Abstract:
A thermal oxidation tester is shown for determining thermal stability of a fluid, particularly hydrocarbons when subjected to elevated temperatures. The tendency of the heated fluid to oxidize and (1) form deposits on a surface of a heater tube and (2) form solids therein, are both measured at a given flow rate, temperature and time. The measured results are used to determine whether a fluid sample passes or fails the test. Sample flow rate is important in the jet fuel thermal oxidation test. Current practice requires manual drop counting or flow confirmation with the use of volumetric glassware. An apparatus is described to precisely measure the flow rate and automatically perform flow rate check using a drip rate method and/or volumetric method.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This is a continuation-in-part of U.S. patent application Ser. No. 12/838,104, filed on Jul. 16, 2010, having at least one overlapping inventor and the same assignee, which application is incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Technical Field 
     This invention relates to a drop counter and flow meter for methods and devices measuring the thermal characteristics of fluids. Specifically, this invention relates to automated flow rate check using a drop counter and/or volumetric flow meter incorporated in methods and devices measuring the thermal oxidation tendencies of fuels used in liquid hydrocarbon-burning engines. 
     2. Background Art 
     The development of higher performance aviation turbine engines has increased the stress placed on the fuels. In addition to being the fuel source, an aviation turbine fuels may also be used as a heat sink to cool engine components. Combined with an overall decreasing quality of feedstock material for production of fuels, the potential for thermally induced deposition formation is high. Deposits within an aviation turbine engine can (1) reduce heat transfer efficiency, (2) block fuel filters, lines, and nozzles, or (3) result in engine failure. 
     While various tests were devised and used in the 1950s and 60s to evaluate the thermal oxidation characteristics of jet fuel prior to being used in jet aircraft, Alf Hundere developed the apparatus and method which became the standard in the industry. In 1970, Alf Hundere filed what became U.S. Pat. No. 3,670,561, titled “Apparatus for Determining the Thermal Stability of Fluids”. This patent was adopted in 1973 as ASTM D3241 Standard, entitled “Standard Test Method for Thermal Oxidation Stability of Aviation Turbine Fuels”, also known as the “JFTOT® Procedure”. This early Hundere patent was designed to test the deposition characteristics of jet fuel by determining (1) deposits on the surface of a heater tube at an elevated temperature and (2) differential pressure across a filter due to collection of particulate matter. To this day, according to ASTM D3241, the two critical measurements are still (1) the deposits collected on a heater tube and (2) differential pressure across the filter due to the collection of particulate matter on the filter. 
     According to ASTM D3241, 450 mL of fuel flows across an aluminum heater tube at a specified rate during a 2.5-hour test period at an elevated temperature. Currently six different models of JFTOT® 1  instruments are approved for use in the ASTM D3241-09 Standard. The “09” refers to the current revision of the ASTM D3241 Standard.  1  JFTOT® is the registered trademark of Petroleum Analyzer Company, LP. 
     While over the years various improvements have been made in the apparatus to run the tests, the basic test remains the same. Improvements in the apparatus can be seen in U.S. Pat. Nos. 5,337,599 and 5,101,658. The current model being sold is the JFTOT® 230 Mark III, which is described in further detail in the “Jet Fuel Thermal Oxidation Tester—User&#39;s Manual”. The determination of the deposits that occur on the heater tube can be made visually by comparing to known color standards or can be made using a “Video Tube Deposit Rater” sold under the Alcor mark. 
     The determination of the amount of deposits formed on the heater tube at an elevated temperature is an important part of the test. The current ASTM D3241 test method requires a visual comparison between the heater tube deposits and known color standard. However, this involves a subjective evaluation with the human eye. To take away the subjectivity of a person, an electronic video tube deposit rater was developed. 
     When performing a test under ASTM D3241 Standard, it is important to accurately measure the rate of flow of the test sample. However, this is difficult to perform manually due to slow rate of flow which is in drops per minute. 
     Once the thermal oxidation stability test has been performed on a batch of fuel, the recorded information and the heater tube are preserved to show the batch of fuel was properly tested. The information that was recorded when testing a batch of fuel is maintained separately from the heater tube itself. This can cause a problem if one or the other is misplaced or lost. Inaccurate information and/or conclusions occur if the wrong set of data is associated with the wrong heater tube. 
     BRIEF SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide an apparatus and method for testing thermal oxidation stability of fluids, particularly aviation fuels with an improved drop counter and flow meter. 
