Abstract:
An integrated module is provided for measuring a flow rate of a fluid, whether gaseous or liquid, with a flow restrictor comprising a plurality of orifices adapted to a flow channel of the integrated module and a sensor mounted to measure a property of the fluid at said flow restrictor corresponding to the flow rate. The integrated module provided may be used in numerous flow systems, such as reactors, ventilators and respirators, and has the benefit of better laminarization of the flow as well as better calibration between the flow sensor and the flow restrictor for more accurate flow measurements.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to flow sensors and, more particularly, to a flow sensor with an integrated flow restrictor. 
     2. Description of Related Technology 
     Flow rate control mechanisms are used in a variety of flow systems as a means for controlling the amount of fluid, gaseous or liquid, traveling through the system. In large-scale processing systems, for example, flow control may be used to affect chemical reactions by ensuring that proper feed stocks, such as catalysts and reacting agents, enter a processing unit at a desired rate of flow. Additionally, flow control mechanisms may be used to regulate flow rates in systems such as ventilators and respirators where, for example, it may be desirable to maintain a sufficient flow of breathable air or provide sufficient anesthetizing gas to a patient in preparation for surgery. 
     Typically, flow rate control occurs through the use of control circuitry responsive to measurements obtained from carefully placed flow sensors. One such flow sensor is a thermal anemometer with a conductive wire extending radially across a flow channel and known as a hot-wire anemometer. These anemometers are connected to constant current sources which cause the temperature of the wire to increase proportionally with an increase in current. In operation, as a fluid flows through the flow channel and, thus, past the anemometer, the wire cools due to convection effects. This cooling affects the resistance of the wire, which is measured and used to derive the flow rate of the fluid. Another form of thermal anemometer flow sensor is a microstructure sensor, either a microbridge, micro-membrane, or micro-brick, disposed at a wall of a flow channel. In this form, the sensors ostensibly measures the flow rate by sampling the fluid along the wall of the flow channel. In either application, the thermal anemometer flow sensor is disposed in the flow channel for measuring rate of flow. 
     There are numerous drawbacks to these and other known flow sensors. One drawback is that the proportional relationship upon which these sensors operate, i.e., that the conductive wire or element will cool linearly with increases in the flow rate of the fluid due to forced convection, does not hold at high flow velocities where the sensors become saturated. This saturation can occur over a range from 10 m/s to above 300 m/s depending on the microstructure sensor, for example. As a result, in high flow regions, measured resistance of an anemometer, or other sensor, no longer correlates to an accurate value of the flow rate. Furthermore, because these sensors reside in the main flow channel, they are susceptible to physical damage and contamination. 
     In addition to these drawbacks, known flow sensors are susceptible to mis-measurement due to turbulent flow effects, i.e., non-uniformity in flow velocity and pressure, both of which exist to some degree in all flow systems. Furthermore, conventional hot-wire anemometers have a slow time response and therefore do not produce accurate flow rate values upon abrupt changes in flow velocity. In addition, they require high input power to keep the entire length of wire at an elevated temperature at zero flow. 
     In contrast, wall-mounted thermal microstructures may have a relatively fast time response, but offer little advantage over the hot-wire anemometers because their response times are too fast, producing flow rate values that fluctuate with turbulence conditions instead of averaging out the noise associated with such turbulence. Therefore, as the flow rate of the fluid increases, turbulence increases and the wall-mounted thermal microstructure will produce increasingly erratic measurements in response thereto. 
     An indirect flow sensor measuring technique that measures flow rate from a sensor positioned outside of the flow channel and improves upon some of the drawbacks of the foregoing, has been designed. In one form, ΔP pressure sensors measure a pressure drop across a flow restrictor, which acts as a diameter reducing element in the flow channel thereby creating a difference in pressure between an entrance end and an exit end of the flow restrictor. These flow restrictors have been in either honeycomb-patterned or porous metal plate flow restrictors. The pressure sensors are disposed in dead-end channels to measure the pressure drop due to the flow restrictor, with this pressure drop being proportional to the flow rate of the fluid. In other forms, the indirect flow mechanism can use a translucent tube disposed near the flow channel with a free-moving ball or indicator that rises and falls with varying flow rate conditions in the flow channel, or a rotameter, such as a small turbine or fan, that operates as would a windmill measuring wind rate. 
