nose piece  70  forming the sniff chamber. 
     FIG. 7 is a block diagram illustrating the microprocessor circuitry  100  for controlling various components of the device  10 . The circuitry  100  includes a main controller  102 , which along with associated firmware, monitors all the system data inputs and provides control and data outputs for various system devices. An on-chip analog to digital (A/D) converter is used to convert critical analog inputs to digital signals for processing. A timer input captures the tachometer pulses from the motorized fan assembly. Digital potentiometers are used to provide voltage controlled outputs for contrast adjust (for the LCD), fan speed and flow sensor offset and span. 
     The device  10  also includes a real time clock chip  104  to keep current date and time information. The real time clock  104  enables test results and errors to be date and time stamped for record keeping purposes. A battery backup keeps the date and time on the clock chip  104  current when power is off. 
     The device  10  further includes a non-volatile memory, preferably at least one electrically erasable, programmable read-only memory chip (EEPROM)  106  to store user information, test location, test records, error logs, and calibration/parameter data. The EEPROM  106  can be uploaded to or downloaded from by a PC  108  using a software package, which will be further described below. This allows for a paperless audit trail. For convenience, user and test location information can be written to the EEPROM  106  by the PC  108  to reduce user data entry in the field. 
     Ambient temperature readings from an onboard temperature sensor  110  are monitored by the microprocessor controller  102  and used to automatically adjust the LCD display contrast with temperature variations. Adjustments are made using a digital potentiometer. 
     The device  10  also includes battery compensation and conditioning circuitry  112 , which automatically compensates for battery voltage level changes. The circuitry  112  can include, e.g., a voltage regulator. Battery compensation allows a wider range of battery voltage to be used while maintaining the calibration accuracy of the flow sensor  92  and increasing the charge life of the battery  16 . 
     The device  10  uses a flow linearization algorithm  93  using a multi-point calibration process of the mass flow sensor readings to provide a linear flow response. The LCD  32  can thus display gas-in-air concentrations, obviating the need for a user to cross-reference readings with a look-up table. 
     A keyboard controller  114  is used as a keyboard decoder. The keyboard controller  114  scans the keyboard  42  for a key press and then communicates the key press to the main controller  102 . If an invalid key is pressed, a beeper  116  is activated to notify the user of the error. 
     The device  10  can be linked to the PC  108  through the data port  54 . The PC  108  includes a software package used as an interface tool to allow uploading and downloading of data to/from the device  10 . Stored test records, user information, test location information, and error logs can be read from the device and used to generate reports, graphs and data archives. Information on users, test locations, and device calibration can be uploaded from the PC  108  to the device  10 . 
     The device  10  can also be configured to be linked through the data port  54  to a printer for printing data stored in the device. 
     FIG. 8 is a state diagram illustrating operation of the device  10  through keypad input. (The term “DTEX” used in the drawings is a trademark for the device  10 .) Initially the user presses the “Power On” button  118  on the keypad  42  causing the system to initialize. If an error occurs during the initialization process, a “Service Unit” message is shown on the LCD  32 . After the device  10  has been initialized, the user can (if desired) connect the device through the data port  54  to the PC  108  in order to transmit and/or receive data. 
     Thereafter, the user is prompted to “sign on,” and after a valid sign on, the main mode is entered. The device includes a user authentication feature for security. For a valid sign on, each user is required to input identification information and a corresponding password. 
     If the user presses the “Test” button  120 , the device  10  switches to the test mode and can be used to conduct testing. For example, threshold odor testing can now be conducted as follows. First, the user enters any needed information on the keypad, including, e.g., information identifying the gas supply location. Next, the inlet hose  20  is connected to the gas supply to be tested. Then, with the fan  94  having been turned on, the user gradually opens the gas flow valve  36 , allowing the gas sample to enter the instrument  10 . At the same time, the user breathes normally with his or her nose placed at the nose piece  70 . When odor in the sample is detected, the user depresses a “Record Test Level” button  121  (shown in FIG.  3 ). The gas concentration reading is displayed on the display  32  and stored in device memory  106 . 
