Method for conductivity calculation in a treatment fluid upstream and downstream a filtration unit in apparatuses for the blood treatment

• US 20070131595A1
• Filed: 12/13/2005
• Published: 06/14/2007
• Est. Priority Date: 12/13/2005
• Status: Active Grant

• Current Assignee: Gambro Lundia AB (Baxter International Inc.)
• Original Assignee: Gambro Lundia AB (Baxter International Inc.)
• Inventors: Jansson, Olof, Sternby, Jan, Persson, Roland
• Granted Patent: US 7,815,809 B2
• Time in Patent Office: Days
• US Class Current: 210/96.200
• CPC Class Codes: A61M 1/16 with membranes
• CPC Class Codes: A61M 1/1607 before use, i.e. upstream of dialyser
• CPC Class Codes: A61M 1/1609 after use, i.e. downstream of dialyser
• CPC Class Codes: A61M 1/1613 Profiling or modelling of patient or predicted treatment evolution or outcome
• CPC Class Codes: A61M 1/1617 using measurements made during a temporary variation of a characteristic of the fresh dialysis fluid
• CPC Class Codes: B01D 2311/243 Electrical conductivity control
• CPC Class Codes: B01D 61/32 Controlling or regulating
• CPC Class Codes: Y10T 137/034 Controlled by conductivity of mixture

Abstract

A method for conductivity calculation in a treatment fluid upstream and downstream a filtration unit in apparatuses for the blood treatment has been provided. The method comprises the steps of: creating a flow of treatment fluid in the filtration unit, imposing a change in the conductivity of the treatment fluid at the inlet of the filtration unit in order thereby to cause an induced conductivity change in the fluid at the outlet of said filtration unit and measuring a predetermined number of conductivity values Cdo downstream from the filtration unit belonging to a conductivity curve. It further comprises the steps of defining one interpolating mathematical function for the purpose of estimating the pattern of the conductivity curve Cdo in an interval of time after the occurrence of the induced conductivity change and determining a characteristic measuring time tcalc_clr; finally the value of the interpolating mathematical function at said characteristic measuring time tcalc_clr is evaluated and represents the conductivity value Cdo2 of the process fluid downstream from the filtration unit after the induced conductivity change.

Claims

1. A method for determining a conductivity of a treatment fluid downstream from a filtration unit in a blood processing machine, said unit comprising a first compartment for the circulation of blood and a second compartment for the circulation of the treatment fluid, said second compartment being separated from the first compartment by interposing at least a semi-permeable membrane;

said method comprising the steps of;

creating a flow of treatment fluid through the second compartment of the filtration unit;

imposing, for a predetermined time interval, a change in the conductivity of the treatment fluid at an inlet of the filtration unit to cause an induced conductivity change in the fluid at an outlet of said filtration unit;

measuring a predetermined number of conductivity values Cdo downstream from the filtration unit, said conductivity values Cdo belonging to a conductivity curve downstream from the filtration unit;

defining at least one interpolating mathematical function for estimating a pattern of the conductivity curve Cdo downstream from the filtration unit in an interval of time after causing the induced conductivity change;

determining a characteristic measuring time tcalc_clr; and

,

calculating a value of the interpolating mathematical function at said characteristic measuring time tcalc_clr, said value representing a conductivity value Cdo2 of the fluid downstream from the filtration unit after the induced conductivity change.

2. A method according to claim 1, wherein the conductivity change imposed in the inlet fluid is a known change in value remaining constant over time, said conductivity change particularly being a positive change in conductivity.

3. A method according to claim 1, wherein the determination of the characteristic measuring time tcalc_clr comprises the following sub steps:

estimation of an intermediate time tA_clr representative of a transient due to the induced conductivity change in the fluid; and

,

correction of the intermediate time tA_clr through the addition of a second time term tcbf, which is a function of blood flow.

4. A method according to claim 3, wherein the characteristic measuring time tcalc_clr is calculated using the equation:

5. A method according to claim 3, wherein the intermediate time tA_clr is calculated using the function:

unit_—

f=ln(sign*(Cdi_step,set−

Cdo)/potential),

and by performing an interpolation with a least squares estimation on the data of the area falling between said function and the x-coordinate time axis in a predetermined time interval, said intermediate time tA_clr coinciding with the instant at which the interpolating function intersects the x-coordinate time axis.

6. A method according to claim 5, wherein the predetermined time interval for the least squares estimation of the function unit_f falls between a first instant of reversal in the blood line (tRev−

1.5 min) and a second instant of reversal in the blood line tRev.