     It is another object of the present invention to provide an apparatus and method to measure the tendency of fuels to form deposits when in contact with heated surfaces, the apparatus including an improved drop counter and flow meter. 
     It is another objective of the present invention to provide a drop counter and flow meter for an apparatus and method for testing the thermal oxidation tendency of fuels utilizing a test sample to determine if solid particles will form in the fuel at an elevated temperature and pressure. 
     It is another objective of the present invention to provide a drop counter and flow meter for an apparatus and method for determining thermal oxidation stability of a batch of fuel by testing a sample at an elevated temperature and pressure to determine (1) deposits that form on a metal surface and (2) solid particles that form in the fuel. 
     It is another objective of the present invention to provide an improved drop counter and flow meter to accurately measure the flow rate of the fuel being tested. 
     It is yet another objective of the present invention to provide a drop counter and flow meter to accurately measure the flow rate of the fuel being tested for thermal oxidation stability. 
     A drop counter and flow meter is provided when testing a fuel for thermal oxidation stability, in which the drop counter and flow meter automatically counts drops and accurately measures the flow rate during testing. The flow rate is controlled by an embedded computer through a sample drive pump during test, and then verified by the drop counter and flow meter. An infrared LED and photodiode pair is used to count the number of drops of fuel and also monitor the fuel level inside of a container. The time to fill a given volume from one level to a second level can be used to determine flow rate. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a general block diagram of a thermal oxidation stability test apparatus illustrating flow and electrical controls. 
         FIGS. 2 and 2A  are a more detailed block diagram showing a thermal oxidation test apparatus used to perform ASTM D3241 Standard. 
         FIG. 3  is a pictorial diagram of the coolant flow for  FIGS. 2 and 2A . 
         FIG. 4  is a pictorial diagram of the airflow in  FIGS. 2 and 2A   
         FIG. 5  is a pictorial diagram showing flow of the test sample in  FIGS. 2 and 2A . 
         FIG. 6  is an elevated, partial sectional view of a drop counter and flow meter with a pictorial block diagram. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       FIG. 1  is a schematic block diagram of a thermal oxidation stability tester referred to generally by the reference numeral  20 . The thermal oxidation stability tester  20  has an embedded computer  21  with a touch screen  23  for user interface. While many different types of programs could be run, in the preferred embodiment, Applicant is running C++ in the embedded computer  21 . The touch screen  23  displays all of the information from the thermal oxidation stability tester  20  that needs to be conveyed to the user. The user communicates back and forth with the embedded computer  21  through the touch screen  23 . If a batch of fuel is to be tested, a test sample is put in the sample delivery system  25 . 
     It is required by the test to ensure the test sample is oxygen saturated through aeration. Therefore, the embedded computer  21  operates a sample aeration control  31  for a period of time to make sure the sample is fully aerated. The aeration of the sample takes place at the beginning of the test. 
     The embedded computer  21  turns on a sample flow control  27 , which is a pump used to deliver the sample throughout the thermal oxidation stability tester  20 . Simultaneous with the sample flow control  27  pumping the test sample throughout the system, sample pressure control  29  maintains a fixed pressure throughout the system. It is important to maintain pressure in the system to prevent boiling of the test sample when at elevated temperatures. In the present thermal oxidation stability tester  20 , the sample is maintained at approximately 500 psi during the thermal oxidation stability test. 
     Also, the embedded computer  21  controls parameters affecting the intelligent heater tube  33  via intelligent heater tube  33 . The test data is recorded to the heater tube writer  35  from the embedded computer  21 . Critical test parameters are recorded on a memory device on an end of the intelligent heater tube via the intelligent heater tube writer  35 . The rating of the deposit formed on the intelligent heater tube  33  will be recorded on the memory device at a later time. 
     In performing the thermal oxidation stability test on a test sample, the intelligent heater tube  33  is heated by tube heater control  37 . The tube heater control  37  causes current to flow through the intelligent heater tube  33 , which causes it to heat up to the temperature setpoint. 
     To prevent the hot intelligent heater tube  33  from heating other parts of the thermal oxidation stability tester  20 , bus-bar coolant control  39  provides coolant to upper and lower bus-bars holding each end of the intelligent heater tube  33 . This results in the center section of the intelligent heater tube  33  reaching the prescribed temperature while the ends of the intelligent heater tube  33  are maintained at a lower temperature. This is accomplished by flowing coolant via the bus-bar coolant control  39  across the ends of the intelligent heater tube  33 . 
