The core technology in Claris is weak acid cation (WAC) exchange softening, which uses a bed of Hydrogen (H+) ions to remove the carbonates from water, as explained in their product literature. This is fundamentally a pH reduction; the trick with Claris is that it can be blended with non-softened water to dial the alkalinity/pH reduction just right. It is perfectly safe when used correctly. When alkalinity is reduced too much, serious corrosion can occur. As discussed in that original thread, this goes with the territory of many products: they must be used correctly, and you use them incorrectly at your peril.
Having said that, a number of people have found that their pH appeared to be reduced significantly more than their alkalinity would suggest. I believe that this has generally been explained away as potentially arising from dissolved CO2 in the water (which is also a byproduct of Claris' ion exchange, as well as the atmosphere). This weak carbonic acid usually doesn't persist in boiler water, and isn't a cause for concern.
However, but I want to call your attention to another potential cause for an unusually low pH reading after using Claris for filtering your water: chloride (Cl-) and sulfates (SO4(2-)). These anions are not typically present in significant concentrations in municipal water, but if they are, they can interact very poorly with the Claris softener (or any other WAC ion exchange softener). The H+ that is bled into the water by the ion exchange forms hyrdochloric (HCl) or sulfuric (H2SO4) acid that can reduce the pH dangerously and lead to corrosion of stainless steel, not to mention copper/brass.
Everpure recommends that if you have water that has chloride levels above 80ppm or sulfate levels above 150ppm, you should not use Claris (or any other WAC ion exchange softener), and should consider a filtration solution that will remove or reduce the chloride and sulfates, such as RO with blending or remineralization.
I have a document from Everpure/Pentair that was given to me by Tim Szejbach of Pentair (representing Everpure and Shurflo). They granted permission for me to post it here on HB, so it is available here. The document contains:
- An example of risky inlet water before and after Claris filtration.
- A table of corrosion thresholds for various SS alloys by temperature and chloride level.
- Summary guidelines for when to use Claris. The summary is reproduced below.
- All weak acid cation (WAC) softening products perform with similar chemistry requirements.
- Claris (like other WAC products) is great for water with bicarbonate levels above 100 & low chloride and sulfate levels (less than 80ppm and 150ppm respectively).
- [Claris] Maintains beneficial minerals for quality coffee - Protects equipment from scale formation.
- Water with high acid risk (high chloride and or sulfate levels) should consider reverse osmosis technology for removal.
- Note chloride and sulfate limits are based on the stainless steel industry and not on Espresso equipment manufacturers specifications.