7. A method according to claim 1, further comprising the steps of:

measuring a predetermined number of conductivity values upstream from the filtration unit, said conductivity values belonging to a conductivity curve upstream from the filtration unit;

determining a characteristic time t0_Cdo of the conductivity curve downstream from the filtration unit;

determining a characteristic time t0_Cdi of the conductivity curve upstream from the filtration unit;

synchronizing the conductivity curves on the basis of the characteristic time of the downstream conductivity curve t0_Cdo and the characteristic time of the upstream conductivity curve t0_Cdi determined by the curves upstream and downstream from the filtration unit, to enable comparison of the respective conductivity values; and

,

comparing the conductivity curves upstream and downstream from the filtration unit after the conductivity curves have been synchronized to determine one or more downstream conductivity values.

8. A method according to claim 7, wherein the characteristic time t0_Cdi of the upstream conductivity curve coincides with the instant at which the change in conductivity occurs in the conductivity curve upstream from the filtration unit, the characteristic time t0_Cdo of the downstream conductivity curve corresponding to the instant at which the induced conductivity change downstream from the filtration unit occurs.

9. A method according to claim 8, wherein the characteristic time t0_Cdi of the upstream conductivity curve is calculated by estimating an area defined below the inlet conductivity curve, the characteristic time t0_Cdi coinciding with the instant at which the area under the curve takes on an average value greater than a predetermined threshold.

10. A method according to claim 8, wherein the determination of the characteristic time t0_Cdo of the conductivity curve downstream from the filtration unit comprises a further step of making a preliminary estimate of the value of the characteristic time t0_Cdo and subsequently correcting the preliminary estimate if incorrect.

11. A method according to claim 10, wherein the preliminary estimate of the value of the characteristic time of the conductivity curve downstream from the filtration unit comprises the following sub steps:

determining the average conductivity at the outlet of the filtration unit Cdo_pre,mean prior to the effects of the change in conductivity, said determination being made on the basis of an average of the measured conductivity values Cdo prior to the effects of the change;

comparing the measured conductivity values at the outlet of the filtration unit Cdo with the average outlet conductivity value Cdo_pre,mean;

estimating the instant at which the measured conductivity values Cdo appear constantly greater than the average outlet conductivity value Cdo_pre,mean.

12. A method according to claim 11, wherein the measured conductivity values Cdo exceed the average outlet conductivity value Cdo_pre,mean a number of times, causing the preliminary estimate of the characteristic time t0_Cdo of the conductivity curve downstream from the filtration unit to coincide with the instant of the last condition in which the measured conductivity values Cdo are greater than the average outlet conductivity value Cdo_pre,mean.

13. A method according to claim 10, wherein the step of correcting the preliminary estimate of the characteristic time t0_Cdo value of the conductivity curve downstream from the filtration unit comprises the following sub steps:

creating an appropriate mathematical expression unit_f, which is a function of the conductivity values measured downstream from the filtration unit Cdo, and of the known change in the conductivity of the inlet fluid, Cdi_step,set, said expression being normalized if necessary; and

,

performing a least squares estimation of said expression unit_f in a predetermined time interval.

14. A method according to claim 13, wherein the mathematical expression is calculated by the following equation:

unit_—

f=ln(sign*(Cdi_step,set−

Cdo)/potential))

where potential=Cdi_step,mean−

Cdo_pre,mean;

Cdi_step,set=value of the known change in conductivity at the filtration unit inlet;

Cdo=conductivity values measured downstream from the filtration unit;

Cdi_step,mean=inlet conductivity value after the change in conductivity and

Cdo_pre,mean=average outlet conductivity value prior to the change in conductivity.

15. A method according to claim 14, wherein the least squares estimation is performed on the mathematical expression unit_f and the estimate is calculated by the following equation:

Cdo_—

1n_clr=Cdi_set,step−

sign*exp(c_clr(1)*t+c_clr (2))

where c_clr(1) and c_clr(2) are coefficients derived from the least squares interpolation of the mathematical expression unit_f.

16. A method according to claim 13, wherein the step of correcting the preliminary estimate of the value of the characteristic time of the conductivity curve downstream from the filtration unit comprises the additional sub steps of:

creating a mathematical function f_norm, which is a function of coefficients c_clr(1) and c_clr(2) derived from the previous least squares interpolation, and a function of the known change in conductivity Cdi_step,set of the inlet fluid and a function of the conductivity values downstream from the filtration unit Cdo, said mathematical function f_norm being normalized if necessary; and

,

performing a least squares interpolation on the mathematical function f_norm within a predetermined interval of values.

