Thermo-blankets Effect On Water Chemistry

[fdegree]

I, sometimes, have a tendency to over analyze things…so I may be way off base on this. But, I have noticed a couple of things, which brought me to a conclusion without actually testing my theories.

Anyway, recently I installed a thermo-blanket on top of the water and noticed 2 things:

1. Higher level of Bromine…15 ppm when it usually stays around 3 – 6 ppm with a floater

2. Lower pH…the pH in my spa has a tendency creep up – today it has dropped to 7.1

These 2 things happened with no bather usage over the course of about 1 – 2 weeks.

So, my question is…can the thermo-blanket trap the bromine, causing the bromine level to continue to rise over time and also trap the carbonation, keeping it from out-gassing, causing the pH to drop?

If the thermo-blanket is the culprit…would folding it in half and just covering half of the water be at all useful? If the thermo-blanket is causing these chemical issues is it even worth having the blanket?

Thanks!!!

[chem geek]

You've made an excellent analysis. Yes, having a cover will tend to keep chemicals in the water so if the main loss of bromine was from it outgassing, then a cover will have that level rise if it continues to dissolve from the feeder at the same rate. Also, if the spa was exposed to sunlight and the cover is opaque, then that would also account for the lower bromine loss as bromine breaks down in sunlight (not as fast as chlorine, but there's no CYA to protect it). And yes, having the cover can prevent the outgassing of carbon dioxide thereby slowing down or stopping the tendency for the pH to rise. It does not, however, directly cause the pH to drop. The pH is dropping because of other additions of chemicals, either from the bromine feeder or from MPS shock or something like that -- and without the offsetting pH rise effect from carbon dioxide outgassing, you are seeing a net drop in pH.

It's probably worth having the blanket partially uncovered to reduce the pH variation and bromine buildup since that will still keep in some of the heat. Or you can keep it on and adjust other parameters. If you can dial down the bromine feeder, then you can adjust the bromine level to not build up as much. I'm not sure what you can do about the dropping of pH. If you continue to add acidic sources of chemical to the water, then you'd have to compensate for that by periodically adding a base. You can raise the TA level (with Alkalinity Up or Arm & Hammer Baking Soda which is Sodium Bicarbonate) which will help buffer these changes so that the pH doesn't drop as much, but you'll still need to add a base to raise the pH. You can raise both the pH and TA together by adding pH Up or Spa Up products which are identical to Arm & Hammer Washing Soda (careful: not the laundry detergent) and are Sodium Carbonate. To raise the pH without as much rise in TA you can use 20 Mule Team Borax in roughly double the amount by weight (about triple the amount by volume) compared to Sodium Carbonate.

Richard