     Though they offer some improvements over sensors disposed directly in the flow channel, all of these indirect flow sensors are hampered by calibration problems. An indirect flow sensor may be calibrated to work generally with certain types of restrictors, e.g., honeycomb restrictors, but imprecise restrictor geometry results in variations in pressure and, therefore, variations in measured flow rate. And, furthermore, the sensors are not calibrated for use with other types of restrictors. 
     In addition to these deficiencies of indirect flow sensors, known flow restrictors further hamper flow measuring mechanisms because they do not produce uniform, laminarizing flow of the fluid. Non-uniformities in the cross-sectional area and position of the orifices in known flow restrictors result in such non-uniform flow, an example of which occurs in a honeycomb restrictor where orifices abutting the outer wall of the flow channel are truncated to conform the restrictor to circular shape of the wall. Moreover, non-linear correlations between pressure and flow rate result from this non-uniformity, especially at higher flow rates. 
     Therefore, to overcome the foregoing shortcomings, it is desirable to have a flow sensor and integrated flow restrictor that reduces calibration errors and that is adapted to reduce the flow rate measurement errors created by turbulence effects at the outer edges of the flow channel and to do so at an affordable cost. 
     SUMMARY OF THE INVENTION 
     In accordance with an aspect of the invention, an integrated module for measuring a flow rate of a fluid in a flow system has a housing that defines a flow channel through which a portion of the fluid flows and a flow restrictor, with an entrance end and an exit end, disposed in the flow channel. The flow restrictor comprises a plurality of orifices adapted to the flow channel to produce substantially uniform flow across the flow channel at the exit end. 
     In accordance with another aspect of the invention, a module for measuring a flow rate of a fluid in a flow system is provided with a flow restrictor disposed in a flow channel; a flow sensor disposed in a sensing channel communicating with the flow channel via a sensing tap with an inlet end and an outlet end such that the flow restrictor creates a pressure drop across the sensing channel allowing a portion of the fluid in the flow channel to flow into the sensing channel; and a housing wherein the flow restrictor, the flow sensor, and the sensing channel are integrally formed. 
     In accordance with yet another aspect of the invention, a flow restrictor, for use in a flow channel, comprises a plurality of orifices adapted to the flow channel to produce a substantially uniform flow across the flow channel at an exit end of the flow restrictor. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross-sectional view of an integrated module in accordance with the invention, wherein a flow restrictor, flow sensor, and housing are shown. 
     FIG. 2 is a front view of the flow restrictor in the preferred embodiment of the invention. 
     FIG. 3 is a top view of an exemplary flow sensor that may be used in the invention. 
     FIG. 4 is a cross-sectional view of the flow sensor of FIG. 3 with an integrated flow tube disposed thereon in accordance with an embodiment of the invention. 
     FIG. 5 is a trimetric view of a pre-assembled molded integrated module in accordance with a first embodiment of the invention in which the module is formed of a singular structure. 
     FIG. 6 is a trimetric view of a pre-assembled integrated module in accordance with a second embodiment of the invention in which the module is formed of adapters for connecting to an existing flow system. 
     FIG. 7 is a cross-sectional view of the housing of FIG. 1 employing extension tubes to allow the module to be used with high flow rates. 
     FIG. 8 is a cross-sectional view of an integrated module with a flow restrictor extending beyond an inlet and an outlet tap of the sensing tap of the flow channel according to a third embodiment of the invention that may be implemented in a similar manner to the first and second embodiments. 
     FIG. 9 is a trimetric view of an inlet or outlet screen of FIG. 1 showing a honeycomb patterned structure. 
     FIG. 10 is a cross-sectional view of a variation of any of the embodiments shown in FIGS. 5-8 in which a pressure sensor is disposed in a channel communicating with a flow channel of the integrated module. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     An integrated module  30  for use in a flow system to measure the rate of flow of a fluid therein is provided in FIG.  1 . As will be apparent from the following description, the integrated module  30  may be used in numerous flow systems in which flow rate measurement is necessary, such as in a ventilator or a respirator. Further, the integrated module  30  is an improvement over the prior art addressing one or more of the problems thereof, as described above, including having a flow channel with an integrated flow restrictor and flow rate sensor that allows for calibrated flow rate measurements. 
