JPH05157601A - Thermal mass flowmeter sensor - Google Patents

Abstract

PURPOSE:To expand the measurable flow rate range of the title sensor so that the sensor can measure large flow rates by forming a sensor pipe to have a relatively large wall thickness by setting the outside and inside diametral dimensions of the pipe to have a ratio larger than a specific ratio. CONSTITUTION:The quantity of heat which is removed from an upstream coil 7 by means of a fluid flowing through a sensor pipe l and the quantity of heat which is given to the pipe l from a downstream coil 8 vary depending upon the heating temperature and mass flow rate, but the temperature change of the pipe 1 caused by the quantity of heat given to the pipe 1 becomes smaller as the wall thickness of the pipe 1 increases and the movement of the temperature distribution from the upstream side to the downstream side also becomes smaller as the wall thickness of the pipe 1 increases. Since the movement of heat can be accurately detected when the movement of the temperature distribution is within the extents of the wound sections of the coils 7 and 8, larger flow rates can be detected as the wall thickness of the pipe 1 is increased. Accordingly, the pipe 1 is formed to have a relatively thick wall thickness by roughly setting the ratio of the outside diametral dimension D0 of the pipe 1 to the inside diametral dimension D1 of the pipe 1 within a range of 1.5<=D0/D1<=4.0 by paying attention to the balance among the kind of fluid, desired flow rate range, and responding speed. Therefore, the allowable flow rate limit can be increased.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention has a heater wound around a sensor pipe through which a fluid to be measured flows, and the flow rate of the fluid flowing inside the sensor pipe is measured based on the temperature of the heater itself or before and after the heater or the energy applied. The present invention relates to an improvement of a thermal mass flowmeter sensor.

[0002]

2. Description of the Related Art A small flow rate measuring device is disclosed in
As disclosed in Japanese Patent Publication No. 1-107114, there is an example in which a sensor pipe through which a fluid flows is cooled, but since heating is generally easier than cooling, a heating type mass flow meter is used. The general structure of a heating type flow meter is to wind a heat-sensitive resistance wire around the upstream and downstream sides of the sensor pipe through which the fluid to be measured flows, heat the sensor pipe, and at the same time change the resistance value of the heat-sensitive wire to the upstream side. One that detects the temperature difference from the downstream side and measures the flow rate of the fluid to be measured from the temperature difference (constant current type), or keeps the temperature of the upstream and downstream heat-sensitive resistance wires constant and There is one (constant temperature type) that detects the difference in applied energy and measures the flow rate of the fluid to be measured from the energy difference. Conventionally, stainless steel has been used for the sensor pipe to provide corrosion resistance, and the inner diameter used is a thin tube of φ0.2 to φ1.0 mm because it is necessary to heat the fluid over the entire cross section of the pipe. ing. Further, the wall thickness of the sensor pipe tends to be remarkably thin in order to improve the responsiveness and output sensitivity of the sensor, and a thickness of about 0.05 to 0.1 mm is used.

The operation of the thermal type flow meter will now be described. When the flow rate is 0, the temperature distribution of the sensor pipe heated by the heating resistors is as shown in b'of FIG. The center of contact is the highest, and at the end of the resistor, the temperature gradually begins to decrease due to the heat dissipated through the sensor pipe, and further to the outside of the resistor winding, the temperature drops to the same temperature as the ambient temperature. .. Further, this temperature distribution is symmetrical with respect to the upstream side and the downstream side about the connection point of both resistors. Therefore, the average temperature and the electric resistance value of both resistors become equal, and the output value of the sensor becomes zero. Next, when the fluid flows, when the fluid flows into the sensor pipe heated to a high temperature by the heating resistor, heat is transferred from the sensor pipe to the fluid, the temperature of the sensor pipe decreases and the temperature of the fluid gradually rises. Begin to. Then, by the time the fluid reaches the connection part of both resistors,
The temperature of the sensor pipe and the temperature of the fluid become equal and equilibrium occurs. Next, when the fluid reaches the downstream end of the resistor, the temperature of the sensor pipe becomes lower than the temperature of the fluid, heat is transferred from the fluid to the sensor pipe, and the temperature of the sensor pipe rises.
The temperature of the fluid gradually begins to drop. Further downstream, the temperature of the fluid and the sensor pipe becomes equal to the ambient temperature, and an equilibrium state is reached. The temperature distribution of the sensor pipe when the fluid flows is shown in Fig. 4a '. Comparing this temperature distribution with the temperature distribution of the sensor pipe when the flow rate is 0, it can be seen that heat is moving from the upstream side to the downstream side due to the fluid. Here, the amount of heat carried by the fluid is proportional to the mass flow rate of the fluid. However, the temperature distribution when the fluid flows is outside the resistor winding part, so even if the temperature difference between the upstream and downstream resistors is measured, it is a value proportional to the mass flow rate to be accurate. Does not mean Here, the relationship between the mass flow rate Qs (cc / min) and the average temperature difference between the upstream side resistor and the downstream side resistor, that is, the output voltage V of the sensor (proportional to the temperature difference) is shown in the graph (Y) of FIG.
Shown in. It can be seen from the figure that the output voltage is proportional to the mass flow rate in the small flow rate range, but the error increases as the flow rate increases, and there is a limit to the flow rate range that can be used as a flow rate sensor.