     The user can enter any comments he or she may have relating to the test such as, e.g., wind conditions at the test site. 
     The user can press a “Review” button  122  to place the device  10  in a review mode, in which test results can be reviewed. If the “Details” button  124  is pressed, specific details of a particular test are displayed. The user can also press “Previous” or “Next” buttons  126  to scroll through other test information. While the user can review test data, the device  10  is programmed to prevent the user from altering critical test data in order to prevent any attempt to falsify data. 
     The “Set” button  128  can be pressed to set up new users and to set the system clock. 
     The “Info” button  130  can be pressed to switch to an information mode to, for example, look at the serial number of the unit, calibration data of the unit or the last reported error of the unit. 
     The device  10  can be powered down by the user or will automatically power down if no request has been made within a given period of time, for example, thirty minutes. 
     After all the desired readings have been taken of gas samples, the instrument can be taken to a central location site and data stored therein can be downloaded to a PC  108 . In accordance with the invention, Windows-based software is provided for the PC to prepare test data into formal reports needed to satisfy the formal reporting requirements. The data from the device can also be logged onto a main data base and retrieved as needed. The software also enables devices to be programmed. FIG. 9 is a high level state diagram illustrating use of the PC  108  for uploading and downloading data to/from a selected device once it has been linked to the PC  108  through the data port  54 . After initialization, a user can view test records and programming information on the PC display. The software provides the user with several options for managing test records and device programming information. The user can use a pointing device such as a mouse to move a cursor on the PC display to select and click on various items. A “Clear” button  132  can be pressed to clear the memory (EEPROM) on the device  10 . A “Close” button  134  can be pressed to close any opened file. A “Download” button  136  can be pressed to download data from the device memory. A “Set COM Port” button  138  can be pressed to change the serial COM port as desired. 
     An “Open File”  140  button can be pressed to open one of three different types of files. The “.MOD” files mirror the files stored in the device. The “.RPT” file contains an archive of test results. The “System Log” file is a master database of all tests run at all sites. FIG. 10 is an exemplary screen shot generated by the PC software of a test log. 
     The “Save File” button  142  can be pressed to save test records in different file types, including “.TXT” files, which can be e-mailed if desired. The files can also be imported into third party developed software. FIG. 11 is an exemplary screen shot illustrating exporting of data to such third party applications. 
     A “Filter Data” button  144  can be pressed to allow stored records to be filtered (i.e., queried) by, e.g., test date, user name, test location and particular unit used. FIG. 12 is an exemplary screen shot illustrating data filtering. 
     A “Print” button  146  can be pressed to print a file. 
     A “Graph” button  148  can be pressed to graph test results. FIG. 13 is an exemplary screen shot of a graph generated from test readings at a given test location. 
     A “View Details” button  150  can be pressed if the user wishes to see complete data of a given test. FIG. 14 is an exemplary screen shot showing details of a given test conducted using the device. 
     A “Unit Information” button  152  can be pressed to view critical parameters of the device, e.g., calibration and error log data. 
     In the “View Programming Information” mode, the user can press an “Open File” button  154  to view a list of all files that have been uploaded to or downloaded from the device. FIG. 15 is an exemplary screen shot illustrating pre-loading of user and test location data on the device. 
     By pressing a “Save File” button  156 , a user can save programming information including, e.g., configuration information. This information can be used to program another device in the same way as the subject device, which may, e.g., be in need of service. 
     The user can press an “Add Item” button  158  to add a programmable item to the master list such as, e.g., a new user and/or location. The user can also press an “Upload” button  160 , which allows data to be uploaded to the device. 
     The device  10  can be used to test the smell sensitivity of technicians working in the industry since individuals will have varying sensitivity to odor. The automatic data recording feature helps prevent any test cheating by users. FIG. 16 is an exemplary screen shot showing details of a qualification test taken by a given user. FIG. 17 is an exemplary screen shot of a graph of the results of multiple qualification tests taken by a given user. 
     The software preferably comprises a set of instructions in a code module resident in a random access memory of the PC  108 . Until required by the computer, the set of instructions may be stored in another computer memory such as a hard disk drive or in a removable memory such as an optical disk for eventual use in a CD ROM or a floppy disk for eventual use in a floppy disk drive, or even downloaded via the Internet. 
