Measuring and testing – Gas analysis – Detector detail
Reexamination Certificate
2001-02-05
2002-04-16
Williams, Hezron (Department: 2856)
Measuring and testing
Gas analysis
Detector detail
C073S053010, C073S061410, C073S023200, C422S082040, C324S438000
Reexamination Certificate
active
06370941
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of Invention
The present invention relates to a gas sensor based on a pH electrode which is used in the fields of medical care and chemical engineering.
2. Related Art
Any one of the conventional gas sensor in which the pH electrode (including an ion sensitive field effect transistor (ISFET)) is its fundamental, has the inner solution suitable for a gas to be measured, the pH electrode, and a reference electrode immersed in the inner solution, and these are covered by a gas permeable membrane. The composition of the inner solution is selected so that, when the gas to be measured is dissolved in the inner solution, the pH is changed.
For example, when the gas to be measured is carbon dioxide or ammonia, H+ ion or OH− ion is generated by the following equilibrium reaction, causing the pH change of the solution. Herein, HCO3− or NH4+ ion which is an ionic species generated when a gas molecule is dissolved in the water, is respectively called a conjugate ion to the carbon dioxide or ammonia. Normally, as the inner solution of the gas sensor based on the pH electrode, a water solution including the excessive conjugate ions to the gas to be measured is used.
CO
2
+H
2
O→HCO
3
−
+H
+
(1)
NH
3
+H
2
O→NH
4
+
+OH
−
(2)
Further, for the purpose that the pH electrode is operated, the reference electrode is necessary, and in many cases, as the reference electrode, a silver wire (Ag/AgCl) whose surface is chlorinated, is used. Because the potential of this electrode is determined by the concentration of the chloride ion in the solution, normally, in the inner solution, the chloride ion of a predetermined concentration is included other than the conjugate ion.
The gas sensor of the above-described principle is generally called Severinghaus type gas sensor. The measurement result of the severinghaus type gas sensor is displayed normally as the gas partial pressure. When the conjugate ion excessively exists in the inner solution, the following relationship exists between the partial pressure of the gas to be measured and the pH of the inner solution. In the case of acidic gas,
pH=A−log Pa (3),
in the case of basic gas,
pH=B+log Pb (4)
Herein, Pa and Pb are respectively the partial pressure of acidic gas and basic gas, and A and B are constants. Hereinafter, the relational expression between the partial pressure of the gas and the output voltage of the pH electrode in the case of the acidic gas will be deriverd.
Generally, between the output voltage V of the pH electrode and the pH, the expression (5) is generally established.
V=V0+S(pH−pH0) (5)
Herein, S is the pH sensitivity of the pH electrode, and pH0 and V0 are the pH of the inner solution and the output voltage of the pH electrode when the partial pressure of the gas is Pa0. Further, pH and V are the pH of the inner solution and the output voltage when the partial pressure of the gas is Pa. From the expressions (3) and (5), it is clear that the following relationship is derived between the pH electrode and the partial pressure of the gas.
V=V0−S(log Pa−log Pa0) (6)
From this,
log Pa=log Pa0−(V−V0)/S (7)
The expression (7) can be generalized as the expression (8).
log Pa=C−V/S (8)
Herein, C is a proper constant of the sensor.
From the expression (8), in order to convert the output V of the pH electrode into the partial pressure Pa of the gas, the following can be understood: it is necessary that the two constants C and S are predetermined. To find C and S respectively corresponds to the zero point calibration and the sensitivity calibration. That is, in the same manner as in each kind of other sensor, also when the partial pressure of the gas is measured by using the severinghaus type gas sensor, it is inevitable to conduct the zero point calibration, and the sensitivity calibration. Further, when the temperature of the sensor changes at the time of the calibration and the measurement, it is also necessary to conduct the temperature compensation.
As described above, in the use of Severinghaus type gas sensor, or generally, of the chemical sensors, the two calibrations are inevitable, and the most troublesome matter for putting it to practical use.
In this connection, generally, the pH sensitivity of the pH electrode is theoretically given by following expression (9) of the Nernst' equation.
S=2.303RT/F (9)
Herein, R is a gas constant, T is absolute temperature, F is a Faraday constant, and when each constant is substituted in this equation, S at 25° C. is 59 mV/pH. Then the equation (9) can be expressed as the expression (10).
S=59(273+t)/298 (mV/pH) (10)
Herein, t is the temperature (° C.). When S is calculated from this expression, for example, it can be found that, when the temperature changes from 0° C. to 40° C., the pH sensitivity increases from 54 to 62 mV/pH. As described above, the sensitivity of the pH electrode is theoretically a function of only the temperature, and in many cases, the actual pH electrode has the sensitivity near the above theoretical value, and its time-dependent change is small. On the one hand, in many cases, there is a case where, for the zero point of the sensor, its time-dependent change can not be negligible.
Based on the above-mentioned trend, there are many cases to apply a method in which, for the sensitivity, the value measured in advance by the manufacturer of the sensor is used, and only the zero point calibration is conducted by the user, or the sensitivity calibration and the zero point calibration are conducted by the user with the sensitivity calibration being less often conducted than the zero point calibration. Specifically, as in the case of a PCO
2
sensor for medical care with the comparatively narrower measuring range of PCO
2
, there is a case in which the necessary accuracy is secured only by the zero point calibration. In JP-A-11-070084, a package for a gas sensor is proposed for simply conducting such a zero point calibration. According to this, by delivering a package of laminated aluminum film in which both of the carbon dioxide of the predetermined partial pressure and the PCO
2
sensor are accommodated as a product, the user can conduct the zero point calibration before the user opens the sensor package.
However, in the above gas package system, when the package is once opened, the zero point calibration can not be conducted. That is, although the zero point calibration can be conducted only once before the start of use, the zero point calibration can not be conducted thereafter. On the other side a sensor system by which the zero point calibration can be repeatedly conducted, is disclosed in JP-B-4-017050. This sensor system is the system in which a carrier solution (corresponds to the inner solution of the Severinghaus type gas sensor) is circulated in a gas exchange catheter located in the blood vessel, and the pH electrode is located at the downstream side of the gas exchange catheter, and the partial pressure of the gas is found from the pH of the carrier solution after the gas exchange. In this method, for example, by making the carrier solution flow at so high flow rate that the gas exchange in the gas exchange catheter may be negligible, the carrier solution in which the gas is hardly solved comes into contact with the pH electrode, thereby, the zero point calibration of the pH electrode can be conducted. That is, by only intermittently heightening the flow rate of the carrier solution, the zero point calibration of the pH electrode can be automatically conducted.
The above gas sensor system of carrier solution flow-through type is a system in which the automation of the zero point calibration can be easily carried out, however, it still has the following problems. (1) When base line drift of the pH electrode is considerable, it is necess
Nagai Yuko
Nakamura Michihiro
Sekiguchi Tetsushi
Nihon Kohden Corporation
Sughrue & Mion, PLLC
Wiggins David J.
Williams Hezron
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