Abstract:
A gas flow sensor receives a low-volume gas flow to be measured in a selectively sealed chamber. A flow rate signal is generated using a measuring device in response to the received gas flow. The flow rate signal is converted into a desired format for purposes of storage, control, and/or display.

Description:
BACKGROUND 
   Conventional devices for accurately measuring (and/or manipulating) various chemical compositions usually maintain an enclosed environment. The enclosed environment helps to ensure that the various chemical compositions are not changed or otherwise affected by the environment. For example, gas chromatography (GC) is a technique used to separate and measure volatile organic compounds. A gas chromatograph typically includes an injector port that is used to introduce a sample into the enclosed environment of the gas chromatograph. The injector port typically includes a septum through which the sample is injected. The septum is typically made of rubber and rubber mixtures containing substances such as silicone, plasticizers, organometallics, and the like, which help the septum to reseal properly. However, the septum degrades over time and with use. 
   The service life of a GC inlet septum is highly unpredictable under all but the most controlled conditions. Even small septum leaks (e.g., as low as 0.1 ml per minute) allow oxygen to enter the flow path. The introduced oxygen can damage sensitive parts in the GC flow path, including degradation of liner deactivations, degradation of GC column stationary phase, and oxidation of sensitive detector parts (such as mass spectrometer detectors). Further, degradates from liner and column phases often interfere with detection of analytes, foul detectors, and the like. Thus, the intrusion of oxygen (especially through a degraded septum) renders the analysis of the sample less effective, and increases costs and frequency of maintenance of the GC. 
   A GC septum is replaced in accordance with a chosen maintenance period for the purpose of lowering the chances of intrusion of oxygen into the GC. However, the GC is unavailable for use while the septum is being replaced. The time required to replace a septum is typically from around 30 minutes to around four hours before systems using the GC septum are re-equilibrated. Maintaining the GC septum, then, poses a dilemma: too frequent changes of the septum result in expensive downtime and too infrequent changes of the septum result in intrusion of oxygen with the resulting decrease in the life of oxygen-sensitive flow path parts. 
   SUMMARY 
   In general terms, this patent relates to isolating a septum from an external environment and detecting a physical characteristic of gas that leaks through the septum. 
   One aspect is an apparatus for measuring gas flow through a septum. The apparatus comprises a housing defining a chamber and an opening. A sensor is positioned within the housing. The sensor is responsive to a gas flowing though the septum, through the opening, and into the chamber. 
   Another aspect is a method of detecting leaks in a septum of a vessel. The method comprises positioning a housing proximal to a septum, the housing defining a chamber and an opening; substantially isolating the chamber from an external environment; and upon gas leaking through the septum, detecting at least one physical characteristic of the gas. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a cross-sectional view of a flow rate detector having a sensor arrangement. 
       FIG. 2  is a cross sectional view of the flow rate detector illustrated in  FIG. 1  in which the sensor arrangement includes pressure and temperature sensors. 
       FIG. 3  is a cross sectional view of an alternative embodiment of the flow rate detector illustrated in  FIG. 1  in which the sensor arrangement includes a rotary vane. 
   

   DETAILED DESCRIPTION 
   Various embodiments will be described in detail with reference to the drawings, wherein like reference numerals represent like parts and assemblies throughout the several views. Reference to various embodiments does not limit the scope of the claims attached hereto. Additionally, any examples set forth in this specification are not intended to be limiting and merely set forth some of the many possible embodiments for the appended claims. 
   Chromatography involves the separation of a mixture of compounds (e.g., solutes) into separate components. Separating the sample into individual components allows identification and measurement of the various sample components. Gas chromatography (GC) is suitable for analyzing 10-20% of known compounds. To be suitable for GC analysis, a compound typically should have sufficient volatility and thermal stability. If all or some molecules of a compound are in the gas or vapor phase at around 400-450° C. (or below), and do not decompose at these temperatures, the compound can usually be analyzed using GC. 
   A GC system typically comprises an injector, a column, and a detector. In operation, one or more high purity gases are supplied to the GC at the injector. One of the gases (called the carrier gas) flows into the injector, through the column and then into the detector. A sample is introduced into the injector using, for example, a syringe or an exterior sampling device. The injector is typically heated to 150-250° C. for the purpose of causing the volatile sample solutes to vaporize. 
