Flow meter

Abstract: Method and device for measuring flows of fluids, such as liquids and gases, flowing through a tube (16). Said tube is heat-conducting and is heated at a certain location (18) , with said heat on the one hand moving by way of the tube wall (17) in a direction contrary to the fluid flow (15) and on the other hand flowing off into a heat-conducting body (20) which preferably extends substantially parallel to the tube. The temperature difference between the tube wall (17) and the conducting body (20) is measured, e.g., by a thermopile (19) and is a measure of the flow gone past. The device preferably comprises a thermopile (19) for carrying out the measurement.

Description: The invention relates to a device for measuring flows of fluids, such as liquids and gases, flowing through a tube, the device being operable with a heat source, and having a thermopile for measuring temperature differences. The measured temperature differences are a measure of the flow. Such devices are generally known. A fluid flows through a heat-conducting tube having a heating element somewhere around it. The temperature difference is then measured between two points on the tube wall, on either side, i.e., upstream and downstream from the heating element and equidistant therefrom. When the fluid does not flow, the curve showing the relationship between the temperature and the location on the tube will indicate a substantially linear relationship and be symmetrical with respect to the heating element, and therefore no temperature difference will be measured. It is assumed that, equidistantly from the heating element, in front and at the rear (upstream and downstream), the temperatures are equal. When the fluid does flow, the said curve upstream with respect to the heating element, seen in the direction of the flow, will sag and downstream it will be upwards convex. The temperature difference measured is a measure of the flow to be measured (= volume or weight flowing past per unit of time). To increase the sensitivity of such measurements and to reduce the space occupation, EP-A 395.126 presents a measuring method by means of a tube bent in a "U", through which the fluid flows, having the heating element as a spiral around the bottom part of the "U". It is then possible to measure with the help of a series of temperature sensors , in particular with a thermopile. The arrangement of the thermopile requires that the bend of the "U" is very narrow. A major drawback of using such a measuring device is in particular that it is hard to clean. The object of the invention is, while retaining the advantages offered by the latter construction - with the tube bent in a "U", and a thermopile for measuring temperature differences, - to enable measuring with a straight tube and therefore with the help of a more easily cleanable device. It has also been proposed - see, e.g., US 3181357, FIG. 12 with associated description, column 5, lines 34-59 - to use a method in which a fluid flows through a conductor bent in a "U" having, between the legs of the "U", a conduit through which no flow takes place. A temperature difference between a point on the fluid-conducting conductor and a point on the conduit is measured and is a measure of the amount of medium flowed past per unit of time. Although the bend of the "U" is less narrow, the reference conductor - the conduit 88 - will first have to be adjusted in such a manner that, at least at the locations where measurements are carried out, the temperature, or (part of) the temperature curve, coincides with that of the fluid conductors. Such adjustment is possible through the choice of materials, the choice of dimensions etc. A drawback of such a method, however, is that the reference conductor - the conduit - must be adjusted. The device according to the invention does involve said drawbacks. To that end, it has the features claimed in claim 1. Compared to the measurement with the help of the tube bent in a "U" one might say that the second tube part of the "U" has now been replaced by a fixed leg. When a fluid flows through the tube in the direction of the heat source, the temperature measured at the tube wall will invariably be lower than when the fluid does not flow. Said temperature curve - mostly linear - does occur in the leg; the temperature difference between those two is now measured, with the help of a thermopile present between the tube and the reference leg. It should be noted that, when measurements are made while using the device according to the invention, the sensitivity is half compared to the one in which the tube is bent in a "U", which is apparent from the curves showing the temperature differences as a function of the distance to the heat source. US 4,107,991 relates to a flow measuring method in which no thermopile is used. Instead, the conduit used has an electrical resistance which varies as a function of temperature. Current is passed through the conduit for heating it. A resistance bridge is joined to the conduit for measuring resistance variations of segments of the conduit (see claim 1). US 4,107,991 shows various conduit shapes: U-shaped (Fig.3); curved (non-straight) (Fig.4); α-shaped (Figs. 6 and 7). Only in Fig.5 a pair of straight conduits is shown, fluid flowing only through one of them. There is no teaching however that the spacing of the conduits of the conduit pair might be made so small as to accommodate a thermopile in thermal contact with each of the conduits. When relatively large flows are measured such as, e.g., a water flow of 55 x 10 -6 kg/s (200 g/h), it may be an advantage to have heat conduction take place between the outer wall and the reference leg, e.g., by making use of heat-conducting materials and/or constructions which are mounted between the two. In the inventive device the heat is fed to the fluid by the tube wall. At their ends away from the heat source, the tube wall and the reference leg have the same temperature, which is achieved, e.g., by having both ends flow out into one and the same heat-conducting body. They may also be simply connected to one another. As mentioned above, the measuring device for the temperature, present between tube wall and leg, is a thermopile. As heat source, there is preferably used a heating spiral. To promote the heat transfer from tube wall to fluidum, the application of an "insert" is possible. Such an insert then consists of, e.g., a cylindrical body having a length which e.g. is equal to that of the tube and having a diameter which is less than the inner diameter of the tube. The cylinder is placed concentrically in the tube and the fluid then flows as a film along the inner wall of the tube. It should be noted that the sensitivity of the device according to the invention may be increased by having a fluid flow through two (or more) parallel tubes, with the reference leg then being located between said two tubes and the temperature differences invariably being measured between the tube parts and the central leg. The invention will be explained in further detail by reference to the drawings, in which FIG. 1 and FIG. 2 schematically show known methods and devices for measuring flows; and FIG. 3 shows a device according to the invention. In FIG. 1, the fluid flow 1 flows through the tube 2 in the direction indicated. Tube 2 is heated with the help of a heating spiral 3, with the generated heat running upstream and downstream by way of the conducting tube wall of the tube 2. When there flows no fluid through the tube 2, the temperature curve, measured at the tube wall, is as shown by 4: a substantially linear curve upstream and downstream. When a fluid does flow through the tube, the temperature curve, measured at the tube wall, is as shown by the curve 5: "sagged" (concave) upstream and convex downstream. The extent to which the temperature curve deviates from the substantially linear situation is a measure of the flow. When, e.g., the temperatures are measured at 6 and 7 - locations on the tube wall which are situated at such a distance from the heat source that in situ the deviation of the temperature from that in the linear situation is significant - the occuring temperature difference ΔT is the measure referred to. FIG. 2 shows how, with the help of the same principle as that according to FIG. 1, a flow may be measured, though with a much smaller space occupation. The tube 8, through which flows the fluid flow 9, is comparable to the tube 2 according to FIG. 1, though this time bent in a "U". The heat-conducting body 10 causes the temperatures of the tube ends which correspond to the ends of the legs of the "U" to be equal. 11 is a thermopile present between the two legs of the "U" with which, along the entire length of the tube walls, the temperature differences are measured and may be supplied in integrated form. The upstream and downstream temperature curves are denoted by 12 and 13, respectively; in the event of an integrated measurement with the help of a thermopile 11, the surface 14 between the curves 12 and 13 is the desired measure of the fluid flow. The device according to FIG. 3 - a device according to the invention - makes possible the method according to the invention. Here, too, there again is a fluid flow 15 flowing through a tube 16 having a conducting wall 17. At 18, the tube 16, and therewith the fluid, is heated. With the help of the thermopile 19, there is measured the temperature difference between the tube wall 17 and a fixed reference leg 20. Said leg 20 extends between the heated part 18 of the tube 16 and the location 21, where tube 16 and leg 20 are in heat-conducting contact with one another. In the same manner as indicated in FIG. 2, the size of the surface 22 - the temperature difference between the tube wall 17 and the reference leg 20 - is integrated along the entire length over which there occurs a temperature difference between the two. Comparison of said surface 22 to the surface 14 as indicated in FIG. 2 shows that the measuring sensitivity amounts to half when measurements are made only on the flow on one side of the heat source. The surface 22, and therefore the signal to be measured, increases in tandem with an increasing fluid flow. In addition, said signal depends on the temperature difference between the temperature at the location where the heat source is situated and the temperature at the location where tube wall and reference leg again have an equal temperature: the measuring range.

