Measuring and testing – Liquid level or depth gauge – Hydrostatic pressure type
Reexamination Certificate
2002-09-27
2003-11-18
Williams, Herzon (Department: 2856)
Measuring and testing
Liquid level or depth gauge
Hydrostatic pressure type
C073S29000R, C073S298000
Reexamination Certificate
active
06647781
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention is related to providing a bubble water depth measuring method for compensating for a total error of a bubble water depth or water level measurement in a reservoir that a water level shifts in a wider range to enable the measurement of the water depth or water level with a higher accuracy, using an industrial pressure transducer, in which the pressure transducer is manufactured in an environment that a circumferential air temperature is changed in the range of −40° C. to +50° C., and a system thereof.
Particularly, the invention is related to providing a bubble water depth measuring method for measuring a water column pressure to enable the remote-measurement of a water depth or water level in a reservoir, a lake, or a river. Herein, it is noted that only a water depth measurement now will be explained below, because a bubble water level meter is a device for measuring a water depth and calculating it into a water level.
PRIOR ART
There has been much interest in a bubble water level meter that can be used for a hydrology observatory, because the bubble water level meter can measure a water depth under an icy condition, if a water surface is frozen in a reservoir, a lake, or a river. The water level meter for the hydrology observatory have characteristics as follows:
In a reservoir where the changing depth of a water level is usually up to 10 m.
There are many areas where a circumferential air temperature of a water level observatory post is in the range of −40° C. to +50° C. in seasons.
Most of the water level observatory posts are not equipped with warming and cooling utilities but also a power source unit.
The demand for a remote water level measuring system is being increased because many water level observatory posts are established in uninhabited places.
In light of these facts, there is a problem in that a bubble type water level meter has a lower accuracy under the conditions of the severe weather change in seasons and a larger water level change depth.
The bubble type water level meter has advantages in that its installing, maintenance and operating costs are relatively lower and can a water level even when a reservoir and a river are frozen in winter, but it is broadly not used as a hydrology observatory because its measuring error is larger.
Also, the bubble type can be operated in more stability compared with other water level meters in a river where the concentration of floating particles is higher. A sand and earth layer is swiftly changed and saves on the maintenance and operating costs, but its measuring accuracy is low.
Referring to
FIG. 1
, error factors that are caused upon measuring a water depth according a bubble generating method will be explained below, and an unit of a water column pressure will be represented as cmH20 or mmH20 for the purpose of consulting the convenience in calling the term “error” as an abbreviate of “absolute error”.
1
is a water column pressure tube, which will be called “a water depth measuring tube”,
10
is a compressed gas generating device and
3
is a pressure transducer. A pressure applied to the lower end portion of the water depth measuring tube
1
is as follows:
Pc=&ggr;hx+Pa,
(1)
Wherein, hx is an altitude difference of water filled in the water depth measuring tube
1
, which is considered as a water depth. &ggr; is a specific gravity (gm/cm
3
) of water. &ggr;hx is a water column pressure. Pa is an atmosphere pressure on a water surface.