     The integrated module  30  comprises a housing  32  defining a flow channel  34  into which an entering fluid may flow from a flow system via an inlet end  36 , with the fluid exiting the flow channel  34  at an outlet end  38 . The flow channel  34  preferably has a cross-sectional shape and size compatible with that of existing flow systems, such as to fit a 22 mm conical connector as used in ventilators and respirators and which allows approximately 200 L/min of flow rate for the entering fluid. Other diameters and varying diameters may be used. However, to ensure that the integrated module  30  is calibrated to measure an accurate flow rate of the fluid and to maintain the flow rate at the outlet end  38  at substantially the same rate as at the inlet end  36 , the flow channel  34  is designed to have substantially the same cross-sectional shape and size throughout a length running along an axis  40 . 
     The integrated module  30  has a flow sensor  42  disposed in a bypass sensing channel  44  parallel to the flow channel  34 . The sensor  42  is preferably a microbridge sensor as exemplarily shown in FIG.  3  and discussed below. In operation, as the fluid flows through the flow channel  34  in the direction indicated, a portion of the fluid flows through the sensing channel  44  so that the flow sensor  42  can measure the flow rate of the fluid in the flow channel  34  indirectly without being exposed to the damage or fluctuating conditions existing in typical flow channels. In FIG. 1, the sensing channel  44  is formed by disposing a flow tube  46  over a sensing tap  48  in the flow channel  34 , communicating therewith, that allows a portion of the entering fluid to flow into the sensing channel  44 . Typically, the flow rate of the fluid in the sensing channel  44  will be a fraction of the flow rate of the fluid in the flow channel  34 . 
     The sensing tap  48  of the embodiment in FIG. 1 is formed from an inlet tap  50  and an outlet tap  52  shaped with a circular cross-sectional shape, disposed on opposing sides of an entrance end (closest to the inlet end  36 ) and an exit end (closest to the outlet end  38 ) of a flow restrictor  54 . The circular inlet and outlet taps  50 ,  52  terminate at mounting indentations  56 ,  58 , respectively, which are fitted to receive O-rings for sealibly mounting the flow tube  46  to the sensing tap  48  to reduce leakage of the tapped portion of the fluid. Though the sensing tap  48  is shown with two taps  50 ,  52 , other numbers of taps could be used and other methods of communicating flow from the channel  34  to channel  44  may be used. And, such other methods are considered within the scope of the invention. 
     The flow restrictor  54  creates a pressure drop across the inlet  50  and outlet  52  of the sensing channel which facilitates fluid flow into the sensing channel  44 . This pressure drop, or pressure differential, is dependent on restrictor geometry and increases with flow rate. Furthermore, the fluid in the flow channel will have an increasingly turbulent flow as the flow rate of the fluid increases, i.e., an increasing non-uniform pressure and velocity across a given plane orthogonal to the direction of flow. In response, the flow restrictor  54 , in addition to creating a pressure drop, straightens and laminarizes the flow in the flow channel  34 , thereby reducing turbulence. The flow restrictor  54  reduces turbulence by forcing the fluid to flow through a series of equally spaced orifices  60 . The pressure drop across the flow restrictor  54  is also dependent on the size and uniformity of these orifices  60 . To further straighten and laminarize the fluid flow in the flow channel  34 , two optional screens, an inlet screen  62  and an outlet screen  64  formed preferably of a honeycomb-patterned structure like those currently available, may be positioned upstream and downstream of the flow restrictor  54 , respectively. 
     With the flow restrictor  54  of FIG. 2, for example, the orifices  60  are circular and concentrically spaced about the axis  40 , which extends normal to FIG.  2 . Due to the large number of orifices  60  in the flow restrictor  54 , only a few are provided with reference numerals in FIG.  2 . Each orifice  60  has a radial distance, R, of approximately 0.085 inches. In the preferred form, the orifices  60  are circular to adapt the flow restrictor  54  to the shape of the flow channel  34 . This shape matching results in more uniform reduction of turbulence across the entire flow channel  34 . Nevertheless, other geometries of orifices adapted to arbitrary cross-sectional shapes of the flow channel  34  and extending substantially parallel to the axis  40  therein may be employed. These could have a uniform repeating pattern of orifices of substantially identical hydraulic diameter throughout or a partially-repeating pattern in which orifices are symmetrically aligned about the axis  40  with other orifices of substantially the same hydraulic diameter. Other variations will be obvious from the disclosure. 
