Abstract:
A miniaturized, inexpensive instrument for measuring the mass flow rate of powder, which is capable of directly outputting the mass flow rate of powder and whose characteristics are not influenced by the kind and physical properties of the powder. The instrument can be easily manufactured, installed, operated, maintained, and color change can be easily done. A constant powder feeder, which is constructed based on this instrument and an electrostatic powder coating system utilizing this feeder has a high performance and a high operational efficiency. A measuring gas with a regulated flow rate is turned into a thin-layer of high-speed gas jets by a dispersion means, thereby powder is cross-dispersed and introduced into a measuring tube, the pressure difference generated therein is measured, and a feeding means is subject to feedback control so that the output of the pressure difference coincides with a set value.

Description:
BACKGROUND OF THE INVENTION 
     1. Technical Field 
     The present invention relates to an instrument for measuring the mass flow rate of powder conveyed together with a gas, a constant powder feeder, which is easily manufactured, operated, maintained, and its operational efficiency is highly enhanced by applying the said instrument, and a high-performance electrostatic powder coating apparatus in which the constant powder feeder is utilized. 
     2. Background Art 
     Conventionally, constant powder feed systems for pneumatically conveyed powder as shown in FIG.  4  and FIG. 7 of the accompanying drawings are known as being used for accurately feeding an expensive powder to each of several to several tens of apparatuses, with a relatively small amount of constant flow rate, e.g., several tens to several hundreds of grams per minute, as in the case of powder feed to powder coating apparatuses, thermal spraying apparatuses and the like. 
     FIG. 4 shows one example of a so-called volumetric type constant powder feeder, wherein powder  102  in a hopper  101  is fluidized by means of compressed air  104  through a porous plate  103  and fed by a screw feeder  106  provided at the bottom of the hopper  101  to an injector  113  where the powder material is fed to a powder gun by means of the injector  113 . 
     At this time, the feeder is able to feed powder with a volumetric flow rate (Fv) represented by the following equation: 
     
       
         Volumetric flow rate  Fv=AR  (cc/min.)  
       
     
     The equation is based on a constant which is determined by the volume of a screw, that is, [an effective volume per one pitch of the screw]=A (cc), and [the motor  108  rotation speed]=R/min. However, the feeding rate essentially required in the powder coating process is not a volumetric flow rate (cc/min.) but a mass flow rate (g/min.). Therefore, it is common to presume the mass flow rate by reflecting the actual bulk density S of the powder in the inside  107  of the screw feeder  106  (hereinafter referred to as feeding bulk density) using the following equation: 
     
       
         Mass flow rate  FM=SAR  (g/min.)  
       
     
     Then, the mass flow rate is presumed by graduating the feeding rate indicator  110  on the motor rotation speed. 
     For the volumetric type constant powder feeder, it is an important point to avoid the influence of the powder level in the powder hopper upon the feeding bulk density S. In the embodiment shown in FIG. 4, a powder level controlling device  105  is provided for this purpose. Besides the above-described one, there are some other methods used for the purpose of insulating the influence of the powder level upon the feeding bulk density, but most of them adopt a method for fluidizing powder in a hopper. This situation is the same with respect to volumetric type constant powder feeders which utilize a table feeder, an eccentric pump, a roll with grooves and the like as a feeding means. 
     The loosened bulk density of a fresh powder that largely affect on the feeding bulk density S varies in the wide range of 0.4 to 0.75 g/cc depending on the kind of the powder. It sometimes varies by 3 to 5% by the production lot even in the same kind of powders. The feeding bulk density S is also affected by 5 to 15% depending on the fluidizing air flow rate. The reason for this is, besides the overall problem of the powder hopper, an air flow resistance that causes fluctuation by several percentage depending on the location of the porous plate  103  in case a plurality of screws are provided to one powder hopper. Since this situation directly affects the feeding bulk density, it necessiates checking the feeding bulk density for each screw, or in other words, an adjustment of the rotation speed for each screw in relation to the rotation speed and the on-site measuring of the feeding rate, as a constant feeding apparatus which is required to have the flow rate accuracy of ±2.5%. 
     In addition, the bulk densities of the fresh powder  111  and recovered powder  112  sometimes varies in the range of 0 to 25%. This situation directly affects on the feeding bulk density when the powder is recycled as common in many powder coating lines. Moreover, since the effect of the mixing ratio of the fresh powder and recovered powder is revealed through the transfer efficiency of the coating line, it is very difficult to maintain the accuracy of the feeding bulk density, or the accuracy of the feeder within ±2.5% especially in a multi-purpose coating line in which a variety of objects to be coated are handled, wherein the transfer efficiency thereof tends to vary. 
