Electrolysis: processes – compositions used therein – and methods – Electrolytic analysis or testing – For oxygen or oxygen containing compound
Patent
1994-11-14
1996-05-07
Wieder, Kenneth A.
Electrolysis: processes, compositions used therein, and methods
Electrolytic analysis or testing
For oxygen or oxygen containing compound
324 712, 324500, 324601, 204415, 73 1G, 2057785, 205781, G01N 2700
Patent
active
055149685
DESCRIPTION:
BRIEF SUMMARY
The invention relates to a method for detecting the need for calibrating probes which probes deliver an electric signal on account of a change of the concentration of a substance to be measured.
BACKGROUND OF THE INVENTION
A general problem with probes as indicated above, for instance, dissolved oxygen measuring probes (DO probes) is that probe failure caused by, for example, fouling of the membrane may be interpreted as a change in the concentration of the substance to be measured.
Normally this problem is solved by calibrating the probe on a regular time basis. However this solution is not without problems, because by periodically calibrating the probe, the calibration can be either too late or too early. When the calibration is done too early it costs too much working-hours and if the calibration is too late the probe had produced false values during an unknown period.
SUMMARY OF THE INVENTION
The above-indicated problem has been solved with the invention presented below. It has been found that the probe response time constant can be calculated periodically and that this parameter can be used for probe diagnosis.
The method is based on the fact that the response of the probe after an instant change in concentration can be modelled as a first order model combined with a ramp function: ##EQU1## wherein t=time (time), t.sub.c =probe response time constant (time), y=substrate probe signal (mass volume.sup.-1), .alpha..sub.0 =substrate concentration at start of response (mass volume.sup.-1), .alpha..sub.1 =rate of change of substrate concentration (mass volume.sup.-1 time.sup.-1). The measured response of the probe is used to calculate the constants in this equation. The probe response time constant (t.sub.c) from this model can be used for probe diagnosis. As long as the probe response time does not change in consecutive measurements of the probe response after an instant change in concentration the probe is accurate. As soon as the probe response time is changed during consecutive measurements the probe should be recalibrated. In the method according to the invention the concentration of the substance to be measured is changed periodically and each time after such a change the probe response time constant is calculated. As long as the probe response time constant doesn't vary there is no problem. As soon however as the probe response time constant changes there is a probe failure and the probe has to be cleaned and calibrated. By using the method according to the invention described a new calibration is always done in time.
To prove the method, Applicant has carried out research with a continuous flow-through respiration meter (EP-B- 257057). The continuous flow-through meter measures the dissolved oxygen (DO) concentration at the inlet and at the outlet of a closed respiration chamber through which the sludge is pumped continuously Spanjets H. and Klapwijk A., (1990). On-line meter for respiration rate and short-term biochemical oxygen demand in the control of the activated sludge process. Advances in Water Pollution Control, Proceedings of the 5th IAWPRC Workshop held in Yokohama and Kyoto, Japan, 26 Jul.-3 Aug. 1990!. The respiration rate is calculated from the difference of the two DO measurements.
The electrochemical oxygen sensor used in the continuous flow-through respiration meter is based upon the electrochemical reduction of oxygen in an amperemetric cell. The electrodes are immersed in an electrolytic solution which is separated from the bulk solution by a semi-permeable membrane. The electrode signal is determined by the diffusion of dissolved oxygen (DO) from the bulk through the membrane. The probe-meter combination will be referred to as "DO-probe".
The respiration meter as described by Spanjets and Klapwijk (1987): Measurement of the toxicity of KCN and some organic compounds for the activated sludge using the Wazu-respiration meter; In: Kolaczkowski S. T. and Crittenden B. D. (eds.) Management of hazaradous and toxic wastes in the process industries; Elsevier, and Spanjars and Klapwi
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patent: 4900422 (1990-02-01), Bryan et al.
patent: 4921582 (1990-05-01), Wang et al.
patent: 5376244 (1994-12-01), Preidel
Do Diep
Ecotechniek B.V.
Wieder Kenneth A.
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