17. A method according to claim 15 wherein the step of correcting the preliminary estimate of the value of the characteristic time of the conductivity curve downstream from the filtration unit comprises the additional sub steps of:

creating a mathematical function f_norm, which is a function of coefficients c_clr(1) and c_clr(2) derived from the previous least squares interpolation, and a function of the known change in conductivity Cdi_step,set of the inlet fluid and a function of the conductivity values downstream from the filtration unit Cdo, said mathematical function f_norm being normalized if necessary; and

,

performing a least squares interpolation on the mathematical function f_norm within a predetermined interval of values,

wherein the mathematical function used is calculated using the following equation;

f_norm=(f2_clr−

min(f2_clr))/max(f2_clr−

min(f2_cir))

where f2_cir=Cdo_—

1n_clr−

Cdo.

18. A method according to claim 16, further comprising the sub step of creating a mathematical estimation function f_norm_est, which is a function of coefficients c_clr (3) and c_clr (4) derived from the a least squares interpolation of the mathematical function.

19. A method according to claim 16, wherein the predetermined interval of values for performing the least squares interpolation on the mathematical function f_norm between 0.1 and 0.9.

20. A method according to claim 18, wherein the corrected value t0_Cdo of the characteristic time of the conductivity curve downstream from the filtration unit coincides with the instant at which the mathematical estimation function f_norm_est takes on a value of 1.0.

21. A method according to claim 18, wherein the mathematical estimation function is calculated using the following equation:

f_norm_—

est=exp(c_clr (3)*t+c_clr (4))

where c_clr (3) and c_clr (4) are a coefficients derived from the least squares interpolation of the natural logarithm of the mathematical function f_norm.

22. A method according to claim 21, wherein the corrected value of the characteristic time t0_Cdo is obtained using the following formula:

t0_Cdo=−

c_clr (4)/c_clr (3).

23. A method according to claim 13, wherein the predetermined time interval of the mathematical expression unit_f in which the interpolation is to be performed essentially falls between a first instant of reversal in the blood line (tRev−

1.5 minutes) and a second instant of reversal in the blood line tRev.

24. A method according to claim 7, wherein the step of synchronizing the conductivity curves entails a relative translation between the conductivity curves upstream and downstream from the filtration unit, ascribed a value D, D being a function of the characteristic times t0_Cdi and t0_Cdo determined by the curves upstream and downstream from the filtration unit.

25. A method according to claim 1, wherein it comprises a step of determining average inlet conductivity after the imposed change in conductivity Cdi_step,mean, said determination being based on an average of the measured conductivity values Cdi after the effects of the imposed change in conductivity.

26. A method according to claim 25, wherein the conductivity values Cdi used to determine the average inlet conductivity after the change in conductivity Cdi_step,mean fall within the time interval between an instant at which the change in conductivity occurs in the conductivity curve upstream from the filtration unit, t0_Cdi+3 minutes, and an instant at which the effects of the change in conductivity cease in the same curve, tf_Cdi−

1 minute.

27. A method according to claim 1, wherein an instant tf_Cdi, at which the effects of the change in conductivity cease in the conductivity curve upstream from the filtration unit, is calculated by estimating an area defined below the imposed conductivity curve, said instant tf_Cdi being identified by the moment at which the area under the curve takes on an average value below a predetermined threshold.

28. A method according to claim 1, wherein the conductivity values measured upstream from the filtration unit Cdi are filtered to eliminate long term fluctuations between such conductivity values.

29. A method according to claim 28, wherein only the inlet conductivity values Cdi measured within a significant time interval are filtered.

30. A method according to claim 28, wherein a mathematical filter is used to eliminate the long term fluctuations between the conductivity values measured upstream from the filtration unit Cdi, said filter being of the exponential type.

31. A method according to claim 30, wherein, after filtering, the conductivity curve upstream from the filtration unit is determined by the following expression:

Cdi_filt,i=(N−

1)/N*Cdi<sub>filt,i−

1</sub>+1/N*Cdi<sub>i−

1</sub>

where N=filtering factor.

32. A method according to claim 31, wherein the filtering factor N is between 150 and 250 and a sampling frequency used to determine conductivity values Cdi ranges from 0.01 to 0.1 min.

33. A method according to claim 1, wherein the measured values of the conductivity curve downstream from the filtration unit Cdo are corrected on the basis of conductivity fluctuations generated in the measured values of the conductivity curve upstream from the filtration unit Cdi.

