Fluid handling – Systems – Dividing into parallel flow paths with recombining
Reexamination Certificate
1999-12-01
2001-06-19
Rivell, John (Department: 3753)
Fluid handling
Systems
Dividing into parallel flow paths with recombining
C137S487500, C137S601180, C138S046000, C073S202000, C073S861580
Reexamination Certificate
active
06247495
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a flow rate detection mechanism for mass flow meters and more particularly to an improved design and mounting a restriction element.
2. Description of Related Art
Flow rate detection mechanism of mass flow meters have been constructed in such a manner as to obtain a laminar flow pattern in a gas passage, and for this type of configuration, an etching plate is used or a plurality of capillary tubes are used at a bypass section, as described in Japanese Patent Publication No. Sho 59-41126.
As shown in
FIG. 9
, a flow rate detection mechanism is described in Japanese Patent Publication No. Hei 6-78926 and introduces gas from a passage
1
P for detecting the gas flow rate and allows gas to flow between a flow rate throttle valve
2
P and a tapered inner circumferential surface
4
P of a hole formed on a housing
3
P in order to obtain laminar gas flow. An inflow port
5
P, provided at the tapered inner circumferential surface
4
P which forms a laminar flow, diverts a part of the gas into a sensor tube, and ejects the gas at an outflow port
6
P. The gas is distributed to a flow rate control section via the circumferential surface
4
P downstream of the outflow port
5
P and before the passage
7
P. On the other hand, the remainder of gas is distributed to the flow rate control section via the circumferential surface
4
P and passage
7
P.
The flow rate detection mechanism of the mass flow controller described in U.S. Pat. No. 5,099,881 obtains laminar gas flow by allowing gas introduced from a passage
1
P′ to flow into an annular passage
4
P′ formed by a plug
2
P′ and a holder
3
P′ both having a tapered portion, as shown in FIG.
10
. And from a through hole
5
P′, provided halfway in the annular passage
4
P′, part of the gas is diverted and introduced from an injection port
6
P′ to a sensor section
7
P′, and the flow rate of this gas is measured by the sensor section
7
P′, discharged from an extracting port
8
P′ to the downstream side of the passage
9
P′. The remainder of gas is distributed to the flow rate control section via the annular passage
4
P′ and passage
9
P′.
In the conventional flow rate detection mechanisms as described above, it is important to stabilize measurement values by forming a stable laminar flow in the tapered inner circumferential surface
4
P or annular passage
4
P′ with a flow rate throttle valve
2
P and plug
2
P′, to achieve uniform pressure distribution, and to divide the passage for measuring the flow rate from a portion in which the stable laminar flow is formed. Such mechanism require high accuracy in manufacturing.
For example, in
FIG. 9
, the flow rate throttle valve
2
P is fastened with a screw
8
P in order to obtain stable laminar flow by accurately aligning the center axis of the flow rate throttle valve
2
P to the center axis of the passage
1
P. However, this configuration has defects in that many components are required and they require high care to reduce the defects in processing, and also generate metallic powders by friction with the screw
8
P when the flow rate throttle valve
2
P rotates. This is a problem for measuring a gas flow rate when, in particular, gas that requires purity is allowed to pass as is the case of the gas used for semiconductor manufacturing processes.
In an example shown in
FIG. 10
, in order to align the center axis of plug
2
P′ to that of passage
1
P′, the basic profile of plug
2
P′ is ground with a lathe, and then the plug
2
P′ must be fixed to the passage
1
P′ by an extremely troublesome fabrication process. That is, with each of the conventional techniques described above, there are problems of high labor and costs in manufacturing a flow rate detection mechanism.
In addition, in the above-mentioned conventional techniques, the maximum flow rate of gas allowed to flow in passages
1
P,
1
P′ must be restricted to a level that would not generate turbulence in the gas flow. That is, in the example of
FIG. 9
, since even a little disturbance that occurs in the pressure distribution of the gas flowing on the tapered inner circumferential surface
4
P directly affects the measurement results, gas is only allowed to flow at a flow rate that would not cause turbulence in the gas flowing on the tapered inner circumferential surface
4
P.
In the example shown in
FIG. 10
, disturbance occurring in the laminar gas flow on the upstream side of the portion with the through hole
5
P′ is able to be absorbed at an annular chamber
12
P′ located on the outer circumference of the holder
3
P′ to some extent, but any turbulence in the outer circumferential portion
13
P′ of the plug
2
P′ in free communication with the extracting port
8
P′ has serious effects on measurement results.
In the flow rate detection mechanism, as in the case of
FIG. 10
, gas is not only restricted to flow at a flow rate that would not cause turbulence around plug
2
P′ on the downstream side
13
P′ of the annular passage
4
P′, but also the center axes of the passage
9
P′, holder
3
P′, and plug
2
P′ must be meticulously aligned in order to prevent any occurrence of turbulence at the outer circumferential portion
13
P′ of plug
2
P′. In addition, because the chamber
12
P′, as shown in
FIG. 10
, is intended to absorb pressure fluctuations by its volumetric capacity, a gas collection portion
14
P′ is generated in a passage causing degradation of replacement characteristics of gas when gas is changed over.
In all of the above-mentioned examples, the profiles of the flow throttle valve
2
P and plug
2
P′ that can form stable laminar flow are those that can suppress the maximum flow rate of gas allowed to flow in passages
1
P,
1
P′ to a specified limit, and it is a practical limit to allow gas to flow at a flow rate of about 20 L/s at flow throttle valve
2
P and plug
2
P′ about 25 mm long. Consequently, to allow gas to flow accurately at a greater flow rate, the precision of the flow rate detection mechanism must be increased and this not only increases the manufacturing cost, but also causes the overall profile of the mass flow controller to be outside the planned dimensions.
SUMMARY OF THE INVENTION
It is an object of this invention to provide a flow rate detection mechanism for a mass flow meter which can achieve stable laminar flow even when pressure on the inlet-side of the sensor section varies and at the same time can greatly improve gas replacement characteristics and enable fine adjustment of the gas flow rate, as well as achieve better affinity to a sensor.
The flow rate detection mechanism for mass flow meters comprises a cylindrical holder inserted into a hole inside a block, an annular groove on the outer circumferential portion of the inlet end of the holder, a through hole for allowing gas to flow into the annular groove from the inlet end of the holder, a resisting member inserted into the holder where in a bypass passage is formed between the resisting member and the holder, a gas introducing hole in free communication with the annular groove, a gas discharging hole in free communication downstream of the holder, a sensor passage tube connected to the gas introducing hole and the gas discharging hole, wherein the gas flowing from the inlet side of the holder collides against the resisting member and is divided, with part of the gas flowing into the through hole, and the remainder flowing into the bypass passage inside the holder.
Consequently, because an annular groove is in free communication with the gas introducing hole, a gas approaching section is provide d by the annular groove and at the same time the pressure distribution can also be alleviated. That is, the gas condition of the gas introducing hole is free from pressure fluctuation by achieving the laminar flow condi
Taniguchi Yoshihiro
Yamamoto Keiichiro
Krishnamurthy Ramesh
Price and Gess
Rivell John
Stec Inc.
LandOfFree
Flow rate detection mechanism with a restriction element for... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Flow rate detection mechanism with a restriction element for..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Flow rate detection mechanism with a restriction element for... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2451170