I wrote to Dupont (makers of Oxone which is the MPS in virtually all brands of non-chlorine shock) and received a response today that answers these issues. As I suspected, hot tubs or spas need a lot more MPS that should be added daily if used daily -- the once a week regimen will not oxidize the organics the rest of the week. So the benefits seen from MPS used on a weekly basis are only partial -- the full benefit should come from more frequent usage. Now keep in mind that this is coming from a manufacturer and that they have incentive to sell more product, but I think with some measurements from user's spas we can get to a reasonable recommendation.

The dosage recommendation for pools is 1 pound per 10,000 gallons one to two times per week depending on bather load which would correspond to 12 ppm of product (for each dosing) and should register as 2.5 ppm in a Combined Chlorine test (according to Taylor, one needs to mix and wait for one minute after adding R-0003; also to distinguish between real Combined Chlorine and MPS, one can use the K-2041 test which has R-0867 reagent). However,

**the non-chlorine shock (MPS) dosage for spas and hot tubs is 1-2 ounces weight (about 4 to 9 teaspoons) per 250 gallons**which is 30-60 ppm and should register as 6.3 to 12.5 ppm Combined Chlorine

**and should be added after each use (or once per week if not used)**. This is much higher (3-5x) and more frequent (up to 7x) than what I had earlier recommended.

The higher MPS recommendation should help eliminate chloramine formation so there should be less "bad" chlorine smell during soaking (when ammonia from sweat combines with chlorine if MPS is not present) and it should reduce chlorine consumption somewhat. Also, if the bigger problem for hot tub covers getting degraded is the monochloramine which outgasses much more than chlorine (hypochlorous acid), then this should help. So, if anyone wants to try this new dosing amount and frequency, please let us know how it works out for you. Remember that MPS will measure as Combined Chlorine (if not separately measured with the K-2041 test) so with this more frequent dosing regimen you should be measuring Combined Chlorine virtually all of the time, but using MPS you will know that this is an MPS measurement and not "real" Combined Chlorine. If you get the Taylor K-2041 test then you can distinguish between the two.

Dupont also gave some broad guidelines for bather load where <1000 gallons/bather/day is considered high bather load; 1000-5000 gallons/bather/day is medium; and > 5000 gallons/bather/day is low.

So the regimen for daily or nearly daily usage of a hot tub or spa is to add both non-chlorine shock and chlorine after each use. The dosage for the non-chlorine shock is 4-9 teaspoons per 250 gallons. The dosage for the chlorine is about 1-1/2 teaspoons of Dichlor per 250 gallons during the first week or two and then 2 fluid ounces of 6% unscented bleach per 250 gallons after the first or second week.

Richard