Method and apparatus for correcting concentration

A concentration correcting apparatus comprising a correction coefficient calculating means and a correcting means is shown and described. The correction coefficient calculating means calculates a correction coefficient for correcting the concentration of the material when the actual density of the medium at the measurement of the material is calculated in terms of the density of the medium under the reference temperature and pressure, from the results of the temperature measuring means and the pressure measuring means. The correcting means corrects the results of the material measuring means to a concentration under the reference temperature and pressure on the basis of the correction coefficient. The measure value of the material in the medium under constant temperature and depressure is virtually calculated by correcting the measured value by using the correction coefficient while calculating the volume of the medium which is regarded as a parameter of a temperature and a pressure in terms of a volume under the constant temperature and pressure. It is therefore possible to calculate the concentration per unit weight under constant conditions even if the temperature and the pressure are varied at the time of measurement.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to method and apparatus for correcting a 
concentration and, more particularly, to the improvement of a mechanism of 
correcting the measured concentration of a sample the volume of which 
varies with a pressure and a temperature. 
2. Description of the Related Art 
Optical means for measuring a predetermined ingredient in a liquid or a gas 
have found spreading use and, in particular, the absorbance measurement 
method is generally adopted for the measurement of concentration because 
it does not require complicated operation. 
In the absorbance measurement method, the absorbance of a sample is 
measured, and the measured value is substituted into the preset equation 
of a calibration curve to obtain the concentration of the sample. This is 
based on the fact that the absorbance of the sample is dependent on the 
quantity (concentration) of a material being measured which is contained 
in a predetermined volume of the sample on the assumption that the volume 
of the sample does not change under any condition. 
In the case of measuring an ingredient which is contained in a refrigerant 
such as flourocarbon and liquified carbon dioxide gas, however, the volume 
of such a medium varies with a pressure, temperature, etc., and the 
content of the material being measured per volume also varies with the 
change. 
As a result, the concentration of the material per volume obtained is not 
proportional to the content per weight, which is required as a more 
substantial quantity, and it is impossible to obtain the accurate 
concentration per unit weight. 
If a sample is measured under strictly constant pressure and temperature, 
this problem is solved. In order to realize this measurement, however, 
very strictly and complicated control of the measuring system is 
necessary. In addition, since the measuring conditions are different in 
apparatuses and measured absorbances, the objectivity of the measured data 
on the concentration of the material can not be expected. 
SUMMARY OF THE INVENTION 
Accordingly, it is an object of the present invention to eliminate the 
above-described problems in the related art and to provide method and 
apparatus for correcting a concentration measurement which are capable of 
obtaining the accurate concentration of the target ingredient contained in 
a medium the volume of which is apt to vary with a temperature and a 
pressure. 
To achieve this aim, the present invention provides a concentration 
correcting apparatus comprising a correction coefficient calculator and a 
correcting means. 
The correction coefficient calculator calculates a correction coefficient 
for correcting the concentration of a material being measured when the 
actual density of the medium at the measurement of the material is 
calculated in terms of the density of the medium under the reference and 
pressure, from the results of a temperature measuring means and a pressure 
measuring means. 
The correcting means corrects the result of a material measuring means to a 
concentration under the reference temperature and pressure on the basis of 
the correction coefficient. 
Since a concentration correcting means according to the present invention 
has the above-described means, it is possible to virtually calculate the 
concentration of a material in a medium under constant temperature and 
pressure by correcting the actual measured value by using the correction 
coefficient while calculating the volume of the medium which is regarded 
as a parameter of a temperature and a pressure in terms of a volume under 
the constant temperature and pressure. 
It is therefore possible to calculate the concentration per unit weight 
under constant conditions even if the temperature and the pressure are 
varied at the time of measurement. 
The correction coefficient calculating means preferably calculates 
correction coefficients a.sub.0 and a.sub.1 in the following equation: 
EQU C=a.sub.0 +a.sub.1 .multidot.A 
wherein C represents a concentration and A the measured density of a 
material, on the basis of the following equations: 
##EQU1## 
wherein P represents a pressure, T a temperature and .alpha.i, .beta.i, 
.gamma.i and .delta.i constants. 
It is preferable that a material measuring means, the temperature measuring 
means and the pressure measuring means are disposed in a refrigerant 
carrier pipe in a freezing refrigeration cycle and that the material 
measuring means measures the density of the oil which is contained in the 
refrigerant. 
It is also preferable that the material measuring means, the temperature 
measuring means and the pressure measuring means are disposed in a 
refrigerant carrier pipe on the excurrent side of a gas-liquid separator 
in the freezing refrigeration cycle. 
The above and other objects, features and advantages of the present 
invention will become clear from the following description of the 
preferred embodiment thereof, taken in conjunction with the accompanying 
drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
An embodiment of the present invention will be explained hereinunder with 
reference to the accompanying drawings. 
FIG. 1 shows an embodiment of a concentration correcting apparatus 
according to the present invention which is applied to the measurement of 
an oil content in a refrigerant. 
The concentration correcting apparatus shown in FIG. 1 is provided with a 
photometer 10 as a material measuring means, a temperature sensor 12 
disposed in the vicinity of the photometer 10 as a temperature measuring 
means, a pressure sensor 14 disposed in the vicinity of the photometer 10 
as a pressure measuring means, a first CPU 16 as a correction coefficient 
calculator and a second CPU 18 as a correcting means. 