Claims: Device for measuring flows of fluids flowing through a tube, comprising: a straight tube piece (16) which is heat conductive and through which a fluid to be measured flows during use, the tube piece (16) having an inlet end and an outlet end; a heat source arranged for co-operation with the outlet end; a second heat-conducting part (20) which extends substantially parallel to, and at a distance from, the tube (16) the temperature of the second heat-conducting part not being affected by the fluid flow; the inlet end of the tube piece (16) being thermally connected to the corresponding end of the second heat-conducting part 20; the outlet end of the tube piece (16) being thermally connected to the corresponding end of the second heat-conducting part (20); and a thermopile (19) being arranged between the tube piece (16) and the second heat-conducting part (20), for measuring temperature differences between the tube (16) and the second heat-conducting part (20) along the length between their respective ends, and for supplying said temperature differences in integrated form. Device as claimed in claim 1, wherein the heat source consists of a heating spiral engaging the outlet end of the tube piece and the corresponding end of the second heat-conducting part.

Inventors: JOUWSMA WIJBREN, VAN DER GRAAF FREDERIK, JOUWSMA, WIJBREN, VAN DER GRAAF, FREDERIK
Applicants: BERKIN BV, BERKIN B.V.
Application year: 1996
Main classification: G01F1
IPC codes: G01P5, G01F1