Abstract:
A detector for monitoring the low levels of differential pressures and the rate of mass flow rate of gas (air, e.g.) in a duct. The sensor&#39;s temperature is maintained at a constant gradient above temperature of the flowing gas, typically 4-7° C. higher. The detector consists of two thermally decoupled sensors—one is the air temperature sensor and the other is a temperature sensor coupled to a heater. The sensors are connected to an electronic servo circuit that controls electric power supplied to the heater. The sensors are positioned outside of the air duct and coupled to the duct via a relatively thin sensing tube protruding inside the duct. The end of the tube has an opening facing downstream of the gas flow, thus being exposed to a static gas pressure. The detector can be employed in fuel burners of the HVAC systems, internal combustion engines, medical equipment to control flow of anesthetic gases, in the security systems to monitor minute changes in air pressure resulted from opening and closing of doors and windows in a protected facility.

Description:
[0001]    The present invention relates generally to devices for measuring and monitoring differential gas pressure and relatively low rates of gas flow. It is based on U.S. Provisional Patent Application No. 60/841,663 filed on 09/01/2006. 
       BACKGROUND OF INVENTION 
       [0002]    In many types of equipment that use movements of air or other gases, measuring the mass flow rate of the gas and/or static gas pressure is very important. An example is a HVAC system of a residential dwelling that incorporates an air filter. Typically, mass of gas (air, in particular) is driven through the dwelling or machinery by means of a forced convection. The purpose of the air filters is to remove airborne contaminants that may adversely effect health of humans and animals, cause malfunction or reduction in efficiency of equipment, and deposits of soiling compounds onto various surfaces. A typical HVAC system is shown in  FIG. 1  where the house  1  has generally atmospheric pressure P 2  due to exposure to atmosphere through doors and windows  80 . The HVAC system is comprised of the blower  5 , air conditioner/burner  6 , and air filter  7 . The blower  5  moves air. The air flow  2  is indicated by arrows. The static negative air pressures (as compared to the atmosphere) are formed across the air filter  7 , adjacent to it sides  8  and  9 . These pressures cause the air flow  2  through the filter and through the air duct  4  connected to the house. The negative air pressures at sides  8  and  9  are shown in  FIG. 2  and the air flow rate is shown in  FIG. 3  as function of the air filter  7  clogging, in percents, where a 100% clogging is a total filter blockage. It is clear from  FIGS. 2 and 3  that the filter contamination can be detected either from monitoring the air flow rate or from one or both static air pressures across the filter  7 . To detect the air filter clogging in a HVAC system, the monitor  10  with a sensor  11  can be installed into an air duct either upstream or downstream from the air filter  7 . Note that the sensor  11  can be either a flow sensor or pressure sensor. 
         [0003]    In the automotive applications and various types of fuel burners, providing a right fuel-to-air ratio is critical for the device efficiency and reduction of pollution. In these devices monitoring either a pressure at the blower or air flow can be very beneficial. 
         [0004]    In medical equipment used for anesthesiology, gases should be efficiently mixed for safety and correct medical effect. A flow monitoring is an important part of the gas delivery control system. 
         [0005]    In security systems, one method of detecting an intrusion into a protected area is monitoring variations in air pressure that may result from closing and opening of doors and window. All the above requires monitoring of very low changes in air pressure and inexpensive and safe monitoring of the air mass flow rate. 
         [0006]    In this patent, we use word “air”, although it should be understood that any gas or mixture of gases can be monitored in a similar manner. 
       DESCRIPTION OF PRIOR ART 
       [0007]    There are two ways of determining the air flow velocity: indirect and direct. In the indirect way, the air flow rate is computed from a differential pressure. Historically, was determined by a calculation that used two values of pressures: the total pressure in a flow and the static pressure. Originally, this method employed the Pitot tubes where the first tube (total pressure) has an opening facing upstream and the other tube (static pressure) had one ore more openings either facing downstream or normal to the flow. Pressures at the Pitot tube outputs could be measured by many types of the pressure sensors, ranging from the water manometers to the solid-state sensors fabricated by the MEMS technologies. 
         [0008]    Air flow always depends on the pressure difference across the tube of flow. When the air flow velocity is computed from a pressure differential P 1-2 , the following equation may be employed: 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       v 
                       
