Welcome to our forum.
First off, I'm a long time lurker and have enjoyed/benefited from the many informative posts. Thanks to all.I have a Marquis Epic and use Lake Michigan water right from the tap. All parameters indicate in acceptable ranges and remain so with only minor tweeking now and then. That is except for the hardness; it always read extremely low. My dealer tells me that its not a problem and not to worry but I'm not convinced. Can anyone shed some light on this and recommend treatments? I use in-line Spa Frog bromine and mineral cartridges.Thanks,Bill
Low pH is damaging to metal, but low hardness or low TA are not. The reason for saturating the water with calcium carbonate by adding calcium (i.e. calcium hardness, CH) and bicarbonate (i.e. total alkalinity, TA) is to protect plaster surfaces. Being undersaturated dissolves such plaster (calcium carbonate) surfaces while being oversaturated can cause scaling. Low CH will not lead to pH bounce -- it is low TA that can cause though it has to get very low unless you are adding strong chemicals that affect pH in the spa (such as Trichlor, but you generally don't use that in spas).The idea of needing calcium carbonate saturation to protect metal surfaces is controversial. You can read a discussion about it from corrosion experts in this link. Even my tap water has a CH of only 55 ppm, TA of 80 ppm, pH of 7.7, TDS of 136 ppm, and temperature of 67ºF for a saturation index of -0.7, yet my pipes are not corroding. The water district does add around 300-500 ppb of phosphates (that's 0.3-0.5 ppm) as a corrosion inhibitor, but they make no attempt to coat the piping with calcium carbonate. It would be difficult to do in a spa anyway since the conditions in a spa change so dramatically with temperature changes of the water and the much higher temperature of the metal in the heat exchanger that is usually 30ºF or so hotter than the bulk water temperature but only when heating.So your dealer is correct, assuming that you do not have a plaster spa or exposed grout in tile (you most likely have an acrylic spa or possibly fiberglass).