Data processing: measuring – calibrating – or testing – Calibration or correction system – Fluid or fluid flow measurement
Reexamination Certificate
2002-12-23
2004-08-17
Shah, Kamini (Department: 2863)
Data processing: measuring, calibrating, or testing
Calibration or correction system
Fluid or fluid flow measurement
C600S553000, C600S553000, C703S007000, C702S045000
Reexamination Certificate
active
06778927
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to a system and method of obtaining measuring characteristic correction factors for a respiratory flow measuring device using a static pressure difference, and more particularly to a system and method of obtaining measuring characteristic correction factors for a pneumotachometer most generally used for respiratory flow measurement.
2. Description of the Prior Art
As well known to those skilled in the art, a pneumotachometer is widely used to examine the capacity of the lungs. The pneumotachometer is constructed to measure the difference between static pressures generated via a fluid resistor in the center of a cylindrical tube using a differential pressure sensor connected to the cylindrical tube. In such a pneumotachometer, when respiratory airflow passes through a fluid resistor, energy is lost due to friction with the fluid resistor, thus generating a differential pressure due to the loss of energy. Accordingly, this device is constructed to calculate respiratory flow by measuring the differential pressure. The pneumotachometer is designed to change the sign of the differential pressure when the direction of the respiratory airflow is reversed, thereby continuously measuring the respiratory flow of a human body, flowing in both directions of exhalation and inhalation. When respiratory flow is measured by the pneumotachometer, the relationship between differential pressure &Dgr;P and respiratory flow F is regulated by an experimentally-derived characteristic equation. Typically, since a fluid resistor R is represented by a linear function of a flow, the differential pressure &Dgr;P is expressed by a quadratic function of the respiratory flow F as indicated in Equation [1] which is Rohrer's equation.
Δ
P
=
R
(
F
)
·
F
=
(
R
0
+
R
1
F
)
·
F
=
R
0
F
+
R
1
F
2
[
1
]
In Equation [1], if coefficients R
0
and R
1
, representing measuring characteristics, are already known, the respiratory flow F can be determined by measuring &Dgr;P using the following Equation [2].
F
=
R
0
2
+
4
R
1
·
Δ
P
-
R
0
2
R
1
[
2
]
In Equation [2], R
0
and R
1
are individually determined with respect to both directional respiratory airflows of the human body (inhalation and exhalation).
Meanwhile, since there are two measuring characteristic coefficients R
0
and R
1
in Equation [1], &Dgr;P and F are not proportional to each other, so the measuring characteristic coefficients can be determined only when a pair of &Dgr;P and F are measured at two or more points.
A rotometer is generally used as a device for determining the characteristic coefficients, which is operated as follows.
First, when respiratory airflow with a certain flow rate is allowed to flow into a respiratory tube of a pneumotachometer by operating a vacuum pump, attractive force is generated by the vacuum pump within a glass tube which is arranged perpendicularly to the respiratory tube to communicate with the vacuum pump. The attractive force lifts a pendulum in the glass tube. At this time, airflow is generated between the pendulum and the wall of the glass tube, so the flow of the airflow moving in the tube is constant.
In this case, the pendulum ascends to a height at which the weight of the pendulum becomes equal to the attractive force, and the attractive force is proportional to the flow generated by the vacuum pump, so the height of the pendulum is proportional to the flow, that is, H∝F. Further, the airflow with the same flow as that of the respiratory tube goes into the pneumotachometer to generate a differential pressure &Dgr;P.
Thereafter, if multivariate regression analysis adopting Equation [1] as a model equation is performed after several data pairs of &Dgr;P and F are measured by varying the attractive force of the vacuum pump to desired intensities, the measuring characteristic coefficients R
0
and R
1
which best satisfy the measured pairs of &Dgr;P and F can be obtained. Since the R
0
and R
1
are determined, the measuring characteristic coefficients R
0
and R
1
are applied to Equation [2], thus measuring new airflow, that is, respiratory flow. In this case, the determined measuring characteristics are characteristics under steady flow, that is, static characteristics.
However, since the measuring characteristics are static characteristics, they are obtained under the measuring environments differing greatly from dynamic airflow in which flow is instantaneously changed, like the respiratory airflow of the human body.
In order to compensate for the static characteristics, the American Thoracic Society (ATS) recommends that a 3 L syringe similar to the volume of the lungs of the human body is connected to a pneumotachometer, and the respiratory flow of the human body is simulated by manually reciprocating the handle of the syringe (refer to Am. J. Respir. Crit. Care Med. Vol. 152: 1107-1136, 1995).
All of airflows generated by the syringe flow into the pneumotachometer, and the volume of the syringe is uniformly 3 L. Therefore, if the flow signal measured by the pneumotachometer is integrated, the integrated result must be 3 L. However, since the measuring characteristic coefficients Ro and R1 which are previously calculated may not be ideal, the following Equation [3] is obtained if a proportional coefficient S is introduced and formulated.
S
=
V
C
∫
F
(
t
)
ⅆ
t
=
3
∫
F
(
t
)
ⅆ
t
[
3
]
In Equation [3], V
C
is the volume of the syringe, and the proportional coefficient S corrects the integrated value of the airflow to be V
C
=3 L by scaling the measured airflow at a predetermined rate regardless of the instantaneous value of the airflow.
The user of a respiratory airflow sensor measures respiratory airflow by using a mean value {overscore (s)} of the values of the proportional coefficient S obtained by repeatedly performing manual reciprocation of the syringe as an ultimate correction coefficient.
This method is widely used because of the convenience of the user's ability to calibrate the pneumotachometer without an additional device except for the syringe. Generally, this method is carried out such that the user derives the following Equation [4], which is an ultimate characteristic equation, by further obtaining the mean value {overscore (s)} of the values of the proportional coefficient S, besides the measuring characteristic coefficients R
0
and R
1
determined by a manufacturing company, and then measures respiratory flow.
F
=
s
_
·
R
0
2
+
4
R
1
·
Δ
P
-
R
0
2
R
1
[
4
]
However, since Equation [1] is an experimentally-derived characteristic equation, Equation [4], which is derived from Equation [1], cannot provide the same accuracy for flow values of all airflows. Referring to fluid resistor-flow characteristics shown in
FIG. 8
, it can be seen that random errors are always present along linear flow values.
Therefore, errors incapable of being expressed by equations according to flow values are generated, and they overlap each other in the case of commercialized respiratory flow measuring devices, such that volume measurement error reaches 8% maximally (Journal of the Korean Society of Medical Informatics Vol. 6, pp 67-78, 2000).
Further, in a method of calibrating the respiratory flow measuring device by calculating the proportional coefficient S using the syringe after obtaining R
0
and R
1
through the static characteristic measuring experiment, R
0
and R
1
are not accurate with respect to flow values of all of airflows as described above, so the proportional coefficient S obtained by the Equation [3] is not constant. Further, since the same coefficients determined under the static environments are actually used under dynamic environments, the di
Cha Un Jong
Kim Hyun Sik
Kim Kyoung A
Birch & Stewart Kolasch & Birch, LLP
Shah Kamini
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