                         1 
                         - 
                         2 
                       
                     
                     = 
                     
                       
                         2 
                          
                         g 
                          
                         
                             
                         
                          
                         
                           
                             P 
                             
                               1 
                               - 
                               2 
                             
                           
                           kd 
                         
                       
                     
                   
                   , 
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
         [0000]    where g is the acceleration of gravity, k is the flow resistance coefficient, and d is the air density. 
         [0009]    A determination of the differential air pressure can be done by placing two static pressure transducers across the air flow restriction, such as the air filter  7  as in  FIG. 1 . One problem in using the indirect method is a difficulty in monitoring very low air flow rates which may be as low as few mm of H 2 O. This requires very sensitive pressure detectors that in many cases may be too complex or too expensive or not sufficiently accurate. 
         [0010]    In a direct way of the air flow measurement, a special air flow sensor (detector) is used. Its output signal is caused by the moving air molecules, rather than by the air pressure differential in the flow. A popular type of a direct sensor is a hot wire anemometer where a wire is heated to high temperatures, 50° C. or higher above the air temperature. The air velocity is measured from the heat loss due a cooling effect by a moving air. A hot wire anemometer is based on the principle of thermo-anemometry. A general principle of thermo-anemometry is well known in industry for measuring flow of fluids. The basic theory of it is described in book:  Jacob Fraden. Handbook of Modern Sensors . Springer Verlag. 3 rd  ed., pp. 359-380. A U.S. Pat. No. 6,543,282 issued to Thompson is an example of a flow sensor based on the thermo-anemometry principle. A low temperature heater of a thermal anemometer is described in U.S. Pat. No. 7,178,410 issued to Fraden et al. 
         [0011]    Another method of gas flow measurement is based on the ultrasonic and electromagnetic techniques. And another method employs a mechanical rotating vane anemometer. The direct methods of air flow monitoring generally may be very sensitive and sufficiently accurate to monitor low flow rates. On the other hand, the direct sensors may require a direct placement into the flow duct that may increase cost and make maintenance more difficult. 
         [0012]    It is therefore the goal of this invention to provide a sensing device for continuous monitoring of gas mass flow rate; 
         [0013]    It is another goal of this invention to provide a sensor that is capable of monitoring low levels of a differential air pressure; 
         [0014]    And another goal of the invention is to provide an air flow sensor that can operate over a broad range of temperatures; 
         [0015]    It is also a goal of this invention to provide an air flow sensor that is simple and inexpensive; 
         [0016]    A further goal of this invention to provide a security indicator responsive to small changes in the air pressure; 
         [0017]    And another goal is to provide a method of monitoring of air flow in an air intake for the fuel burners and combustive engines. 
       SUMMARY OF INVENTION 
       [0018]    The invention is based on a combination of the principle of a thermo anemometry and the classical Pitot tubes arrangement. The air flow detector is comprised of a sensing tube having an opening and two sensors where one is the air temperature sensor and the other is a similar temperature sensor being thermally coupled to a heater, whereas both sensors and the heater and connected to an electronic servo control loop. The servo circuit output represents both the air mass flow rate and the differential air pressure between the sensing tube opening and the opposite side of the flow detector. 
     
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0019]      FIG. 1  is a schematic diagram of a dwelling with the HVAC system. 
           [0020]      FIG. 2  is the graphs of pressures at air filter sides as function of its clogging. 
           [0021]      FIG. 3  is a graph showing dependence of the air flow rate as a function of the air filter clogging. 
           [0022]      FIG. 4  show an air ducts with a bypass tube with the air flow sensor. 
           [0023]      FIG. 5  shows a sensing tube with multiple openings facing downstream. 
           [0024]      FIG. 6  shows a sensing tube with an opening normal to the flow direction. 
           [0025]      FIG. 7  shows a flow/pressure sensor with the sensing tube inside the air duct. 
           [0026]      FIG. 8  represents a thermo-anemometer sensor with a thermistor. 
           [0027]      FIG. 9  is a thermo-anemometer sensor with a thermo-couple 
           [0028]      FIG. 10  depicts a thermo-anemometer sensor fabricated with the MEMS technology. 
           [0029]      FIG. 11  is circuit diagram of a servo loop for the air flow sensor with thermistor sensors. 
           [0030]      FIG. 12  shows a graph of the servo circuit output voltage as function of mass flow rate. 
           [0031]      FIG. 13  shows a pressure differential detector for a security system. 
           [0032]      FIG. 14  illustrates a security sensor installed into a wall between two adjacent rooms. 
           [0033]      FIG. 15  shows is circuit diagram of a servo loop for the air flow sensor with thermo-couples. 
           [0034]      FIG. 16  is an illustration of a flow-sensor assembled on a circuit board. 
           [0035]      FIG. 17  depicts an air flow rate near the sensor in a security system. 
           [0036]      FIG. 18  shows a flow probe arrangement for an internal combustion engine. 
       