     The mechanism and problems of the volumetric type constant powder feeder are summarized in FIG.  5 . FIG. 6 shows the relation numerically. When the screw rotation speed is set to be constant, the relation between the volumetric flow rate and mass flow rate varies in a wide range of the area indicated between the lines  115  and  116 . When taking into consideration of the influence of the state of fluidization and transfer efficiency of the coating line, probably the values vary in a wider range of the area indicated between the lines  118  and  117 . For the purpose of determining the feeding bulk density that prescribes the volumetric flow rate, i.e., the relation between the rotation speed of the screw and the actually required mass flow rate, there is nothing for it but to regulate the actual flow rate of the practically used powder for each feeder, or for each screw in case of a multiple feeder, at a working site. 
     While a long period of working time and proficiency are required for this regulating work, it is not easy to always maintain a feeding accuracy of about ±2.5%. Thus, it is the first crucial problem common in volumetric type constant powder feeders. The volumetric type constant powder feeder, which can not be operated without regulating each apparatus individually depending on the practical working conditions on-site, can be hardly recognized as an industrial measuring and controlling equipment, therefore the creation of a new system that enables a direct detection and control of the mass flow rate of powder has been waited. 
     The screw feeder  106  as an apparatus having a volumetric type constant powder feeding function (hereinafter referred to as a volumetric type constant powder feeder) shown in FIG. 4 has such as complicated structure that the screw feeder requires dismounting and disassembling for having a cleaning at the time of color change of the powder together with the hopper. This requires a long period of working time and many hands. When a prompt color change is required, spare machines have to be prepared, which calls for a large amount of investment of equipment. The said difficulty in dealing with color change is the second problem of a volumetric type constant powder feeder. The same is true with respect to other systems which utilize a volumetric type constant powder feeder other than a screw feeder. 
     The third problem of the volumetric type constant powder feeder is that the screw feeder shown in FIG.  4  and other volumetric type constant powder feeding apparatuses are always large in size, heavy in weight, and expensive without exception. Furthermore, the space between neighboring volumetric type constant powder feeding apparatuses becomes so large that hoppers are required to be strongly constructed in a square shape with a high accuracy. As hoppers tend to be expensive, heavy and large-sized, they are inconvenient to carry, thereby hinder the operations such as color change at working sites. It is also practically impossible to streamline the coating process inexpensively by adding the above-mentioned volumetric type constant powder feeder to the hoppers of already installed powder coating facilities. 
     FIG. 7 shows another example of the conventional art other than the above-mentioned volumetric type constant powder feeder. A desired powder feeding rate is obtained by the steps comprising: flowing a measuring gas  114  at a constant velocity into a measuring tube  120  by means of a nozzle  121 ; introducing a fluidized powder  102  in a hopper  101  into the measuring tube  120  so as to accelerate the powder; detecting the pressure difference generated between the inlet and outlet of the measuring tube  120  by means of a pressure difference sensor  123  which constitutes a fixed small volume blind tube via a filter  122 ; obtaining an output signal by an amplified signal processor  124 ; detecting the powder flow rate by observing the indicated feed rate of the powder (g/min.) on a display device  125 ; and automatically controlling the driving gas  128 , from an injector  129  comprising a throat  127  and a nozzle  126 , by an automatic control means (not shown in the figure). 
     In the conventional art shown in FIG. 7, the sensing characteristics of the powder feeding rate is not affected by the variation of the fluidizing air flow rate, since the flow rate of the air accompanied by the fluidized powder is by  1 / 100  or less in comparison with the flow rate of the measuring gas  114 . Accordingly, the characteristics do not show practical changes depending on the attached position of the hopper or the hopper itself. However, with respect to common powder materials, the working curve showing the relation between the feeding rate at a set value and the actually measured feeding rate varies in the range of about ±7.5% as shown in the working curves  130  and  131  in FIG.  8 . This depends on the kind of the powder, or the variation of the mixing ratio of the fresh powder and recovered powder, as a result an on-site regulation is necessary. Incidentally, when the flow rate of the measuring gas  114  has to be reduced due to some circumstances at a working site, there is also a problem that the sensor output declines downwardly, and deviates from a linear relation, and then shows a non-linear characteristic in the region where the powder feeding rate is high as shown in the working curve  132 . 
     In the above-described conventional art with reference to FIG.  7  and FIG. 8, the powder is merely introduced and accelerated by the measuring gas in the measuring tube. Therefore, the degree of powder dispersion differs depending on the kind of the powder, thereby causing a difference in acceleration, as a result different working curves are obtained depending on the kind and physical properties of the powder as shown by  130  and  131  in FIG.  8 . The reason why a nonlinear working curve as shown by  132  in FIG. 8 will be explained as follows. A linear characteristic curve is obtained in the range where a powder flow rate is small because powder dispersion is sufficiently done. But when the powder flow rate is high, the powder is not dispersed so sufficiently that the acceleration efficiency decreases, thereby the working curve deviates from the linear relation and bends toward the x-axis with respect to the constant powder feeder which is controlled by an automatic control system in which the pressure difference generated by the powder flow is kept constant. Additionally, in the conventional art illustrated in FIG.  7  and FIG. 8, because of insufficient powder dispersion, the powder flow in the measuring tube is downwardly biased in the horizontally placed measuring tube by the influence of the gravity, thereby creating a large velocity difference between the powder flow and the measuring gas flow. In other words, it results in an insufficient acceleratinon of the powder flow. In this case, there is a problem that a working curve varies depending on physical properties of the powder or operational conditions. 