The pressure transducer
3
measures a surplus pressure, not for an atmosphere pressure (P≈Pab−Pa; Pab-Absolute pressure). The water depth measuring tube
1
filled with water generates bubbles at the lower end portion with compressed gas being supplied to the upper end portion thereof. Upon generating of the bubbles, the pressure transducer
3
measures the compressed gas pressure, and its result is as follows:
P
mx
=&ggr;h
x
+&Dgr;
px
+(
P
a
−P
ao
)−&ggr;
g
H
o
+&Dgr;
pb
; (2)
Wherein, &Dgr;px is an absolute error of the pressure transducer
3
at the time of measuring Pmx. &Dgr;Pa=(Pa−Pao) is a difference between an atmosphere Pa on the water surface and an atmosphere Pao applied to the pressure transducer
3
. Generally, Pao≠Pa because a water level observatory post is placed on a much higher position than a water surface. Ho is an altitude difference between the lower end portion of the water depth measuring tube
1
and the mounting position of the pressure transducer
3
. &ggr;g is a density of compressed gas to be supplied to the water depth measuring tube
1
. &Dgr;pb is a pressure of a supplementary compressed gas changed according to the bubble pressure that is formed at the lower end portion of the water depth measuring tube
1
. &Dgr;pb will be ignored because its reducing method is now developed. In expression (2), all items are a measuring error of a water column pressure &ggr;hx except for the water column pressure &ggr;hx. What the measuring errors are summed up is assumed as total absolute error &Sgr;&Dgr;x of the water column pressure &ggr;hx as follows:
&Sgr;&Dgr;
x
=&Dgr;
px
+&Dgr;P
a
−&ggr;
g
H
o
, (3)
Wherein, &Dgr;Pa=Pa−Pao=&ggr;&agr;(H
o
−hx), &ggr;a is an air density, which can be seen as
γ
g
≈
γ
go
P
mx
+
P
a
P
a
.
&ggr;go is a density of a compressed gas to be used at an atmosphere. Un the condition that a water depth is changed over 10 m in a reservoir, the altitude difference between the lower end portion of the water depth measuring tube
1
and the mounting position of the pressure transducer
3
usually becomes Ho≧20 m.
Looking into &Dgr;Pa and &ggr;gHo, if Ho=20 m=2000 cm, hx is changed in the range of 200 to 1000 cm, an air density &ggr;
&agr;
=1.2·10
−3
gm/cm
3
and a water depth is measured using a compressed gas, the change of &Dgr;Pa is as follows:
&Dgr;
Pa=
1.2·10
−3
(2000-200)=2.16
gm/cm
2
≈2.16
cm
H
2
O(if
h
x
=200
cm
)
&Dgr;
Pa=
1.2·10
−3
(2000-200)=1.2
gm/cm
2
≈1.2
cm
H
2
O(if
h
x
=1000
cm
)
&Dgr;
gHo=
1.2·10
−3
·1.2·2000=2.88
gm/cm
2
≈2.9
cm
H
2
O(if
h
x
=200
cm
)
&Dgr;
gHo=
1.2·10
−3
·2·2000=4.8
gm/cm
2
≈4.8
cm
H
2
O(if
h
x
=1000
cm
)
Herein, it is known that when hx=200~1000 cm, the error changing range is 2.16-2.9≈−0.74 cm to 1.2-4.8≈−3.6 cm due to &Dgr;Pa−&ggr;gHo. If an allowance error of the water depth, water level is ±1 cm, the error component of &Dgr;Pa−&ggr;gHo cannot be ignored. Of course, &ggr;a was ignored though it is changed according to a temperature.
The pressure transducer
3
includes a temperature compensation circuit for correcting the property that a pressure sensor mounted therein is changed according to a temperature. But the temperature compensation circuit is lacking of compensating for an error changed according to a temperature t and a pressure P to be measured, perfectly. An error of a curve &Dgr;p=f(t;P) always happens.
For example, when the pressure transducer (Model PTX1000) that secures a pressure error &dgr;p (=±0.25% fs) at room temperature (t≈20-24° C.) measures a pressure of P=1000 cm H2O with being cooled at −10° C., its measuring error &Dgr;p≅−31 cm H2O, even though its use temperature range is introduced as −40° C.~+90° C. More precisional pressure transducer (Model PDCR862) has a useable temperature range of −54° C.~+125° C. and a measuring error &dgr;p of ±0.1% fs. Therefore, &Dgr;p=8 gm/cm
2
, and an absolute error hx is ~8 cm. Of course, if only a gas temperature is changed without cooling or heating the pressure transducer on the whole, the error is reduced. But, when a bubble water level meter is installed outdoor, the pressure transducer is cooled or heated
Fleshner & Kim LLP
Frank Rodney
Hydrosonic International Co., Ltd.
Williams Herzon
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