     The flow restrictor  54  is made from a material strong enough to withstand high flow rates of over 6000 L/min, and can be formed from multiple pieces affixed together or from a single molded structure. It is desirable to have the flow restrictor  54  formed of a temperature resistant material of high modulus of elasticity. Furthermore, for use in applications such as anesthetizing apparatus where high concentrations of potentially damaging chemicals are used or in medical applications generally where potentially damaging cleaning agents are used, the flow restrictor  54  should be made from a material that is also chemically resistant. In addition, the flow restrictor  54  requires thin walls to minimize turbulence. To this end, the flow restrictor  54  is preferably made of a nylon 6/6 resin, e.g., RTP 201™ resin available from RTP Corporation of Illinois. Nylon resins are processible by injection molding, foam molding or extrusion techniques and demonstrate minimal creep defects under high load. Therefore, the use of this resin allows the flow restrictor  54  to be easily manufactured and to achieve very long useful lifetimes under heavy load conditions. In particular, nylon resins have proven more manufacturable than other materials because of their ability to uniformly fill all intricate details of the mold used to form the flow restrictor  54  and/or integrated module  30 , before hardening into the final structure. The RTP 201™ resin with 10% glass fill has a tensile strength of approximately 14,000 psi and an elastic modulus of approximately 700,000 psi. Those of ordinary skill in the art will appreciate that other similar materials can be used to form the flow restrictor  54 . For example, a polyetherimide resin such as an Ultem® 2210 resin with 20% glass by weight, available from General Electric Co., could be used. This polyetherimide resin forms a material more amorphous than semi-crystalline plastics which produce non-uniform shrinkage in the mold causing slightly warped structures. In addition, the flow restrictor  54  may be formed of various metals, plastics, resins, ceramics, or liquid crystal polymers (LCPS) which exhibit similar properties as those provided above. 
     As discussed above, the orifices  60  of the flow restrictor  54  have been shaped to match the curvature of an outer wall  66  of the flow channel  34 . By utilizing the shape of the outer wall to create orifices of substantially the same circular shape, the flow restrictor  54  is adapted to the flow channel to reduce the non-uniform velocity effects present at the outer walls with previously known flow restrictors. Each orifice  60  is disposed concentrically with each other, with the outer wall  66 , and with the axis  40 . Moreover, the flow restrictor  54  has outer orifices  68  which are partially formed by the flow channel wall  66  (FIG. 1) and are uniform in shape and concentric with orifices  60 . Two main support rods  70 ,  72  extend radially across the flow restrictor  54  to support the same and to give points of contact for affixing the flow restrictor  54  rigidly into the flow channel  34 , if necessary. 
     Alternatively, and preferably, the flow restrictor  54  can be formed integrally with the housing  32  and outer wall  66  by using a single casting mold for the entire structure. In this form, the integrated module  30  could be entirely formed of a moldable nylon resin or the other materials referenced above. In fact, the integrated module  30  is preferably formed entirely of a nylon resin. The use of this material also gives the outer wall  66  sufficient smoothness to allow the fluid to travel through the flow channel  34  with a reduced amount of turbulence. 
     In addition to the two main support rods  70 ,  72 , numerous support pairs formed of two smaller support rods aligned radially with one another are used (shown as pairs  74   a ,  74   b ,  76   a ,  76   b ,  78   a ,  78   b , and  80   a ,  80   b ). The support pairs  74   a ,  74   b  through  80   a ,  80   b  provide additional support to the flow restrictor  54  preventing those portions of the orifices  60  between the main support rods from moving during high flow rate usage, whereas without such support pairs, the flow restrictor  54  could move, or rattle, resulting in less reduction in turbulence of the fluid and more noise measured by the sensor  42 . Thus, this rigidity reduces noise in the system thereby allowing the use of a higher sensitivity sensor  42 . The support pairs are formed of these smaller support rods  74   a ,  74   b  through  80   a ,  80   b , in such a way as to form identical, mirrored orifices on each side of the axis  40  along a given radial distance, with uniformity in orifice structure and diameter being important to laminarizing the flow fluid. That is, by using the main support rods  70 ,  72  and support pairs  74   a ,  74   b  through  80   a ,  80   b  in this symmetrical way, any orifice  60  will be symmetric with an identical orifice  60  of equal hydraulic diameter. As is known, when the width of an opening is much greater than the distance (r) across the opening, the hydraulic diameter is approximately equal to 2r. The symmetrical and uniform hydraulic diameters of the orifices  60 ,  68  create uniform flow velocity at the exit end  38 . 