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0037]    As it follows from Eq. (1), a differential air pressure can be computed from the air flow rate from the following formula: 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       P 
                       
                         1 
                         - 
                         2 
                       
                     
                     = 
                     
                       
                         v 
                         
                           1 
                           - 
                           2 
                         
                         2 
                       
                        
                       
                         kd 
                         
                           2 
                            
                           
                               
                           
                            
                           g 
                         
                       
                     
                   
                   , 
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
     
         [0000]    and therefore, a differential pressure measurement may be substituted by measurement of the air flow as shown in a general diagram of  FIG. 4 . The air duct  26  contains an air flow restriction  27  that can be caused, for example, by an air duct geometry, the air filter, or other components. The air flow  22  is produced by the air blower  5 . A bypass tube  28  circumvents the flow restriction  27 , thus diverting a small portion of air flow  29  to go through the bypass tube  28  and exit as flow  30  at the other side of restriction  27 . The respective ends of the bypass tube  28  are exposed to two air pressures, P 1  and P 2 . An air flow monitor  10  is installed at the bypass tube  28  with its flow sensor  11  being exposed to the interior of the bypass tube  28 . Since the airflow  30  inside the bypass tube  28  related to a differential pressure P 1-2 =P 1 −P 2 , this pressure can be computed with the use of Eq. (2). 
         [0038]    While  FIG. 4  illustrates a general operating principle of the present invention, the practical implementations can take various forms. A preferred embodiment of the monitor  10  with a flow sensor is depicted in  FIG. 7 . The air duct  4  conducts air flow  22 . At a particular spot of its inner cross-section, a static air pressure P 1  exists (with respect to the external pressure). The sensing tube  15  is inserted into the air duct  4  to be positioned near a spot of the interest (the one with pressure P 1 ). The tube has an opening  36  facing downstream from the flow  22 . At the other side of the sensing tube  15 , there is an inlet tube  31  with the opening  16  exposed to external pressure P 2  that may be the atmospheric pressure. In-between the sensing tube  15  and the inlet tube  31 , there are two sensors: the reference temperature sensor  18  supported by wires  25  and the TA sensor  17  that is supported by wires  23  and  24  (TA stands for “thermo-anemometer”). Both sensors are mounted on a printed circuit board  19  that has an opening  68  to allow air flow  20  to pass by the sensors  17  and  18 . Note that this arrangement responds to an absolute pressure differential and absolute value of air flow (regardless of the direction of the air flow  22 ). This means that the air flow  20  may go in either direction inside the interior  14  of the sensing tube  15 , depending whether P 1-2  is positive or negative. It is important that sensors  17  and  18  are thermally decoupled from one another. The sensing tube  15  may have various types of openings. If the opening faces downstream, the measured pressure will be static. If it faces upstream, the measured pressure will be static plus dynamic.  FIG. 5  shows the multiple openings  33   a ,  33   b , etc. which allow exposing the tube&#39;s  15   a  interior to different points of the air flow  22  and thus to different static air pressures. The integral air flow through the sensing tube  15   a  will be the function of all these pressures. Another practical type of an opening is depicted in  FIG. 6 , where at least one opening  35  is made at the end of the sensing tube  15   b . This opening(s)  35  is normal to the air flow  22  and thus is exposed to the static pressure. Note that optionally an additional side opening  33  may be combined with the end opening(s)  35 . 
