JPH11316144A - Differential pressure type flow meter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an inexpensive differential pressure type flow meter of simple structure with high measuring pecision. SOLUTION: Hollow probes 3, 5 orthogonal to a flow passage 1a are provided substantially in the center of a pipe body 1, and an upperstream slit 3a opened toward an upperstream ranging over at least substantially a radius of the pipe body 1 including substantially the center of the pipe body 1 and a downstream slit 5a opened toward a downstream ranging over at least substantially a radius of the pipe body 1 including substantially the center of the pipe body 1 are provided in the proves 3, 5. Differential pressure between fluids flowing into the respective proves 3, 5 through respective slits 3a, 5a is detected by a differential pressure sensor 7 to measure a flow rate of the fluid flowing in the flow passage 1a of the pipe body 1.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a differential pressure type flow meter for a fluid, and more particularly to a structure of a differential pressure type flow meter.

[0002]

2. Description of the Related Art Conventionally, as a differential pressure type flowmeter, a flowmeter using an orifice, a venturi nozzle or the like, or a differential pressure type flowmeter using a pitot tube is known.

[0003]

A flow meter using an orifice or a venturi nozzle has a large flow pressure loss and a large foreign matter stagnation portion in the orifice or the like. There is a problem that is not suitable. In the orifice etc.,
Since high accuracy is required for the edge processing accuracy, there is a problem that the processing cost is increased and it is difficult to apply the method to a flow path having a large pipe diameter.

On the other hand, a flow meter using a pitot tube follows the principle of dynamic pressure measurement, is not affected by the viscosity coefficient, and solves the above problem. Since it becomes necessary, there is a problem that the measurement accuracy is reduced accordingly.

It is an object of the present invention to provide a differential pressure type flow meter which has a simple structure, is inexpensive, and has high measurement accuracy, while solving the above-mentioned problems of the prior art.

[0006]

According to the first aspect of the present invention,
A hollow probe that is orthogonal to the flow path is provided substantially at the center of the tube, and an upstream slit that opens to the upstream side at least over a substantially radius of the tube including the substantially center of the tube, and the probe has a hollow probe. A downstream slit that opens downstream at least over the approximate radius of the tube including the approximate center, and detects the differential pressure of the fluid flowing into each of the probes through each slit, whereby the flow of the tube is detected. It is characterized in that the flow rate of the fluid flowing through the path can be measured.

According to a second aspect of the present invention, in the first aspect of the present invention, the hollow probe is formed by two, an upstream probe and a downstream probe, and the upstream probe is provided with an upstream slit, and the downstream probe is provided. Is provided with a downstream slit.

According to a third aspect of the present invention, in the first aspect, the hollow probe is formed by one, and the one upstream probe and the downstream slit are provided at substantially the center of the tubular body. Are provided for point objects with the center as the center.

According to a fourth aspect of the present invention, a hollow probe is provided substantially at the center of the tube at right angles to the flow path, and the probe is provided on the upstream side over at least a portion of the radius of the tube including the center of the tube. An upstream slit that opens at the bottom and a downstream slit that opens at the downstream side over at least a substantial radius of the tube including the substantially center of the tube are provided, and a number of through holes are formed through the outer ends of each probe. A chamber having a cylindrical body penetrating in the direction is connected, and an inlet-side chamber located at the inlet side of the through-hole in the chamber is connected to an outer end of a probe communicating with the upstream slit. The outlet chamber located on the outlet side of the hole is connected to the outer end of the probe communicating with the downstream slit, and the flow path of the tubular body is detected by detecting the differential pressure of the fluid in the inlet chamber and the outlet chamber. Flowing fluid Wherein the amounts were to be measured.

According to a fifth aspect of the present invention, an orifice is provided in a flow path of a tubular body, and a pair of thin tubes communicating with the flow path are provided before and after the orifice. A hole is connected by a chamber having a cylindrical body penetrating in the axial direction therein, and an inlet-side chamber of the chamber located on the inlet side of the through-hole is connected to an outer end of one of the thin tubes located in front of the orifice. And connecting the outlet side chamber located on the outlet side of the through hole to the outer end of the other thin tube located behind the orifice, and detecting the pressure difference between the fluids of the inlet side chamber and the outlet side chamber. The flow rate of the fluid flowing through the flow path of the pipe can be measured.