     The test parameters, such as the dimension of the heater tube, pressure of the test sample or flow rate are fixed by ASTM D3241. However, the control of the equipment meeting these parameters are the focus of this invention. 
     Referring now to  FIGS. 2 and 2A  in combination, a schematic flow diagram is shown connecting the mechanical and electrical functions. The embedded computer  21  and the touch screen  23  provide electrical signals as indicated by the arrows. A test sample is contained in the sample container  41 . To make sure the sample in the sample container  41  is fully aerated, an aeration pump  43  is turned ON. The aeration pump  43  pumps air through a dryer  45  where the air is dehumidified to remove moisture. From the dryer  45 , a percent relative humidify sensor  47  determines the humidity level of the pumped air and provides that information to the embedded computer  21 . Assuming the percent humidity of the pumped air is sufficiently low, the test procedure will continue pumping air through the flow meter  49  and aeration check valve  50  into the sample container  41 . During aeration, flow meter  49  should record approximately 1.5 liters of air per minute. Since the flow meter  49  runs for approximately six minutes, the aeration pump  43  will sparge approximately nine liters of air into the test sample. This is sufficient time to saturate the test sample with dry air. 
     Within the sample container  41 , a sample temperature measurement  51  is taken and provided to the embedded computer  21 . The sample temperature measurement  51  is to ensure that the test sample is between 15°-32° C. If the test sample is outside of this temperature range, results can be impacted. Therefore, if the test sample is outside this temperature range, the embedded computer  21  would not let the test start. 
     Once the test sample has been aerated and if all the other parameters are within tolerance, then the sample drive pump  53  will turn ON. The sample drive pump  53  is a single piston HPLC pump, also known as a metering pump. With every stroke of the piston, a fixed volume of the sample is delivered. The speed of the sample drive pump  53  is controlled so that it pumps 3 mL/min of the test sample. The sample drive pump  53  is configured for fast refill which minimizes the need for manual pump priming. Pulsations, associated with pumps of this design are minimized with the use of a pulse dampener and a coil tubing on the outlet side as will be subsequently described. 
     To get air out of the tubing between the sample container  41  and the sample drive pump  53  at the start of the test, an auto pump priming valve  55  is opened, a sample vent valve  54  is closed and the aeration pump  43  is turned ON by the embedded computer  21 . The auto pump priming valve  55  opens and remains open while a combination of sample and air is discharged into waste container  57 . At the same time, the aeration pump  43  provides positive pressure in the sample container  41  to force test sample from the sample container  41  to the sample drive pump  53 . The sample vent valve  54  closes to prevent venting of the air pressure to atmosphere to maintain a pressure of 2 to 3 psi. A sample vent check valve  56  across the sample vent valve  54  opens at 5 psi to prevent the pressure in the sample container  41  from exceeding 5 psi. Once the sample drive pump  53  starts pumping the test sample, auto pump priming valve  55  will close and the sample vent valve  54  will open. Thereafter, the sample drive pump  53  will pump the test sample through check valve  59  to the prefilter  61 . The check valve  59  prevents fluid from flowing backwards through the sample drive pump  53 . The check valve  59  operates at a pressure of approximately 5 psi. The check valve  59  prevents siphoning when the sample drive pump  53  is not pumping. Also, check valve  59  prevents fluid from being pushed backwards into the sample drive pump  53 . 
     The prefilter  61  removes solid particles in the test sample that could affect the test. The prefilter  61  is a very fine filter, normally in the order of 0.45 micron in size. The purpose of the prefilter  61  is to make sure particles do not get into the test filter as will be described. The prefilter  61  is replaced before every test 
     From the prefilter  61 , the test sample flows through an inlet  63  into the cylindrical heater tube test section  65 . Outlet  67 , while illustrated as two separate outlets, is actually a single outlet at the upper end of the cylindrical heater tube test section  65 . Extending through the cylindrical heater tube test section  65  is the intelligent heater tube  69 , sealed at each end with ceramic bushings and an o-ring (not shown). While the test sample flows through the cylindrical heater tube test section  65  via inlet  63  and outlet  67  and around the intelligent heater tube  69 , the housing of the cylindrical heater tube test section  65  is electrically isolated from the intelligent heater tube  69 . Only the test sample comes in contact with the center section of the intelligent heater tuber  69 . Inside of the intelligent heater tube  69  is a thermocouple  71  that sends a signal back to the embedded computer  21  as to the temperature of the center section of the intelligent heater tube  69 . 
