Flow meter

Abstract: A flow sensor has a flow passage 4, a rectifying element such as a plurality of screens 2 or a honeycomb structure 8, arranged in the flow passage, a flow passage reducer 3 for reducing the flow passage behind the rectifying element, and a thermal flow sensor 1 mounted at a location immediately behind the reducer. This structure can largely reduce disturbance in flow to thereby enable highly accurate measurements of a wide range of flow without a low pass filter.

Description: FLOW METER This invention relates to a flow meter, and more particularly to a flow meter of the kind which is capable of eliminating flow disturbance and generating stable detected signals even with a high flow rate. Description of the Prior Art In conventional flow meters employing a microbridge sensor, the sensor is simply placed at a location on the inner wall of a flow passage of a predetermined shape such as a circular or rectangular pipe. Such an arrangement allows measurement of flow at a low rate. However, as the flow rate is increased, disturbance is produced in the flow due to the difference in the passage diameter at the upstream side curvature of the passage, the wall surface, and so on, whereby a rapidly responding sensor such as the microbridge sensor responds to such disturbance and accordingly its output signal is correspondingly disturbed. It is therefore necessary to provide a strong low pass filter to remove noise components and measure an average flow. Also, if available electric power is limited, the sensor has to be driven intermittently, so that a disturbed output makes the measurement quite difficult. There is disclosed in French patent specification Number FR A 2450 347 a flow sensor which has a flow passage, a rectifying means arranged in the flow passage, a flow passage reducing means for reducing the flow passage behind the rectifying means and a thermal sensor in the form of a filament downstream of the reducing means which extends across the flow passage between diametrically opposite sides. This disposition is inconvenient and does not provide optimum measurement characteristics over a wide flow range. In view of the above problems, it is the object of the present invention to provide a flow meter which can eliminate disturbance in flow and generate stable output signals even with a high flow rate. According to the invention there is provided a flow sensor comprising a flow passage, rectifying means arranged in the flow passage, a flow passage reducing means for reducing the flow passage behind the rectifying means and a thermal flow sensor downstream of the reducing means characterised in that the thermal flow sensor is disposed on the inner wall of the flow passage and positioned wholly to one side of the passage at a location immediately behind the reducing means. The thermal flow sensor is preferably a mircrobridge sensor. It is advantageous if the peripheral shape of the flow passage reducing means has a surface curvature of cosine form. Preferably, the rectifying means is a plurality of screens laterally extending and mutually spaced in the flow passage or a honeycomb structure. The flow passage may have an enlarged portion in which the rectifying means is arranged. The above and other objects and features of the present invention will become apparent from the following detailed description of the preferred embodiments which are described by way of example only as with reference to the accompanying drawings. Figures 1 and 3 are cross-sectional views respectively showing first, second and third embodiments of flow meters according to the present invention; Figures 4 and 5 are cross-sectional views used for explaining how screens are arranged; Figure 6 is a cross-sectional view used for explaining how a screen is attached to the entrance of a pipe; and Figure 7 is a graph showing the relationship between the flow and output voltage of the embodiments shown in Figures 1-3. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Figure 1 shows a cross-sectional view of a first embodiment of a flow meter according to the present invention. The flow meter of Figure 1 includes a microbridge sensor 1, a screen 2, a reducer 3 for reducing the flow passage, a flow passage 4 of a 20-inch circular pipe and a flow passage 5 of a 10-inch circular pipe. The circular flow passage 4 is provided with a rectifying element formed, for example, of three sheets of 30-mesh screens inserted therein. A back portion of the flow passage 4 is reduced, as shown by the reducer 3, to be connected to the 10-inch flow passage 5. The microbridge sensor 1 is mounted at a location on the inner surface of the flow passage 5. In the present embodiment, the diameter of the flow passage 5 in which the microbridge sensor is mounted is chosen to be 10 inches, however, the passage diameter generally depends upon the measuring range. Specifically, a small diameter passage is suitable for measuring a small amount of flow since the flow rate is high in such narrow passage and the output signal level is also high. However, a large amount of flow in this same narrow passage causes the output from the microbridge sensor 1 to be saturated and a pressure loss to be increased. Therefore, a larger diameter of flow passage is required to measure up to a large amount of flow. The reducer 3 is useful in suppressing disturbance in flow and making the flow distribution even, thereby increasing the flow rate in the vicinity of the location on which the microbridge sensor 1 is mounted, with the result that the output signal level is also extended. Figure 2 shows a cross-sectional view of a second embodiment of the present invention. The parts corresponding to those in Figure 1 are designated the same reference numerals and the explanation thereof will be omitted. In the second embodiment, a sheet of 30-mesh screen is arranged at the entrance of the 20-inch flow passage 4. The diameter of the passage is enlarged by a diffuser 6, for example, to 34 inches to form an enlarged passage 7. Then two sheets of 30-mesh screens are inserted in the enlarged passage 7. Also, a back portion of the enlarged passage 7 is reduced by the reducer 3 to be connected to the 10-inch circular flow passage 5 in which the microbridge sensor 1 is mounted. The enlarged passage 7 is provided for decreasing the flow rate and improving a rectifying effect. It is experimentally found that a shorter diffuser 6 (in the flow direction) shows a better characteristic. It is important to arrange the screens 2 immediately behind the diffuser 6. Figure 3 shows an example which has a bundle of capillaries 8, as a rectifying element, in place of the screens 2. It is better to select smaller diameter capillaries rather than long capillaries to effectively decrease the disturbance. Also, the capillary thickness should be sufficiently thin. The mounted position of the microbridge sensor 1 in Figures 1 - 3, is preferably as close as possible to the reducer 3. This is because the disturbance is developed larger at a position further away from the reducer 3. Also preferably, the inner peripheral shape of the reducer 3 is a cosine curve. Figures 4 and 5 are cross-sectional views showing how to arrange the screens 2 in the flow passage. More specifically, Figure 4 shows that two sheets of the screens 2 are respectively inserted into grooves 12, 13 formed on the inner surface of the enlarged passage 7. The enlarged passage 7 is provided with O-rings 9 and screws 10 for a fluid tight structure. Figure 5 shows that one of the screens 2 is attached to the entrance of the flow passage 4 by means of a screen attachment ring 11. Preferably, the screen 2 attached to the entrance of the flow passage, as shown in Figures 2 and 3, is disposed at a location slightly downstream, for example, at the boundary between the circular flow passage 4 and the diffuser 6, as illustrated in Figure 6. Figure 7 shows a typical output characteristic of the flow meters shown in Figures 1 - 3, wherein the abscissa represents the flow amount and the ordinate the output voltage. In each of the above embodiments, a circular pipe is employed for the flow passage . Alternatively, a rectangular pipe may be employed. However, the rectangular pipe causes vortex or disturbance to be produced at its corners, so that the circular pipe shows better characteristics. As described above, the flow meter according to the present invention is provided with a rectifying element, such as a plurality of screens suitably spaced with each other and a honeycomb structure, a reducer for reducing the flow passage behind the rectifying element, and a microbridge sensor disposed immediately behind the reducer, whereby disturbance in flow is suppressed and accordingly the flow is quite stable. It is therefore possible to highly accurately measure a wide range of flow, from small to large quantities, even without a low pass filter, or in an intermittently driving condition. Incidentally, as the rectifying element, three sheets of screens with relatively large meshes, for example 30-mesh per inch, are preferably employed to reduce a pressure loss.