According to a sixth aspect of the present invention, there is provided a fuel cell system comprising:
A hollow probe is provided, and the probe has an upstream slit that opens to the upstream side from the approximate center of the tubular body to the approximate radius of the tubular body, and the slit extends from the approximate center of the tubular body to the approximate radius of the tubular body. A downstream slit that opens to the downstream side and connects the outer end of the probe that communicates with the upstream slit to a flow path downstream of the probe via an upstream connection pipe, and communicates with the downstream slit. The outer end of the probe to be connected is connected to a flow path upstream of the probe via a downstream connection pipe, and a hot wire mass flow meter is attached to each of the upstream connection pipe and the downstream connection pipe. .

[0012]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the differential pressure type flow meter according to the present invention will be described below with reference to the drawings.

1A and 1B, reference numeral 1 denotes a tube having a flow path 1a through which a fluid flows. Male threads 2 and 2 are formed at both ends of the tubular body 1, and the tubular body 1 is configured to be in-line connected in a pipeline by using, for example, a pipe joint (not shown) or the like.

At substantially the center of the tube 1, a tube 2 orthogonal to the flow path 1a is provided.
The hollow probes 3 and 5 are provided, and these probes 3 and 5 are fixed to the tube 1 by welding 4. The upstream probe 3 located on the upstream side is formed with a narrow upstream slit 3a which opens to the upstream side over substantially the inner diameter of the tube 1, and the downstream probe 5 located on the downstream side has a tube slit. A narrow downstream slit 5a that opens to the downstream side is formed over substantially the inside diameter of one. The outer ends 3b, 5b of the two hollow probes 3, 5 are connected to a differential pressure sensor 7 for detecting a differential pressure of a fluid flowing into each probe 3, 5 through each slit 3a, 5a. The differential pressure sensor 7 has, for example, a built-in pressure film (not shown) to which a strain gauge is attached, and detects a displacement of the pressure film with the strain gauge to detect a differential pressure.

Next, the measurement principle of the differential pressure type flow meter will be described.

When the suffix U is on the upstream side and the suffix D is on the downstream side, the total pressure Pt at each of the slits 3a and 5a is given by the following equation as a static pressure Ps, an average flow velocity V, and a density ρ, respectively. The average flow velocity and average density are the same between the two points.)

The _{upstream: Pt U = Ps U + ρV} 2/2 ... (1) downstream: Pt _D = Ps _D - since the diameter of ρV ^2/2 ... ⁽²⁾ inserting probes 3,5 are small, Since the pressure drop due to viscosity and friction in the pipe is considered to be small, it is considered that “Ps _U = Ps _D ” in the above equation. As a result,
The pressure difference ΔP between the two points is obtained from the difference between the equations (1) and (2).
It is given by the following equation.

ΔP = ρV ² (3) Assuming that the cross-sectional area of the flow path 1a is “A”, the flow rate “Q” flowing through the flow path 1a is measured based on the following equation.

Q = A (ΔP / ρ) ^1/2 (4) That is, if the cross-sectional area A of the applicable flow path 1a and the density ρ of the fluid are clear, the flow rate “Q” is measured by measuring ΔP.
Is calculated.

While the conventional "Pitot tube flowmeter" measures the flow velocity at one point in the flow, the flowmeter according to this embodiment comprises "the probe 3 having the slit 3a facing the flow of the fluid" and " The probe 5 "having the slit 5a in the same direction as the flow of the fluid differs in that the integrated average value (= average flow velocity) is measured.

That is, in this embodiment, (1) the average flow rate can be measured simply and accurately; (2)
It has excellent features such as no stagnation, like a JIS, DIN, or a throttle flow meter represented by a round orifice, and (3) it is not affected by the viscosity of the fluid and has a small pressure loss. ing.

Next, another embodiment will be described with reference to FIGS. 2A and 2B.

Reference numeral 11 denotes a tube having a flow path 11a through which a fluid flows. At both ends of this tube 11, a male screw 12,
A pipe 12 is formed, and the pipe 11 can be connected in-line in a pipe using, for example, a pipe joint (not shown).