     Test sample flowing from the cylindrical heater tube test section  65  flows through a differential pressure filter  73 , commonly called the “test filter”. In a manner as will be explained in more detail, the intelligent heater tube  69  heats up the test sample inside of the cylindrical heater tube test section  65  to the test parameter set point. Heating of the test sample may result in degradation of the test sample, or cause solid particles to form. The solid particles may deposit on the center section of the intelligent heater tube  69 , and/or may collect on the differential pressure filter  73 . The pressure drop across the differential pressure filter  73  is measured by differential pressure sensor  75 . Pressure across the differential pressure filter  73  is continuously monitored by the embedded computer  21  through the differential pressure sensor  75 . When the pressure across the differential pressure filter  73  exceeds a predefined differential of approximately 250 mm to 280 mm of mercury, the differential pressure bypass valve  77  opens to relieve the pressure. By test definition, exceeding a differential pressure of 25 mm Hg results in failure of the test. 
     For this test to be performed, the test sample must remain as liquid. At typical testing temperatures of 250° C. to 350° C., many hydrocarbon fuels will transition to the vapor phase at ambient pressures. To keep the test sample in the liquid phase, the back pressure regulator  79  maintains approximately 500 psi pressure in the system. This system pressure is monitored by the system pressure sensor  81 , which reports information to the embedded computer  21 . During a test, normal flow of a test sample is through differential pressure filter  73  and through the back pressure regulator  79 . From the back pressure regulator  79 , the test sample flows through sample drop counter and flow meter  83  to waste container  57 . The sample drop counter and flow meter  83  automatically counts drops and accurately measures the flow rate of the test sample during the test. The sample drop counter and flow meter  83  provides sample drop counts and flow rate information to the embedded computer  21 . 
     A system/safety vent valve  85  is connected into the system and controlled via the embedded computer  21 . The system/safety vent valve  85  acts to relieve excess system pressure in the case of power loss, improperly functioning system components or other system failures. In the event of this occurrence, the system pressure sensor  81  sends a signal to the embedded computer  21 , triggering the system/safety vent valve  85  to open and relieve excess pressure. Also, at the completion of a test, the system/safety vent valve  85  opens to vent pressure from the system. The system/safety vent valve  85  is normally set to the open position requiring a program command from the embedded computer  21  to close the system/safety vent valve  85 . Therefore, if power is lost, the system/safety vent valve  85  automatically opens. 
     At the end of the test, after the system/safety vent valve  85  is opened and system pressure is relieved, the flush air pump  87  turns ON and flushes air through flush check valve  89  to remove the test sample from the system. The flush air pump  87  pushes most of the test sample out of the system via the system/safety vent valve  85  into the waste container  57 . 
     The system may not operate properly if there are air pockets or air bubbles in the system. During a test, it is important to maintain an air-free system. Therefore, at the beginning of each test, the solenoid operated differential pressure plus vent valve  91  and the differential pressure minus vent valve  93  are opened, flushed with test sample, and vented to remove any air pockets that may be present. During the beginning of each test, the position of the differential pressure vent valves  91  and  93  ensure there is no air in the differential pressure lines. 
     If the waste container  57  is properly installed in position, a signal will be fed back to the embedded computer  21  indicating the waste container  57  is correctly connected. This also applies for the sample container  41  which sends a signal to the embedded computer  21  when it is properly connected. The system will not operate unless both the waste container  57  and the sample container  41  are properly positioned. 
     The center portion of the intelligent heater tube  69  is heated to the test parameter set point by flowing current through the intelligent heater tube  69 . Instrument power supplied for current generation and all other instrument controls is provided through local available power  95 . Depending on local power availability, local available power  95  may vary drastically. In some areas it is 50 cycles/sec. and in other areas it is 60 cycles/sec. The voltage range may vary from a high of 240 Volts down to 80 Volts or less. A universal AC/DC converter  97  takes the local available power  95  and converts it to 48 Volts DC. With the universal AC/DC converter  97 , a good, reliable, constant 48 Volts DC is generated. 
