Method and device for measuring the purity of ultrapure water

1. Method for analyzing the quantity of organic compound existing in a liquid, at the outlet from a purification device including in series filter means, oxidation means and polishing means and further comprising means for measuring the resistivity of the water at the outlet of said filter means and at the outlet of said oxidation means, including the following steps:

a) measuring the resistivity ρ UPW of the liquid at the outlet of the filter means ( 1 );

b) establishing a reference mode by exposing a portion of the liquid to said oxidation means for a predetermined number of significantly different time periods and measuring the corresponding resistivity values of the liquid after its exposure to said oxidation means;

c) determining in this reference mode the resistivity at infinity ρ ∞REF of said liquid by establishing by regression a formula expressing its resistivity ρ as a function of time t, said formula including at least one exponential function of the type ρ=ρ ∞REF +(ρ UPW −ρ ∞REF )e −t/T ;

d) establishing an analysis mode by passing said liquid to be analyzed through one of said resistivity measuring means to determine its resistivity ρ at the outlet from the oxidation means;

e) determining the purity of the water from the measured resistivity ρ, the resistivity ρ UPW of the water at the outlet from the filter means and the resistivity at infinity ρ ∞REF estimated in the reference mode;

wherein said step e) of determining the purity includes the following sub-steps:

determining the time constant T of the exponential function established in the step c);

determining the resistivity at infinity ρ ∞ of said liquid in this analysis mode by successive iterations until this parameter converges, expressing the measured resistivity ρ as a function of this resistivity at infinity ρ ∞ by a formula including at least one exponential function of the type ρ=ρ ∞ +(ρ UPW −ρ ∞ )e −t/T , t being an iteration variable, and by varying the iteration variable t toward infinity; and

determining the quantity of organic compounds contained in said purified liquid from this resistivity at infinity ρ ∞ .

2. Analysis method according to claim 1 wherein the formula used in the step c) is the sum of an exponential function of t and a linear function of t.

3. Analysis method according to claim 2 wherein the number of different time periods used in the step b) is greater than or equal to six.

4. Analysis method according to any one of claims 1 to 3 wherein the formula used in the step e) to express the resistivity at infinity in analysis mode is a formula including a single function of t of exponential form.

5. Analysis method according to any one of claims 1 to 3 wherein the formula used in the step e) to express the resistivity at infinity in analysis mode is the sum of an exponential function of t and a linear function of t.

6. Analysis method according to any one of claims 1 to 3 wherein all the resistivity measurements are effected in the same resistivity measuring means.

7. Device for analyzing the quantity of organic compounds existing in a liquid, including in series filter means, oxidation means and polishing means, including means for measuring the resistivity of the water at the outlet from said filter means and at the outlet from said oxidation means, and further including:

control means for exposing a portion of the liquid to said oxidation means during a determined number of significantly different time periods;

means for determining by regression the resistivity at infinity ρ ∞REF of said liquid from the resistivities measured at the outlet from the oxidation means after said exposure time periods;

wherein it also includes

means for determining, from the resistivities measured at the outlet from the oxidation means after said exposure time periods, the time constant T of the curve of evolution as a function of time of the resistivity ρ at the outlet from the oxidation means, expressed by a formula including at least one exponential function of the type ρ=ρ ∞REF +(ρ UPW −ρ ∞REF )e −t/T , being the resistivity of the liquid measured at the outlet from the filter means;

means for determining by successive iterations, until this parameter converges, the resistivity at infinity ρ ∞ , of the liquid at the outlet from the oxidation means, expressing the measured resistivity ρ as a function of this resistivity at infinity ρ ∞ by a formula including at least one exponential function of the type ρ=ρ ∞ +(ρ UPW −ρ ∞ )e −t/T , t being an iteration variable, and causing the iteration variable t to vary toward infinity;

means for determining the quantity of organic compounds contained in said purified liquid from this resistivity at infinity ρ ∞ ;

and means for displaying the result obtained.

8. Analysis method according to claim 4 wherein all the resistivity measurements are effected in the same resistivity measuring means.

9. The method of claim 1 , wherein said liquid is ultrapure water.

10. Method for analyzing the quantity of organic compound existing in a liquid, at the outlet from a purification device including in series, a filter, an oxidizer and a polisher, and further comprising a resistivity measuring cell for measuring the resistivity of the water at the outlet of said filter and at the outlet of said oxidizer, including the following steps:

a) measuring the resistivity ρ UPW of the liquid at the outlet of said filter;

b) establishing a reference mode by exposing a portion of the liquid to said oxidizer for a predetermined number of significantly different time periods and measuring the corresponding resistivity values of the liquid after its exposure to said oxidizer;

c) determining in this reference mode the resistivity at infinity ρ ∞REF of said liquid by establishing by regression a formula expressing its resistivity ρ as a function of time t, said formula including at least one exponential function of the type ρ=ρ ∞REF +(ρ UPW −ρ ∞REF )e −t/T ;

d) establishing an analysis mode by passing said liquid to be analyzed through said resistivity measuring cell to determine its resistivity ρ at the outlet from the oxidizer;

e) determining the purity of the water from the measured resistivity ρ, the resistivity ρ UPW of the water at the outlet from the filter and the resistivity at infinity ρ ∞REF estimated in the reference mode;

wherein said step e) of determining the purity includes the following sub-steps:

determining the time constant T of the exponential function established in the step c);

determining the resistivity at infinity ρ ∞ of said liquid in this analysis mode by successive iterations until this parameter converges, expressing the measured resistivity ρ as a function of this resistivity at infinity ρ ∞ by a formula including at least one exponential function of the type ρ=ρ ∞ +(ρ UPW −ρ ∞ )e −t/T , t being an iteration variable, and by varying the iteration variable t toward infinity; and

determining the quantity of organic compounds contained in said purified liquid from this resistivity at infinity ρ ∞ .

11. Analysis method according to claim 10 , wherein the formula used in step c) is the sum of an exponential function of t and a linear function of t.

12. Analysis method according to claim 11 wherein the number of different time periods used in the step b) is greater than or equal to six.

13. Analysis method according to claim 10 , wherein the formula used in step e) to express the resistivity at infinity in analysis mode is a formula including a single function of t of exponential form.

14. Analysis method according to claim 10 , wherein the formula used in step e) to express the resistivity at infinity in analysis mode is the sum of an exponential function of t and a linear function of t.

15. Analysis method according to claim 10 , wherein all the resistivity measurements are effected in the same resistivity measuring cell.