Measuring and testing – Volume or rate of flow – Thermal type
Reexamination Certificate
2001-02-06
2003-03-25
Williams, Hezron (Department: 2855)
Measuring and testing
Volume or rate of flow
Thermal type
Reexamination Certificate
active
06536273
ABSTRACT:
The invention relates to a thermal flow-rate sensor and a method for determining the flow rate of a fluid.
The practice of determining the flow rate of a fluid (i.e., a liquid or a gas) through, for example, a pipeline, by means of a thermal flow sensor is known in the art. Such a flow sensor comprises a heating device and a temperature detector which reacts to the temperature of the heating device. For the purpose of determining the flow rate, use is made of the cooling a effect on the flow sensor caused by the inflowing fluid. The greater the velocity of flow and, consequently, the flow rate of the fluid, the greater the quantity of heat that is taken away from the flow sensor by the fluid per unit of time through heat transfer and convection. Thus, for example, if a constant heating power is supplied to the heating device, the temperature detected by the temperature detector is lower in the case of a high flow rate than in the case of a low flow rate. Accordingly, if the heating power is regulated in such a way that the flow sensor is at a constant temperature, a higher heating power is required in the case of a greater flow rate than in the case of a lesser flow rate. Calibration is required in order to permit measurement of absolute flow quantities with such a flow sensor. This takes account, for example, of effects of the geometry of the flow sensor and the material properties of the fluid, which are substantial contributory factors in the determination of the heat dissipation capacity. Thus, for example, the heat dissipation capacity of the fluid depends on its composition.
Measurement of the flow rate by means of such thermal flow sensors is described in, for example, the textbooks of O. Fiedler, “Strömungs- und Durchflu&bgr;me&bgr;technik” [“Flow and flow-rate measuring systems”] (Oldenbourg-Verlag 1992), and H. Eckelmann, “Einführung in die Strömungsme&bgr;technik” [“Introduction to flow measuring techniques”] (Teubner-Verlag 1997).
In order to render possible the use of a flow sensor of the type outlined, the composition of the fluid whose flow rate is to be determined must not vary, or it must vary only within very narrow limits. This is because a different fluid composition could result in an alteration of the heat dissipation capacity, so that changes in the temperature or in the heating power of the flow sensor are not necessarily attributable to flow rate variations.
The object of the invention is to provide a thermal flow-rate sensor and a method for determining the flow rate of a fluid with the use of a thermal flow-rate sensor which also render possible flow-rate measurement on a fluid of variable composition.
This object is achieved by a thermal flow-rate sensor having the features of claim 1 and by a method, having the features of claim 14, for determining the flow rate of a fluid. Advantageous embodiments are disclosed by the dependent claims.
The thermal flow-rate sensor according to the invention comprises a flow sensor with a first heating device and a first temperature detector, reacting to the temperature of the first heating device, and can be exposed to a flowing fluid whose flow rate is to be determined. This flow sensor is a thermal flow sensor, based on the principle described above. In addition, the thermal flow-rate sensor has a thermal-conductivity measuring cell which comprises a measuring-cell casing, a second heating device and a second temperature detector, reacting to the temperature of the second heating device, the measuring-cell casing having at least one opening arranged for entry of the fluid into the measuring-cell casing.
A portion of the fluid can enter the thermal-conductivity measuring cell via the opening. Within the thermal-conductivity measuring cell, virtually no thermal transfer occurs through convection, since this measuring cell is designed for measuring the thermal conductivity of a fluid contained in it and its interior is therefore largely protected against disturbance by fluid flows. The thermal conductivity of the fluid can be determined by means of the second heating device and the second temperature detector, which reacts to the temperature of the second heating device. The fundamental measuring principle of the thermal-conductivity measuring cell is similar to that of the thermal flow sensor, but since no convection occurs within the measuring-cell casing, the heat removed from the second heating device by the fluid is transferred essentially through thermal conduction. Through selection of an appropriate geometry (particularly a small spacing between the second heating device and the inside of the measuring-cell casing) and an appropriate temperature range for the second heating device (relatively low temperatures are advantageous), the proportion of radiation in the heat transfer can be kept low. Such thermal-conductivity measuring cells, and the measuring principle used with them, are known in the art. The thermal conductivity of the fluid can therefore be determined by means of the thermal-conductivity measuring cell.
The thermal conductivity is a measure of the composition of the fluid. If the fluid has two components with different thermal conductivities, the proportions of the components in the fluid can be determined through a measurement of the thermal conductivity of the fluid. However, the thermal flow-rate sensor according to the invention can also be used, advantageously, for fluids with more than two components, as illustrated by the following example. If, for example, the fluid is a mixture of air and the hydrocarbons propane and butane, in respect of its thermal conductivity it consists essentially of two components, namely air and hydrocarbon. This is because, in comparison with air, propane and butane differ only slightly from one another in respect of their thermal conductivity properties and also have a similar heat dissipation capacity in respect of the flow sensor. In the case of this fluid, for example, the concentration of hydrocarbon in air can be determined, in the range from 0% to 100%, with an accuracy of better than 5%, by means of the thermal-conductivity measuring cell. The composition of the fluid is thus known with sufficient accuracy to enable it to be taken into account in the behaviour of the flow sensor.
The thermal-conductivity measuring cell thus makes it possible, by simple means, for the composition of the fluid to be determined or at least estimated to the extent that composition-dependent differences in the heat dissipation capacity of the fluid can be taken into account in the operation of the flow sensor in order to achieve a reliable flow-rate measurement. Since the thermal flow-rate sensor according to the invention comprises both the flow sensor and the thermal-conductivity measuring cell, it is ensured that all measurements are always performed on the same fluid.
The thermal flow-rate sensor preferably comprises a control and evaluation device which is arranged for the purpose of generating a first measuring signal, characterizing the heat dissipation capacity of the fluid, by means of the heating power supplied to the second heating device and the temperature of the second temperature detector, and generating a second measuring signal, characterizing the flow rate of the fluid, by means of the heating power supplied to the first heating device, the temperature of the first temperature detector and the first measuring signal. This first measuring signal is preferably assigned to the composition of the fluid. The control and evaluation device renders possible a preferably fully automatic operation of the flow sensor and thermal-conductivity measuring cell under the conditions explained above. In a preferred embodiment, the control and evaluation device is arranged for the purpose of determining the flow rate of the fluid by means of parameters, determined in calibration measurements, and the second measuring signal. By means of such calibration measurements, as already mentioned above, the quantities entering into the evaluation of the measureme
Fafnir GmbH
Mack Corey D.
Nixon & Vanderhye
Williams Hezron
LandOfFree
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