     The 48 Volts DC from the universal AC/DC converter  97  is distributed throughout the system to components that need power through the DC power distribution  99 . If some of the components need a voltage level other than 48 Volts DC, the DC power distribution  99  will change the 48 Volts DC to the required voltage level. 
     To heat the intelligent heater tube  69 , the 48 Volts from the universal AC/DC converter  97  is converted to 115 Volts AC through 48 Volt DC/115 Volts AC inverter  101 . While taking any local available power  95 , running it through a universal AC/DC converter  97  and then changing the power back to 115 Volts AC through a 48 Volts DC/115 Volts AC inverter  101 , a stable power supply is created. From the 48 Volts DC/115 Volts AC inverter  101 , power is supplied to the heater tube module  103 . The heater tube module  103  then supplies current that flows through the intelligent heater tube  69  via upper clamp  105  and lower clamp  107 . The heater tube module  103  is controlled by the embedded computer  21  so that during a normal test, the thermocouple  71  inside of the intelligent heater tube  69  will indicate when the intelligent heater tube  69  has reached the desired temperature. 
     While the center section of the intelligent heater tube  69  heats to desired test set point, the ends of the intelligent heater tube  69  should be maintained near room temperature. To maintain the ends of the intelligent heater tube  69  near room temperature, a coolant flows through an upper bus-bar  109  and lower bus-bar  111 . The coolant inside the upper bus-bar  109  and lower bus-bar  111  cools the upper clamp  105  and lower clamp  107  which are attached to the ends of the intelligent heater tube  69 . The preferred cooling solution is a mixture of approximately 50% water and 50% antifreeze (ethylene glycol). As the coolant flows to the coolant container  115 , the flow is measured by flow meter  113 . To circulate the coolant, a cooling pump  117  pumps the coolant solution into a radiator assembly  119 . Inside of the radiator assembly  119 , the coolant is maintained at room temperature. The radiator fan  121  helps remove heat from the coolant by drawing air through the radiator assembly  119 . From the radiator assembly  119 , the coolant flows into the lower bus-bar  111  then through upper bus-bar  109  prior to returning via the flow meter  113 . 
     The flow meter  113  is adjustable so that it can ensure a flow of approximately 10 gal./hr. The check valve  123  helps ensure the cooling system will not be over pressurized. Check valve  123  will open at around 7 psi, but normally 3-4 psi will be developed when running the coolant through the entire system. 
     To determine if the intelligent heater tube  69  is shorted out to the housing (not shown in  FIGS. 2 and 2A ), a heater tube short detector  110  monitors a short condition. If a short is detected, the embedded computer  21  is notified and the test is stopped. 
     On one end of the intelligent heater tube  69  there is a memory device  125  to which information concerning the test can be recorded by IHT writer  127 . While a test is being run on a test sample, the IHT writer  127  will record information into the memory device  125 . At the end of the test, all electronic information will be recorded onto the memory device  125  of the intelligent heater tube  69 , except for the manual tube deposit rating. To record this information, the intelligent heater tube  69  will have to be moved to another location to record the deposit rating either (a) visually or (b) through a Video Tube Deposit Rater. At that time, a second IHT writer will write onto the memory device  125 . The Video Tube Deposit Rater may be built into the system or may be a standalone unit. 
     The intelligent heater tube  69  is approximately 6-¾″ long. The ends are approximately 3/16″ in diameter, but the center portion that is heated is approximately ⅛″ in diameter. Due to very low electrical resistance of aluminum, approximately 200 to 250 amps of current flows through the intelligent heater tube  69 . Both the voltage and the current through the intelligent heater tube  69  is monitored by the embedded computer  21 , but also the temperature of the center section of the intelligent heater tube  69  is monitored by the thermocouple  71  which is also connected to the embedded computer  21 . The objective is to have the center section of the intelligent heater tube  69  at the required temperature. To generate that type of stable temperature, a stable source of power is provided through the universal AC/DC converter  97  and then the 48 VDC/115 VAC inverter  101 . By using such a stable source of power, the temperature on the center section of the intelligent heater tube  69  can be controlled within a couple of degrees of the required temperature. 