   The vaporized solutes are transported into the column by the carrier gas. The column is typically arranged within a temperature-controlled oven. The solutes travel through the column at a rate in accordance with physical properties of the solutes, the temperature, and composition of the column. The various solutes typically travel through the column at rates that vary in accordance with differing physical properties of the solutes. Typically the fastest moving solute exits (“elutes”) the column first, and is followed by the remaining solutes in an order that is associated with physical properties of the different kinds of solutes. As each solute elutes from the column, the solute enters the detector, which is typically heated. An electronic signal is generated upon interaction of the solute with the detector. Signal parameters are recorded by a data system and typically plotted over time to produce a chromatogram. 
   In an example GC system, a capillary GC column comprises a tubing and stationary phase. A thin film (0.1-10.0 μm) of a high molecular weight, thermally stable polymer is typically coated onto the inner wall of small diameter (0.05-0.53 mm I.D.) tubing to provide a polymer coating (that is associated with the stationary phase). Gas flows through the tubing and is called the carrier gas (and is associated with the mobile phase). 
   Upon introduction into the column, solute molecules typically distribute between the stationary and mobile phases. The molecules in the mobile phase are carried down the column; the molecules in the stationary phase are temporarily immobile and as such do not move down the column. As the molecules in the mobile phase move through the column, some of them eventually collide with and re-enter the stationary phase. During the same time span, some of the solute molecules leave the stationary phase and enter the mobile phase. This often occurs thousands of times for each solute molecule as it passes through the column. 
   The molecules corresponding to a specific compound usually travel through the column at nearly the same rate and appear as a band of molecules (called the sample band) at the detector. Avoiding an overlap between adjacent sample bands as they exit the column can be accomplished by having each sample band travel at a different rate and by minimizing the width of the sample band. The rate at which each sample band moves through the column is a function of the structure of the compound, the chemical structure of the stationary phase, and the column temperature. The width of the sample band is dependent on the operating conditions and the dimensions of the column. An unknown substance can be identified by comparing the retention and peak size with the retention and peak size of a known substance as long as the retention and/or peak sizes are not the same. Accordingly, peak co-elution can be minimized by selecting a proper column and the operating conditions. 
     FIG. 1  illustrates an exemplary embodiment of a septum leak detector  100 . The exemplary embodiment engages a septum nut  112  that defines an opening  118  and is fixed to a vessel  104  for holding a gaseous sample. Septum  110  is attached to and covers the opening  118  in septum nut  112 . The septum nut  112  attaches to the vessel  104  in any conventional manner. Examples can include threading to the vessel or fitting into and frictionally engaging the vessel such as a stopper. 
   Septum nut  112  and septum  110  seal the vessel  104  and provide an environmentally closed system (such as in a gas chromatograph) that contains flowing gases. In various applications, the septum  110  provides an injection site for introducing substances to the environmentally closed system. However, the septum  110  develops leaks  114  over time as the material forming the septum  110  degrades because of circumstances such as oxidation in the presence of oxygen and repeatedly traversing the septum with a needle (as in during injections). Although a particular septum nut  112  and septum  110  arrangement is illustrated, septum leak detectors within the scope of this document can be used with alternative structures of septums and alternative retaining structures for the septums. 
   In the exemplary embodiment, the septum leak detector  100  includes a housing  102  having a base portion  120  and defining a chamber  130 , a seal  122 , and a sensor arrangement  132 . The base portion  120  is annular and defines an inner passage  124  that is open to the chamber  130 . An annular seal  122  such as an o-ring is attached to the base portion and circumscribes the inner passage  124 . The base portion  120  is configured to be removably attached to the septum nut  112  and/or vessel  104 . In various embodiments, the base portion  120  can be attached to the septum nut  112  with different mechanisms such as threads, a frictional fit, clips, clamps, or the like. 
   The sensor arrangement  132  can be any type of sensor that responds to one or more physical characteristics of gas so that gas within the chamber gas can be quantified. Examples of physical characteristics include temperature, the number of moles, pressure within a chamber, flow rate, and the like. Some embodiments calculate the flow rate of gas based on measured physical characteristics or by directly measuring the flow rate. In the exemplary embodiments disclosed herein, for example, the flow rate of gas is determined from calculating the number of moles of gas or from measuring the rate at which the gas rotates a vane. Yet other embodiments might determine whether a leak exists by analyzing a physical characteristic other than flow rate. 