     The object of the present invention is to solve the problems in the conventional art summarized in FIGS. 4,  5 ,  6 ,  7  and  8 . 
     Another object of the present invention is to develop an instrument for measuring the mass flow rate of powder which is capable of obtaining a constant linear relational output against the powder mass flowing per unit time (g/min.), that is, powder mass flow rate without the influence due to the kind and physical properties of the powder, thereby obtaining a sensor based feed-back system, or a constant powder feeder, for automatically controlling the output to coincide with a prescribed value. 
     SUMMARY OF THE INVENTION 
     The first method for attaining the object of the present invention is to provide a powder flow rate measuring system comprising the steps of: 
     introducing powder to be measured into a measuring gas flow formed in a measuring tube with a constant flow rate, 
     measuring a mass flow rate of the powder by a pressure difference between the inlet and outlet of the measuring tube, the pressure difference being generated by allowing the measuring gas to accelerate the powder flow in the measuring tube, 
     wherein the measuring gas accompanying the powder is introduced into the measuring tube by dispersion means which is capable of dispersing the powder into the measuring gas sufficiently at just in front of the inlet of the measuring tube. 
     In the second method, a pressure difference sensor system in which a small volume blind tube is formed at both sides of a substantially solid pressure receiving diaphragm is utilized as measuring means by a filter. 
     In the third method, the inner surface of the measuring tube is formed with a non-adhesive material such as a fluororesin. 
     In the fourth method, the gas flow distribution and the powder dispersion in the measuring tube is made to be axially symmetrical and stable, wherein a uniform turbulence is formed and the powder is uniformly dispersed along the entire length in the measuring tube by generating a turbulent flow of the measuring gas by the dispersion means in the first method described above at the inlet of the measuring tube along with by adjusting the Reynolds number of the air in the measuring tube to be 2,300 or more. 
     The speed of the powder particles at the outlet of the measuring tube is not affected by the physical properties of the powder, thus enabling to be accelerated at a substantially same speed as the speed of the measuring gas in a common powder, and generating the same pressure difference against the same flow rate (g/min.) of the powder by applying the four methods described above. In other words, it became possible to measure the mass flow rate of the powder conveyed by the air. 
     In the dispersion means as the main of the four methods described above for sufficiently dispersing the powder material with a measuring gas at just in front of the measuring tube, the measuring gas is cross-sprayed as high-speed thin-layer gas streams with a speed of 15 to 50 m/sec from the circumference of the cylindrical powder column to its center axis at near the outlet of the powder material introduction means that is coaxial with the measuring tube and usually comprises a circular pipe to feed the powder material as a bulk or in a fluidized state via a small space. Consequently, the powder can be dispersed into independent single particles in the measuring gas even when the measuring gas flows at a small velocity required according to the circumstances of the overall structure of the constant powder feed system, by successively shearing off the terminal end of the column of the powder material fed at a slow feeding speed of about 5 to 20 cm/sec into thin layers by a turbulent shear force of the measuring gas. Because the powder is sent into the measuring tube by being accompanied by the measuring gas and is uniformly distributed in the measuring tube in cooperation with the foregoing fourth method, all the fine particles with a particle size of 75 μm or less, which comprises the particle size of most common powders, and having a net specific gravity in the range of 0.9 to 1.65 are substantially accelerated up to the same velocity as the velocity of the measuring gas in the measuring tube. Consequently, the pressure difference between the inlet and outlet of the measuring tube having one-to-one correspondence with the mass flow rate of the powder is generated irrespective of the kind of the powder, or physical properties of the powder, thereby completing the present invention. 
     According to the present invention, with respect to common powders, an instrument for measuring the mass flow rate of powder, which is capable of measuring the mass flow rate and whose characteristics are not influenced by the physical properties or kind of the conveyed powder can be obtained. By combining this instrument with an automatic powder feeding control system, it is possible to obtain a constant powder feeder, which is not influenced by the operational conditions and physical properties of the powder, and which is capable of constantly maintaining a high precision without requiring an on-site checking and adjustment. The instrument of this invention is simple in structure and small in size, comprising a connection of circular pipes made of a non-adhesive material without any built-in members or movable parts, and the instrument is detachably connected with the hopper. Therefore, it is possible to clean the interior of the instrument by detaching the instrument from the hopper, then sucking the fresh air and let the air go through the inside of the instrument at the high speed. As the result, disassembling operation becomes unneccesary and a one-touch color change operation becomes possible. Moreover, since the instrument is small in size, and it does not require a hopper to have any special structure, it is possible to introduce the instrument into an already operating powder coating line for streamlining purpose without requiring a big change to the existing product system. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     FIG. 1 is a front view of the instrument for measuring the mass flow rate of powder and a part of the electrostatic powder coating apparatus utilizing the instrument illustrated by cross sections according to the present invention. 