Claims: A flow sensor comprising a flow passage (4), rectifying means (2,8) arranged in the flow passage, a flow passage reducing means (3) for reducing the flow passage behind the rectifying means and a thermal flow sensor (1) downstream of the reducing means characterised in that the thermal flow sensor (1) is disposed on the inner wall of the flow passage and positioned wholly to one side of the passage at a location immediately behind the reducing means (3). A flow sensor as claimed in claim 1, characterised in that the thermal flow sensor (1) is a microbridge sensor. A flow sensor as claimed in claim 2, characterised in that the microbridge sensor (1) has a flat surface which extends substantially parallel to the flow axis of the passage. A flow sensor as claimed in any one of claims 1 to 3, characterised in that the peripheral shape of the flow passage reducing means (3) has a surface curvature of cosine form. A flow sensor as claimed in any one of the preceding claims, characterised in that the rectifying means is a plurality of screens (2) laterally extending and mutually spaced in the flow passage. A flow sensor as claimed in any one of the preceding claims, characterised in that the rectifying means is a honeycomb structure (8). A flow sensor as claimed in any one of the preceding claims, characterised in that the flow passage has an enlarged portion (7) in which the rectifying means is disposed.

Inventors: AOSHIMA SHIGERU, INABA TAKASHI, KAMIUNTEN SHOJI, OCHIAI KOUICHI, TSUMURA TAKASHI, AOSHIMA, SHIGERU, INABA, TAKASHI, KAMIUNTEN, SHOJI, OCHIAI, KOUICHI, TSUMURA, TAKASHI
Applicants: YAMATAKE HONEYWELL CO LTD, YAMATAKE-HONEYWELL CO. LTD.
Application year: 1989
Main classification: G01F1
IPC codes: G01F1