   When the base portion  120  is attached to the septum nut  112 , the annular seal  122  engages the septum nut, circumscribes the septum  110 , and seals any space between the base portion and the septum nut  112 . The septum  110  is in fluid communication with the chamber  130  through the inner passage  124  of the base portion  120 . The seal  122  isolates the chamber  130 , inner passage  124  of the base portion  120 , and septum  110  from the external environment and substantially prevents any gas associated with leak  114  from leaking to the environment, and likewise substantially prevents entry of any environmental gas into the chamber  130 . Although an o-ring type of seal is illustrated, other embodiments can include any type of seal and related structure that isolates the septum  110 , inner passage  124 , and chamber  130  from the external environment. 
   The leak detector  100  can be attached to the nut  112  for a single measurement and then immediately removed. Alternatively, the leak detector  100  can be mounted on the septum nut  112  and left there for convenience, for repeated testing for leaks in the septum, or for continuous testing for leaks in the septum  110 . 
   In the exemplary embodiment, a sensor arrangement  132  that responds to the leaking gas is positioned within the chamber  130  and is in electrical communication via leads  116  with programmable circuit  106  that include a display  140 . The programmable circuit  106  receives a signal generated from the sensor arrangement  132 , determines a flow rate and displays the flow rate on the display  140 . The exemplary embodiment includes any type of conventional programmable circuit configured to processes signals from the sensor arrangement  132  and generates a display in response thereto. Other embodiments could include nonprogrammable circuits that are responsive to the sensor arrangement. 
   In various embodiments, the programmable circuit  106  and display  140  are physically attached to base portion  120 . In other embodiments, the programmable circuit  106  and display  140  are physically separated from base portion  120 , and are in electrical communication with the sensor arrangement  132  via a data link that communicates according to any suitable communication protocol and can be either wired or wireless (e.g., radio frequency, infrared, and acoustic). The display  140  can be a liquid crystal display, a CRT, light emitting diodes (LED&#39;s), or any other component or device that can communicate information. The programmable circuit  106  also includes a power supply  108 . The power supply  108  can provide power for any active circuitry used in the sensor arrangement  132 . 
   In use, the sensor arrangement  132  responds to the gas  114  and provides a signal to the programmable circuit  106 . The programmable circuit  106  in the exemplary embodiment processes the signal and determines the flow rate of gas. In the exemplary embodiments, the programmable circuit  106  determines the flow rate of gas by either measuring it directly or by calculating the leak rate in real-time by measuring a physical characteristic of the gas and using this information to calculate the flow rate using any of desired units of measurement such as moles per second, mils per second, and the like. In various embodiments, the programmable circuit  106  can present the information and generate warning signals. For example, if an LCD or CRT display is used, the display  140  can present the calculated flow rate for the user to read. 
   In other embodiments, the programmable circuit  106  is programmed with a failure threshold value that indicates the failure point for a septum. In these embodiments, the programmable circuit  106  compares the calculated flow rate to the threshold value and generates a failure signal if the calculated flow rate reaches or exceeds the failure threshold value. In yet other embodiments, the programmable circuit  106  is programmed with a warning threshold value that indicates the flow rate of gas through the septum  110  is approaching the failure point, but has not yet failed. In this embodiment, the programmable circuit  106  compares the calculated flow rate to the warning and failure threshold values and generates a warning signal if the flow rate reaches or exceeds the warning threshold value and a failure signal if the flow rate reaches or exceeds the failure threshold value. 
   The failure and warning signals can take a variety of forms depending on the embodiment. Examples include certain words (e.g., “failure” or “warning”) or other indicia if the display is an LCD or CRT. In other possible embodiments, the failure and warning signals are generated by illuminating LEDs. For example, a red LED could be used for a failure signal and a yellow LED can be used for a warning signal. 
   As discussed in more detail herein, the programmable circuit  106  determines the flow rate of gas leaking through the septum  110  in any appropriate units of measurement such as moles per second, milliliters per second, and the like. Furthermore, the sensor arrangement  132  and programmable circuit  106  are capable of measuring gas leaks in the range of about 0.01 ml to about 2 ml per minute with an accuracy of about 15% or better. In other embodiments, the sensor and programmable circuit  106  are capable of measuring gas leaks in the range of about 0.6 ml per minute to about 6 ml per minute. These ranges and level of accuracy have several advantages over conventional methods. 
   For example, soap bubble flow meters rely on the creation of a soap bubble in the flow in a tube connected to a flow source. The soap bubble flow meter typically does not measure accurately below about 1-5 mL/min due in part to variations in water and bubble pressure and other inaccuracies in estimating the volume of the bubble. 