     The flow rate sensor  42  is shown in FIG. 3 for exemplary purposes only because, as will be appreciated by persons of ordinary skill in the art, many types of flow sensors may be used in addition to those shown by Bohrer in U.S. Pat. No. 4,478,076 and Johnson et al. in U.S. Pat. No. 4,651,564 including optical flow sensors, orifice-based ΔP sensors, and pitot tubes. With the thermal microstructure form of sensor  42  being only exemplary, the operation of the flow rate sensor  42  will only be generally described below. 
     As a microbridge structure, the sensor  42  comprises a microchannel  88  that defines an air space into which some of the fluid flowing into the sensing channel  44  will flow, in the direction indicated. The fluid will flow across an upstream sensor  90 , a heater  92 , and a downstream sensor  94 . In principle, the heater  92  is heated to approximately 160° C. above ambient temperature via the application of a current to a conductive wire or pattern through two heater leads  96   a ,  96   b . Under no flow conditions, the upstream sensor  90  and the downstream sensor  94  would both read the same temperature due to the heater  92 , i.e., both sensors would have the same measured resistance values because both are equally-spaced from the heater  92 . The resistance values of the sensors  90 ,  94  are measured from the application of a constant current to a conductive wire or pattern forming the sensors  90 ,  94 . As the fluid enters the flow channel  34 , a portion thereof enters the sensing channel  44  with an even smaller portion thereof enters the microchannel  88  (FIG.  4 ), creating a flow path across the top and bottom surfaces of the heater  92  and sensors  90 ,  94 . Convection from the flow of the fluid moves heat produced by the heater  92  away from the upstream sensor  90  and towards the downstream sensor  94 , resulting in a reduction of temperature of the former and an increase in temperature in the latter. The change in temperatures produces a corresponding change in the resistance values of each sensor, with the upstream sensor  90  seeing a lower temperature therefore lower resistance and the downstream sensor  94  seeing higher temperature therefore a higher resistance. The difference between the resistance values for the two sensors  90 ,  94  is measured by circuitry (not shown) and used to determine the flow rate of the fluid in the sensing channel  44  from which the flow rate of the fluid in channel  34  can be determined. 
     The entire sensor  42  may be formed on a silicon, or other insulating semiconductor, substrate  98  upon which sits heater leads  96   a ,  96   b , upstream sensor leads  100   a ,  100   b , and downstream sensor leads  104   a ,  104   b  for connecting to control or biasing circuitry (not shown) through wire leads like the exemplary three-wire lead  104  of FIG. 5, with input, output, and ground leads that are used to control the heating and sensing functions of the sensor  42 . 
     The sensor  42  is integrated with the flow channel  34  containing the flow restrictor  54  by affixibly mounting the sensor  42  onto the housing  32  to form the sensing channel  44 . Preferably, the sensor  42  is first adhesively mounted to a substrate base  106 , supporting the lead  104  and providing a wire-bond connection thereto, which is then adhesively mounted to the flow tube  46  creating the structure of FIG.  4 . The substrate  106  can be formed of an alumina ceramic, for example. An epoxy is used for the adhesive mounting. Wire bonds  108 ,  110  extending from the sensor  42  are attached to conductive pads on the substrate  106  before the substrate is mounted to a top opening in the flow tube  46  (FIG.  5 ). These wire bonds  108 ,  110 . connect directly to the wire leads  104  via the conductive pads depending on the design of the sensor  42  and substrate  106 . 
     The flow tube  46  is compatible with attachment to the sensing tap  48  for forming an air tight sensing channel parallel to the flow channel. The flow tube  46  can be made of an nylon resin, ceramic or other above-mentioned materials. By epoxying the flow tube  46  to the substrate  106 , the microchannel  88  will be exposed to receive a portion of the fluid that flows into the sensing channel  44 . An advantage of affixing the flow tube  46  directly to the substrate  106  before mounting it onto the housing  32  is that the combined structure can be more easily attached to the integrated module  30  in an aligned position with the more sensitive alignment of the microchannel  88  having already occurred. This method also helps protect the wire bonds  108 ,  110  during assembly. 