At the approximate center of the tube 11, two hollow probes 13, 15 orthogonal to the flow path 11a are provided, and these probes 13, 15 are fixed to the tube 11 by welding 14. The upstream probe 13 located on the upstream side has a tube 11
A narrow upstream slit 13a that opens to the upstream side over the substantially inner diameter of the tube 11 is formed, and the downstream side probe 15 that is located at the downstream side has a width that opens to the downstream side over the approximately inner diameter of the tube 11. A narrow downstream slit 15a is formed. The outer ends 13b, 15b of the two hollow probes 13, 15 are respectively passed through the slits 13a, 15a.
A laminar flow meter 17 for detecting the flow rate of the fluid flowing into the inside is connected. A throttle 20 is provided on one side of the probe. The laminar flow meter 17 includes a chamber 18 connecting the outer ends 13b and 15b of each probe.
A cylindrical body 19 is inserted into the chamber 18 through a partition 16.
The cylindrical body 19 is provided with a large number of through holes penetrating in the axial direction.

The outer end 13b of the probe 13 communicating with the upstream slit 13a is connected to the inlet side chamber A located on the inlet side of the through hole, and the outlet side chamber B located on the outlet side of the through hole. An outer end 15b of the probe 15 communicating with the downstream slit 15a, and a pressure port 18a communicating with the inlet chamber A;
A differential pressure sensor (not shown) is connected between the pressure chamber 18 and the pressure port 18b communicating with the outlet side chamber B.

Next, the measurement principle of the differential pressure type flow meter will be described.

When the flow path 1a of the tube 1 is a main flow path "M" and the chamber 18 connected to the outer ends of the probes 13 and 15 is a sub flow path "B", the flow rate flowing through each flow path If the main flow path is “Qm (liter / min)” and the sub flow path is “Qb (liter / min)”, the total flow rate Qt
(Liter / min) is given by the following equation.

[0028] Qt = Qb + Qm = Qb ( 1 + Qm / Qb) ... (5) each slit 13a at this time, the total pressure in 15a, respectively P _in, when the P _out, flow through the respective flow paths, the following equation Given by

Here, “Sm” and “Sb” in the formula are the minimum cross-sectional areas of the main flow path “M” and the sub flow path “B”, respectively, and “ξ” and “ζ” are flow coefficient. It is.

Qm = ξ · Sm · (P _out −P _in ) ^1/2 (6) Qb = ζ · Sb · (P _out −P _in ) ^1/2 (7) Equations (6), (6) When “Qm / Qb” is obtained from 7), Qm / Qb = (ξ · Sm) / (ζ · Sb) (8) “Qb” is measured by the laminar flow meter 17. In this laminar flow meter 17, since the flow in the chamber 18 becomes laminar due to the cylindrical body 19, when the pressure difference before and after the laminar flow element is set to ΔP, “Qb” is obtained based on the following equation. a is a constant.

Qb = a · ΔP (9) Then, by substituting equations (8) and (9) into equation (5) and rearranging, the following equation is obtained.

Qt = a · ｛1+ (ξ · Sm) / (ζ · Sb)｝ ΔP (10) In the above equation, ξ, Sm, ζ, and Sb can all be considered as constants. “B = (ξ · Sm) / (ζ · S
b) ", which is given by a constant" b ",
0) is represented by the following equation.

Qt = a · {1 + b} ΔP (11) According to this measurement principle, by disposing a small laminar flow meter 17 in the sub flow path “B”, the measured differential pressure “ΔP” is reduced. A proportional flow meter is constructed. For example, in the flow meter according to the embodiment (FIG. 1), the equation (4) “Q = A (ΔP / ρ)
^As is clear from " ^1/2 ", " ^1/2 " is included in the arithmetic expression when calculating the flow rate "Q". Although an arithmetic unit for performing an operation including “ ^1/2 ” is expensive, in the present embodiment, the expression (1) is used.
As is clear from 1), the flow rate “Qt” is obtained in proportion to the measured differential pressure “ΔP”, so that a low-cost calculator can be used.

In this embodiment, (1) the average flow rate can be measured simply and accurately, and (2) JIS and D
IN or small amount of stagnation as in a throttle flow meter represented by a round orifice; (3) a flow rate can be calculated linearly only by measuring a differential pressure by the above sub flow path structure;
And so on.

This principle of measurement can be applied to, for example, a tube body provided with an orifice 22 as shown in FIG.

