[Epson Salt In Spa]

See this thread for a discussion of various additives to water including aroma therapy and bath salts (including Epsom Salt). The main problem is that the 2% (20,000 ppm) level of salt that is normally recommended to get the main benefit is a level of salt that is corrosive to metal. If you have a copper heat exchanger in your gas heater for the spa, then I would say that this level of salt would be out of the question. If it's titanium or a cupro-nickel alloy more resistant to corrosion, then that's safer though there is still the pump to consider. If you use a lower level of salt staying below the 3000 ppm level found in saltwater chlorine generator (SWG) pools, then that would be safer. Any level of salt up to around 1500 ppm is probably OK. Also, if you keep the pH higher -- always above 7.5 -- then that reduces the risk of corrosion.

To get to the 3000 ppm salt (sodium chloride) equivalent for Epsom Salt (Magnesium Sulfate Heptahydrate) in 350 gallons which is equivalent to 8.7 pounds of sodium chloride salt would be 36.7 pounds of Epsom Salt (again, in 350 gallons). I would add half that amount and see how it feels and I wouldn't add more than the 36.7 pounds per 350 gallons. You can, of course, add even less to see if smaller amounts give any benefit. If an average sized bathtub holds 50 gallons, then 3000 ppm would be 5.2 pounds of Epsom Salt so you can see if that is what was recommended for your bath and if less is required, then you can scale down accordingly. I've seen recommendations (here for example) of 2 cups of Epsom Salt added to a bath (say, 50 gallons) so that would result in around 1 pound so that's around 600 ppm (sodium chloride equivalent) which is a low amount of salt and should be no problem in a spa (so that would be equivalent to around 14 cups of Epsom Salt in 350 gallons).

You are able to use Epsom salt in a bathtub without problems because you regularly drain the water out of the tub. The spa, however, will keep the salt until you next drain/refill which may be many months and if you always use salt then the exposure will be continuous.

One final word of caution. Be careful not to splash out or drip the water onto unsealed concrete or stone surfaces (or if you do, rinse it off to dilute it). When the water evaporates, it will leave salt and if there are any cracks or crevices, the evaporation will cause the salt to crystallize in those cracks which creates high pressures. Unfortunately, Epsom Salt (magnesium sulfate) creates a very high crystallization pressure (higher than regular sodium chloride salt) so could damage concrete and stone surfaces over time.

Richard

Thanks, Richard. I'm new to the Pool and Spa Forum and this info is exactly what i needed!