Measuring and testing – Volume or rate of flow – Thermal type
Reexamination Certificate
1999-09-08
2002-01-15
Fuller, Benjamin R. (Department: 2855)
Measuring and testing
Volume or rate of flow
Thermal type
C374S054000
Reexamination Certificate
active
06338271
ABSTRACT:
FIELD OF THE INVENTION
The present invention pertains to a device for measuring the velocity of flow of a fluid with at least one thermocouple with a junction point arranged in the fluid, with an a.c. power source heating the thermocouple to an increased working temperature compared with the temperature of the fluid and with an evaluating circuit processing the thermocouple voltage. The present invention also pertains to a process for measuring the velocity of flow of a fluid with a thermocouple.
BACKGROUND OF THE INVENTION
A suitable measuring method for measuring the velocity of flow of a fluid is, e.g., the hot wire anemometry, which is based on measuring the amount of heat released from a heating element to the fluid to be investigated. The velocity of flow can be inferred by taking this variable into account with the temperature difference prevailing between the heating element and the fluid.
The arrangements used for this type of measurement usually determine the temperature difference by the separate measurement of the temperature values for the heating element and the fluid and taking them mutually into account by means of the downstream electronic unit, which leads to problems due to the lead-in and contact resistances because of the further processing of the measured signal being separated in space. Since the lead-in and contact resistances of the plug contacts also affect the measured value proper at the time of the measurement of the hot wire resistances of interest, depending on the measuring circuit, the measurement may be interfered with by changes in the arrangement of the wiring. The substraction also increases the relative error in the signal difference compared with the initial variables, because the measurement of the individual resistance results in an absolute error that is not eliminated during the subtraction, while the measured signal decreases at the same time. The quotient of the absolute error to the measured value, i.e., the relative error, is increased as a result.
However, it is advantageous in this principle of measurement for the temperature of the fluid to be known. Any temperature-related drifts of the fluid parameters, e.g., the heat conductivity, can thus be compensated at least partially. The change in temperature is measured for this purpose in order to infer the change in the signal caused by parameter drift and to correct this change by taking the measured value into account.
However, arrangements have also been known in which temperature differences are measured directly at the site of the measurement. This may be done, e.g., by the hot wire located in the fluid to be investigated not having been made of a homogeneous wire material but of a series of at least two partial wires consisting of different materials, which are connected to one another at their junction point in an electrically conducting manner, e.g., by soldering or welding. A thermocouple is thus formed at the junction point of the wires, while another heat transition is formed at the transitions between the hot wire and the support element. Such a device has become known from EP 0 187 723 A2. The thermocouple voltage delivered by this device is a direct indicator of the temperature difference between the hot wire and the environment, which is gaseous in this case, if it is ensured by design measures that the area of the support elements is at ambient temperature. If this temperature difference is maintained at a constant value by control technology by adjusting the heating power, the device may be used optionally for measuring the velocity of flow or the heat conductivity of the surrounding medium if the respective other parameter is known.
Problems are caused by the fact that the heat conductivity of gases depends on the temperature and increases with increasing temperature. This means that a measuring device of the type described, which only maintains the temperature difference between the measuring wire and the fluid at a constant value and evaluates the heating power needed for this as a measured variable, also detects these temperature drifts as an apparent measured signal. The consequence of an increasing fluid temperature is that the heat conductivity will increase as well and it brings about increased heat dissipation from the hot wire, which would reduce its excess temperature relative to the fluid without control. The additional heating power that is needed to bring the excess temperature of the wire relative to the fluid to the old difference set point now mimics a change in the measured value of the velocity of flow, from which a corresponding drift of the measured signal results.
SUMMARY AND OBJECTS OF THE INVENTION
The primary object of the present invention is to improve a device of the type described such that the temperature dependence of the heat conductivity of the fluid to be investigated will be compensated in a simple manner.
According to the invention, a device for measuring the velocity of flow of a fluid is provided with at least one thermocouple. The thermocouple has a junction point arranged in the fluid, with an a.c. power source heating the thermocouple to an increased working temperature compared with the temperature of the fluid. An evaluating circuit processes the thermocouple voltage. The temperature coefficient of the ohmic resistance of the thermocouple is adapted to the temperature dependence of the heat conductivity of the fluid such that the heating power converted in the thermocouple during a change in temperature and unchanged a.c. current varies by exactly the same amount as the amount of heat released to the fluid due to the changed heat conduction.
According to another aspect of the invention, a process for measuring the velocity of flow of a fluid with a thermocouple is provided with a junction point arranged in the fluid and heated with the a.c. power source to a working temperature that is increased compared with the fluid temperature. The temperature coefficient of the ohmic resistance of the thermocouple is adapted to the temperature dependence of the heat conductivity of the fluid such that the heating power converted in the thermocouple in the case of a change in temperature and unchanged a.c. current varies by exactly the same amount as the amount of heat released to the fluid due to the changed heat conduction. Corresponding conditions prevail in the case of a decrease in temperature.
Even though the selection of materials is already limited due to the necessity to use thermocouple wires for the heating element, the temperature characteristic of the ohmic resistance of the thermocouple can be affected by a number of parameters so that various design possibilities may be considered for the required adaptation of the temperature coefficient. Helpful is the fact that even though the two partial wires of the thermocouple possess different physical properties, such as resistivity, temperature coefficient and heat conductivity, the properties pass over into each other in the area of the junction point. A fine tuning may therefore already be achieved by varying the geometric conditions, without the need to use novel wire alloys. This may be done in this case by welding together partial wire sections of different thickness.
Another advantageous embodiment is based on the use of a continuous carrier, to which the thermoelectrically active materials may be applied as desired in a layer thickness that can be freely determined. The effect of the physical properties of the individual materials on the behavior of the thermocouple can thus be set within a broad range. Besides the use of insulating materials, such as glass fibers, especially the use of a continuous metal wire as a carrier is conceivable.
Also advantageous is a structure in which a hot wire, whose temperature coefficient is optimally adapted to the fluid, is provided with an insulating intermediate layer to subsequently apply to the latter the thermoelectrically active layers needed for the temperature measurement by vapor deposition.
The mode of action of the temperatu
Dräger Aktiengesellschaft
Fuller Benjamin R.
Mack Corey D.
McGlew and Tuttle , P.C.
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