Abstract:
An air flow rate meter and method for monitoring the flow of air in high-pressure compressed air system includes a low pressure gauge contained within a sealed, high pressure vessel that is connected to the high pressure compressed air line. The low pressure gauge is pneumatically connected to opposite ends of a tube disposed within the meter through which the main air pressure line flows in order to detect the pressure drop across the restricted orifice defined by the tube. The measured pressure drop is then used by the gauge to determined the rate of air flowing through the line. To avoid damage to the meter, the air flow rate meter further includes a pressure regulating valve for regulating the input air flow pressure during startup of the system, and a check valve for closing the flow path through the pressure vessel during shut down.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This application relates to an air flow gauge, more particularly to an air flow gauge intended for use in conjunction with a paint spraying gun atomization circuit. 
     2. Description of Related Art 
     Many factories use compressed air as a source of power for operating various types of production equipment. “Compressed air,” which is sometimes referred to as “pressurized air” or referred in spray paint operations as “atomization air,” is defined as free air that has been compressed into a volume that is smaller than the volume the air normally occupies at normal atmospheric pressure. Controlled expansion of the compressed air can be used as a source of power to operate a wide range of pneumatically powered tools. Compressed air is typically supplied from onsite or nearby compressors and piped through a distribution system to a downstream point of use. Paint spraying operations for painting various types of manufactured products, such as automobiles, is one typical use of compressed air. 
     In a spray paint operation, a paint fluid, which can be in the form of either a liquid or a fine powder, is mixed together with compressed air in a spray gun nozzle in order to atomize the paint into extremely fine particles and to transfer the paint particles onto the surface of the item being painted. One commonly used spray paint gun, referred to in the industry as a high volume low pressure (HVLP) spray gun, generates high volumes of low pressure air that propel the paint particles from the nozzle of the gun toward the surface of the article being painted. Other uses of compressed air include pneumatically powered drills, wrenches and other types of machine tools. The optimum operation of such tools depends upon providing a specified flow rate of compressed air to the tool. 
     Industrial compressed air systems are commonly controlled by pressure regulation, meaning by regulating the nominal air pressure at a certain point in the system. A pressure regulator might be placed, for example, at or near the compressor, at one or more points on the distribution line, or on a hose used to supply air to the tool. The major disadvantage with this method is that measuring air pressure at only one particular point within the system is not necessarily a good indicator of the volume of air flowing though that particular point in the system. Air pressure drops as it flows through the system, and the amount that the air pressure drops from one point to the next varies greatly depending on the specific system installation and also on varying conditions of usage occurring during the course of the day. For example, in many cases a compressed air system supplies not only spray guns but also other devices used in a paint shop such as sanders, polishers, screw drivers, drills and so forth. 
     For paint spray operations in particular, one commonly used method for determining whether a sufficient amount of compressed air is being delivered to the spray gun is to place a pressure gage on the cap of the spray gun immediately after the painter has set the spray gun for proper atomization of the paint, but before he actually begins painting. Many operators, however, find this extra step to be a great inconvenience as it interrupts their painting operations. Therefore, this procedure is often disregarded. Another method of checking whether a sufficient amount of compressed air is being delivered to the spray gun is to attach an air gauge to the handle of the gun. However, attaching a pressure gauge to the gun naturally increases its weight. Over a period of time, muscle fatigue sets in, thereby causing the operator to use unnatural arm and wrist actions which, in turn, cause over spray or under spray conditions and other flaws in the paint job. 
     More importantly, regulating the nominal pressure at any one point in the system does not necessarily mean that the proper amount of air, or even any air is flowing at another point downstream. For example, there may be blockage in the spray nozzle of a paint gun, or a break in the line or some other problem in the system. 