     As stated above, the integrated module  30  is designed to be installed into existing flow systems, and to facilitate this installation in one embodiment the integrated module  30  is formed of a housing with molded inlet and outlet mounts  120 , 122  as shown in FIG.  5 . The inlet and outlet mounts  120 ,  122  are preferably  22  mm conical connectors which are easily adapted for use in a flow system. The entire structure, i.e., including the housing  32 , the inlet mount  120 , and the outlet mount  122 , can be formed of the nylon resin or other materials referenced above. Alternatively, the conical mounts  120 ,  122  can be replaced by threaded screw mounts or rubber O-ring connections to sealibly connect with an existing flow system. Furthermore, the inlet and outlet mounts  120 , 122  can have a screen recess space  124  for optionally placing the upstream inlet screen  62  and a downstream outlet screen  64  of FIG.  1 . 
     The integrated module  30  of FIG. 5 is further depicted with a rectangularly shaped sensing tap recess  126  into which the flow tube  46  will be sealed, with the recess  126  extending from the inlet end  50  (not shown) to the outlet end  52  of the sensing tap  48 . The mounting indentation  58  for the outlet end  52  is also depicted and, along with the mounting indentation  56  for the inlet end  50 , is sized to receive rubber O-rings for sealibly attaching the flow tube  46  to the recess  126 . 
     A cover  128 , disposed against the rear side of the substrate  106  opposite the sensor  42  (shown in phantom), protects the sensor  42  from environmental effects and clamps the sensor  42  and flow tube  46  against the sensing tap  48  of the housing  32 . Clamping is achieved primarily in two ways. First, notches  130  on the cover  128  are disposed to snap in place over pegs  132  of the housing  32 . Second, the cover  128  has two oppositely disposed, horseshoe-=shaped tabs  134 ,  136  that are flexible in a spring-like manner. As the cover  128  is snapped into place over the pegs  132 , the two tabs  134 ,  136  will contact the top surface of the substrate  106  and thereby protrude slightly upward while applying a restraining downward force onto the substrate  106 . The cover  128  can be made from similar resins as the other components. Additionally, the cover  128  also has an upper connection portion  138  which forms an electrical receptacle with a lower connection portion  140  of the housing  32 , both of which house the wire lead  104  when the cover is snapped in place to form an electrical receptacle for connecting the lead  104  to external circuitry. 
     An alternative embodiment of an integrated module  146  is shown in FIG. 6, wherein a housing  150  defining the flow channel  34  is formed with an inlet mount  152  and an outlet mount  154  which are attached to adaptors  156 ,  158 , respectively. Thus, the housing  150 , inlet mount  152 , and outlet mount  154  are not formed of a single molded structure like that of the embodiment in FIG.  5 . The integrated module  146  uses the sensor  42 . The embodiment of FIG. 6, however, shows an alternative means of mounting the sensor, whereby the flow tube  46  is affixed to the housing  150  before the substrate  106  is epoxied to the flow tube  46 . The mounting steps of FIG. 5 could also be used. 
     The integrated module  146  requires less manufacturing expense; is packaged in a smaller form; and allows the customer greater versatility in mounting than the embodiment of FIG.  5 . For example, a customer may prefer that the mounts  152  and  154  take the form of rubber O-rings which can be placed into adaptors  156 ,  158  or tapered threaded screw mounts, either of various sizes. Alternatively, the customer could choose to clamp the module  146  to a manifold. 
     Screw mounting through holes  160   a , 160   b  and  162   a ,  162   b  may be used to attach the adaptors  156 ,  158 , respectively, to the housing  150  at the manufacturing facility. An epoxy, heat-stake, or other method may also be used instead. Further, O-rings  164 ,  168  may used to form a better contact between the adaptors  156 ,  158  and the housing  150 . Optional screens  62 ,  64  may be placed on either an inlet end or an outlet end of the flow channel  34 , or both. Moreover, the adaptors  156 ,  158  have industry standard end connections that can receive standard 22 mm conical connectors (ISO 5356), such as those used in ventilators and respirators. A cover  170  is similar to that of the cover  128  except that notches  172  are used to fasten the cover  170  in place. And while it is not shown, the cover  170  could have tabs similar to those of FIG.  5 . 
     In any of the previous embodiments, the flow channel  34  can be used to pass fluid of very high flow rates. For example, flow rate above 5,500 L/min can be achieved with a 3 inch diameter flow channel. It may be desirable, under high flow operating conditions however, to deploy extensions to the inlet end  50  and outlet end  52  of the sensing tap  48 . In FIG. 7 exemplary extensions are shown as narrow hollow tubes  174 , 176 , respectively. These tubes  174 , 176  magnify the pressure differential across the sensing channel  44  and further restrict the flow entering the sensing channel to prevent damage and reduce noise that could result from high flow rates. The tubes  174 ,  176  have a hollow core with a diameter approximately equal to that of the ends  50 ,  52  and are aligned to extend the same distance into the channel  34  from each respective end. 