   Acoustic displacement technology measures by exposing a flexible membrane to the force created by the flow path in an open system. The technology is designed for an open system (which allows introduction of contaminants), requires frequent recalibration to maintain accuracy, and typically does not measure accurately below about 1.0 mL/min. 
   Acoustic flowmeters measure the phase shift of a sound wave propagating though a duct containing a flowing gas, where the gas (and the equation of the gas state) is known. Acoustic phase measurements, thus, require “before-hand” (i.e., a priori) knowledge of the composition of the gas to be measured and typically do not measure accurately below about 0.1 mL/min. 
   In some possible embodiments, a user can optionally select a desired format for the output generated by the programmable circuit  106  and displayed on the display  140 . Examples of formats that the user might be able to select in these embodiments include whether to display the flow rate, a failure signal, and a warning signal, or any combination thereof. The user also might be able to select the units of measurement for displaying the flow rate. 
     FIG. 2  illustrates an exemplary embodiment of a septum leak detector  100  that includes a housing  102  defining a chamber  130  and having a base portion  120 , a seal  122 , a sensor arrangement  132 , and programmable circuit  106  with a display  140 . The housing  102  includes a port  128  that the user can selectively open and close with a plug  134 . When the port  128  is open, it provides an opening between the chamber  130  and the atmosphere external to the housing  102 . 
   The sensor arrangement  132  includes an internal temperature sensor  136  and an internal pressure sensor  138  positioned within the chamber and in electrical communication with the programmable circuit. An external pressure sensor  142  is located on the outer surface of the housing  102  and is also in electrical communication with the programmable circuit. 
   In operation, the housing  102  is attached to a septum nut  112  for testing the septum  110 . After the housing  102  is attached to the septum nut  112 , the port  128  is opened to equalize internal and external pressures associated with chamber  130 . As gas leaks through (i.e., traverses) the septum  110 , the pressure and temperature of chamber  130  increases in accordance with the Ideal Gas Equation:
 
PV=nRT  (1)
 
where P is the pressure internal to the camber  130 , V is the volume of the chamber  130  and the inner passage  124  of the base portion  120 , n is the quantity of gas in the chamber  130  expressed in moles, R is the gas constant, and T is the temperature in the chamber  130 . The internal pressure and temperature sensors  138  and  136  generate pressure and temperature signals, respectively, corresponding to the pressure and temperature within the chamber  130  and input these signal to the programmable circuit  106 . The programmable circuit  106  them uses the ideal gas equation to calculate the quantity of gas in the chamber  130  at least two different points in time and uses this information to calculate the flow rate using the equation:
 
 F=n/Δt   (2)
 
where F is the flow rate of gas in moles per second, n is the number of moles calculated according to the ideal gas equation (Equation 1), Δt is the lapsed time between the start of the measurement and the moment that measurement of the internal pressure and temperature is concluded. Depending on the embodiment as described herein, the calculated flow rate is displayed on the display  140 . In other embodiments, the programmable circuit  106  compares the calculated flow rate to a predetermined warning threshold value and/or a predetermined failure threshold value and generates warning and/or failure signals and displays as described above.
 
   For repeated measurements (as in an industrial manufacturing process), the port is opened to equalize pressures between the interior and exterior of chamber  130 , and then closed again, which allows successive measurements to be made. 
   In various embodiments, the internal pressure  138 , internal temperature  136 , and external pressure  142  sensors are used in different ways to determine flow rates. In one embodiment, the step of re-equalizing the pressure can be replaced (or combined with) with the step of “re-zeroing” the measurements made with the internal pressure  138  and internal temperature  136  sensors. The internal pressure  138  and temperature sensors  136  can be re-zeroed by considering a previous set of measurements to be a reference point from which a subsequent measurement set is used to determine flow rate. 
   In an alternate embodiment, the external pressure sensor  142  can be used as reference point to determine (and/or verify) increases in pressure in the interior of the chamber  130 . For example, the exterior pressure sensor  142  can be used to determine the instantaneous pressure difference (by taking the difference of the interior  138  and exterior pressure sensors  142 ), which can be further used to validate the calculation of the change in internal pressure (over time) made by successive measurements using the interior pressure sensor  138 . 
   In another alternate embodiment, the port  238  is not opened to equalize the internal chamber pressure and the external pressure. Instead, for example, septum leak detector  100  itself can be removed from septum nut  112 , which equalizes the pressure. 