     Another problem with the traditional method of using a single pressure gauge to monitor air flow in a high pressure line is that the particular gauge being used must be able to withstand the high pressure of the spray system. In paint spray systems, the liquid paint is atomized under high pressure, typically in the range of about 10 p.s.i. for a HVLD spray gun, 25 to 60 p.s.i. for a dynamic air spray and 100-125 p.s.i. for a static air spray. Thus, pressure gauges used on such systems are therefore typically made from very heavy and bulky components and consequently lack the resolution necessary to accurately measure the difference between, for example, 8 and 9 p.s.i. Traditional monitors also lack repeatability as mass and hysteresis of the moving components of the gauge effect the movement of gauge needle. Thus, accurately regulating the pressure and flow rate of air in a spray system is extremely difficult. 
     It is advantageous to monitor airflow through a spray gun to assure proper performance. Restrictions in the air delivery hose, gun body, and spray gun cap can greatly affect the airflow through them. Varying conditions of hose length, delivery pressure, and supply air temperature also affect pressure and flow rates. For monitoring the air flow it is desirable to use an inline circuit monitor which can be connected to the components to be monitored rather than to disassemble a spray gun system and take the component to a test bench to test the flow rate. An inline flow gauge is especially important when the spray gun is part of an automated machine and is not designed to be removed easily from the control system. 
     Various types of gauges such as floating ball, turbine, thermal, ultra-sonic, and differential pressure gauges have been used to measure the flow rate of air in high pressure air systems. Such devices are commonly calibrated so that their scales read in terms of cubic feet or liters per minute. They must be carefully made so as to be accurate, yet at the same time, they must be able to withstand the high pressures and also sudden pressure changes or surges that commonly occur in industrial paint spray air systems. As a result, these flow meters typically constitute the most expensive single element in an inline monitor. 
     Virtually all known monitoring devices for high pressure systems have a number of drawbacks. As mentioned, heavy gauges can withstand sudden pressure changes, yet they cannot accurately measure small pressure differences. Additionally, the narrow resolution of heavy gauges makes them less effective when the equipment is working at low pressures. Light gauges can accurately measure small pressure differences, but they cannot withstand the high pressures and sudden pressure changes in typical paint spray lines. Finally, known differential pressure measuring devices such as floating ball gauges and other devices mentioned above are too expensive to use on a plant-wide basis for many users. 
     Accordingly, there is a need for an improved air flow rate monitoring device or gauge that can accurately measure the air flow of sprays within high-pressure paint spray systems which can also be manufactured at a considerably lower cost than other flow gauges using differential measuring methods. 
     SUMMARY OF THE INVENTION 
     An improved flow rate meter than can accurately measure and regulate flow of sprays in high-pressure paint spray systems is disclosed. The invention, which is defined by the claims set out at the end of this disclosure, is especially designed and adapted to address several of the drawbacks noted above with respect to the use of conventional, heavy-duty high pressure gauges. Specifically, the air flow rate meter disclosed herein provides an accurate measurement of air flow rates operating at high pressure values and can also withstand sudden pressure drops and surges. 
     The air flow rate meter disclosed herein comprises a means for providing a low pressure flow meter which can be used in conjunction with gas lines of much higher pressure than the meter or gauge is rated for. The means comprises a meter or gauge mounted within a sealed housing that is connected to the gas line. By mounting the low pressure gauge within the housing, and having the pressure within the gauge and housing equalized with the air pressure in the line, the gauge can be used to detect any pressure drops or surges within the gas line. The gauge is connected to a restricted orifice to measure the pressure drops or surges in the gas line, which directly correspond to the rate (i.e., volume) of air flowing through the line. 
     More specifically, the air flow rate meter of the present invention comprises a low-pressure meter, (e.g., 0 to 5 p.s.i.), encapsulated within a high-pressure vessel, with both the low pressure meter and the high pressure vessel pneumatically connected to the high pressure air line. Although the system pressure may be extremely high (e.g., 100 to 125 p.s.i. or more), the gauge of the present invention is designed to measure a relatively small pressure drop through a known restriction at a specific point in the gauge. Due to the relationship between a pressure drop between opposed ends of a restriction having known dimensions and the flow rate through the restriction, the flow rate can be measured by calculating the pressure drop across the known restriction. 