A pair of small tubes 23, 25 communicating with the flow path 21a is provided in the tube body 21 before and after the orifice 22, and outer ends 23a, 25a of the pair of small tubes 23, 25 are connected to a chamber 26. ing. This chamber 26
A cylindrical body 27 supported by the partition 24 is provided inside the cylindrical body 27. The cylindrical body 27 has a large number of through holes penetrating in the axial direction. The outer end 23a of one of the thin tubes 23 located in front of the orifice 22 is connected to the inlet side chamber A located on the inlet side of the through hole, and is connected to the outlet side chamber B located on the outlet side of the through hole. On the other hand, an outer end 25a of the other thin tube 25 located behind the orifice 22 is connected, and a pressure port 26a communicating with the inlet-side chamber A is formed.
And a pressure port 26b communicating with the outlet side chamber B, a differential pressure sensor (not shown) is connected. The measurement principle in this case is substantially the same as that shown in FIG.

Further, another embodiment will be described.

As shown in FIG. 4, one hollow probe 32 orthogonal to the flow path 31a is provided substantially at the center of the tube 31. The inside of the probe 32 is partitioned at a substantially center of the tube 31 through a partition 33, and the probe 32 located above the partition 33 has a radius substantially equal to the radius of the tube 31 from substantially the center of the tube 31. A narrow upstream slit 32a that opens to the upstream side is formed over the probe 31. The probe 32 that is located below the partitioning body 33 has a downstream portion extending from the approximate center of the tubular body 31 to the approximate radius of the tubular body 31. A narrow downstream slit 32b that opens to the side is formed. Upstream slit 32
The outer end 32c of the probe 32 communicating with the probe a is connected to the flow path 31 on the downstream side of the probe 32 via the upstream connecting pipe 34.
a, and an outer end 32d of the probe 32 communicating with the downstream slit 32b is connected to a flow path 31a upstream of the probe 32 via a downstream connection pipe 35. Then, hot wire mass flow meters 36 and 37 are attached to the upstream connection pipe 34 and the downstream connection pipe 35, respectively. Both probes are provided with a throttle section 38.

Next, the measurement principle of the differential pressure type flow meter will be described.

When the flow path 31a of the pipe 31 is a main flow path "M" and the upstream connection pipe 34 and the downstream connection pipe 35 are a sub flow path "B", the flow rate flowing through each flow path is determined as the main flow path. Is "Qm (liter / min)" and the sub flow path is "Qb
(Liter / min) ", the total flow rate Qt (liter / min) is given by the following equation.

[0041] Qt = Qb + Qm = Qb ( 1 + Qm / Qb) ... (12) each slit 32a at this time, the total pressure in 32b, respectively P _in, when the P _out, flow through the respective flow paths, the following equation Given by

Here, “Sm” and “Sb” in the formulas are the minimum cross-sectional areas of the main flow path “M” and the sub flow path “B”, respectively, and “、” and “ζ” are flow rate coefficients. It is.

Qm = ξ · Sm · (P _out −P _in ) ^1/2 (13) Qb = ζ · Sb · (P _out −P _in ) ^1/2 (14) Equations (13) and (13) 14), “Qm / Qb” is obtained as follows: Qm / Qb = (ξ · Sm) / (ζ · Sb) (15) By substituting this relationship into equation (12) and rearranging, Qt = ｛ 1+ (ξ · Sm) / (ζ · Sb)｝ Qb (16) “Qb” is measured by the hot-wire mass flow meters 36 and 37, and is obtained based on the following equation. Here, an analog voltage output of V = 0 to 5 (V) and a voltage pulse output of H = 0 to 10 (KHz).

Qb = a · V = b · H (17) Then, by substituting equations (16) and (17) into equation (12) and rearranging, the following equation is obtained.

Qt = a ｛1+ (ξSm) / (ζSb)｝ V (18) = b ｛1+ (ξSm) / (ζSb)｝ H (19) In the equation, ξ, Sm, ζ, and Sb can all be considered as constants, so “c = (ξ · Sm) / (ζ · S
b) ”, which is given by a constant“ c ”, and is obtained by the above equation (1)
8) and (19) are represented by the following equations.

Qt = a ｛1 + c｝ V (20) = b ｛1 + c｝ H (21) As described above, a small “hot wire mass flow meter” is provided in the sub flow path “B”.
, It is possible to configure a large-scale mass flow meter at low cost.

While the conventional "Pitot tube flow meter" measures the flow velocity at one point in the flow, the flow meter according to this embodiment uses the "integrated average value (= (Average flow velocity).