34. A method according to claim 1, wherein the conductivity values measured upstream from the filtration unit Cdi are filtered to eliminate long term fluctuations between the upstream conductivity values and wherein the measured values of the conductivity curve downstream from the filtration unit Cdo are corrected on the basis of the conductivity fluctuations generated in the measured values of the conductivity curve upstream from the filtration unit Cdi, the measured values of the downstream conductivity curve Cdo being corrected on the basis of the filtered conductivity values Cdi_filt measured upstream.

35. A method according to claims 31, wherein the measured values of the conductivity curve downstream from the filtration unit Cdo are corrected on the basis of fluctuations generated in the measured values of the conductivity curve upstream from the filtration unit Cdi and wherein the corrected conductivity curve downstream from the filtration unit is determined by the expression:

Cdo_corr=Cdo+(Cdo2−

Cdo_pre,mean)*Cdi_diff/step size

where Cdi_diff=Cdi_filt−

Cdi_step,mean;

step size=Cdi_pre,mean−

Cdi_step,mean

Cdo2=conductivity value downstream from the filtration unit after the effects of the change in conductivity.

36. A method according to claim 15, wherein the measured values of the conductivity curve downstream from the filtration unit Cdo are corrected on the basis of the conductivity fluctuations generated in the measured values of the conductivity curve upstream from the filtration unit Cdi, wherein the corrected conductivity curve downstream from the filtration unit fits the expression:

Cdo_corr=Cdo+(Cdo2−

CdO_pre,mean)*Cdi_diff/step size

where Cdi_diff=Cdi_filt−

Cdi_step,mean;

step size=Cdi_pre,mean−

Cdi_step,mean

Cdo2=conductivity value downstream from the filtration unit after the effects of the change in conductivity; and

,

and wherein, for the purpose of calculating Cdo_corr, a preliminary estimate value of Cdo2 is used, said estimate being defined by the value that the function Cdo_ln_clr assumes at the instant t=t0_Cdo

37. A method according to claim 1, further comprising the step of computing the conductivity curve Cdo downstream from the filtration unit to determine a characteristic time tStartCF, beyond which the conductivity curve has stabilized after undergoing the effects of the imposed change in conductivity.

38. A method according to claim 37, wherein said characteristic time tStartCF is given by the sum of two terms, a first term t_target, which is a function of the conductivity curve, and a second term tcbf, which is a function of blood flow.

39. A method according to claim 38, wherein the second term tcbf is determined by the following relation:

tcbf=16/60+260/Qb

where Qb=blood flow.

40. A method according to claim 7, further comprising the step of computing the conductivity curve Cdo downstream from the filtration unit to determine a characteristic time tStartCF, beyond which the conductivity curve has stabilized after undergoing the effects of the imposed change in conductivity, and said characteristic time tStartCF is given by the sum of two terms, a first term t_target, which is a function of the conductivity curve, and a second term tcbf, which is a function of blood flow, the determination of the first term t_target for calculating the characteristic time entailing the sub steps of:

estimating an intermediate time tA_clr

deriving the first term t_target, said first term t_taget coinciding with the time that makes a straight line passing through points (t0_Cdo;

Cdo0) and (tA_clr;

Cd0A) take on the value Cdo_pre,mean+step size,

where Cdo0 and CdoA are the values taken on by curve Cdo at instants t0_Cdo and tA_clr where Cdo_pre,mean is the average outlet conductivity prior to the effects of the change in conductivity.

41. A method according to claim 40, wherein the intermediate time tA_clr is calculated using the function:

unit_—

f=ln(sign*(Cdi_step,set−

Cdo)/potential)

and performing an interpolation with a least squares estimation on the data of the area between said function and the time axis of the x-coordinates in a predetermined interval of time, said intermediate time tA_clr coinciding with the instant at which the interpolating function intersects the time axis, i.e. takes on a value of zero.

42. A method according to claim 41, wherein the predetermined time interval for the least squares estimation of the function unit f falls between a first instant of reversal in the blood line (tRev−

1.5 min) and a second instant of reversal in the blood line tRev.

43. A method according to claim 1, further comprising the step of calculating conductivity at the outlet of the filtration unit after the imposed change in conductivity.

44. A method according to claim 43, wherein the step of calculating conductivity at the outlet of the filtration unit further comprises a sub step consisting in the least squares interpolation of a mathematical curve f, which is a function of Cdi_step,mean and Cdo_corr, in a predetermined time interval.