         [0039]      FIG. 8  depicts the TA sensor  17  built on a substrate  40  which can be a ceramic, plastic or metal. If metal, the substrate  40  should have electrically isolated front surface  37 . On the front surface  37 , a resistive layer (heater  42 ) is deposited. It has a typical resistance between 10 and 100 Ohms. The heater  42  is connected to terminals  55  and  56 . The heater  42  temperature can be elevated by passing electric current through terminals  55  and  56 . A temperature sensor  41  is attached and thermally coupled to the heater  42  so that temperature of the heater may be measured. As the temperature sensor  41 , various types of temperature sensors can be employed. One example is an NTC thermistor with the top-bottom electrodes  59  and  60  attached to the conductors  53  and  54 . Since the TA sensor  17  is exposed to the air flow, for a better protection from the airborne contaminants, it may be enveloped by a protective coating (not shown), such as glass, epoxy, etc. Thermal conductivity of such a layer should be as high as practical.  FIG. 9  illustrates another design of a temperature sensor with a thermo-couple joint  113  of two dissimilar wires  111  and  112 . 
         [0040]    A reference sensor  18  is a small conventional temperature sensor fabricated, for example, in a bead shape and is not depicted here. It shall be positioned in the same air flow as the TA sensor  17  but must be thermally decoupled from the TA sensor  17 . The location of both sensors in the air flow  20  is illustrated in  FIG. 7 . 
         [0041]    An air flow detector design which is a combination of a reference sensor  18  and the TA sensor  17  fabricated with the MEMS technology is shown in  FIG. 10 . The combined sensor is fabricated as a silicon frame  61  with opening  63  where air flow can pass though. All electrical parts are formed and deposited on the front surface  62 . A thin membrane  64  is etched in the center of the opening  63  and is supported by the silicon links  65 . A thickness of the membrane  64  may me on the range of 1 micrometer. A resistive heater  42  is formed on the membrane  64  while the temperature sensor  41  is also located on the same membrane  64 . The heater  42  and temperature sensor  41  may be either on top of one another or inter-digitized side by side. It is important that they are thermally coupled. The reference temperature sensor  18  is positioned on the frame  61  and exposed to the same air flow. The sensor  18  is connected to the terminal pads  57  and  58 . The temperature sensors  18  and  41  can be resistive, semiconductive or thermoelectric. The resistive heater  42  is connected to terminals  55  and  56  while the second temperature sensor  41  is connected to conductors  53  and  54  via the conductive paths  66 . The combined sensor of  FIG. 9  can be positioned at the opening  68  ( FIG. 7 ) in place of the discrete sensors  17  and  18 . An alternative design of the MEMS sensor is without the opening  63  where the air flow hoe parallel to membrane  64  which is directly supported by the frame  61 . 
         [0042]      FIG. 11  shows a servo circuit diagram where the temperature sensors are the NTC thermistors. The TA sensor  17  consists of thermally coupled thermistor temperature sensor  41  and heater  42 . Along with the reference temperature sensor  18  they are exposed to air flow  22  passing through the sensing tube  15 . A thermal insulator  100  is positioned between the sensors  17  and  18 . A thermal insulator may be an air gap between the sensors as illustrated in  FIGS. 7 and 10 . The reference temperature sensor  18  measures the air temperature while the TA sensor measures the heat loss resulted from the air flow. These two sensors  18  and  17  along with two resistors  43  and  44  form a Wheatstone bridge circuit having the outputs  48  and  49  connected to the servo amplifier  46 . Two additional resistors  51  and  52  can be connected to the reference temperature sensor  18  for improving its operation over a broader range of the air temperatures. The ratio of the resistors  43  and  44  is such as to correspond to the second temperature sensor  41  be warmer than the reference temperature sensor  18  by a constant thermal gradient of several degree C., typically, 4-7° C. 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       
                         R 
                         a 
                       
                       
                         R 
                         g 
                       
                     
                     = 
                     
                       
                         R 
                         43 
                       
                       
                         R 
                         44 
                       
                     
                   
                   , 
                 
               
               
                 
                   ( 
                   3 
                   ) 
                 
               
             
           
         
       
     