That is, in this embodiment, (1) the average flow rate can be measured simply and accurately, and (2)
It has excellent features such as the absence of a stagnation portion as in a throttle flow meter represented by JIS, DIN or a round orifice, and (3) a linear mass flow rate can be calculated by the above sub-path structure. doing.

Further, in the embodiment shown in FIG. 4, since the forward path and the reverse path have the same configuration, the mass flow rate measurement with the reverse flow can be performed.

"Forward" is replaced by the suffix "n", "backflow"
Is the suffix “b”, the total pressure at each position in the flow path 31a is given by the following equation.

In the case of “forward flow” Position− (B): ΔP = (P ₁ −P ₂ ) ≒ 0 (22) Position− ′ (N): ΔP = (P ₂ −P _{1 ′} ) ≒ ρVn ^2/2 ... (23) when the position of "reflux"'- (N _{section): ΔP = (P 1'} -P 2) ≒ 0 ... (24) position - '(B section): ΔP = (P _{1' ^{-P 2) ≒ ρVb 2/2}} ... (25) the entire flow through in the case of "order flow""Qnt (liters /
min) is the flow rate of the main flow path “Qm (liter / mi)
n) "," N part ", the sub-flow rate" Qn (liter / mi)
n) ", the flow rate" Qb (liter / min) "of the" backflow "measurement flow path" part B "is Qnt (liter / mi)
n) is given by the following equation.

Qnt = Qm + Qn + Qb = Qn (1 + Qm / Qn + Qb / Qn) (26) However, since the differential pressure is small as shown in Expression (22), Qb << Qn, and approximately Qb ≒ 0 And can be put.

Therefore, equation (26) becomes as follows.

Qnt = Qn (1 + Qm / Qn) (27) Assuming that the pressure difference before and after the throttle portion at this time is ΔP, the flow rate flowing through each flow path is given by the following equation. Here, “Sm” and “Sn” in the formulas are the minimum cross-sectional areas of the respective flow paths, and “ξ” and “流量” are flow coefficient.

Qm = ξ · Sm · ΔP ^1/2 (28) Qn = ζ · Sn · ΔP ^1/2 (29) When “Qm / Qn” is obtained from Equations (28) and (29), / Qn = (ξ · Sm) / (ζ · Sn) (30) Then, the following equation is obtained by substituting equation (30) into equation (27).

Qt = ｛1+ (ξ · Sm) / (ζ · Sn)｝ Qn (31) In the above equation, ξ, Sm, ζ, and Sn can all be considered constants. By measuring the flow rate “Qn”, the “forward flow” flow rate is measured.

This relationship is the same in the case of "backflow".
Therefore, by measuring the difference between the output from the hot-wire mass flow meter 36 in the “N” portion and the output from the hot-wire mass flow meter 37 in the “B” portion, simultaneous “forward” and “backflow” Measurement becomes possible.

According to this, for example, a measurement effect excellent in elucidating the blowback phenomenon of the intake system in an automobile engine is exhibited.

In each of the above embodiments, a slit is formed in the probe. However, in order to measure the average flow velocity, the slit may be opened at least over a substantially radius of the tube including the substantially center of the tube. is important.

Although the present invention has been described based on one embodiment, it is apparent that the present invention is not limited to this.

In FIG. 1, the number of hollow probes 3, 5 provided in the tube 1 is not limited to two. For example, as shown in FIG. 5, the probe 51 may be formed by one. The inside of this one probe 51 is partitioned at a substantially center of the tube 1 via a partition 53, and the probe 51 located above the partition 53 is provided with a probe 51 from the substantially center of the tube 1. A narrow upstream slit 51a that opens to the upstream side over a substantially radius portion is formed, and the probe 51 located below the partition body 53 has a width substantially equal to the radius of the tube body 1 from the approximate center of the tube body 1. A narrow downstream slit 51b that opens to the downstream side is formed. A connecting pipe 55 is connected to an outer end 51c of the probe 51 communicating with the downstream slit 51b. The connecting pipe 55 extends outside the tubular body 1, and flows into the probe 51 and the connecting pipe 55 through the slits 51a, 51a at the outer end 55a of the connecting pipe 55 and the outer end 51d of the probe 51. A differential pressure sensor 7 for detecting a differential pressure of the fluid is connected. According to this embodiment, the same effect as that shown in FIG. 1 can be obtained.

