Measuring and testing – Liquid level or depth gauge
Reexamination Certificate
2001-01-03
2002-11-05
Williams, Hezron (Department: 2856)
Measuring and testing
Liquid level or depth gauge
C073S29000R
Reexamination Certificate
active
06474157
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention is related to a sonic level measuring technology, and particularly, to a sonic level measuring method of measuring a water level in a high accuracy using a sonic wave in a reservoir, a river, underground water or subterranean, etc. a water level range of which is highly changed.
PRIOR ART
A changing range of a water level in an artificial reservoir is a few tens of meters. There are many rivers that have a water level changing range of 10 m. It is requested for the hydrology observation so that an absolute error of a water level measurement is below ±5~10 m independent of a water level changing range. An original point of a level meter is often placed at a higher position over a water surface according to the mounting condition of the level meter. In this case, even through the changing range of the water level is small, the water measuring range may become greater. In this case, a distance from the original point of the water level to a water surface is normally 10 to 20 m. If the water level change is the range of ±5 m, the water level measuring range becomes 10 to 15 m or 15 to 25 m. In case that a level meter is mounted on a dam in a reservoir, a water level measuring range normally becomes 40 to 80 m. Further, when it is intended to measure a level of the underground water, even though the underground water level is not changed at a high range, the water level is measured by the reference of an original point constituted as a top of an underground water observing tube, which is near the ground surface. A case that the water level measuring range is greater often happened.
There were sonic level meters developed to measure a water level, accurately, if the water level measuring range is great. Typical some sonic level measuring method and apparatus having the relatively higher accuracy are disclosed in patents as follows:
U.S. Pat. No. 5,842,374 published on Dec. 1, 1998
Germany Patent No. 19511234 published on Sep. 11, 1997
Japanese Patent No. 2,756,647 published on Mar. 13, 1998
Korean Patent No. 150714 published on Jun. 16, 1998
These patents are commonly entitled as Measuring method of wide range level and apparatus thereof.
A conventional sonic level measuring method previously disclosed is illustrated in
FIG. 1. 1
is a sonic generator,
2
is a waveguide tube and
5
1
,
5
2
,
5
3
, . . .
5
n
,
5
n+1
are sonic receivers that are arranged in a constant interval l along the waveguide tube
2
. The position of the sonic receiver
5
1
is an original point or zero point to measure the water level. A distance Lx from the original point to a water surface is measured as follows: as the sonic generator
1
generates sonic pulses, the sonic pulse is transited or propagated toward the water surface, reflected on the water surface and then transited upward. At the moment that the sonic pulses reach the original point, the sonic receiver
5
1
generates an outputting signal. Similarly, as the sonic pulses are advanced, the sonic receiver
5
n
nearest to the water surface generates the outputting signal and also receives reflected sonic pulses. Therefore, the water level Lx is subject to being measured using four signals received like this. A time interval t
1
between time points that the sonic receiver
5
1
receives the advancing pulse and the reflected pulse is as follows:
t
1
=
2
Lx
c
1
(
1
)
A time interval t
2
between time points that the sonic receiver
5
1
and
5
n
receive the advancing pulse, respectively, is as follows:
t
2
=
L
0
C
2
=
(
n
-
1
)
l
C
2
(
2
)
Wherein, L
0
=(n−1) l is a distance that is accurately measured, previously, L
0
=const, C
1
is a sound velocity in the interval Lx, C
2
is a sound velocity in the interval L
0
, and n is the number of the sonic receivers.