45. A method according to claim 44, wherein the mathematical curve is determined by the following equation:

f=ln(sign*(Cdi_step,mean−

Cdo_corr)).

46. A method according to claim 44, wherein said predetermined time interval falls between tstartCF_clr and tstartCF_clr+1.5 min.

47. A method according to claim 44, wherein the step of calculating conductivity at the outlet of the filtration unit further comprises the sub step of creating an equation Cdo_—

1n_clr, which is a function of Cdi_set,mean and c(1) and c(2), where c(1) and c(2)=coefficients derived from the least squares interpolation of function f.

48. A method according to claim 47, wherein the equation is determined by:

Cdo_—

1n_clr=Cdi_set,mean−

sign*exp(c(1)*t+c(2))

and the conductivity value Cdo2 s the value of the equation at t=tCalc_clr.

49. A method of calculating the filter clearance by means of a function relation of Cdo2, Cdo1, Cdi2 and Cdi1, said conductivities being calculated using the method according to claim 1.

50. A method according to claim 49, wherein the filter clearance is determined by the following equation:

51. A method for calculating fistula flow, comprising the steps of:

determining a filter clearance according to the method of claim 44;

reversing blood flow inside the filtration unit, said reversal causing recirculation in the fistula and a consequent change in conductivity in the conductivity curve downstream from the filtration unit;

determining, by means of mathematical calculations, an outlet conductivity prior to the reversal of blood flow Cdon;

determining a conductivity of the outlet process fluid Cdor following the reversal in blood flow; and

,

setting or estimating an inlet conductivity Cdi after the imposition of a change in conductivity.

52. A method according to claim 51, wherein the fistula flow is determined by the following formula:

53. A method according to claim 52, wherein an actual fistula flow is obtained by correcting the value Q_aw according to the following formula:

54. A method according to claim 51, wherein the step of determining, by means of mathematical calculations, an outlet conductivity prior to the reversal in blood flow Cdon is achieved by creating a second function f as a function of Cdi_step,set and Cdo.

55. A method according to claim 54, wherein the second function f is interpolated with a least squares estimation in a predetermined time interval.

56. A method according to claim 54, wherein the second function f is determined by the following equation:

f=ln(sign*(Cdi_step,set−

Cdo)).

57. A method according to claim 54, wherein the step of determining outlet conductivity prior to the reversal in blood flow Cdon further comprises the sub step of estimating the outlet conductivity curve of the filter as a function of Cdi_step,mean, c_pre,acc(1) and c_pre,acc (2), where c_pre,acc(1) and c_pre,acc(2) are coefficients of the least squares interpolation performed on the second function f.

58. A method according to claim 57, wherein the sub step of estimation is made based on the relation:

Cdo_—

1n_pre,acc=Cdi_step,mean−

sign*exp(c_pre,acc(1)*t+c_pre,acc (2)).

59. A method according to claim 55, wherein said predetermined time interval falls between t=tstartCF_crl and t=t0_acc−

½

min

60. A method according to claim 58, wherein the value Cdon is calculated by estimating the value of the function Cdo_—

1n_pre,acc at the instant t=tCalc_acc.

61. A method according to claim 51, wherein the step of determining outlet conductivity after the reversal in blood flow Cdor is achieved by creating a second relation f as a function of Cdi_step,set or Cdi_step,mean and Cdo.

62. A method according to claim 61, wherein the second relation f is interpolated with a least squares estimation in a predetermined time interval.

63. A method according to claim 61, wherein the second relation f is determined by the following equation:

f=ln(sign*(Cdi_step,set−

Cdo)) or f=ln(sign*(Cdi_step,mean−

Cdo)).

64. A method according to claim 61, wherein the step of determining, by means of mathematical calculations, the outlet conductivity after the reversal in blood flow Cdor further comprises the sub step of estimating the conductivity curve at the filter outlet as a function of Cdi_step,mean, c_acc(1) and c_acc(²), where c_acc(1) and c_acc(2) are coefficients of the least squares interpolation performed on the second relation f.

65. A method according to claim 64, wherein the sub step of estimating the conductivity curve is performed by means of the relation:

Cdo1n_acc=Cdi_step,mean−

sign*exp(c_acc(1 )*t+c_acc (2)).

66. A method according to claim 62, wherein said predetermined time interval falls between t=tstartCF_acc and t=tfc_Cdi−

1 minute.