[0062]

According to the present invention, there is provided a flow meter according to the principle of measuring dynamic pressure, which is not affected by the viscosity coefficient and does not need to measure the flow velocity distribution of the fluid flowing in the pipeline. It is possible to obtain an inexpensive differential pressure type flowmeter with improved accuracy and a simple structure.

[Brief description of the drawings]

1A is a longitudinal sectional view showing an embodiment of a differential pressure type flow meter according to the present invention, and FIG.

2A is a longitudinal sectional view showing another embodiment, and FIG. 2B is a transverse sectional view showing the same.

FIG. 3 is a longitudinal sectional view showing another embodiment.

FIG. 4 is a longitudinal sectional view showing another embodiment.

FIG. 5 is a longitudinal sectional view showing another embodiment.

[Explanation of symbols]

 1,11,21,31 Tube 1a, 11a, 21a, 31a Channel 3,5,13,15 Hollow probe 3a, 5a, 13a, 15a Slit 7 Differential pressure sensor 17 Laminar flow meter 32,51 Hollow probe 32a, 32b, 51a, 51b Slit 36, 37 Hot wire mass flow meter

Claims (6)

[Claims] 1. A hollow probe which is provided substantially at the center of a tubular body and which is orthogonal to the flow path, and which has an upstream slit which is open to the upstream side at least over a substantial radius of the tubular body including the substantial center of the tubular body. And a downstream slit that opens to the downstream side over at least a portion of a radius of the tube including a substantially center of the tube, and detects a differential pressure of a fluid flowing into each probe through each slit. A differential pressure type flow meter characterized in that a flow rate of a fluid flowing through a flow path of the tube can be measured. 2. The method according to claim 1, wherein the hollow probe is formed by two probes, an upstream probe and a downstream probe, the upstream probe is provided with an upstream slit, and the downstream probe is provided with a downstream slit. Item 6. A differential pressure type flow meter according to Item 1. 3. The method according to claim 1, wherein the hollow probe is formed as a single piece, and the upstream side slit and the downstream side slit are provided symmetrically with respect to a substantially center of the tubular body. The differential pressure type flow meter according to claim 1. 4. An upstream slit which is provided substantially at the center of the tubular body and which is orthogonal to the flow path, and which is open to the upstream side at least over a substantial radius of the tubular body including the substantially center of the tubular body. And a downstream slit that opens downstream at least over a portion of the radius of the tube including the approximate center of the tube, and a number of through holes axially penetrate the outer ends of the probes. Connected to an outer end of a probe communicating with the upstream slit, and an inlet side chamber located on the inlet side of the through hole in the chamber is connected to an outer end of a probe communicating with the upstream slit. The outlet side chamber to be connected to the outer end of the probe communicating with the downstream side slit, and the flow rate of the fluid flowing through the flow path of the pipe body is measured by detecting the differential pressure of the fluid in the inlet side chamber and the outlet side chamber. Made it possible And a differential pressure flow meter. 5. An orifice is provided in a flow path of a tubular body, and a pair of small tubes communicating with the flow path are provided before and after the orifice. A large number of through holes penetrate outer ends of the pair of thin tubes in an axial direction. A chamber having a cylindrical body therein, and an inlet-side chamber of the chamber located on the inlet side of the through-hole is connected to an outer end of one of the narrow tubes located in front of the orifice, and the through-hole is provided. Is connected to the outer end of the other thin tube located after the orifice, and the flow path of the tubular body is detected by detecting the pressure difference between the fluids of the inlet chamber and the outlet chamber. A differential pressure type flow meter characterized in that a flow rate of a flowing fluid can be measured. 6. A hollow probe which is provided substantially at the center of the tube and which is orthogonal to the flow path and whose interior is partitioned at the center of the tube, wherein the probe has a radius substantially equal to the center of the tube from the center of the tube. An upstream slit that opens upstream and a downstream slit that opens downstream from substantially the center of the tube over substantially the radius of the tube, and a probe that communicates with the upstream slit. The outer end is connected to the flow path on the downstream side of the probe via the upstream connection pipe, and the outer end of the probe communicating with the downstream slit is connected to the flow path on the upstream side of the probe via the downstream connection pipe. A differential pressure type flow meter, wherein a hot wire mass flow meter is attached to each of an upstream connection pipe and a downstream connection pipe.