     An advantage of the embodiments depicted in FIGS. 5-7 is that the sensor  42  can be calibrated to produce accurate flow rate measurements for a given flow restrictor. To calibrate the integrated module  30  or  146 , first, testing is performed at no flow conditions to measure the null output of the sensor  42 . The gain stage feedback resistor of a gain amplifier (not shown but disposed on control circuitry connected to either the sensor  42  or the substrate  98 ) is trimmed until the sensor output is at a predetermined voltage, such as 1V. Second, a fluid flowing at a known flow rate such as 200 L/min is passed through the flow channel  34  producing a voltage output from the sensor  42 . The gain stage of the sensor  42  is further trimmed at this operating output to a predetermined voltage, such as 6V. In this way, the sensor is calibrated to produce output voltages over a defined range compatible with the functional voltage input ranges of any connecting circuitry. Furthermore, by calibrating the output range in this manner, sensitive flow sensors may be used thus allowing the flow rate of gases to be readily measured. 
     While in the preferred embodiment the flow restrictor  54  is that of FIG. 2, in any of the embodiments of FIGS. 5-7 other known flow restrictors, such as a orifice-based flow restrictor or a honeycomb flow restrictor, can be used. However, these flow restrictors may create pressure drops across the sensing channel but are not adapted for use with a particular flow channel in that they do not reduce turbulence uniformly across the entire flow channel  34 , as occurs with the concentrically orificed flow restrictor  54 . 
     In addition, in any of the above embodiments a flow restrictor  180 , which is identical to the flow restrictor  54  but longer, may extend beyond the inlet end  50  and outlet end  52  of the sensing tap  48  as shown in FIG.  8 . In such an embodiment, the sensor  42  (not shown) measures the flow rate based on the pressure drop between the inlet end  50  and the outlet end  52  of the sensing channel  44 . Though these two ends  50 ,  52  are not disposed outside of the flow restrictor  180 , there will be a pressure drop between the two (as well as a laminarizing of the flow), from which the sensor  42  can measure flow rate. Pressure drop, i.e., pressure differential, is created in primarily three ways by a flow restrictor. First, a pressure drop is created as the fluid enters the restrictor. Second, a further pressure drop is created as the fluid exits the restrictor. Third, a still further pressure drop occurs as the fluid is flowing through the flow restrictor. In the embodiment of FIG. 1, the sensor  42  measures flow rate based upon a pressure drop across the sensing channel  44  from all three factors because the inlet tap  50  and outlet tap  52  are disposed on opposite sides of the flow restrictor  54 . Whereas, with the longer flow restrictor  180 , the sensor  42  will measure a flow rate based upon a smaller pressure drop that includes the drop in pressure due to the first and third factors described above, because the flow restrictor  180  extends beyond the taps  50 ,  52 . An advantage of using this longer flow restrictor arrangement, is that a longer flow restrictor produces a more laminar flow with less noise measured by the sensor  42 . 
     As shown by example in FIGS. 1,  5  and  6 , the inlet screen  62  and the outlet screen  64  may be used to further straighten and laminarize the flow as well as reduce the Reynold&#39;s number in the flow channel  34 . The inlet and outlet screens  62 ,  64  may be any type of flow laminarizer, including a honeycomb-patterned screen  190  (FIG.  9 ), a woven polyester, or a structure similar to the flow restrictors  54   180 . 
     In the foregoing embodiments, a flow sensor in the form of a thermal microstructure is used. Alternatively as shown in FIG. 10, a differential pressure (ΔP) sensor  192  can be deployed in the channel  44  for measuring the pressure of the flowing fluid. The pressure differential created by the flow restrictor  54  between the inlet  50  and the outlet  52  is measured by the sensor  192  from which a flow rate of the fluid can be derived. The sensor  192  is positioned across the sensing channel  44 , thus precluding the flow of fluid across the channel  44 . 
     Those of ordinary skill in the art will appreciate that, although the teachings of the invention have been illustrated in connection with certain embodiments, there is no intent to limit the invention to such embodiments. On the contrary, the intention of this patent is to cover all modifications and embodiments fairly falling within the scope of the appended claims either literally or under the doctrine of equivalents.