     In addition, although the various methods described are conveniently implemented in a general purpose computer selectively activated or reconfigured by software, one of ordinary skill in the art would also recognize that such methods may be carried out in hardware, in firmware, or in more specialized apparatus constructed to perform the method steps. 
     As has been described, the present invention provides a microprocessor-based odorometer wherein a motorized fan is maintained at a substantially constant speed to facilitate a substantially constant air flow into the sniff chamber. A further enhancement to the inventive approach is now described. In particular, when the apparatus is used at high altitudes, the fact that there is lower air density means that less air is provided to the blower. In addition, variations in ambient temperature also impact air density. Thus, according to the present invention, in conjunction with the fan motor speed correction, the microprocessor-based control system also accounts for air density variations (whether caused by altitude or temperature variations, or both). In general, when and where lower air density is encountered, the fan speed is adjusted upwards. On the contrary, when and where higher air density is encountered, the fan speed is adjusted downwards. In either case, the goal of maintaining a constant air flow is maintained. 
     Thus, according to an alternate embodiment of the present invention, a target fan speed is established upon initiation of the test as a function of relative air density at the given location where the test is being undertaken. In particular, during initialization, the user may enter a new location  9  or recall a stored location) at which the test is being performed. One of the given fields of information that may be entered about the location is the altitude of the location. Another field of information is the ambient temperature. The keypad is used to enter such information. With the altitude and temperature information, the microprocessor (during setup) calculates a target fan speed by multiplying a given fan speed by an adjustment factor. Preferably, the adjustment factor is a Relative Air Density (RAD) that is obtained from a look-up table stored in the device. (Alternatively, the look-up table may be evaluated manually by the user, with the RAD factor then input through the keypad). FIG. 18 is a table illustrating a Relative Air Density Comparison for given elevations at given temperatures. Using the elevation and temperature inputs, an RAD value is computed (or manually entered). That value is then multiplied by the given fan speed to derive the target fan speed. The controller then drives the fan motor either at a higher speed or lower speed as compared to the given fan speed, but still maintains the constant air flow that is desired. 
     Accounting for changes in air density provides significantly-enhanced operation of the device. The apparatus may be taken to various locations at various elevations yet still provide consistent and accurate readings across all tests. By applying a relative air density compensation to the fan speed motor, the test results fully account for variations in altitude and temperature. The device thus enables the operator to accurately quantify the gas concentration in air irrespective of where the test is being performed, all with a single integrated device. Operators of such devices are assured of consistent, useful results, despite variations in testing conditions. 
     As noted above, the RAD compensation is typically effected only once, namely, before initiation of the actual sniff test. One of ordinary skill in the art will appreciate, however, that dynamic adjustments to the fan motor target speed may be provided as a function of ambient conditions that might change during the test. Further, it may be desirable to further compensate fan motor speed to take into consideration changes in barometric pressure (e.g., due to changing weather conditions across a set of tests). With a microprocessor-based controller, such modifications and variations may be readily implemented. 
     Thus, according to the present invention, the fan delivers an appropriate amount of air (for the sniff test) and the controller holds the fan speed constant, irrespective of the altitude and/or temperature conditions at the location where the device is being used. This is accomplished by normalizing the air flow control algorithm (i.e., the algorithm that controls the fan motor speed) to temperature and altitude variations through the RAD factor. If desired, the fan motor speed may only be normalized as a function of temperature or as a function of altitude, but improved results are provided when both factors are considered. 
     As noted, the particular method by which the temperature or altitude inputs are provided is not restricted to any give approach. In the preferred embodiment as discussed above, the calibration is performed in response to input of both values, but this is not a limitation. The system may store a set of elevations cross-referenced to given locations, or the user may enter the RAD manually after calculating the necessary value from the table in FIG.  18 . 
     As a corollary, the various interface screens and output reports previously described preferably also include appropriate fields identifying the altitude and temperature data. 
     Having thus described my invention, what I claim as new and desire to secure by letters patent is set forth in the following claims: