JPH0334653Y2 - - Google Patents

Description

[Detailed Description of the Invention] Industrial Application Field The present invention relates to a thermal mass flowmeter that can accurately measure the flow rate of a fluid to be measured without being affected by outside air.

Problems to be solved by conventional techniques and ideas Conventional thermal mass flowmeters heat the metal resistance wires that are wound around the upstream and downstream sides of the flow tube through which the fluid to be measured flows. , due to the difference in the amount of heat removed from each metal resistance wire by flowing the fluid to be measured, a difference in temperature occurs between the upstream and downstream metal resistance wires, resulting in a difference in resistance value. Since the difference changes as the flow velocity of the fluid to be measured changes, the output voltage between the two metal resistance wires was detected and the flow rate of the fluid to be measured was measured from this detected value. In such a thermal mass flowmeter, the metal resistance wire is exposed to the outside air and heat is taken away not only by the fluid being measured but also by the surrounding air, so changes in the resistance value of the metal resistance wire are caused by changes in the outside air. As a result, the flow rate of the fluid to be measured could not be accurately measured. As a countermeasure for this,
Some devices are designed to prevent heat from being taken away by the outside air by creating a vacuum around the metal resistance wire or covering it with an insulating material, but creating a vacuum increases the complexity of the device and increases manufacturing costs. If it is covered with an insulating material, the insulating material itself will be heated, increasing the overall heat capacity and reducing the response.
It has also been proposed to relatively reduce the amount of heat transferred to the outside air by providing fins inside the flow tube to increase the amount of heat transferred to the fluid to be measured, but this method would increase manufacturing costs. There was a problem.

Means for Solving the Problems The present invention, as a means for solving the above-mentioned problems, provides that the outer periphery of the flow tube made of a material with high thermal conductivity through which the fluid to be measured flows is made of a material with low thermal conductivity. The base body is closely fixed, and a heater is arranged at a reference position of the base body far from the flow tube to maintain and heat the reference position at a constant temperature, and at a position of the base body close to the flow tube. A temperature sensor is placed isolated from the outside air, and the flow rate of the fluid to be measured is measured from the temperature value detected by the temperature sensor.

Functions and Effects The present invention has the above-mentioned configuration, and when the reference position is heated to a constant temperature by making the heater generate heat while adjusting the calorific value, and the fluid to be measured that is lower temperature than the reference position is circulated through the flow tube, As heat is transferred from the reference position to the measured fluid through the inside of the base and the flow tube, a large temperature gradient is formed inside the base, which has low thermal conductivity, and the temperature gets lower as it approaches the temperature sensor. A small temperature gradient is formed inside the flow tube where the flow rate is low from the outer circumference to the inner circumference, and when the flow rate of the fluid to be measured increases, the amount of heat transferred from the flow tube to the fluid to be measured increases. increases, the temperature at the inner circumference of the flow tube decreases, and when the flow velocity of the fluid to be measured decreases, the temperature at the inner circumference of the flow tube increases, and the temperature at the inner circumference of the flow tube changes. Then, since the temperature at the reference position far from the flow tube is kept constant, the temperature gradient inside the flow tube and the inside of the base changes. The temperature of the temperature sensor located on and near the outer circumference of the thin tube and isolated from the outside air changes by approximately the same amount as the temperature change on the inner circumference of the flow tube, and this change corresponds to the change in the flow velocity of the fluid to be measured. Therefore, the flow rate of the fluid to be measured can be measured with high accuracy using the temperature sensor.In addition, a heater is placed at the reference position of the base to heat the reference position to a constant temperature, and the temperature sensor is isolated from the outside air. Because I placed it,
The temperature gradient formed through the temperature sensor is determined only by the flow rate of the fluid being measured, making it possible to accurately measure the flow rate of the fluid being measured without being influenced by outside air. Since no equipment or processing is required, manufacturing costs can be kept low and the volume can be reduced.

Embodiment Hereinafter, an embodiment of the present invention will be described based on the accompanying drawings.

In FIG. 1, 1 is a rectangular base body made of an inorganic material such as quartz, glass, or sapphire, which has electrical insulation properties and low thermal conductivity, and has a smooth surface and a uniform thickness. A heater 2 is formed on the upper surface 1a by attaching a metal heating element 3 having a thin strip shape and high electrical resistance along a certain pattern and connecting it to a power source (not shown) through a lead wire 4. A first temperature sensor 5 is formed by arranging a metal resistor 6 along the heating element 3 and connecting it to a temperature detection device (not shown) via a lead wire 7. 3, the temperature of the upper surface 1a is detected from the electrical resistance value of the resistor 6 of the first temperature sensor 5, and the amount of current is adjusted according to this detected value, thereby heating the upper surface 1a to a desired temperature. On the lower surface 1b of the base 1, a thin metal resistor 9 having a width narrower than the heating element 3 of the heater 2 on the upper surface 1a is arranged in a pattern corresponding to the heating element 3. A second
A temperature sensor 8 is formed to detect the temperature of the lower surface 1b from the electrical resistance value of the resistor 9, and the heater 2, the first temperature sensor 5, and the second temperature sensor 8 are formed. The base body 1 is fixed to a flow tube 11 through which a fluid to be measured flows.
The flow tube 11 is made of thin-walled stainless steel with high thermal conductivity, and has an oval cross-sectional shape with straight sections at the top and bottom, as shown in FIG. flat surface 12, 12
are formed, and the lower surface 1b of the base 1 is bonded to the upper flat surface 12 so as to correspond parallel to each other with an adhesive 13 made of a material having substantially the same thermal conductivity as the flow tube 11. , the resistor 9 of the second temperature sensor 8 is surrounded by an adhesive 13 and shielded from the outside air.

Next, the operation of this embodiment will be explained.

The upper surface 1a of the base 1 is exposed to the outside air and when heated by the heater 2, heat is taken away by the surrounding air.
By adjusting the amount of electricity applied to the heating element 3 of the heater 2, the upper surface 1a is kept at a predetermined constant temperature, and in this state, when the fluid to be measured at a constant temperature lower than the upper surface 1a is flowed into the flow tube 11, the base 1 , the adhesive 13, the flow tube 11, and the fluid to be measured.
As shown by the solid line in the graph of the figure, inside the base 1, which has low thermal conductivity and slow heat transfer, there is a temperature that decreases significantly from the upper surface 1a toward the lower surface 1b where the second temperature sensor 8 is located. At the same time as the gradient A is formed, the adhesive 13 with high thermal conductivity and rapid heat transfer and the inside of the flow tube 11 are continuous with the temperature gradient A of the base 1 and have a temperature gradient of the adhesive 13 of the base 1. A temperature gradient B is formed in which the temperature slightly decreases from the surface in contact with the lower surface 1b toward the inner circumference of the flow tube 11, and when the flow velocity of the fluid to be measured increases, As the amount of heat transferred to the fluid increases, the temperature of the inner peripheral surface of the flow tube 11 decreases, and the temperature of the lower surface 1b of the base 1 also decreases, and since the temperature of the upper surface 1a is kept constant, both temperature gradients A, If B changes as shown by the dashed-dotted line in the graph of FIG. 3, and conversely, the flow velocity of the fluid to be measured decreases, the amount of heat transferred to the fluid to be measured decreases, and the temperature gradient A, B changes as shown by the two-dot chain line in the same graph, but because the thermal conductivity of the base 1 is low and the thermal conductivity of the adhesive 13 and the flow tube 11 is high, the temperature change on the lower surface 1b of the base 1 is caused by the flow. tubule 1
1, and the temperature of the lower surface 1b detected by the second temperature sensor 8 isolated from the outside air is almost equal to the temperature of the inner peripheral surface of the flow tube 11. , and the flow tube 11
changes in response to changes in the temperature of the inner circumferential surface of the
The temperature detected by the second temperature sensor 8 changes in accordance with the flow rate of the fluid to be measured, and the flow rate of the fluid to be measured is measured from this detected temperature.

In this embodiment, the temperature of the inner circumferential surface of the flow tube 11 does not become higher than the upper surface 1a of the base 1, which is kept at a constant temperature.
There is no risk that the fluid to be measured flowing through the chamber becomes abnormally high temperature.

Similarly, in this embodiment, since the cross-sectional shape of the flow tube 11 is oval, the base body 1 is reliably fixed by forming the flat surface 12, and the cross-sectional shape is circular. Compared to the case where the flow rate is the same, the cross-sectional area through which the fluid to be measured flows is smaller and the flow rate is increased even if the flow rate is the same, so the amount of heat transferred from the flow capillary tube 11 increases and there is an advantage that it is less susceptible to the influence of outside air.