     The durability of the proposed invention is dependent upon equalizing the pressure within the pressure vessel with the outside system pressure. Once the entire system is up and running and fully pressurized, the air pressure in the low pressure gauge and the pressure vessel naturally becomes equalized to the system pressure. However, during startup and shut down, the difference in pressure between the airline and the vessel may exceed the capacity of low-pressure meter and, unless the meter is effectively isolated, damage the internal parts of the meter. Therefore, the vessel further comprises a pressure regulator for regulating the input pressure during startup, and a check value for equalizing the pressure across the gauge during inlet pressurization. 
    
    
     Accordingly, the air flow rate meter of the present invention provides a highly accurate measurement of the air flow rate to the paint spray gun or other air tool connected to the system, yet at the same time is able to withstand the high pressures and sudden pressure changes, i.e., drops or surges, that commonly occur in industrial paint spray air systems. Other objects and advantages of the invention will become apparent from the following description which, taken in connection with accompanying drawings, set forth by illustration and example certain embodiments of the present invention. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The drawings, wherein a certain presently preferred embodiment of the invention are illustrated, include the following: 
     FIG. 1 is a top plan view of an air flow rate meter of the present invention; 
     FIG. 2 is a cross-sectional view along line  2 — 2  of FIG. 1; 
     FIG. 3 is a circular sectional view along line  3 — 3  of FIG. 2; 
     FIG. 4 is a circular sectional view along line  4 — 4  of FIG. 2; 
     FIG. 5 is a circular sectional view along line  5 — 5  of FIG. 2; 
     FIG. 6 is a cross-sectional view similar to FIG. 2 illustrating the operation of the air flow rate meter in a normal flow mode; 
     FIG. 7 is a cross-sectional view similar to FIG. 2 illustrating the operation of the air flow rate meter in an over maximum pressure flow mode; 
     FIG. 8 is a cross-sectional view similar to FIG. 2 illustrating the operation of the air flow rate meter in a reverse flow mode; and 
     FIG. 9 is a cross-sectional view similar to FIG. 2 illustrating a second embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Before explaining the preferred embodiments invention in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of the components set forth in the following description or illustrated in the drawings. The invention is capable of other embodiments or being practiced or carried out in various ways. Also, it is to be understood that the phraseology and terminology employed herein is for the purpose of description and should not be regarded as limiting. 
     In the drawings, a first preferred embodiment of the air flow rate meter in accordance with the invention is illustrated in FIG. 1 at the reference numeral  10 . The air flow rate meter  10  includes a flow gauge  11 , which determines flow based on the pressure differential between the upstream side  12   a  and downstream side  12   b  of a tube  13 . The flow gauge  11  is, relative to the overall system pressure, a low-pressure gauge. For a typical paint spray system that operates on a system pressure on the order of 100 to 120 p.s.i., the flow gauge  11  is preferably capable of measuring pressure differentials on the order of about 0 to 3 p.s.i. The low pressure gauge is preferably a conventional diaphragm gauge that includes a diaphragm (not shown), which is an elastic pressure sensing element formed from a thin wall of an elastic or flexible material. A rotating rod and arm (not shown) rest on the thin walled diaphragm. The rotating rod is connected to a geared movement (not shown) through a linkage system (not shown). The rotating rod and geared movement are in turn coupled to a shaft (not shown) which actuates the needle  17  when the diaphragm senses a pressure differential in the air flow, indicating the air flow on a scale  18  marked in appropriate pressure units, preferably in cubic feet per minute. Any conventional diaphragm flow gauge having the required properties is acceptable for use in the air flow rate meter  10  of the present invention. 
     As is explained in detail below, one side of the diaphragm is subjected to upstream pressure and other side is subject to the downstream pressure of the tube  13 . The change in pressure on opposite sides of the tube  13  is related to flow rate of the air through the tube  13 . Thus, the pressure differential sensed by the diaphragm is used to calculate the flow rate based on conversions known in the art. 