     FIG. 2 is a graph indicating one example of the working curve for the instrument for measuring the mass flow rate of powder having the structure shown in FIG.  1 . 
     FIG. 3 is a detailed drawing of a part in FIG.  1 . 
     FIG. 4 is an illustrative drawing of a volumetric type powder feeder according to the prior art. 
     FIG. 5 is an illustrative chart of a volumetric type powder feeder according to the prior art. 
     FIG. 6 is a graph indicating the relation between the volumetric flow rate and mass flow rate of the volumetric powder feeder according to the prior art. 
     FIG. 7 is a part of the front view of the main part of the powder feeder according to another prior art illustrated by a cross section. 
     FIG. 8 is a graph indicating one example of the working curve of the powder feeder shown in FIG.  7 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1 shows the structure of the constant powder feed system for powder coating, utilizing the instrument for measuring the mass flow rate of pneumatically conveyed powder (=a powder mass flow meter) according to the present invention, and one example of the method for cleaning the interior thereof. FIG. 2 shows the characteristics of the constant powder feed system according to the present invention. 
     As shown in FIG. 1, an injector  24  comprising a throat  22  and nozzle  23  driven by a controlling gas  16  is connected to the outlet  8  of the measuring tube  1  whose inner surface is made of a non-adhesive material such as a fluororesin. A powder  32  in the inner cylinder  35  of a hopper  31  fluidized via a porous plate  33  is elevated by means of a compressed air  34  into the powder introduction means  5  through a suction pipe  36  as a bulk powder column containing a small amount of air. A measuring gas  2  with a prescribed flow rate is supplied to a dispersion means  3  provided in the vicinity of the top end of powder introduction means  5 , and the measuring gas is blown and intersected with the powder, thereby forming thin-layer of high-speed air streams  6  (illustrated by twelve arrows (3×4) in the cross section A—A) with a flow speed of 15 to 50 m/sec, blown through very thin slits  4  provided at the top end of the dispersion means  3 , in the direction from the circumference to the center of the top end of the bulk powder column elevating through the powder introduction means  5 . In this way, the terminal end of the column of the powder as described above is successively sheared off into thin layers by a turbulent shear force. Then the powder assumes a dispersed state as independent single particles suspended in the measuring gas, then blown into the measuring tube  1 . When the inner diameter of the measuring tube  1  and the flow speed of the measuring gas  2  is appropriately selected to adjust the Reynolds number in the measuring tube to be 2,300 or more, a linear relation is observed between the pressure difference between the inlet  7  and outlet  8  of the measuring tube  1  and the mass flow rate (g/min.) of the powder passing through the measuring tube  1 , irrespective of the kind and physical properties of the powder with an accuracy of ±2.5% as shown in FIG. 2 with respect to common powders. In other words, measurement of the mass flow rate is made possible. Even in case that the Reynolds number of the measuring gas in the measuring tube has to be set to 2,300 or below in relation to other auxiliary equipment, the powder mass flow meter according to the present invention is hardly affected by the physical properties of the powder or by the operational conditions as compared with the conventional art. While the embodiment in FIG. 1 shows the case where the thin-layer high-speed gas streams  6  are blown so as to cross orthogonally with the flow of the powder, another appropriate crossing angle can be selected, if necessary. 
     In FIG. 2, the actually measured values (g/min.) of the mass flow rate, which are gravimetrically measured by sampling the powder passing through the measuring tube  1  within a given period of time, are plotted on the x-axis. The indicated values are plotted on the y-axis, which are obtained by the values of the pressure difference between the inlet  7  and outlet  8  of the measuring tube  1  detected by a pressure difference sensor  11  via a filter  9  and displayed by a display device  13  via an amplifier  12 . The mass flow rate can be obtained from the indicated value by means of the working curves  51  and  52  showing the relation between the measured values and the indicated values. 
     In FIG. 2, with respect to common powders, the characteristics of the powder mass flow meter according to the present invention can be represented by a single working curve  50  of y=x. Each working curve corresponding to respective powder is laid in the area surrounded by the working curve  51  having a slope by +2.5% and the working curve  52  having a slope by −2.5% against the working curve  50 , thereby making it possible to apply the working curve  50  as a representative working curve without necessitating any on-site actual measuring. According to the present invention, the mass flow rate of common powders can be measured with an accuracy of ±2.5% by measuring the pressure difference between the inlet and outlet of the measuring tube, this accuracy being sufficient for making a great effect on saving the powder in the powder coating process, for making the coated film thickness uniform, for improving the powder transfer efficiency and for saving labor. 
