Troubles using the Mettler-Toledo_M400 PH online analyzer?
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This unit is equipped with a Mettler-Toledo_M400PH online analyzer. There is no feed material supplied to the unit; instead, a water circulation system is in use. The water used is deoxygenated water, with a pH value of around 7.8. However, since the unit started operating, the readings from the online analyzer have been decreasing continuously. The relevant pipelines were inspected, and actual water samples were tested on-site; test strips indicated that the pH was around 7, whereas the analyzer showed a value of less than 3.8! The water sample flow was interrupted, and tests were conducted using backwash water (fresh water with a pH of around 6.8). It was found that the measurement values remained within this range and were quite stable, with no downward trend observed! The process flow diagram is as follows: Question: 1. Why is there such a large difference between the actual measured pH value and the measured pH value? 2. Why is the pH almost the same as the measured value and remains relatively stable when backwash water is used? 3. Is there any way to deal with it?
This post was last edited by qugd on 2018-5-16 at 17:55, addressed to liaifeng. Based on the information you provided, your process medium should be pure water (water that has undergone strict deionization). Such water has an extremely low conductivity. The pH measurement system you have ordered is a standard pH meter, not one designed for measuring pure water; this can result in significant fluctuations in the measurement values. Another reason for drift is that the measurement electrode system is affected by nearby stray electromagnetic signals. The water I use for backwashing is likely the regular water used in the process equipment, that is, municipal supply water; its conductivity falls within the range measurable by ordinary pH electrodes, so the measurement values become normal again. The Mettler family should have pH systems for pure water measurement, which generally consist of pH electrodes designed for use with pure water and special flow cells for such purposes; there are also certain technical considerations involved in their installation. If no suitable product from Mettler is available, Yokogawa, Hach, E+H, and DKK also offer similar products.
I have to say that the senior is truly an expert in this field! The reasons you analyzed are the same ones I learned from the supplier yesterday; they’re not very different from what you said! In other words, the current operating conditions and water quality are not those for which the online meter is designed; especially regarding water quality, the supplier assured me that the downward trend indicates the accuracy of their meters, as these meters function properly when calibrated using fresh water I would like to express my gratitude once again; sometimes a single sentence is enough to wake someone up from their illusions. When the supplier asked what the conductivity of the deoxygenated water was, I immediately understood the whole situation and it also strengthened some of my opinions!
Indeed, they are all experts. . . I learned from all the seniors*
Learning!* Thank you to the teacher for their guidance!