     Referring now to FIGS. 2 and 4, the low pressure gauge  11  is contained within a windowed pressure vessel or chamber  14  comprised of a transparent cover  15 , preferably formed of Plexiglas, secured to a main body  16 . The body  16  of the vessel  14  includes a base  18  and an open end  19  opposite the base  18 . The open end  19  defines a recess  20  that extends into the body  16  of the pressure vessel  14  towards the base  18 . The recess  20  receives and retains the low pressure gauge  11  such that the gauge  11  rests within the vessel  14  at a level below the open end  19 . To enclose the gauge  11  within the pressure vessel  14  and make the vessel  14  air tight, an elastic O-ring  21  is disposed within a circumferential groove  22  extending around the open end  19 . The O-ring  21  contacts the cover  15  when the cover  15  is positioned over the open end  19  and provides an air tight seal between the body  16  and the cover  15 . 
     To retain the cover  15  in position over the recess  20  in sealing engagement with the O-ring  21 , a cap  23  is secured to the open end  19  over the cover  15 . The cap  23  has an outer dimension approximately equal to that of the body  16  and includes a notch  24  that receives the edge of the cover  15  when the cap  23  is positioned over the cover  15  and open end  19  of the body  16 . The cap  23  also has a number of bores  25  disposed along the periphery of the cap  23  and spaced from the notch  24 . The bores  25  extend completely through the cap  23  and are alignable with a number of apertures  26  disposed along the periphery of the open end  19 . When the bores  25  are aligned with the apertures  26 , a plurality of bolts  27  can be engaged within the bores  25  and apertures  26  to secure the cap  23  and cover  15  to the body  16 . The engagement of the cap  23  and the cover  15  over the open end  19  encloses the recess  20  forming an enclosed chamber  28  within the body  16 . 
     Opposite the cap  23 , the body  16  also includes the tube  13 . The tube  13  is disposed within the body  16  adjacent the base  18 , with the upstream side  12   a  and downstream side  12   b  positioned on opposite sides of the body  16 . The upstream side  12   a  and downstream side  12   b  are each separately connected to the chamber  28  in a manner to be described later. Between the upstream side  12   a  and downstream side  12   b , the tube  13  includes a restricted diameter portion or throat orifice  29 . The throat orifice  29  has a diameter less than that of each side  12   a  and  12   b  of the tube  13  and generates a pressure drop in the flow of air between the respective sides of the tube  13  by increasing the speed of the air flow. This pressure drop can be measured by the gauge  11  and used to determine the air flow rate through the system. 
     The inline flow meter  10  is capable of being used in spraying systems in which the air flows through the system at high flow rates, e.g., flows on the order of twenty-five (25) to fifty (50) cubic feet per minute through the orifice  29 . Also, using a low pressure gauge  11  having a construction, as specified previously in the chamber  28  permits the measurement of fine variations of the pressure drop across the tube  13 . To measure the pressure drop across the tube  13 , the gauge  11  is connected on one side of the diaphragm to the upstream end  12   a  and to the downstream end  12   b  on the opposite side of the diaphragm. In the chamber  28 , air is maintained under a pressure equal to the pressure in the downstream end  12   b  leading to the spray gun (not shown) or other tool that is attached to the system. This downstream end pressure in the chamber  28  contacts the diaphragm within the gauge  11  in order to provide one half of the pressure differential across the diaphragm. The upstream end  12   a  is connected to the opposite side of the diaphragm to complete the pressure differential and enable the measurement to be made. To protect the low pressure gauge  11  from extreme changes in pressure within the system, a downstream safety valve  31  and an upstream safety valve  32  are housed within the body  16  and are operably connected between the chamber  28  and the downstream end  12   b , and between the gauge  11  and the upstream end  12   a , respectively, as described in detail below. 