     The most crucial point enabling the practice of the present invention is that, with respect to common powders, i.e., the powder having an average particle size of 10 to 50 μm and the net specific gravity of 0.9 to 1.7, all the powder particles can be accelerated to the same speed as the speed of the measuring gas in the measuring tube  1  at the inlet of the measuring tube  1  provided that the powder is sufficiently dispersed by the measuring gas  2 . However, in the practical equipment, for example, as shown in FIG. 1, the flow rate of the measuring gas  2  is limited within 5 to 20 liters/min since the performance of the injector  24  is deteriorated when the flow rate of the measuring gas  2  is increased. Accordingly, it is required to realize a perfect dispersion within this range. For the purpose of dispersing the powder with a flow speed of 250 to 500 g/min that is required for the usual powder coating process under the measuring gas flow rate as small as described above, acceleration of the flow speed of the measuring gas is first of all required. There is no choice but to spout out the measuring gas so as to form a thin-layer gas stream so that the powder is made to contact with the measuring gas with a wide contact area, and cross-dispersed with the measuring gas, thereby realizing a desired dispersion ability to achieve the present invention. The problem of an impact fusion can be avoided in this dispersion means because there is no obstacle, such as a tube wall, where the powder material dispersed in the measuring tube and conveyed by the measuring gas would directly collide with. Moreover, the initial speed of the dispersed powder at the inlet of the measuring tube  1  because the measuring gas  2  does not have the speed distribution along the axial direction of the measuring tube  1 , thereby diminishing a margin of error in the pressure difference generated at the measuring tube to a degree of practically negligible level. 
     A vortex air flow method and a parallel air flow method are generally used for dispersing powder or liquid by a high speed air flow other than the method according to the present invention. In the dispersion method using a vortex air flow, the measuring gas  2  forms a vortex air flow in the measuring tube  1  which generates a large pressure difference that is harmful for measuring the mass flow rate of powder, and also the powder is biased in the vicinity of the tube wall due to the centrifugal force generated by the vortex air flow. Then a one-to-one correspondence between the energy consumption for accelerating the powder and the pressure difference generated in the measuring tube  1  is lost, thereby making it difficult to measure the mass flow rate. In the parallel air flow method that is the other dispersion method, the contact area between the air flow and the powder can not be made so large that a sufficient dispersion effect can not be expected, thereby the effect due to the kind and physical properties of the powder can not be avoided. The conventional art shown in FIG.  7  and FIG. 8 belong to this category. While a collision-dispersion method by a high-speed gas flow can be also used for dispersing powder by a gas, application of this method is not recommended in the powder having a low glass transition point because it may cause an impact fusion. 
     In FIG. 1, a pressure difference sensor  11  inside of which minute volume blind tubes are formed with a filter  9 , at the both sides of a pressure receiving membrane  10  made of a solid material is applied for detecting the pressure difference at the inlet  7  and outlet  8  of the measuring tube  1 . The controlling gas  16  supplied to the injector nozzle  23  is adjusted by a feed back control of the control valve  17  using an amplified output of the difference obtained by comparing the output of an amplifier  12  with its set value of a reference circuit  15  at a regulator  14 . By adjusting the pressure difference between the inlet and outlet of the measuring tube, which corresponds to the mass flow rate (g/min.) of the powder equal to the set value of the reference circuit  15 , the mass flow rate of the powder can be always adjusted equal to the set value. 
     While the regulator  14 , control valves  17  and  19 , injector  24  and hopper  31  are assembled as parts of the powder mass flow meter system in the foregoing description, the system comprising the parts indicated by the reference numerals  1  to  20 ,  31  and  37  is generically called a constant powder feeder. There may be a case that the flow rate of the controlling gas  16  may be fluctuated during control in the constant powder feeder in FIG. 1 causing undesirable effects for operating the powder gun  25 , or may be another case that the combined volume of the measuring gas  2  and controlling gas  16  may be insufficient for the gas volume for conveying the powder material in the pipe  20 . For solving these problems, auxiliary air  18  is blown into the outlet of the injector  24  via the control valve  19 . For such cases, in the embodiment as shown in FIG. 1, the constant flow valve  27  is applied for maintaining the conveying air flow in the pipe  20  at a prescribed value so that the conveyed air volume in the pipe  20  is always kept constant in compliance with the set value of the constant flow valve  27  even when the air flow resistance at the downstream of the valve is changed by the action of the control valves  17  and  19 . Since the control valve  19  is desirably operated in a reverse relation to the control valve  17 , the control valve  19  may occasionally be utilized with a reverse motion interlocked with the motion of the control valve  17  by means of an interlock means  26 . The reverse motion means that, when the control valve  19  is opened, the control valve  17  is closed responding to the motion of the control valve  19 . In the valves  17  and  19  interlocked with each other by spur gears  17 ′ and  19 ′ as shown in FIG. 3, for example, when one of the valve member  17 ″ is rotated by being driven with a motor responding to the output of the controller  14  to close the valve by means of a screw in the valve member  17 ″, the other valve member  19 ″ is inversely rotated by the spur gear  19 ′ to open the valve, thereby the compressed air  27 ′ from the constant flow valve is controlled to increase either one of the output flow  16  or  18  and to decrease the other, or vice versa, thus readily realizing the reverse motion. In other words, the interlock means in FIG. 1 is embodied in a pair of the spur gears  17 ′ and  19 ′ in FIG.  3 . However, when the control valve  19  is previously adjusted to have an appropriate fixed aperture, the conveying air flow in the pipe  20  can be adjusted to a constant value in a wide range of the powder mass flow not to affect practical purposes. When the convey speed is not required to be strictly adjusted, the constant flow valve  27  is not always necessary. The compressed air  30  to be used in this system is usually supplied from a constant pressure source via an accurately controlled constant pressure valve  29 , making the operation of the constant flow valve  27  easy. The flow rate of the measuring gas  2  to be used as a reference of the operation of this system is set to a desired flow rate by a setting valve  28  connected to the pressure source described above. 