         [0000]    where R a  is the combined resistance of the reference temperature sensor  18  at the air temperature T a , and the resistors  52  and  51 , R g  is the resistance of the temperature sensor (inside the TA sensor  17 ) when its temperature T g =T a +g, R 43  and R 44  are the resistances of the resistors  43  and  44  respectively, and g is the constant temperature gradient. The output of the servo amplifier  46  drives the current amplifier  47  that is capable of pushing a sufficient electric current through the heater  42 . The purpose of the servo circuit is to balance the Whetstone bridge by elevating temperature of the heater  42  and, subsequently, of the second temperature sensor  41 . When air flow  22  cools down the TA sensor  17 , more current is required through heater  42  to maintain the constant temperature gradient above ambient temperature that is measured by the reference temperature sensor  18 . The servo amplifier  46  may be substituted with a micro-controller having a software that provides a PID function to control the heater  42 . The voltage  50  across the heater  42  is the output of the measurement circuit that represents the magnitude of the mass flow rate through the test tube  15 .  FIG. 11  illustrates dependence of the output voltage  50  from the air mass flow rate. Note that at zero flow rate, the output has a bias of V 0 . 
         [0043]    Another embodiment of the flow sensor can use thermo-couples as temperature sensors. This is illustrated by the servo-circuit of  FIG. 15  where a “hot” thermo-couple  113  is connected in series with a “cold” thermocouple  114  and, in turn, to a pre-amplifier  115 . A reference signal  117  is applied to the servo amplifier  46 . The rest of the circuit operates similarly to the circuit of  FIG. 11 . The “cold” thermocouple  114  measures the air temperature. Note that an additional heating element  115  may be added. It&#39;s function is to compensate for the conductive heat losses from heater  113  via the supporting structure. This idea is further illustrated in  FIG. 16  which shows an air flow sensor fabricated on a miniature circuit board  118 . The board may also carry an electronic circuit  119 . Note that different parts of the circuit board  118  has cut-outs  120 - 124  to reduce a conductive heat flow from the sensing heater  42  toward the reference (“cold”) thermocouple junction  125 . The thermocouple wires  127  and  128  pass under the additional heating element  115  before forming a “cold” junction  114 . The heating element conductors are not shown in  FIG. 16 . 
         [0044]    Security System Applications 
         [0045]    To illustrate how the present invention can be employed in a security system, consider  FIG. 13 . The purpose of the security device is to respond to relatively rapid changes in the air pressure inside a building. Normally, air pressure in a protected facility changes relatively slowly, along with the external atmospheric pressure. When a door or window is being closed or opened, the air pressure may vary. This can be detected by the device of  FIG. 13 . The arrangement is similar to one shown in  FIG. 7  with the following differences. A short tube  90  (between 0.5 and 5″ long) is exposed to the room pressure P 1 . The reference sensor  18  and TA sensor  17  are positioned at the other end of the tube  90  at the opening  68  of the board  19  and supported by wires  25  and  69 . The other side of the board  19  is covered by enclosure  74  which has the internal pressure P 2 . When pressure P 1  changes, air flow  22  goes through the tube  90 , the opening  68  to the enclosure  72 . At least one hole  72  in the enclosure helps to facilitate the air movement. The servo circuit  71  is connected to the board  19  and generates the output signal  101  that is fed into the processor  73 . The variable pressure differential is shown in  FIG. 17 . The servo circuit output signal has a shape similar the pressure signal of  FIG. 17 . The processor  73  analyzes rates of the differential pressure changes and identifies if the rate of change is higher than a pre-set threshold value. It is seeing that the rate Δ b  is greater than Δ a . When the rate of change is sufficiently high, the alarm  70  is initiated. 
         [0046]      FIG. 14  shows how the similar principle can be employed for two adjacent rooms in a building. The rooms A and B are separated by a wall  75  and have different air pressures P 1  and P 2 , respectively. The sensors  17  and  18  are positioned between two receptive tubes  76  and  78  that respectively face the rooms A and B. The variable air flow  22  is resulted from the variations in pressures in one or both rooms and can be processed in the circuits similar to  FIGS. 11 and 15 . 
         [0047]    Burners and Internal Combustion Engine Applications 
         [0048]    A sensor based on the present invention as described above has a natural application for the fuel burners and automotive machinery where the internal combustion engines are in use.  FIG. 18  illustrates parts of a gasoline engine with the air filter assembly  84 . Air inlet  82  is positioned upstream from the air filter  84  and carries air flow sensing tube  15 . The tube  15  is connected through a flexible tubing  83  to the air flow monitor  10 . The monitor contain an air flow sensor that is built in accordance to one of the described or implied embodiments of this invention. The monitor  10  is further connected to a signal processor (not shown) that makes use of data received from the air flow monitor  10 . One possible use of such monitoring is the detecting of an air filter clogging. The other use is controlling the rate of air intake and control the air-to-fuel mixing ratio to increase the engine or burner efficiency.