     Air under pressure from an atomization air supply, e.g. an air compressor, (not shown) that is coupled to the system enters the flow meter  10  through the upstream end  12   a  of tube  13 . An inlet passageway  33  having a narrowing cross-section leads from the upstream end  12   a  to a throat orifice  29 . Opposite the inlet passage  33  the diameter of the orifice  29  expands to form an outlet passageway  34  that leads to the downstream end  12   b . The inlet passageway  33  is coupled to an adaptor (not shown), such as by a threaded connection, that is further connected opposite the inlet passageway  33  to the source of compressed air. Similarly, the outlet passageway  34  is coupled to an adaptor (not shown) that is connected opposite the outlet passageway  34  to a hose (not shown) leading to the point of usage, namely, a spray gun (not shown) or other pneumatically operated tool. 
     Immediately upstream of the restricted orifice  29  of the tube  13  is disposed an upstream tap  36  leading into the body  18  towards the gauge  11 . The upstream tap  36  enables a portion of the pressurized air flow at the upstream end  12   a  to be directed into the gauge  11 . When air flowing through the tube  13  reaches the restricted orifice  29 , the flow rate increases due to the constriction of space in the orifice  29  of the tube  13 . Concurrently, a drop in pressure occurs as the air flows through the throat  29  of the tube  13 . The air pressure of the flow through the tube  13  is also sampled downstream of the orifice  29  at a downstream tap  30  in order to enable the gauge  11  to determine flow rate in the manner explained below. 
     Air from the inlet passage  33  on the upstream side of the orifice  29  is routed to the flow gauge  11  via the upstream tap  36 , which is formed of a first segment  40  that extends upwardly from and perpendicular to the inlet passageway  33  and a second segment  42  which extends inwardly towards the center of the gauge  11  parallel to the inlet passageway  33 . The first segment  40  and second segment  42  of the upstream tap  36  have a smaller diameter than the inlet passageway  33  and are operably connected to one another by a valve well  44  that encloses the upstream safety valve  32 , whose construction is described in detail below. The safety valve  32  limits the pressure differential applied to the diaphragm within the low pressure gauge  11  through the upstream tap  36  when the pressure differential exceeds the maximum capacity of the gauge  11 , i.e., above 3 p.s.i. When the pressure differential is beneath the maximum capacity of the gauge  11 , air is permitted to travel from the first segment  40  through the valve  32  and into the second segment  42 , which is connected to the low pressure gauge  11  by a gauge inlet passageway  48  that is in fluid communication with one side of the diaphragm inside the gauge  11  opposite the chamber  28 . 
     Air entering the outlet passage  34  or the downstream side of the orifice  29  is routed to the chamber  28  and into contact with the flow gauge  11  via the downstream tap  30 . The downstream tap  30  is formed of a conduit  38  that extends upwardly from and perpendicular to outlet passageway  34 . The downstream tap  30  also has a smaller diameter that of outlet passageway  34  and is connected to the downstream safety valve  31  (described in detail below) that operates to protect the gauge  11  from reverse pressure flows, e.g., when the source of compressed air is switched off and the pressure within the inlet passage  33  is exhausted while the pressure within the outlet passage  34  remains constant. Opposite the downstream tap  30 , the downstream safety valve  31  is also connected to the gauge inlet passageway  48 . 
     As best shown in FIG. 3, the upstream safety valve  32  disposed within the valve well  44  contains a flow controlling diaphragm  54  extending across the well  44 , a hollow stem  55  extending downwardly from one side of the diaphragm  54 , a flow controlling ball  56  which is releasably engageable with the stem  55  and a flow controlling ball seat  58  formed in the first segment  40 , and a pressure spring  60  engaged with the diaphragm  54  opposite the stem  55 , which urges the flow control ball  56  out of engagement with the ball seat  58 . The diaphragm  54  is in fluid communication with the upstream side  12   a  on one side via the first segment  40  of the tap  36  and the chamber  28  on the opposite side by an opening  61 . Thus, the diaphragm  54  can operate to either close or open the valve  32  as necessary based upon the pressure differential between the upstream side  12   a  and the chamber  28  as applied to each side of the diaphragm  54 . 