     Since the detection characteristic of the powder flow rate is not affected by the physical properties of the powder in the constant powder feeder according to the present invention, the powder feeding rate never varies beyond an accuracy of ±2.5% even when the physical properties of the fresh powder  38  and recovered powder  39  fed into the hopper  31  is different with each other and the mixing ratio between them is changed. Even when the flow rate of the compressed air  34  for use in fluidization is changed and the change is directly transferred into the air accompanied by the powder material in the bulk powder introduction means  5 , the change is a negligible level of as low as 0.5% or less as compared with the speed of the measuring gas in the measuring tube. When the air for fluidization is changed along with changing the expansion coefficient of the fluidized powder in the range of 0 to 50%, the change below {fraction (1/200)} relative to the flow rate of the measuring gas—usually about 10 liters/min—does not influence on the speed of the measuring gas in the measuring tube. For these two reasons described above, neither the detection characteristics of the measurement nor the feeding characteristics are influenced by the fluidized air with respect to the mass flow meter and the constant powder feeder according to the present invention. By the same reasons above, the constant powder feeder according to the present invention can be operated by utilizing a hopper using no fluidizing air, that is, merely a bulk type hopper can be utilized with respect to common powders, provided that the powder is securely fed to the inlet of the suction pipe by a vibration means of the hopper. 
     For detecting the pressure difference between the inlet  7  and outlet  8  of the measuring tube  1 , in FIG. 1, it is advantageous to use a semiconductor type pressure difference sensor by forming minute volume blind tubes, of which inside volume is invariant, at both sides of a pressure receiving membrane  10  made of a substantially solid substance such as silicon membrane via a filter  9  comprising a non-adhesive substance such as a fluororesin having a lot of very fine air permeable holes. Owing to this structure, the volume of the air passing through the filter during pressure changes is very small and the average time interval for the pressure change approaches zero, the powder never invades into the filter but deposits on the outside of the filter and forms a thin layer. As the volume of the inside of the sensor is so small that it is also made possible to securely detect the pressure difference of the gas including the powder for a long period of time through the thin deposit layer of the powder and the filter. 
     In FIG. 1, it has been already mentioned that the inner surface of the measuring tube  1  and filter  9  is composed of a non-adhesive substance such as a fluororesin, the portion making a contact with the powder in the dispersion means  3 , inlet  7  and outlet  8 , and inside of the injector  24  are also in general composed of a non-adhesive substance as well, making the time required for changing the color of the powder of the entire constant powder feeder very short. The constant powder feeder under operation is sucking up the powder material from the hopper by being closely connected to the suction pipe  36  through a seal device  37  by means of a fastener (not shown in the figure). A color change operation of the powder is carried out by the steps comprising: 
     halting a compressed air source  30  of the constant powder feeder for a while; 
     releasing the fastener to separate the suction pipe  36  and seal device  37  off the bottom end of the powder material introduction means  5  as shown by the suction pipe  36 ′ and seal device  37 ′ in a different hopper  31 ′ as shown in FIG. 1; 
     full-opening the control valve  17  while opening the valve  42  that is closed during the operation; and 
     starting to feed through the compressed air source  30  again. The hopper  31 ′ is separated from the system by the procedure described above, preventing the overall system from conveying the powder material, thereby largely increasing the suction power of the injector  24 . A large volume of clean air  40 ,  41  is sucked from the bottom end of the powder introduction means  5  to pass through the interior of the nozzle, measuring tube and injector in high speed together with the measuring gas  2 , controlling gas  16  and auxiliary air  18 . Since each part described above is formed into a circular pipe structure made of a non-adhesive material and connected in series with each other with no internal structures therein, the remaining powder after the operation can be cleaned off without disassembling the system, thereby immediately completing the preparation for color change of the powder. Then, the hopper for the last operation is replaced with the hopper containing powder for use in the next operation (hereinafter abbreviated as “next powder”) followed by connecting the powder introduction means  5  of the constant powder feeder with the suction pipe of the next powder hopper, immediately starting the operation by using the next powder. Accordingly, no spare facilities for changing the color of the powder are needed. When the inside of the pipe  20  between the injector  24  and powder gun  25  is made of a non-adhesive substance such as a fluororesin and the pipe is not so long in the method for changing the color of the powder according to the present invention described above, the color change of the paint of the constant powder feeder and pipe  20  can be simultaneously carried out at one time. However, when the pipe is very long despite its interior is made of a non-adhesive substance, or when the substance is not non-adhesive, a pipe cleaning means  21  is provided separately. It means that another means not hindering the cleaning of the foregoing constant powder feeder is added, for example, blow-in of a compressed air for use in the pipe cleaning, suction of a separately prepared air for use in the pipe cleaning, or replacement of the pipe  20 . While these means are applied between the injector and the tip of the gun, in FIG. 1 they were illustrated at the outlet of the injector for convenience. 