[0004]

In the conventional sensor, the wall thickness of the sensor pipe is remarkably reduced to increase the temperature change of the sensor pipe with respect to the flow rate, thereby improving the output sensitivity and responsiveness. On the other hand, however, there is a problem that the temperature change of the sensor pipe is too large as described above, and the flow rate detection limit is reached with a small flow rate, which is not suitable for a flow meter that allows a large flow rate. In view of the above, it is an object of the present invention to provide a thermal mass flowmeter sensor capable of widening the linear range of the output voltage with respect to the flow rate and increasing the measurable maximum flow rate range.

[0005]

Means for Solving the Problems The present invention is, that the ratio D ₀ / D ₁ of the outer diameter D ₀ and the inner diameter D ₁ of the sensor pipe is 1.5 or more, formed by a relatively thick sensor pipe Thus, the above object is achieved. The present invention winds an upstream coil and a downstream coil, which are formed by heat-sensitive resistance wires, on the outer peripheral surfaces of the upstream side and the downstream side of the sensor pipe, respectively, and these upstream side coil, downstream side coil, and other resistors are wound. A constant current type heat that constitutes a bridge circuit, keeps the current flowing to the upstream and downstream coils constant, and measures the flow rate of the fluid flowing in the sensor pipe by detecting the unbalanced voltage of the bridge circuit. Type mass flow sensor and a constant temperature for measuring the flow rate of fluid flowing in the sensor pipe by controlling the temperature of the upstream and downstream coils to be constant and detecting the difference in energy supplied to both coils. Type thermal mass flow sensor.

[0006]

According to the present invention, the sensor pipe is made thicker so that the temperature change of the sensor pipe with respect to the flow rate is moderated and the sensitivity is lowered, but a sensor having a wide flow rate measuring range is obtained. This will be explained below. FIG. 6 shows a sensor model with one heating resistor. In FIG. 6, 11 is a sensor pipe, the diameter of this sensor pipe is R, the wall thickness is D, the flow rate of the gas flowing through this pipe is Q, and the specific heat of the gas is Cg. The thermal conductivity of the pipe is α ₁ . It is assumed that a current I is passed through the sensor coil 12 and heat of P ₀ is applied every second. Assuming that heat radiated through the sensor pipe 11 is P ₁ , heat radiated from the sensor coil to the atmosphere P ₂ and heat lost by heating the fluid gas is P ₃ , the heat that heats the sensor is P ₀ − (P ₁ + P ₂ + P ₃ ). These P ₀ , P ₁ , P ₂ and P ₃ can be expressed by the following equation.

[Equation 1]

[Equation 2]

[Equation 3]

[Equation 4] Here, symbols other than the symbols described above have the following meanings. T: Average temperature of heater part T ₀ : Ambient temperature α ₂ : Thermal conductivity of air L: Length of heater part R ₀ : Resistance value at room temperature T ₀ α: Temperature coefficient of resistance C: Heat capacity of heater part Can be expressed as C = πR · D · L.
The rate of temperature rise of the heater part is expressed by the following equation.

[Equation 5] Solving this differential equation gives:

[Equation 6] From the solution of this differential equation, the time constant is as follows.

