Mixing time?

[Curmudgeon]

I have been using my hot tub for a month now. Thanks to information learned here I am finding the BBB system producing excellent results.

When adding chemicals I dilute them in a qt container and pour about half the solution across the surface of the tub with both pumps running. I then dip the container and fill it from the tub and again pour half the solution into the tub. The third time I just pour it all in and rinse the container in the water.

Does anyone have a suggestion regarding how long it takes the water in the tub to mix thoroughly? How long I need to keep the pumps running?

The first time I empty the tub I plan to add some dye just before dumping to determine the mix time.

Curmudgeon

350 gal Infinity, BBB

 

[Guest]

IF you are adding predissolved or liquid chemicals they should mix fairly fast in a tub because of the amount of circulation with all the jets going. 10 minuetes should be fine unless you are shocking. Then you want ot leave the tub uncovered and running for a few hours.

 

[Curmudgeon]

I agree than continuous mixing is required to bring fresh oxidant (chlorine) to the surface of particulate impurities, and time may be required for the oxidation.

But 10 minutes seems much longer than I would expect for complete mixing of dissolved components (e.g., HCl added for pH adjustment) in a hot tub. I was thinking a few minutes at most.

If I can find a suitable dye I will report my results when I drain the tub in a few months.

Thanks,

Curmudgeon

350 gal Infinity using BBB

 

[spishex]

If you're running two pumps on 2" lines, you're probably moving close to 200 GPM so you've essentially got a 2 minute "turnover" of the water. At that point the equivalent of all the water in the tub would have moved through the pumps. However, there might be very little movement in some spots. Your footwell, for instance, could be mixing much slower. So it will depend on the number and orientation of your jets.

My tub, for example, has just four jets and a skimmer. I might have a similar turnover (I don't, but let's say I do), but the corners of my tub and the footwell take several times longer to move than if I had 30 dual-port spinners and suction lines in the footwell.

So it's individual, and your dye experiment will really be the only way to tell. Take some pics!

 

[Curmudgeon]

I had not thought of taking pictures during the mixing experiment. Good suggestion. I'll try.

Tub has a two speed pump with a cartridge filtered return and a single speed pump with three unfiltered footwell returns. With 50+ jets there are probably few dead spots.

Thanks for your reply.

Curmudgeon

350 gal Infinity, BBB

 

[Guest]

The 10 minutes I gave you was a very conservative estimate. It should cover all bases.

 

[PaulR]

When adding chemicals I dilute them in a qt container and pour about half the solution across the surface of the tub with both pumps running. I then dip the container and fill it from the tub and again pour half the solution into the tub. The third time I just pour it all in and rinse the container in the water.

I mix mine in a 2gal bucket and just dump it all in. Let the pump run while I put everything away. For bleach sometimes I get lazy and just pour it in straight--it's only about 1oz at a time, dribble it into the path of a jet and voila all mixed.
--paulr

 

[Curmudgeon]

When adding some cold water to make up for evaporation I realized I don't need dye to get an estimate of mixing time.

After sitting unpumped overnight after two days of no heating I measured temperatures at several locations with a Rat Shack thermometer (0.1 *F readout) with a probe on a wire. I got a 19 *F spread from the floor to the top of the tub.

I suspended the probe 6" below the surface in the center of the tub. Within 35 seconds of turning the pumps on high the temperature reading became constant.

The thermometer appears to sample every 12 seconds and the probe is covered with plastic. Thus the time required to reach equilibrium is probably shorter than indicated.

Two conclusions:

Complete mixing requires no more than one minute.

I need to get some more insulation under the tub (the water at floor level was the coldest).


Curmudgeon

350 gal Infinity, BBB

 

[spishex]

Good idea!