     The safety valve  32  also includes a backpressure poppet valve  62  that includes a backpressure poppet spring  64  which rests on a backpressure poppet seat  66 . The poppet spring  64  urges the flow controlling diaphragm  54  and ball  56  upwardly against the bias of the pressure spring  60  into engagement with the seat  58 . Thus, the pressure spring  60  and poppet spring  64  are directly opposed to one another and operate in concert with the pressure differential applied to the diaphragm  54  to open or close the upstream valve  32 . 
     The purpose of the valve well  44  and safety valve  32  is to limit the pressure differential applied to the diaphragm in the gauge  11  when the flow rate through the tube  13  exceeds the maximum displayed on the flow gauge  11 , i.e., the maximum capacity of the pressure gauge  11 , which in this application has been specified to be the rate equal to a pressure drop of 3 p.s.i. across the tube  13 . As air flows through the inlet passage  33 , the flow control diaphragm  54  limits the air flow past the flow-controlling ball  56  and flow-controlling ball seat  58  by modulating the flow rate through the upstream tap  36  based on the pressure differential across the flow control diaphragm  54 . For example, as shown in FIG. 6, in a normal air flow condition a portion of the incoming air flow through the upstream side is diverted upwardly into the upstream tap  36 . A portion of the air flow flowing through the downstream side  12   b  is also diverted upwardly through the downstream tap  30  into the chamber  28 . The air flow within the chamber  28  then flows downwardly through the opening  61  to contact one side of the diaphragm  54  opposite the upstream tap  36 . The air flow contacting the diaphragm  54  opposite the upstream tap  36  opposes the pressure exerted on the diaphragm  54  by the flow of air through the upstream tap  36  and past the ball  56 . After flowing past the ball  56 , the air flow through the upstream tab  36  is at a pressure even further reduced from that found at the downstream end  12   b , such that the air flow through the opening  61  pushes downwardly on the diaphragm  54  in concert with the spring  60 , allowing the ball  56  to be unseated from the seat  58  against the bias of the poppet spring  64  and the pressure of the incoming air, and allowing the air flow from the upstream end  12   a  to flow through the valve well  44  to the gauge  11  as shown in FIG.  6 . 
     The tension of the pressure spring  60  and the poppet spring  64  are balanced to limit the air flow past the seat  58  when the pressure drop across the flow-control diaphragm  54  exceeds the maximum for the gauge  11 . As a result, the safety valve  32  limits the pressure differential that is allowed to exist across the windowed pressure vessel  14  which equals the tension of the flow control spring  60 . For example, in the situation where the incoming air flow through the upstream end  12   a  is at a pressure higher than the pressure maximum for the gauge  11 , the upstream safety valve  32  will operate to close off the flow of air through the upstream tap  36  in order to prevent damage from being done to the gauge  11 . As best shown in FIG. 7, when the incoming air at the upstream end  12   a  creates a pressure differential higher than the maximum rated pressure differential for the gauge  11 , the force of this air urges the ball  56  upwardly into engagement with the ball seat  58  in conjunction with the poppet spring  64  against the downward force on the diaphragm  54  provided by the pressure spring  60  and air pressure generated by the air flow at the downstream end  12   b . Thus, none of the high pressure air is allowed to flow through the upstream tap  36  to the gauge  11 , thereby preventing the gauge  11  from being damaged. 
     Referring now to FIGS. 2 and 5, as noted above, air from the outlet passage  34  is routed to the flow gauge  11  via the downstream tap  30  and the conduit  38 . The downstream tap  30  also includes an offshoot  67  connected between the conduit  38  and the gauge inlet passageway  48 , allowing the downstream tap  30  to communicate with the gauge inlet  48 . The offshoot  67  includes an enlarged diameter section that forms a second valve well  68  and seat  69  that encloses the downstream safety valve  31 , which protects the low pressure gauge  11  from reverse pressure flows through the meter  10 . The downstream valve  31  includes a ball  70  releasably engageable with the seat  69  and a check spring  72  disposed in engagement with the ball  70  opposite the seat  69  that urges the ball  70  into engagement with the seat  69 . The spring  72  is configured to retain the ball  70  in engagement with the seat  69  within the range of pressure differentials measurable by the gauge  11 . 
