Measuring and testing – With fluid pressure – Leakage
Reexamination Certificate
1998-10-15
2001-02-06
Larkin, Daniel S. (Department: 2856)
Measuring and testing
With fluid pressure
Leakage
C073S049300
Reexamination Certificate
active
06182501
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a leak test method and apparatus which are used to check various containers or vessels for leaks.
In the manufacture of products or parts required to be free of leaks, it is general practice in the prior art to inspect them in succession on production lines and compare the inspection data with preset reference values to determine if they are leak-free or not. With the conventional method, products under test for leaks (hereinafter referred to as works), such as vessels or containers, are tested successively on production lines by introducing thereinto compressed gas and making a check to see if the compressed gas leaks out thereof or if the amount of leakage is smaller than a reference value. A leak testing apparatus that has been used in the past is a differential pressure type leak tester that detects leaks of the compressed gas from the work based on variations in the pressure difference between a master tank (hereinafter referred to as a master) and the work.
Referring now to
FIGS. 1 and 2
, the differential pressure type leak tester will be described in brief.
FIG. 1
is a piping diagram of a prior art example and
FIG. 2
a graph showing how the abovementioned pressure difference changes with the lapse of time.
In
FIG. 1
, a pressurized gas source
10
is piped via a pressure regulation valve
12
and a three-way electromagnetic valve
14
to electromagnetic valves
16
W and
16
M leading to a work
22
W and a master
22
M, respectively. Connected between the work
22
W and the master
22
M is a differential pressure sensor
18
. The three-way electromagnetic valve
14
normally vents the conduits on both its sodes to the atmospheric pressure, but in response to a drive voltage, conducts both the conduits to each other.
The actual leak test starts with opening the valves
12
,
14
,
16
W, and
16
M to introduce pressurized gas into the work
22
W and the master
22
M from the pressurized gas source
10
, followed by regulating the regulation valve
12
to set a pressure gauge
13
at a desired testing pressure. Next, the electromagnetic valves
16
W and
16
M are closed and after a certain elapsed time the pressure difference between the work
22
W and the master
22
M is measured by the differential pressure sensor
18
. If the work
22
W leaks, the pressure on the work side becomes gradually lower than the pressure on the master side. The detected pressure difference is compared with a threshold value L
th
in a decision part
19
. When the detected pressure difference is smaller than the threshold value L
th
, it is decided that the work
22
W is leak-free or that the leak is negligibly small, and when the pressure difference is larger than the threshold value L
th
, the work
22
W is decided to be leaky. In this case, if the work
22
W does not leak, the pressure difference ought to be zero. In practice, however, the pressure difference frequently develops even if the work
22
W does not leak. Such a situation might be the case wherein when the temperature of the work
22
W heated on the production line is still higher than the temperature of the master
22
M (room temperature, for instance), the measurement of the pressure difference is started and thereafter the pressure in the work
22
W drops as its temperature is gradually reduced toward that of the master
22
M as a result of thermal radiation. Even if no leaks are detected, the pressure difference usually varies due to temporal changes in the temperature difference between the work
22
W and the master
22
M. A description will be given below, with reference to
FIG. 2
, of how the pressure difference varies during measurements.
In
FIG. 2
, the curve
2
A indicates the pressure difference when the work
22
W does not leak and the curve
2
B the pressure difference when the work
22
W leaks. As shown, the pressure difference develops at and after time t
s
when the electromagnetic valves
16
W and
16
M depicted in
FIG. 1
are closed, and thereafter the pressure difference varies unstably until time t
a
. This is primarily due to a shock resulting from the closing of the electromagnetic valves
16
W and
16
M. Then, the pressure difference undergoes substantially linear variation during the time interval from t
a
to t
b
. The reason for this is, for example, that the temperature of the pressurized gas introduced into the work
22
W as mentioned above is gradually lowered. And the pressure difference varies in a smooth curve from time t
b
to t
c
, because the cooling rate of the pressurized gas reduces as its temperature approaches room temperature.
After time t
c
, the pressure difference does not vary when the work is free of leaks, but in the case of a leaky work, the pressure difference further undergoes linear variation. This period will hereinafter be referred to as a “stabilization period.” Since the gas temperature in either of the work and the master is considered to be equal to room temperature during this period, pressure difference variation per unit time is in proportion to the amount of leakage (cm
3
/sec). The amount of leakage that is intended to be detected is appreciably small, and it can be regarded as substantially constant from time t
s
to the stabilization period after time t
c
. Through utilization of this phenomenon it is possible to decide that the work is leak-free or leaky, depending upon whether the detected pressure difference variation per unit time after time t
c
is close to zero or not.
However, the conventional method is time-consuming since the measurement cannot be started until after time t
c
. A differential pressure type leak tester that has been proposed as a solution to this problem is disclosed in Japanese Patent Application Laid-Open Gazette No. 4-506262. With this leak tester, the pressure difference variation per unit time is premeasured using a leak-free, non-defective work in the time interval t
a
to t
b
depicted in
FIG. 2
during which the pressure difference varies linearly after time t
a
when it ceases from sharp variations. The abovementioned period from time t
a
to t
b
will hereinafter be referred to as a “measurement period.”
In the actual leak test, pressure difference variation per unit time, &Dgr;p/&Dgr;t, is detected in the measuring period t
a
to t
b
, and is compared with the pressure difference variation premeasured using the leak-free, non-defective work. It is possible to decide that the product under test is leak-free or leaky, depending upon whether the pressure difference values compared are nearly equal or not. This enables the measurement to start prior to time t
c
and hence permits reduction of the measurement time.
This prior art method is effective in reducing the measurement time but requires the preparation of a non-defective work. To solve this problem there has been proposed such a method as described below.
To begin with, two pressure differences p
1
and p
2
are measured, using a sample work, at a predetermined time interval &Dgr;t (two seconds, for instance) in the measurement period from time t
a
to t
b
shown in
FIG. 3
, and pressure difference variation per unit time, &dgr;p
1
=(p
2
−p
1
)/&Dgr;t=&dgr;p
1
/&dgr;t, is calculated from the two pressure differences p
1
and p
2
. This is followed by calculating pressure difference variation per unit time, &dgr;p
2
, from two pressure differences p
3
and p
4
similarly measured at the predetermined time interval &Dgr;t in the stabilization period after time t
c
. The pressure difference variations per unit time, &dgr;p
1
and &dgr;p
2
, are calculated as follows:
&dgr;p
1
=(p
2
−p
1
)/&Dgr;t=&Dgr;p
1
/&Dgr;t
&dgr;p
2
=(p
4
−p
3
)/&Dgr;t=&Dgr;p
2
/&Dgr;t
The pressure difference variation &dgr;p
2
in the stable period can be regarded as a variation attributable to leaks of the work. The pressure difference variation &dgr;p
1
in the measurement period can be regarded as the sum of the abovementioned variation &dgr;p
2
and the amount of variation wh
Furuse Akio
Horikawa Hiroshi
Nakagomi Masayuki
Cosmo Instruments Co., Ltd.
Larkin Daniel S.
Pollock Vande Sande & Amernick
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