     Referring now to  FIG. 3  of the drawings, a pictorial representation of the coolant flow during a test is illustrated. Like numbers will be used to designate similar components as previously described. A pictorial illustration of the heater tube test section  129  is illustrated on the lower left portion of  FIG. 3 . Coolant from the radiator assembly  119  is provided to the lower bus-bar  111  and upper bus-bar  109  via conduit  131 . From the upper bus-bar  109 , the coolant flows via conduit  133  to flow meter  113 . From flow meter  113 , the coolant flows through conduit  135  to the coolant container  115 . The cooling pump  117  receives the coolant through conduit  137  from the coolant container  115  and pumps the coolant into radiator assembly  119 . If the pressure from the cooling pump  117  is too high, check valve  123  will allow some of the coolant to recirculate around the cooling pump  117 .  FIG. 3  is intended to be a pictorial representation illustrating how the coolant flows during a test. 
     Likewise,  FIG. 4  is a pictorial representation of the aeration system for the test sample. Similar numbers will be used to designate like components as previously described. An aeration pump  43  pumps air through conduit  139  to a dryer  45 . The dryer  45  removes moisture from the air to prevent the moisture from contaminating the test sample during aeration. From the dryer  45 , the dried air will flow through conduit  141  to humidity sensor  47 . If the percent relative humidity of the dried air blowing through conduit  141  exceeds a predetermined amount of 20% relative humidity, the system will shut down. While different types of dryers  45  can be used, it was found that Dry-Rite silica gel desiccant is an effective material for producing the desired relative humidity. 
     From the percent humidity sensor  47 , the dried air flows through conduit  143  to flow meter  49 , which measures the air flow through conduit  143  and air supply conduit  145 . From air supply conduit  145 , the dried air flows through aeration check valve  50  and conduit  146  sample container arm mounting clamp  147  and sample container arm  149  to aeration conduit  151  located inside of sample container  41 . In the bottom of sample container  141 , a glass frit  153  connects to aeration conduit  151  to cause the dried air to sparge through the test sample in sample container  41 . When the sample container  41  is in place and the sample container arm  149  is connected to the sample container arm mounted clamp  47 , contact  155  sends a signal to the embedded computer  21  (see  FIG. 2 ) indicating the sample container  41  is properly installed. 
     Referring now to  FIG. 5 , a pictorial illustration of the flow of the test sample in connection with  FIGS. 2 and 2A  is shown in a schematic flow diagram. The test sample is contained in sample container  41 , which is connected via sample container arm  149  to the sample container arm mounting clamp  147 . Vapors given off by the test sample are discharged through a vent  157 , normally through a vent hood to atmosphere. Simultaneously, the sample drive pump  53  draws some of the test sample out of the sample container  41 . The sample drive pump  53  is a single stroke HPLC pump connected to a pulse dampener  159 . While the pulse dampener  159  may be configured a number of ways, the pulse dampener  159  in the preferred configuration has a diaphragm with a semi-compressible fluid on one side of the diaphragm. This fluid is more compressible than the test sample thereby reducing pressure changes on the test sample flow discharged from the sample drive pump  53 . The sample drive pump  53  is connected to auto pump priming valve  55 . During start-up, the closed auto pump priming valve  55  opens until all of the air contained in the pump and the lines are discharged into the waste container  57 . In case it is needed, a manual priming valve  161  is also provided. Additionally, the aeration pump  43  (see  FIG. 2 ) is turned ON to provide a slight pressure in the sample container  41  of about 2 to 3 psi. The sample vent valve  54  closes to prevent this pressure from escaping to atmosphere. This pressure will help push the fluid sample from the sample container  41  to the inlet of the sample drive pump  53 . The 5 psi check valve  56  prevents the pressure in the sample container exceeding 5 psi. During the test, coil  163  also provides further dampening in addition to the pulse dampener  159 . Check valve  59  ensures there is no back flow of the sample fuel to the sample drive pump  53 . However, at the end of a test, flush check valve  89  receives air from flush air pump  87  to flush the test sample out of the system. 
     During normal operation of a test, the sample fuel will flow through check valve  59  and through a prefilter  61  removing most solid particles. Following the prefilter  61 , the test sample flows into the heater tube test section  129  and then through the differential pressure filter  73 . Each side of the differential pressure filter  73  connects to the differential pressure sensor  75 . Also connected to the differential pressure filter  73  is the back pressure regulator  79 . The pressure on the system is continuously monitored through the system pressure transducer  81 . If for any reason pressure on the system needs to be released, system/safety vent valve  85  is energized and the pressurized test sample is vented through the four-way cross connection  165  to the waste container  57 . 