The photometer 10, the temperature sensor 12 and the pressure sensor 14 are 
connected to a refrigerant carrier pipe 20, and the photometer 10 measures 
the oil content per unit volume in the refrigerant by an ultraviolet 
absorbance. The temperature sensor 12 and the pressure sensor 14 measure 
the temperature and the pressure, respectively, of the refrigerant in the 
refrigerant carrier pipe 20. Since both sensors 12, 14 are disposed in the 
vicinity of the photometer 10 and the refrigerant carrier pipe 20 has a 
uniform diameter, the temperature and the pressure detected by the sensors 
12, 14, respectively, agree with the temperature and the pressure of the 
refrigerant which is being measured by the photometer 10. 
The first CPU 16 is provided with an A/D converter for converting the 
analog data output from the sensors 12, 14 into digital data, and 
calculates the correction coefficient for correcting the concentration of 
the oil when the actual density of the medium is calculated in terms of 
the density of the medium under the reference temperature and pressure. 
The second CPU 18 corrects the data measured by the photometer 10 to a 
concentration under the reference temperature and pressure by using the 
correction coefficient. 
The corrected result is displayed by a printer, plotter or the like 22 as a 
concentration (wt%). 
FIG. 2 is a flowchart of the operation of the concentration correcting 
apparatus shown in FIG. 1. 
As shown in FIG. 2, when a measurement is started (step 50), a time 
function t is first reset to zero (step 52). 
An absorbance A, a temperature T and a pressure P are then obtained from 
the photometer 10, the temperature sensor 12 and the pressure sensor 14, 
respectively (step 54). 
A correction coefficient a.sub.i(P, T), which is dependent on the 
temperature T and the pressure P are next calculated by the first CPU 16 
(step 56). 
The second CPU 18 calculates the concentration C(t) at the time t on the 
basis of the correction coefficient a.sub.i(P, T) (step 58) and displays 
the result in the form of a chart or the like (step 60). 
When this correction is finished, next measurement is started so as to 
measure the data after the lapse of a time .DELTA.t (step 62). 
In this way, according to the concentration correcting apparatus of this 
embodiment, it is possible to accurately calculate a trace ingredient in 
medium the volume of which greatly changes with a temperature and a 
pressure, for example, a refrigerant as a concentration per weight. In 
addition, it is also possible to serially measure the concentration (at a 
sampling interval of .DELTA.t) under constant conditions even if the 
temperature and the pressure vary moment by moment. 
The correction coefficient used in the present invention will now be 
explained. 
In this embodiment, on the basis of the fact that the concentration is 
proportional to the absorbance under constant temperature and pressure, 
the calibration curve represented by the following function system is 
adopted: 
##EQU2## 
wherein C represents a concentration, A an absorbance, P a pressure and T 
a temperature. 
Since it is possible to treat the volume and the temperature dependence of 
a liquid (including a supercritical fluid) which is treated in the present 
invention in the same way as those of a gas, the following equation holds: 
EQU PV=K.sub.1 .multidot.T 
wherein K.sub.1 represents a constant. 
Since C.multidot.V is constant in the same sample and the absorbance A is 
proportional to the concentration, A.about.C, hence, A.multidot.V is 
constant (K.sub.2). 
In order that the concentration obtained as the measured value is constant 
in the same sample irrespective of P and T, it is necessary that the 
following equation holds independently of P and T: 
##EQU3## 
That is, a.sub.1(P, T) must be proportional to T/P. 
On the basis of the equation (2), the experimental formulas of a.sub.0, 
a.sub.1 are represented as follows: 
##EQU4## 
.alpha..sub.j, .beta..sub.j, .gamma..sub.i and .delta..sub.i in the 
equations (3) and (4) are constants and they are determined, for example, 
by the least square of the values which are obtained by measuring a sample 
having a known concentration. 
The above-described conclusions are also reached by another process. 
Under certain pressure P and temperature T, the following equations hold: 
EQU C.sub.(P, T) =a.sub.0 +a.sub.1 .multidot.A.sub.(P, T) (5) 
EQU C.sub.(P0, T0) .multidot.V.sub.(P0, T0) =C.sub.(P, T) .multidot.V.sub.(P, 
T)(6) 
wherein P.sub.0, T.sub.0 represent reference pressure and temperature, 
respectively. 
The equation (6) therefore becomes as follows: 
##EQU5## 
If the following equation is assumed to be the experimental formula: 
##EQU6## 
the same function system represented by the equations (1), (3) and (4) are 
finally obtained. 
Although an impurity in a refrigerant is measured in this embodiment, the 
present invention is also applicable to the measurement of supercritical 
fluid, for example. 
As described above, according to a concentration correcting apparatus of 
this embodiment, since a correction coefficient calculating means and a 
correcting means are provided, it is possible to obtain the accurate 
concentration per weight irrespective of a temperature and a pressure. 
While there has been described what is at present considered to be a 
preferred embodiment of the invention, it will be understood that various 
modifications may be made thereto, and it is intended that the appended 
claims cover all such modifications as fall within the true spirit and 
scope of the invention.