The theoretical maximum for water at 25C is 18.18 MOhm-cm. When it was brand new, my water system could produce water that was this clean. These days, even after complete maintenance of the system (sanitization, cartridge replacement, resistivity cell cleaning) the best I can get is 17.8. Is this just something that has to be accepted as normal, or it is a malfunction?
We have a MilliQ Integral 5 unit that produces 2 types of water. The first is around 13-14 MOhm-cm, which is stored in a reservoir for a 2nd pass treatment to produce the 18.2 MOhm-cm reagent quality water. The pods for delivering each type of water measure resistivity in real time. After 6 years of operation, there is little fluctuation in the higher quality water (still measures at 18.2), but the lower quality water varies a bit +/- 0.5 MOhm-cm with the age of the cartridge.
I don't know if you have a 2-pass system like ours but the things for you to consider include the following:
1. Feedwater quality variation since the time your unit started operating.
2. Are you measuring resistivity after storage? Equilibrium with CO2 over time will drop your resistivity.
18.2 MOhm-cm is the standard for high purity water, though a little less may be tolerated. If you´ve got problems, it might be useful to check thw water you´re feeding your processor. You see, even if the manual might state it is a "complete" processor, it might be worth (in terms of cost and of dutarion of your filters) to pre-process your water, like prefiltering/active charbon/AgNO3 bactericide/silica removing etc. (one or more of these processes).
Doing so might mean not only you´re going to get a better MOhm-cm result at the end, but also longer life for your processor and filters (though some commercial vendors wouldn´t admit this).
By the way, 18.2 MOhm-cm might not be the only standard you need to obey: it doesn´t guarantee 100% microbe purity, and neither does it guarantee low silica content. Silica ("sand", SO2, or pure "glass") is a common contaminant in certain waters, and it is a bad electricity conductor, so you might need to check silica content on your pre- and post-filtering water as well.
You must be sure of what degree of purity you need in your water for the best results, generally the purer the water the less stable the pure processed water and the more careful you must be in order to preserve it.
why this value is 18.2 mega ohm. cm exactly.
if I want keep the specification, shall I keep the resistivity greater than or equal to 18.2 mega ohm.cm for Ultra pure water.?
- Badges
- Science topic
- Similar topics
- Agricultural Science
- Horticulture