In the above embodiment, the case where the temperature of the fluid to be measured is constant has been described, but even if the flow rate is constant, if the temperature of the fluid to be measured is high, the amount of heat transferred from the flow tube 11 will be small, and conversely, the temperature will be low. As the amount of heat transferred increases, the second temperature sensor 8
When the temperature of the fluid to be measured changes because the temperature value detected by
A compensating temperature sensor (not shown) is provided upstream of the base 1 to detect the temperature of the fluid to be measured before the heat is transferred, and the detected value from the second temperature sensor 8 is determined based on this detected value. By compensating or compensating the temperature of the heating element 3, the flow rate of the fluid to be measured whose temperature changes can be accurately measured.

Similarly, in the above embodiment, the upper surface 1 of the base 1
In order to maintain the temperature of the heater 2 at a constant temperature, the temperature of the upper surface 1a is detected by the first temperature sensor 5 disposed along the heater 2, and the amount of heat generated by the heater 2 is adjusted based on this detected value. Alternatively, the temperature of the upper surface 1a may be kept constant by keeping the temperature of the heater 2 constant without using the first temperature sensor 5. To do this, as shown in FIG. By incorporating the heater 2 on one side of the bridge of the bridge circuit, when the temperature of the heater 2 changes and its resistance value changes, the output voltage of the bridge changes,
In response to this change in output voltage, the output voltage of the differential amplifier 20 may be amplified and applied to the input terminal of the bridge, and as a result, the current flowing through the heater 2 changes, causing resistance heating of the heater 2. The amount of heat generated by the heater 2 is adjusted, and automatic control is performed so that the temperature of the heater 2 is always kept constant.

Note that when the size of the base 1 is reduced,
Since the heat capacity is small, the temperature gradient A changes quickly when the flow rate of the fluid to be measured changes, and there is an advantage that the delay in change of the measured value with respect to the change in the flow rate is reduced.

The amount of heat transferred from the flow tube 11 to the fluid to be measured varies depending on the cross-sectional shape and size of the flow tube 11 and the type and flow rate of the fluid to be measured. The thin metal wire is not wound around the base 1 on which the heater 2 and the first and second temperature sensors 5 and 8 are installed, but the cross-sectional shape and size of the flow tube 11 are fixed. can be easily changed,
By changing the cross-sectional shape and size according to the type and flow rate of the fluid to be measured,
It is possible to adjust the amount of heat transferred from the fluid to the fluid to be measured.

Note that the material that can be used as the substrate 1 is not limited to the inorganic substances shown in the above embodiments, but may also be organic materials such as ceramics made of nitrides, carbides, etc., and polymer films.

Furthermore, an LSI manufacturing process can be used to integrally manufacture the base 1, the heating element 3 of the heater 2, the resistor 6 of the first temperature sensor 5, and the resistor 9 of the second temperature sensor 8. According to this method,
Part or all of the Si substrate is etched to make it thinner, and a SiO ₂ or Si ₃ N ₄ film is formed by CVD (chemical vapor deposition), thermal oxidation, or nitridation, and a second temperature sensor is installed here. After attaching the resistor 9 of 8,
A film with low thermal conductivity made of SiO ₂ or Si ₃ N ₄ is deposited thickly by CVD, and the heating element 3 of the heater 2 is placed on top of this film.
and the resistor 6 of the first temperature sensor 5.

[Brief explanation of drawings]

The accompanying drawings show an embodiment of the present invention, and FIG. 1 is a perspective view of a state in which a heater, a first temperature sensor, and a second temperature sensor are arranged on a base, and FIG. 2 is a state in which the base is fixed to a flow tube. The cross-sectional view of Fig. 3 is the 2nd
A partial enlarged view of the figure and a graph showing the temperature gradient inside the base and the flow tube are shown side by side. Figure 4 shows the case where the top surface of the base is kept at a constant temperature using only the heater without using the first temperature sensor. FIG. DESCRIPTION OF SYMBOLS 1... Base body, 2... Heater, 5... First temperature sensor, 8... Second temperature sensor, 11... Distribution thin tube.

Claims (1)

[Scope of utility model registration request] A base body made of a material with low thermal conductivity is closely fixed to the outer periphery of a flow tube made of a material with high thermal conductivity through which the fluid to be measured flows, and a heater is arranged at a reference position of the base body far from the flow thin tube. At the same time, a temperature sensor is placed in a position close to the flow tube on the base body, shielded from the outside air, and the temperature detected by the temperature sensor is heated. A thermal mass flowmeter characterized in that the flow rate of a fluid to be measured is measured from the value.