About chlorine levels

[johnnyk]

Hi,

I've had a hottub for 6 months now, and I'm still confused with how I should measure chlorine levels. I have understood that the free chlorine level should be somewhere around 3-5 ppm. Ok, I can measure that, but my question is that should the level by that high all the time or only when measuring the level after adding chlorine?

My local dealer said that the free chlorine level should be measured almost immediately after adding the chlorine and not for example after half an hour. Usually this leads to that the level is ok immediately after I have added the chlorine, but quickly drops to 0-1 after ½-1 hours.

To me it feels wrong that the level is measured immediately after adding the chlorine. Isn't the whole idea that the free chlorine level should always be at 3-5 ppm so that there is always some "spare" chlorine that can react with crap that builds up in the tub water?

I've been dosing the chlorine according to the instruction booklet (0,5-1 teaspoon after taking a bath and 1,5 teaspoons when shocking the water each week). However, this amount of chlorine does not keep the free chlorine level constantly at 3-5 ppm. Usually it's somewhere between 0 and 1 if I randomly open the cover and check the levels with a test strip. To get the levels to stick at 3-5 ppm, it feels that I have to at least triple the amount of chlorine, which in turns seems weird. I'm also using an ozonator and silver ion cartridge.

I usually use the tub alone, maybe 1-2 times a week, so there is no heavy usage. Naturally I wash myself and my swim pants thoroughly before entering the tub.

I've also had this problem with the yellow sticky stuff building up on the spa interior just at the water level and sometimes I also get these gross yellow foam bubbles. So probably there is something wrong.

Any ideas?

[chem geek]

0.5 to 1 teaspoon of Dichlor would only handle the bather waste from 9 to 17 minutes of soaking in a hot (104ºF) spa if there were no ozonator.  A typical ozonator (they vary) handles roughly half the bather waste so perhaps you'd get 18 to 34 minutes of soaking handled.  Are you soaking longer than that?  If so, then you simply aren't adding enough chlorine.  Also realize that ozone reacts with chlorine so in between soaks it may be doubling your regular 24-hour chlorine demand from 20-25% to 40-50% or even more.  That means you may need to add more chlorine in between soaks as well.

As for when to measure, you measure BEFORE your soak and you measure 24 hours AFTER your soak or other times in between soaks.  You do NOT measure chlorine levels right after adding chlorine since as you point out it will be oxidizing your bather waste so is expected to first form CC and then further get depleted over the next hours.

[Sam]

If you have an ozonator and mineral stick you don't need 3-5 ppm.  1-3 should be sufficient.  Of course I am over simplifying.  Chemgeek can give a much more thorough explanation.

[Tman122]

I like to soak in close to 0-.5 I go in mostly clean and it's private. I kill after I am out and rock it up to 3-5. Couple days later it is perfect. That's with dichlor only and baking soda, water change 3-4 months. Ozone and N2 to maintain if I go 3-4 days. Sometimes longer but I put a couple tsp if it goes more than 4-5. I'm not religious with N2 and 03. It works with or without. Not sure if it's safe or right but it works for me and our local water. I used bromine for a few years and it seemed chlorine smellier. The level is constant, tough to dial down before a soak. Many have different ways to success.

[chem geek]

For a residential spa there's not much need to soak in a lot of disinfectant so starting with 1-2 ppm FC should be plenty.  Depending on how long you soak, the chlorine will quickly combine with the ammonia from your sweat and form monochloramine which is the same as found with chloraminated water.  That will still disinfect though more slowly, but for a residential spa the risk of person-to-person transmission of disease is low.  It's commercial/public spas that try and keep a higher and consistent FC level when people are in the spa since one ill person can infect many if there were no fast-acting disinfectant in the water.

Since one adds a lot of chlorine after the soak to handle the bather waste, it also kills off any remaining bacteria as well as oxidizing bather waste.  The key is to not let the chlorine get to zero for an extended period of time (more than an hour or two) which is why it is best to measure the level just before your soak to make sure you have been adding enough to still have a residual at that time.

[johnnyk]

Hi,

thanks for your replies! I'll try to keep the level around 1-2 in normal use. With friends coming over, I'll probably have to raise the level to 3-5. We'll see how it works out.