[Equation 7] From the above, the temperature change ΔT when the temperature T is kept stable at the flow rate Q is ΔT = −I ² R ₀ / (A + C
gQ). The voltage change at this time is

[Equation 8] Becomes On the other hand, the temperature change ΔT when the flow rate Q is zero is -I ² R
_{Since 0} / A, the voltage change ΔV at this time is B / A. This changes the sensor voltage

[Equation 9] Indicated by. From FIG. 6 and the above description, the sensor output voltage Δ
In order to reduce V and reduce the temperature change of the resistor with respect to the flow rate, the wall thickness D is increased, the sensor pipe inner diameter R is increased, and the winding length L is increased. It can be seen that it is effective to increase the heat radiation and the heat radiation transmitted through the pipe. In other words, the ratio of the heat transfer P3 to the fluid is reduced with respect to the amount of heat Po to be applied, and the temperature change of the sensor pipe with respect to the flow rate when the fluid flows is reduced. Of the above R, D, and L, L has only the effect of radiating heat to the atmosphere and increasing the heat capacity of the sensor pipe, and is less effective than R and D. On the other hand, if R is too large, the fluid flowing in the pipe cannot be heated uniformly over the entire cross section of the pipe, resulting in a decrease in accuracy. Therefore, increasing the pipe wall thickness D is an effective means. If the wall thickness is increased, the heat radiation through the pipe will be increased, and as a result, the pipe temperature T will be lowered and the sensitivity at the maximum flow rate will be reduced, but T can be adjusted by the current I passing through the heating resistor. , Can be set to a desired temperature. However, the output voltage per unit flow rate,
The response time constant of the sensor decreases as the wall thickness increases. The above is the operation of the model having one heating resistor, but this model corresponds to the upstream resistor winding portion in the actual sensor. On the downstream side, a similar model can be applied although the heat transfer directions of the sensor pipe and the fluid are opposite. To summarize the above, the amount of heat taken by the fluid from the sensor pipe in the upstream resistor part and the amount of heat given to the sensor pipe in the downstream resistor part depends on the sensor pipe heating temperature and mass flow rate, not on the sensor pipe wall thickness. Although the temperature change of the sensor pipe is smaller, the thicker the pipe, the smaller the change in temperature distribution of the sensor pipe from the upstream side to the downstream side. The flow rate detection limit arises from the fact that the temperature distribution of the sensor pipe extends beyond the resistor winding portion and the amount of heat transfer by the fluid cannot be accurately detected. Therefore, the thick-walled pipe sensor in which the movement of the sensor pipe temperature distribution with respect to the flow rate is small can detect a large flow rate as compared with the thin-walled pipe sensor.
On the other hand, however, increasing the wall thickness slows down the response speed. Therefore, considering the kind of fluid, the desired flow rate range, and the response speed, approximately 1.5 ≦ D ₀ / D ₁
It is preferable to select from the range of ≦ 4.0.

[0007]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a front view of a thermal type flow meter sensor according to an embodiment of the present invention. The case 4 is assembled in a groove 5 formed in the mating surface 3 in a half-divided state so that a heat insulating material 6 made of ceramic paper in which a ceramic fiber is formed into a paper shape is installed to hold the sensor pipe 1. There is. The sensor pipe 1 is a thin tube bent in a substantially U shape, and is fixed so that both ends 1a and 1b penetrate the substrate 2. Both ends 1a, 1b of this sensor pipe are connected to a main pipe (not shown), and a bypass flow passage is formed in parallel with the sensor pipe 1, and the flow rate QS of the sensor pipe 1 and the bypass flow passage are formed. The ratio with the flow rate is set to be constant. Thus, the flow rate Q of the sensor pipe 1
The total flow rate of the main pipe is required from S. On the outer circumference of the sensor pipe 1, a pair of coils 7 and 8
Is wound and stored in the groove 5 in the case 4. The coils 7 and 8 are formed of an extremely fine enamel-coated metal wire having a core wire of platinum, iron-nickel, or the like.
An insulating material prepared by diluting polyimide resin with toluene is thinly applied to the outer surface of the sensor pipe 1, and the heater / sensor coils 7 and 8 are wound 100 to 200 times in the length direction of the sensor pipe 1 on the insulating material. The above insulating material is applied to form an insulating coating to insulate the coils and the coils from the sensor pipe.

FIG. 2 shows a means for detecting the temperature difference between the upstream side and the large flow side of the sensor pipe 1, and assuming that the electric resistance values of the heater / sensor coils 7 and 8 are R ₇ and R ₈ , this resistance Forms a bridge circuit with another pair of fixed resistors R ₁ and R ₂ . Constant current from constant current source 10 in the bridge circuit
Ib flows and the Joule heat of the coils 7 and 8 causes the sensor pipe 1 to reach 50 to 100 ° C above room temperature or fluid temperature.
The temperature rises. Sensor flow rate QS is QS = 0
, The bridge is balanced and the unbalanced voltage ΔV is ΔV = 0. When the flow rate QS flows through the sensor pipe 1, heat is transferred from the coil 7 to the coil 8 side, and the coil 7
Average temperature of the coil 8 decreases and the average temperature of the coil 8 increases. Due to this temperature change, the resistance values R ₇ R _{8 of the} coils 7 and ₈ change and bring an unbalanced voltage ΔV to the bridge circuit, and ΔV is taken out as the output of the differential amplifier 11, and this output is sent to the sensor flow rate QS. Since it is proportional, the mass flow rate of the main pipe can be obtained.