A value of Lx to be measured in the expressions (1) and (2) as follows:
L
x
=
t
1
2
t
2
×
L
0
×
C
2
C
1
(
3
)
Wherein, L
0
is a previously known value, t
1
and t
2
and measured and substituted into the expression (3), and C
1
and C
2
are not known. Assuming that Lx is approximately equal to L
0
, and C
1
≈C
2
, L′x is as follows:
L
x
′
=
t
1
2
t
2
×
L
0
(
4
)
In case that C
1
≠C
2
, Lx≠L
0
. A measuring error &dgr;
Lx
of Lx occurs as follows:
δ
x
=
L
x
′
L
x
-
1
=
C
2
C
1
-
1
(
5
)
When Lx is measured, it is assumed that each of the sound velocity C
1
and C
2
is changed in the interval's Lx and L
0
as follows:
C
1
=C
0
+&agr;(
{overscore (T)}
L
x
)
C
2
=C
0
+&agr;(
{overscore (T)}
L
G
) (6)
Wherein, &agr; is a temperature coefficient of a sound velocity in air, &agr;≈0.6. C
0
is a sound velocity, when an air temperature is zero.
In order to evaluate the error &dgr;
Lx
in the patents described above, assuming that the air temperature in the waveguide tube from the original point
0
to the water surface is changed in a straight gradient of
T
0
-
T
Lx
Lx
as shown in
FIG. 2
, when C
1
and C
2
are calculated and then the results are substituted into the error expression (5), the error &dgr;
L′x
is as follows:
δ
L
x
′
=
0.5
a
(
T
0
-
T
L
x
)
C
0
+
0.5
a
(
T
0
-
T
L
x
)
×
Δ
L
L
x
Wherein, T
0
is a temperature at the original point and T
Lx
is a temperature at the water surface.
A maximum error &dgr;
Lxmax
appears when &Dgr;
Lmax
≈l.
Δ
Lx
max
=
0.5
a
(
T
0
-
T
L
x
)
C
0
+
0.5
a
(
T
0
-
T
L
x
)
×
l
(
7
)
An absolute error &Dgr;
lmax
is as follows:
δ
Lx
max
=
0.5
a
(
T
0
-
T
L
x
)
C
0
+
0.5
a
(
T
0
-
T
L
x
)
×
l
L
x
(
8
)
If a water level measuring allowance absolute error &Dgr;′
L′x
is given, an interval l between the sonic receivers
5
i
and
5
i+1
is obtained from the expression (8). Assuming that C
0
=331.6 m and &agr;=0.6, the value of l is as follows:
l
=
Δ
L
x
′
×
331.6
+
0.3
(
T
0
+
T
L
x
)
0.3
(
T
0
-
T
L
x
)
(
9
)
Considering that T
0
=40° C., T
Lx
=25° C. in summer, and T
0
=0° C., T
Lx
=15° C. in winter, in order that &Dgr;
Lx
=0.01 m (1 cm), l is as follow:
l=0.78 m in summer
l=0.74 m in winter
If the interval l between sonic receivers is secured to get smaller, the water level measuring absolute error becomes small more and more. Therefore, the conventional sonic level measuring method has great advantages in that the water level absolute error &Dgr;
L′x
is equal throughout a full range to measure the water level independent of the water level measuring range and can be secured to be smaller.
The sonic level measuring method has another method as follows: it saves the mounting cost by which the waveguide tube can be mounted along a gradient surface of a river bank and a reservoir bank unlike other sonic level meters. In this case, a length of the waveguide tube is the multiplication of a value Lx measured by the sonic level meter and sin=45°, and a water level changing range of a reservoir is 50 m, the length of the wave guide tube must be over 70.7 m by 50 m/sin45°.
But, the conventional sonic level measuring method has problems as follows: in case that T
0
and T
Lx
in the expression (9) are often changed, and the absolute allowance error &Dgr;′
L′x
=±0.001 m, l=0.74-0.78 m must be secured. If it is necessary to measure the water level of a reservoir, more accurately, l=0.37-0.39 m must be secured, so that &Dgr;′
L′x
=±0.0005 m. In this case, if the maximum water level measuring range is 70 m, the number of the sonic receivers is as follows:
n
≈
70
m
0.37
m
=
189
≈
190
Even if &Dgr;′
L′x
=±0.01 m is secured, n
Frank Rodney
International Hydrosonic Co., Ltd.
Lee & Sterba, P.C.
Williams Hezron
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