FI75046B - TERMISTORGIVARE FOER MAETNING AV GASENS STROEMNINGSHASTIGHET. - Google Patents

Description

1 75046

THERMISTOR SENSOR FOR GAS FLOW RATE MEASUREMENT

The invention relates to a thermistor sensor which can be used as a sensor for a gas flow meter, i.e. an anemometer, when measuring low flow rates, for example in rooms.

5 To measure low air flow rates, there are meters that work on a number of different principles. The most common group of measuring devices in this area is the so-called thermal Anemometers, in which a sensor heated to a temperature higher than the air temperature acts as a sensor, which can be 10 e.g. a wire or a thermistor. A wide range of thermal anemometers is commercially available. The number of related publications is very high. For example, the series publications QUQTERLY, A Quarterly Publication of TSI Incorporated or DISA Information, Measurement and Analysis 15, as well as several publications in the field of measurement, deal with the topic.

All thermal anemometers are to some extent direction-dependent, i.e. the incoming direction of the flow facing the sensor affects the measurement result obtained. Hot wire sensors are particularly sensitive to the flow inlet direction due to their straight, wire-like structure. There are also spherical sensors that have been designed to minimize directional dependence. However, there is a large blind spot in the direction of the mounting rod of the fire sensor, which causes an error of up to 25 35 ... 50% in the measured flow rate value, even with the best sensors, e.g.

Fitzner K., Richtungsabhängige Anemometer in Raumströmungen, Klima-Kälte-Heizung, no 11, 1980, pp. 441-6; Finn E. Jorgensen, An Omnidirectional Thin-film Probe for Indoor Climate Research, DISA information no 24, May 1979, 30 ss 24-9. In measuring applications where the flow direction is unknown and possibly even changes during the measurement, the complete possible direction independence of the measuring sensor is a very valuable feature.

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For example, the measurement of air flow rates in a room requires the aforementioned feature of the anemometer sensor to be used.

The cunning sensor of the invention provides a substantial improvement over the drawback described above. To achieve this, the device according to the invention is characterized by what is set forth in the characterizing part of claim 1.

The advantage of the invention compared to current measuring sensors is that the directional dependence of the sensor has been made very small with the construction according to the invention. At its maximum, the effect of flow direction variation is only about ± 3% of the measured velocity value, and no specific blind spots are affected at all. This makes it possible to measure the flow rate without the direction of the flow practically affecting the result obtained.

In the following, the invention will be explained in detail with reference to the accompanying drawings.

20 Figure 1 shows a thermistor with wires in a rectangular coordinate system.

Figure 2 shows the measuring sensor in its entirety.

Figure 3 shows the effect of flow direction variation on the measurement result.

According to Fig. 1, the electrically conductive metal wires 2, 3, k of the thermistor 1 are bent on both sides of the thermistor in sections parallel to the coordinate axes, the lengths of which are a, b and c from the thermistor (a <b <c). When measuring the flow rate, 30 electric currents are passed through the thermistor. The thermal power generated in the thermistor causes the temperature of the 11 3 75046 thermistor to rise to a higher temperature than the surrounding medium. Some of the developing power is transferred directly from the thermistor to the medium, e.g. air, by the effect of convection. Since the thermistor is approximately spherical in shape, the direction of flow facing the thermistor has no effect on this event. The rest of the thermal power is transferred from the thermistor to the metal wires and from there on to the medium by convection. Since the heat transfer coefficient on the surface of the wire and thus also the heat output from the wire 10 depends on the incoming flow direction, the thermistor sensor wires are bent parallel to the coordinate axes and the lengths a, b and c are selected so that the heat output of the sensor is constant regardless of the flow direction. Pi-15 values have been calculated using rib theory, e.g., Ryti H., Stationary Heat Transfer, Technical Manual 5, 1970, pp. 1-76.

Figure 2 shows the measuring sensor in its entirety. The thermistor 1, the wires 2, 3 * 4 of which are bent according to the principle described above, is attached to a frame 5 made of metal wire. The opposite sides of the frame are galvanically connected to each other only by means of a thermistor. In this way, the frame acts as an electrically conductive part when connecting the sensor to the anemometer. When the frame is made large enough, there are no structures interfering with the flow in the vicinity of the thermistor, and thus no blind spots are created which cause a measurement error.

The effect of the input direction of the flow encountering the sensor on a sensor constructed as described above and connected to a constant temperature anemometer is shown in Figure 3. The vertical axis of the figure shows the relative difference between the flow velocity v * and the reference velocity vq. The horizontal axis has an angle of attack <x. . It can be seen from the figure that the effect of the flow inlet-35 direction on the measurement result is only about Jt 3% of the measured velocity value.

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The following is the basic implementation of the sensor dimensioning.

Both wires leaving the thermistor can be considered as a rib. Then for each wire section 2, 3 and U the superheat-5 state 0 with respect to the environment can be represented in the form 0 = Acosh (wx) + Bsinh (wx), where = -j ^ and further x is the distance from the thermistor along the wire, h is the heat transfer coefficient of the wire and between the surrounding medium, P is the cross-sectional area of the wire 10, L is the thermal conductivity of the wire, and A is the cross-sectional area of the wire.

Continuity of temperature and its derivative is used as boundary conditions at the bends between the wire portions 2, 3 and A.

When the boundary conditions are placed in the above temperature equation, a group of equations of six equations is obtained, from which the temperature of each wire section can be solved. Equations derived for temperatures can now be placed in the expression q = jhP0 (x) dx of the heat flow from the wire to the environment, whereby:: 20 the integration of the above expression is possible.

The principle of dimensioning is now to select a length for each section 2, 3 and 4 such that the total heat flux leaving the wires: ·. q “« 2 + q3 + “U · 25, which is the sum of the heat flows of the different wire sections, would remain constant in the flow direction and with it the heat transfer coefficient. fluctuating. This sizing step must be done by experimenting with different lengths for the 5 75046 wire sections and then calculating the total heat flux as a function of the flow direction as described above, as the solution cannot be obtained in analytical form. The combination of lengths that gives the smallest range for the value of the total heat flux as a function of the input direction can be considered as the solution sought.

The dimensioning is therefore mainly influenced by the thickness and material of the wire and, to a lesser extent, the heat transfer coefficient. The temperature of the thermistor does not matter. For example, with a wire diameter of 0.2 mm and a material of copper, the optimal length of the wire portion 2 is about 3 mm and the length of the wire portion 3 is about 5 mm. The length of the last wire section 4 is not critical, as long as it is long enough, in this case about $ cm, because the heat flux far from the ter-15 mistor is small

Claims (2)

6 75046 A thermistor sensor for measuring the gas flow rate, especially in cases where the flow direction of the flow encountering the sensor is unknown or variable, characterized in that both metal wires leaving the thermistor (1) are bent into three mutually perpendicular portions (2, 3 »λ), that when the thermistor (1) is located at the origin of the right-handed rectangular coordinate system, the portions (2) closest to the thermistor coincide with the positive and negative y-axes, the subsequent portions (3) are parallel to the x-axis and the last portions (4.) are parallel to the z-axis. Thermistor according to Claim 1, characterized in that it is mounted on a frame (5) of such dimensions, which acts as both a support structure and an electrically conductive part, that it does not interfere with the flow in the vicinity of the thermistor, whatever the direction of incoming flow. il