   Septum leak detector  100  is used to calculate leak rates from a septum  110  under which there is a pressure of either ambient or vacuum below ambient pressure. Experimental test results have shown septum leak detector  100  is capable of measuring leaks between 0.01-2.0 ml per minute, with a plus or minus 15 percent accuracy or better. The volume parameter (from the Ideal Gas Equation) is determined by the volume of chamber  130  and the volume, if any, provided by septum  110  recessed within (or protruding from) septum nut  112 . 
     FIG. 3  illustrates another exemplary embodiment of a septum leak detector that includes a housing  102  defining a chamber  130  and having a base portion  120 , a seal  122 , a sensor arrangement  132 , and programmable circuit  106  with a display  140 . In this embodiment, the sensor arrangement  132  includes a rotary vane  144  having vane members  146  disposed radially about a shaft  148 . Vane members  146  are configured to move and rotate the shaft  148  in response to the gas flow from leak  114  exerting a force against them. 
   The shaft  148  of rotary vane  144  is captivated by “frictionless” bearings  150  and  152 . Frictionless bearings  150  and  152  impart no substantial static friction to the shaft  148  of rotary vane  144 . The lack of substantial static friction allows the rotary vane  144  to begin rotating without the gas flow from the gas leak  114  having to overcome the static friction that is initially present when the shaft  148  is not rotating. The frictionless bearings  150  and  152  comprise permanent magnets and/or electromagnets that draw power from the power supply  108  in the programmable circuit  106 . 
   The rotary vane  144  also comprises a light source  156  positioned on one side of the vane members  146  and a light sensitive transducer  154  on the opposite side of the vane members  146 . The light source  156  shines a light beam  158  onto the light sensitive transducer  154 , which generates a signal in response thereto and inputs that signal into the programmable circuit  106 . As the vane shaft  148  rotates, the vane members  146  repeatedly break the light beam  158  thereby interrupting the signal input from the light-sensitive transducer  144  to the programmable circuit  106 . The programmable circuit  106  uses the time between successive pulses in the signal (and the angles subtended by the vane members  146  about the axis of rotation) to determine the angular speed of the rotation of rotary vane  144 . 
   In operation, the housing  102  is attached to the septum nut  112  for testing the septum  110 . The port  134  is open to exhaust the gas flowing through chamber  130  so that a substantial back pressure does not result within chamber  130  after the leak detector  100  is attached. A substantial amount of back pressure is a pressure that builds up in the chamber  130  and impedes the flow of gas leaking through septum  110 , which adversely affects accuracy of the measurements. Some embodiments also include a pressure relief valve (not shown) in the wall of the housing  102  to protect rotary vane  144 . The pressure relief valve opens and further vents the chamber  130  if pressure within the chamber exceeds a predetermined level. 
   As gas from the gas leak  114  flows through the chamber  130  it exerts a force against the vane members  146  and causes the rotary vane  144  to rotate and periodically interrupt the light traveling from the light source  156  to the light-sensitive transducer  154 . In turn, the signal from the light-sensitive transducer  154  to the programmable circuit  106  is repeatedly interrupted and the programmable circuit  106  determines the length of time for each interruption. The programmable circuit  106  uses this length of time for the interruption to determine the angular velocity of the rotary vane  144 . The angular velocity of the rotary vane  144  can be determined, for example, by comparing the rate of interruptions with the angular displacement (about the axis of rotation) of the vane members  146 . 
   The programmable circuit  106  then determines the rate of the gas flow through the septum  110 . The rate of the gas flow can be determined, for example, by correlating the measured angular velocity with stored empirical measurements made using a similar rotary vane sensor arrangement. A lookup table can be addressed using the angular velocity as an index and thereby obtaining the gas rate for the indexed angular velocity. Rates falling between intermediate index values can be determined by interpolation of values (such by using linear interpolation, least squares, curve fitting, and the like). Gas flow rates can be displayed in any convenient form, including moles per second, mils per second, and the like. In various embodiments as described herein, the programmable circuit  106  displays the flow rate of gas, generates a failure signal, and/or generates a warning signal. 
   The various embodiments described above are provided by way of illustration only and should not be construed to limit the claims attached hereto. Those skilled in the art will readily recognize various modifications and changes that may be made without following the example embodiments and applications illustrated and described herein, and without departing from the true spirit and scope of the following claims.