The sensor pipe 1 in this embodiment is made of stainless steel (SUS316L) having an outer diameter of 1.1 mm and an inner diameter of 0.52 mm. FIG. 3 shows the temperature distribution of the heater / sensor coils 7 and 8 (outer diameter 0.05 mm) wound around the sensor pipe as described above. The temperature distribution when the flow rate QS flowing in the sensor pipe 1 is QS = 0 is shown by the line b, and the temperature distribution when QS = 10 cc / min (N ₂ gas) is shown by the line a. On the other hand, the temperature distribution shown in FIG. 4 was measured under the same conditions as above except that the outer diameter of the sensor pipe was 0.6 mm and the inner diameter was 0.52 mm. However, this temperature distribution has a large temperature change of the a'line when a fluid is flown,
In addition, the temperature distribution generally moves to the right and tends to shift from the position of the sensor coil. However, in the case of FIG. 3, the temperature change is small, and the change is suppressed within the installation range of the sensor coil. Therefore, it is possible to set a large flow rate limit.

Next, a graph (X) in FIG. 5 shows the result of measuring the unbalanced voltage ΔV with respect to the flow rate QS flowing in the sensor pipe 1 in this embodiment. For comparison, the graph (Y) in the case of FIG. ) Is also shown. As can be seen from this figure, in this embodiment, the proportional range in which the flow rate QS and the unbalanced voltage ΔV change linearly increases to QS = 35 cc / mm, and ΔV at that time is 60 mV. On the other hand, in the comparative example, the proportional range that changes linearly is up to QS = 7 cc / mm, and ΔV at that time is
= 50 mV. That is, in this embodiment, although the detection sensitivity per unit flow rate decreased, the sensitivity at the maximum flow rate did not decrease, and the upper limit flow rate in the linearly changing range increased five times.

[0011]

As described above, according to the thermal mass flow sensor of the present invention, the measurable flow rate range is about 5 times that of the conventional one.
It can be doubled and is suitable for a flow rate sensor for a large flow rate.

[Brief description of drawings]

FIG. 1 is a front view showing an embodiment of the present invention.

[Fig. 2] Circuit diagram of bridge circuit

FIG. 3 is a diagram showing a temperature distribution of a sensor coil unit according to the embodiment of the invention.

FIG. 4 is a diagram showing a temperature distribution of a sensor coil portion in a conventional example.

FIG. 5 is a characteristic diagram showing changes in the unbalanced voltage ΔV with respect to the sensor flow rate QS.

FIG. 6 is a diagram illustrating characteristics of a sensor.

[Explanation of Codes] 1 ... Sensor pipe 2 ... Substrate 3 ... Mating surface 4 ... Case 5 ... Groove 6 ... Heat insulating material 7 ... Upstream coil 8 ... Downstream coil

Claims (2)

[Claims] 1. A sensor pipe through which a fluid to be measured flows,
An upstream side coil and a downstream side coil formed by heat-sensitive resistance wires are wound around the outer peripheral surfaces of the upstream side and the downstream side of the sensor pipe, respectively, and a bridge circuit is formed by the upstream side coil, the downstream side coil and other resistors. Constant current type thermal mass flow rate configured to measure the flow rate of the fluid flowing in the sensor pipe by detecting the unbalanced voltage of the bridge circuit by keeping the currents flowing through the upstream and downstream coils constant. In the meter sensor, the thermal mass flow meter sensor is characterized in that the ratio D ₀ / D ₁ of the outer dimension D ₀ and the inner diameter dimension D ₁ of the sensor pipe is set to 1.5 or more. 2. A sensor pipe through which a fluid to be measured flows,
The upstream side coil and the downstream side coil, which are formed by heat-sensitive resistance wires, are wound on the outer peripheral surfaces of the upstream side and the downstream side of the sensor pipe, respectively, and the temperature of the upstream side coil and the downstream side coil is controlled to be kept constant. However, at this time, in the constant temperature type thermal mass flowmeter sensor for measuring the flow rate of the fluid flowing in the sensor pipe by detecting the difference in energy applied to both coils, the outer diameter dimension D _{0 of the} sensor pipe The thermal mass flowmeter sensor is characterized in that the ratio D ₀ / D ₁ of the inner diameter dimension D ₁ to 1.5 or more.