     At the beginning of the test, to ensure there is no air contained in the system, the differential pressure plus vent valve  91  and the differential pressure minus vent valve  93  are opened to vent any pressurized fluid through the four-way cross connection  165  to the waste container  57 . 
     In case the differential pressure filter  73  clogs so that the differential pressure exceeds a predetermined value, differential pressure bypass valve  77  will open to relieve the pressure. 
     To determine the exact flow rate of the test sample through the system, the sample flow meter  83  measures the flow rate of test sample from the back pressure regulator  79  before being discharged through the waste container arm  167  and the waste container clamp  169  into the waste container  57 . The waste container  57  is vented all the time through vent  171 . 
     Sample Drop Counter and Flow Meter 
     The sample drop counter and flow meter  83  as shown in  FIG. 2  is shown in more detail in  FIG. 6 . A sample in  300  is from the back pressure regulator  79  shown in  FIG. 2 . Since the flow rate is very low, the sample in  300  will be in the form of drops  302 . As the drops  302  are released, the drops will interrupt the light from the infrared LED  306  that is being received by the photodiode  304 . When that occurs, LED  308  lights up indicating that a drop has been detected. Also, the signal from photodiode  304  that indicates a drop has been received is also fed to counter  310 . Since the counter  310  is a straight binary counter, converter  312  changes the parallel signal to a serial signal. The serial signal connects over a four-wire bus  314  to connector  316 . Connector  316  connects to embedded computer  21  previously shown in connection with  FIG. 2 . 
     At the beginning of a volumetric flow rate measurement, the valve  318  is closed by a signal from embedded computer  21  via connector  316 . The signal also lights up LED  320  indicating a flow rate measurement is taking place. Thereafter, drops  302  of the test sample accumulates in the sample flow container  322 . Once enough drops  302  of the test sample have accumulated in the sample flow container  322 , level zero measured by infrared LED  324  and photodiode  326  will be reached. A level zero of fluid will interrupt light from the infrared LED  324  so that it is not received by a photodiode  326  which sends a signal through the converter  312  and connector  316  to the embedded computer  21 . Simultaneously, the LED  328  is lit up indicating the level zero of sample liquid has been detected. This also starts the beginning of a timing cycle to determine the flow rate. 
     Thereafter, as drops  302  of the test liquid continue to accumulate inside of sample flow container  322 , level one of the fluid will be detected when the infrared signal from the infrared LED  330  is interrupted so that it is no longer received by photodiode  332 . This sends a signal through converter  312  and connector  316  to the embedded computer  21 . Simultaneously, LED  334  will be lit indicating the sample liquid has reached level one. 
     By knowing the exact size of a sample flow container  322  between level zero and level one, the flow rate of the fuel under test can be accurately determined. In a prototype built by Applicant, there was a 9 mL volume between level zero and level one. 
     To keep the drops  302  from interfering with the signals from either (a) infrared LED  330  and photodiode  332  or (b) infrared LED  324  and photodiode  326 , a deflector  336  is contained within sample flow container  332  below the forming of the drops  302 , but above infrared LED  330  and photodiode  332 . The deflector  336  directs the drops  302  to the inside wall of the sample flow container  322  so that the drops run down the inside wall thereof. In this manner, the drops  302  will not interfere with the infrared signals being picked up by either photodiode  326  or photodiode  332 . The deflector  336  may be made from glass or any other material that is not corrosive when coming in contact with fuels or similar materials. 
     Once level one of the liquid under test has been detected by photodiode  332  and the signal sent through converter  312  and connector  316  to embedded computer  21 , the embedded computer  21  can then send a signal back through connector  316  to open valve  318 . The sample out  338  received from valve  318  goes to the waste container  57  shown in  FIG. 2 . To prevent pressure build up in the sample flow container  322 , overflow connection  340  is provided to vent line  342  which connects to vent out  344 . Vent out  344  is the same as the waste container vent shown in  FIG. 2 . To supply power to the components of sample flow meter  83 , a positive 3.3 VDC is provided along with a ground connection from DC power distribution  99  as shown in  FIG. 2A . When power is applied, LED  346  will light up indicating power has been received. 
     By use of the sample drop counter and flow meter  83  as just described in connection with  FIG. 6 , the flow rate of the test sample is accurately determined. Therefore, the flow rate of the fluid being tested is accurately checked. The sample drop counter and flow meter  83  is located after the heater tube test section, and therefore does not influence the results of the test.