     Usually, a constant powder feeder is often applied to a multi-gun system in a large scale coating line. The constant powder feeder according to the present invention can be arranged into a package of connection/separation system by assembling a plurality of suction pipes  36  together with respective seal means  37  into one unit, or by collecting the lower end of respective powder introduction means  5  into one pipe to serve as respective manifold, making it easy to automatically control the system by an air pressure operation or electromagnetic operation. In general, the powder may be simply sucked up by a hose from the top of the fluidized or vibrated powder hopper. As shown in FIG. 1, in many cases it is not necessary to fixedly connect the constant powder feeder with the hopper, as the shape, size, accuracy and strength of the hopper to be used in the present invention can be quite freely designed so that the hopper has a simple structure with a light weight and low cost. Accordingly, no heavy economic load is imposed on preparing a hopper for each of powder colors along with making a color change operation easy and the operation can be easily automated. Moreover, neither remodeling nor replacement of the hopper is needed in adding the constant powder feeder according to the present invention to already installed hoppers in the facilities under operation for the purpose of rationalization, resulting in a quite easy work and low cost. With respect to the powder hopper besides a conventional fluidization type hopper having a porous plate over the entire bottom, any hoppers may be used, for example, as shown in FIG. 1, a hopper having a porous plate  33  partly over the bottom, wherein an inner cylinder  35  is provided over the plate with a space  43  between the bottom end of the cylinder and the porous plate forming a fluidizing region mainly limited to inside and lower part of the cylinder so that the top end of the inner cylinder  35  is positioned lower than the powder level under operation to save the fluidizing air, thereby making dust-collection treatment in the fluidizing air unnecessary. (Fluidization is mainly carried out inside of the inner cylinder  35  and fine powders included in the fluidizing air is collected by the bulk powder layer over the inner cylinder  35 , practically making dust-collection treatment of the exhaust air from the hopper unnecessary. Mixing of the powder can be facilitated by the circulation  44  due to the bulk density difference between the fluidized part and the bulk part in the hopper. A vibrator may be added, if necessary.) A hopper in which a Partial fluidizing device is provided at the bottom end of the suction pipe, and a hopper in which the powder material is always constantly fed at the inlet of the suction pipe by means of a vibration device or a stirring device with no use of fluidizing means may be also available. Although the powder material is usually taken out by sucking up the powder from the top portion of the hopper for convenience as described in the forgoing examples, the method is not necessarily limited thereto but it is possible to take out from the side face or bottom part of the hopper. The dispersion means  3  and measuring tube  1  can be installed along any direction besides the vertical direction, if necessary. A high-precision instrument for measuring the mass flow rate of powder conveyed together with a gas and a constant powder feeder based on the instrument can be obtained by providing the measuring tube  1 , measuring gas  2  and dispersion means  3  so as to satisfy the principles of the present invention. 
     The measuring tube  1  to be used in the present invention is required to always have a constant cross sectional area. Change of the cross sectional area due to adhesion can be prevented with respect to common powders by forming its inner surface with a non-adhesive substance such as a fluororesin while a countermeasure for abrasion can be achieved by replacing the measuring tube  1  in every 200 to 500 hours. When a higher durability is required, a surface treatment in which a porous alumina membrane is formed on the inner surface of the measuring tube  1  followed by impregnating with a fluororesin in vacuum, or a composite plating in which fine particles of a fluororesin (PTFE or fluorinated pitch) is dispersed in a metal film such as a nickel film is applied as a countermeasure. These preventive treatments comprise important constructive elements of the present invention. Preventive measures against adhesion and abrasion at the inner surface of the throat  22  of the injector  24  is necessary as well as for the inside of the measuring tube  1 . Conventionally, it used to be difficult to solve these problems at the same time. However, these problems can be solved by applying the impregnation surface treatment of the porous alumina with an fluororesin in vacuum, or the composite plating surface treatment using fine particles of a fluororesin, obtaining an injector throat having 3 to 10 times or more of durability. The velocity distribution is stabilized by keeping the Reynolds number of the measuring gas in the measuring tube  1  at 2,300 or more and by generating a turbulent flow over the entire length of the measuring tube  1  for sufficiently dispersing the powder material. Uniform particle distribution in the measuring tube  1  can prevent partial abrasion of the tube and is effective for a long term stabilization of the measuring accuracy. This effect can be more improved by vertically standing the measuring tube. 