     Looking now at FIG. 6, in a reverse air flow mode where the pressure of the air at the outlet passage  34  and flowing upwardly along the conduit  38  and offshoot  67  is much larger than the pressure of the air at the inlet passage  33  and flowing into the gauge inlet passageway  48 , i.e., is greater than the maximum for the gauge  11 , the ball  70  within the downstream valve  31  is urged away from the seat  69  by the pressure of the air in the conduit  38  and offshoot  67  because the air pressure exceeds the bias force of the spring  72 . This enables the high pressure air flow from the downstream tap  30  to enter both the chamber  28  and the gauge inlet passageway  48  and contact both sides of the diaphragm within the gauge  11 . This ensures that the pressure differential across the diaphragm within the gauge  11  will not exceed the limit for the gauge  11  because the pressure acting on each side of the diaphragm is coming from the same source and should be at the same pressure. Further, the high pressure air flows from the gauge inlet passageway  48  through the second segment  42  and into the first valve wall  44 . In the wall  44 , the air contacts the diaphragm  54  and ball  56  to urge the diaphragm  54  and ball  56  away from one another. Once the stem  55  of the diaphragm  54  separates from the ball  56 , the high pressure air can flow upwardly through the hollow stem  55  and into the chamber  28  to relieve any pressure building that may occur in the gauge inlet passageway  48 . 
     In operation, after connecting the gauge  11  to the compressed air system, compressed air flows into the tube  13  through the inlet passageway  33 . The air flow is sampled at the upstream tap  36 . The sampled air travels through the upstream tap  36  and through the upstream safety valve  32  to contact one side of the diaphragm contained within the low pressure gauge  11 . The unsampled air flow travels through the throat orifice  29  of the tube  13 , which increases the velocity of the air flow while also reducing its pressure. The air flow is then sampled again at the downstream tap  30  located on the downstream side of the throat orifice  29 . Air moves through the downstream tap  30  into the chamber  28  and contacts the low pressure side of the diaphragm within the low pressure gauge  11 . In response to the pressure differential between the upstream and downstream air flow samples, the diaphragm moves within the gauge  11 . This movement is then transferred to the rotating rod and arm that are connected to the diaphragm. The rotating rod, connected to a geared movement through a linkage system, moves the shaft and in turn, drives the needle  17  over the scale  18  to generate an air flow rate reading. 
     The flow gauge  11  described herein has a rugged construction and can be incorporated in a spray finishing system to measure flows produced over a wide range of pressures. The flow gauge  11  is of simpler construction than flow gauges heretofore used and, therefore, can be manufactured and assembled at considerably lower cost than such conventional gauges. This factor will consequently decrease considerably the overall cost of an air flow monitor. The flow gauge  11  described herein is particularly useful monitoring finishing components, which operate in pressure ranges without large pressure drops across the tube  13 . 
     Referring now to FIG. 9, if it is necessary to provide a digital readout or value for the pressure differential measured by the flow rate meter  10 , the gauge  11  can also take the form of a digital pressure transducer  80 . The transducer  80  is positioned in the same location as the gauge  11  in order to be in contact with the different air flows on opposite sides of the meter  10 . The transducer  80  provides a digital output signal that can be displayed on the transducer  80  or transmitted to a separate monitor (not shown) that can be connected to an automated air flow controller capable of automatically altering the air flow through the system in response to the signal from the transducer  80 . 
     It is understood that the various preferred embodiments are shown and described above to illustrate different possible features of the invention and the varying ways in which these features may be combined. Apart from combining the different features of the above embodiments in varying ways, other modifications are also considered to be within the scope of the invention. 
     The invention is not intended to be limited to the preferred embodiments described above but rather is intended to be limited only by the claims set out below. Thus, the invention encompasses all alternate embodiments that fall literally or equivalently within the scope of these claims.