Here’s the fact. Demineralised water is not good for you.
Measuring demin – what’s so hard?
Well, demineralised water is just that – it’s water, which has probably been treated, to remove the natural minerals that occur in nature. The lack of these additives leaves the pure water actively looking for minerals to replace them. Whilst demineralised water might be a necessary component of many processes, if left to its own devices it searches out ions and minerals.
The most common effect of this is leaching out of it’s containment (such as vessels and piping) of various impurities from both metal and plastic materials. If the purpose of demineralisation is further down the process, then this has to be avoided by careful design of wetted parts.
If not, then the side-effect will be weakening the structure of these items.
Choose your materials carefully
If you want to keep the demineralised properties of the liquid then choose your materials carefully. In some semiconductor processes where ppb is the goal then all the wetted parts will be plastic and be of an ultra-high purity grade such as PFA440. In lesser ppm processes then metals might be avoided. If they are allowed then chose the grades with equal care.
Sometimes tungsten is selected as a bearing material. The traditional binder is cobalt – this keeps the tungsten in good shape by filling the gaps. Given the searching nature of demin, it’s the cobalt that is removed leaving a weak, crumbly material which is no longer suitable as a bearing. Nowadays, tungsten can be specified nickel filler giving it better lifetime properties. Ceramic and sapphire make good alternatives for ball materials.
Note that USP Class VI is not dedicated to demineralised water at all. It is a US standard (United States Pharmacopeia) aimed at selecting materials suitable for use in medical devices including parenteral devices where the leaching out of impurities in plastic must be avoided. Class VI is the most rigorous. The testing certifies that there are no harmful effects caused by the leached out chemicals.
Good meter technologies for measuring demineralised water to ppb standards
- Ultrasonic, time of flight
- All plastic turbine meters including Pelton Wheel
- All plastic VA
Good meter technologies for measuring demineralised water to ppm standards
- Orifice plate
- Averaging Pitot
- Thermal Mass
Not so good meter technologies for measuring demineralised water
Bad meter technologies for measuring demineralised water
- Gear meters- positive displacement
- Most other positive displacement meters.
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