     The method for measuring the pressure difference between the inlet  7  and outlet  8  of the measuring tube  1  is not limited to the method shown in FIG. 1, but the pressure difference can be also detected by an electric means by providing a pressure sensor at the inlet  7  and outlet  8 , respectively, in order to simplify the mechanical structure by omitting to use a pressure guide tube. In some cases, the pressure at the inlet of the measuring tube can be measured in the powder upstream of the dispersion means  4 . 
     Although the conveying air flow rate for the powder required to flow in the pipe  20  for conveying the powder to the gun  25  may be changed in a wide range depending on the conditions such as the shape of an object to be coated or required coating film thickness in the electrostatic powder coating system according to the present invention as shown in FIG. 1, the flow rate can be always maintained at a constant speed irrespective of the overall operational conditions of the system by forming the inner diameter of the measuring tube  1  by ½ to ¼ as small as the inner diameter of the pipe  20 , thereby the flow rate of the measuring gas  2  is reduced to ⅓ to {fraction (1/10)} of the air flow rate required in the pipe  20 . 
     Characteristics of the powder mass flow meter according to the present invention do not necessarily need to be checked by on-site measurements in the calibration of characteristics at the manufacturing process or in the periodic inspection and maintenance after installation. Calibration and adjustment can be performed by connecting an air flow meter to the powder introduction means  5  in FIG. 1, operating the system by sucking the air instead of the powder and verifying the relation between the sucked air flow rate and the output of the flow meter. This is practically very convenient. 
     The powder dispersion means applied in the powder mass flow meter according to the present invention is not limited to the embodiment of the mechanism shown in FIG. 1, but a variety of methods based on the principles of the present invention are available. For example, the slits for creating thin-layer high-speed gas streams can be replaced with a closely-arrayed series of small holes. The powder can be cross-dispersed by allowing high-speed gas streams to form thin-layers by providing a diffuser at the top end of the powder introduction means or at the outlet of the measuring gas. 
     While the embodiment in which the powder feeder is provided upstream of the powder mass flow meter according to the present invention is hitherto described, the present invention is not limited to this, but another method is also included within the scope of the present invention, wherein the powder mass flow meter according to the present invention is provided downstream of a volumetric type constant powder feeder, which can be any one of a vibration type, roll groove type, table type or screw type so that the volumetric type constant powder feeder is controlled by feed-back control by the output of the powder mass flow meter, thereby enabling to obtain a mass flow rate constant powder feeder. 
     When the powder is fed by conveying it with a gas as shown in the embodiment in FIG. 1, the feeding rate of the powder can be accurately controlled within the set value along with suppressing the fluctuation of the conveying gas flow rate so as to be smaller than that of in the prior art. It is also possible to keep the conveying gas flow rate at a constant set value. By applying the powder feeder according to the present invention to a paint feeder of an electrostatic powder coating apparatus, making it possible to accurately keep the powder discharge rate of an electrostatic powder gun at a constant value, at the same time, the discharge speed and dispersion state of the discharged powder, that is, the discharge pattern, can be maintained in a constant optimum condition, making it possible to obtain a high transfer efficiency and uniform film thickness distribution. Thus, a high-performance electrostatic powder coating apparatus enabling to largely save the consumption of powder and to ensure a high product quality is obtained. The present invention is applicable to a variety of electrostatic powder coating guns for electrostatically coating powder by charging the powder with a corona discharge or tribo-electric charging. In FIG. 1, illustrations of a power source for a gun  25 , discharge pattern control and operation system is omitted. The powder mass flow meter hitherto described can be used for various ways in the same purpose besides electrostatic powder coating. 
     Industrial Applicability 
     According to the present invention, a gas convey type mass flow meter that can measure the mass flow rate of the powder conveyed by a gas without being affected by the kind and physical properties of the powder is obtained, which used to be impossible to obtain in the prior art. By combining this mass flow meter with a powder feeder of which feeding rate is adjustable, it is possible to obtain a constant powder feeder controlled by a sensor-based feed-back control by the mass flow meter. Since the characteristics and accuracy of the system according to the present invention are practically not influenced by the kind and physical properties of the powder, or by the operational conditions along with no need of on-site actual measuring, resulting in obtaining the systems having uniform characteristics. The system is especially suitable for parallel running of a plurality of systems or for multiple-color changes or powder changes in powder coating where a variety of powder materials are alternately used. The system itself has a small-sized and simple structure besides requiring no on-site calibration, so that manufacturing process of the system can be managed only by adjusting its size and electrical characteristics. Moreover, the system is so cheap along with being able to largely reduce the cost for installation, adjustment and maintenance. 
     Cleaning of the powder mass flow meter, the constant powder feeder and the electrostatic powder coating apparatus according to the present invention can be performed in a short period of time without being disassembled. Therefore, the time required for the color change of the powder becomes so short that an effective net working rate of the system can be enhanced.