67. A method according to claim 58, wherein the value Cdon is calculated by estimating the value of the function Cdo_ln_pre,acc at the instant t=tCalc_acc

68. A method according to claim 59 or 67, wherein the reversal in blood flow is effected manually or automatically.

69. A method according to claim 68, wherein the calculation of tCalc_acc comprises the following steps:

filtering the current conductivity curves downstream from the filtration unit.

creating a mathematical relation f_flt, which is a function of Cdi_step,mean and Cdo_flt; and

,

performing a least squares interpolation of said function in an established time interval.

70. A method according to claim 69, wherein the calculation of tCalc_acc additionally comprises the steps of:

creating a relation Cdo_ln_acc,est, which is a function of Cdi_step,mean, c_accest (1) and c_acc,est (2), c_accest (1) and c_acc,est (2) being coefficients derived from the interpolation of the mathematical relation f_flt; and

,

constructing a further relation f²_acc, which is the difference between Cdo_ln_acc,est and Cdo_flt.

71. A method according to claim 70, wherein the calculation of tCalc_acc comprises the further sub steps of creating a difference function diff_f2_acc=f2_acc (i)−

f2_acc(i−

1) and standardizing said difference function, if necessary, to obtain the function;

f²_der,norm=diff2_acc/min(diff_—

f2_acc).

72. A method according to claim 71, wherein

t0_acc,est is the instant at which f²_der,norm first takes on a value greater than 0.5,

t0_acc preferably being defined as t0_acc=t0_acc,est−

0.1 min.

73. A method according to claim 72, wherein in the case of a normal reversal in blood flow t0_acc coincides with tCalc_acc.

74. A method according to claim 71, wherein tStartCF is given by the sum of the instant at which f2_der,norm, after peaking, falls below the value of 0.2, plus an increment of 0.5 min.

77. A blood treatment machine comprising:

at least a filtration unit with a first compartment for the circulation of blood and a second compartment for the circulation of the treatment fluid, said first and second compartments being separated by interposing at least a semi-permeable membrane;

means for changing the conductivity of the treatment fluid upstream from the filtration unit;

at least a first sensor installed upstream from the filtration unit and a second sensor installed downstream from the filtration unit, wherein said first sensor measures the conductivity of a process fluid upstream from the filtration unit and said second sensor measures the conductivity of the process fluid downstream from the filtration unit; and

,

a control unit governing said devices in order to change the conductivity of the process fluid, said control unit configured to receive conductivity signals from the first and second sensors, wherein the control unit carries out the steps described in claim 1 for calculating the conductivity of the fluid downstream from the filtration unit.

78. A method according to claim 16, wherein the predetermined interval of values for performing the least squares interpolation on the mathematical function f_norm is between 0.2 and 0.8.

79. A method according to claim 28, wherein the significant time interval corresponds to the interval falling between an instant at which the change in conductivity occurs in the conductivity curve upstream from the filtration unit, t0_Cdi+0.5 minutes, and an instant at which the effects of the change in conductivity cease in the same curve, tf_Cdi−

0.5 minutes.

80. A method according to claim 31, wherein the filtering factor N is 200 and a sampling frequency used to determine conductivity values Cdi is 0.033 min.

75. A method of determining the conductivity of a process fluid downstream from a filtration unit in blood processing machines, said unit comprising a first compartment for the circulation of blood and a second compartment for the circulation of the process fluid;

said method comprising the following steps;

creating a flow of fluid through the second compartment of a filtration unit;

imposing, for a predetermined time interval, a change in the conductivity of the process fluid at the inlet of the filtration unit in order to cause an induced conductivity change at the outlet;

measuring a predetermined number of conductivity values upstream and downstream from the filtration unit belonging respectively to conductivity curves upstream and downstream from the filtration unit, further comprising the steps of;

determining, by means of mathematical calculations, at least a characteristic time t0_Cdi of the conductivity curve upstream from the filtration unit;

determining, by means of mathematical calculations, a corresponding characteristic time t0_Cdo of the conductivity curve downstream from the filtration unit;

synchronizing the conductivity curves on the basis of the characteristic times t0_Cdi and t0_Cdo determined by the curves upstream and downstream from the filtration unit to enable a comparison of the respective conductivity values; and

,

comparing the conductivity curve upstream from the filtration unit and the conductivity curve downstream from the filtration unit after the respective conductivity curves have been synchronized to determine one or more downstream conductivity values.

76. A method according to claim 75, wherein the measured values of the conductivity curve downstream from the filtration unit Cdo are corrected on the basis of conductivity fluctuations generated in the measured values of the conductivity curve upstream from the filtration unit Cdi.