Issue of the difference between online meter readings and surface measurements for demineralized water
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Our head plant and branch plants use two desalination water production systems respectively: an anion-exchange bed + cation-exchange bed + mixed-bed system, and an ultrafiltration + reverse osmosis + EDI system. The online measurement of the water produced by the mixed-bed system is around 0.1 us/cm, while the surface measurement is around 0.8 us/cm. For EDI, the resistivity of the produced water is only 17 MΩ·CM, which corresponds to approximately 0.05 us/cm; however, the value measured at the surface is around 0.4 us/cm. I know that due to the influence of air, there is definitely a difference between the values measured online and those taken at the surface level. However, I’m not sure exactly how large this difference is. Could someone please analyze whether the water produced by our equipment is normal? In the other two plant sites, there are no online pH monitors; the pH values measured at the surface are all below 6, or exactly 6
I’m not very familiar with this issue. In my opinion, under normal circumstances, the water quality at the first outlet of a water treatment system truly reflects the system’s processing capacity and operational condition. So the produced water should be normal, unless there is something wrong with the measuring instrument. Sampling for testing at subsequent water usage points may show elevated analysis values due to the influence of air or other factors, which should be normal. This is likely related to the path and environment through which the water flows between the first and second detection points; as for the difference in the degree of influence, it may not be comparable. If only the suitability of water quality for boilers or other process operations is considered, can other methods be used to verify that although the conductivity increases, it will not actually have an impact on the production process? For example, boil water with a high conductivity detected in the second test, seal it and let it cool, then measure its conductivity to see what’s going on. Perhaps my reply is incorrect; please use it with caution.
For the quality monitoring of pure water, online continuous analysis should be preferred, as pure water has a high degree of purity. Even trace amounts of impurities, such as carbon dioxide and oxygen in the air, or impurities introduced by sampling containers and pipelines, as well as contamination that occurs during sample transfer and packaging, can significantly affect the results of water quality measurements. The conductivity is such, the pH is such, and the dissolved oxygen is such as well. This level of contamination cannot be measured precisely using quantitative indicators; it can only be said qualitatively that the impact is extremely significant.