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Ask 'Dr. Spa' Q & A Message Board

Ask 'Dr. Spa' Q & A Message Board archives 3

Re: Free Chlorine vs. Total Chlorine


In case you are interested in a bit more of an in-depth discussion of dichlor sanitizing, I am reposting a response I made a couple of months ago on "dichlor dosing". It goes through what I think is an easy and effective dichlor treatment approach (don't be dismayed by the length of the post - it really is a very easy approach that may involve a total of 10 minutes of time per week) - so although it isn't an exact answer to your specific question, I think it may help you with what I think you ultimately are asking - namely, about how to use dichlor effectively. I have added a couple of things in red see section # 4 of the unabridged verson to give a few more specifics on pH and chlorine.

What isn't addressed until near the end is the fact that "total" chlorine is made up of "free" and "combined" chlorine - so I'll summarize here in case that's all you want. "Free" chlorine is made up of two species - hypochlorous acid (which you want) and hypochlorite ion (which is not anywhere near as effective). Any measurement you or I make will not differentiate between the two forms of "free" (your measurement will pick up both forms) - it is determined by the pH of your water (see #3 in the Reader's Digest version and #4 in the longer version - below). To keep the hypochlorous form as the dominant form, you want your pH below 7.5 and close to 7.2 (the low end of the recommended spa pH range). The "combined" chlorine (which actually may take the form of at least three forms of chlorine compounds)is typically in the form of chloramines which are the smelly, eye-iritating, skin irritating, etc. forms of chlorine that are formed when free chlorine interacts with nitrogen (in particular). The nitrogen results from many sources, most importantly, human use of the spa. When you "shock" a tub, you break down the chloramines and, thoeretically, immediately after a complete shock, your free and total chlorine will briefly be the same (no "combined" chlorine).

Here it the earlier post:

Dichlor dosing - Vermont Style!!

Posted By: Vermonter
Date: Saturday, 3 August 2002, at 5:52 p.m.

In Response To: Vermonter (zang)


The process is actually quite simple. I'll give you the Reader's Digest version first - and you can stop with that. Or, you can read below that version and get into more details and explanations.
It really is very simple and you will get into a routine that takes just a few minutes a week.

One note - I think your choice to use dichlor vs. MPS is wise. I see practically no advantages to MPS - and many disadvantages.

Reader's Digest Version - how to dose dichlor, Vermont Style (draft #1!!):

1) Add dichlor to achieve an initial free chlorine residual of about 2- 3 ppm. Add your dichlor after use and, ideally, even on days you don't use the tub. The latter (dosing on off days) is not critical if you are adequately disinfecting after use. Measure your free chlorine after you have gotten a thorough mix of the chlorine in your tub. Personally, I would try to not go below 2 ppm. The volume of dichlor needed will have to initially be determined by trial and error. Once you have established this volume (e.g. 1 teaspoon or 2 teaspoons, etc.), as long as your use habits stay the same, you won't need to measure the chlorine more than once a week or two (as a check) and your normal dose can probably stay consistent.

2) If you change your use pattern (more people, longer tub use, etc.) you should check to see if you need to adjust your dose. Do this by measuring your free chlorine and confirming that you are in the 2 + ppm range.

3) Ideally, keep your pH below 7.5 so that the more efficient form of free chlorine is present.

4) Shock weekly (plus or minus a few days is OK) by adding about 7 times your normal daily dose. This will take care of chloramines and will "super disinfect" your tub on a regular basis.

5) During any daily dosing or weekly shocking, make sure that all pumps, valves, jets, etc. are on/running/open so that you are getting the chlorinated water in contact with all wetted surfaces of your tub.

My unabridged recommendations - how to dose dichlor, Vermont Style (draft #1!!) - are as follow:

1) Add dichlor after you use the tub. This way the chlorine level will be at a minimum the next time you use the tub. It may not matter to you, but some people may be sensitive to chlorine (I'm not totally convinced of this) and by dosing after use you will be assured of minimal levels when you use the tub.

If you don't use the tub every day, you can check the chlorine levels (see #3 below) and see if they are maintained. If they are, you can skip dosing if you don't use the tub - if they drop a lot (they probably will), I think it is a good idea to add dichlor even if you don't use the tub. It isn't as important to do this as after you use the tub, because if you dose appropriately after use, then there should be very few pathogens hanging around within a few hours of your dosing. I don't dose every day, but, if I was really practicing what I preach (and going for the ultimate in disinfection), I would probably do so.

2) After you dose, make sure you run all pumps and jets, etc. for at least 5 or 10 minutes to make sure that you have achieved a good mix of the chlorine (dichlor) throughout the tub and that all contacted surfaces have been hit with the chlorine. If you shut down a pump and it contains chlorinated water it should be sufficient to provide disinfection (assuming your does is high enough - see below). But you need to make sure you get the chlorinated water to the pump (or valve, or jet, etc.) to allow this to statement to be true.

3) Chlorine dose - I am a firm believer in adjusting your dichlor dose so that you achieve a target level of chlorine rather than use a simple volume dose (e.g. 1 teaspoon or 2 teaspoons, etc.). After you get familiar with your tub and its "chlorine demand" you can probably use a standard volumetric approach, but you first need to determine what volume is needed. I strongly recommend that you dose dichlor so that you achieve a minimum of 2 ppm of free chlorine after the dose. You can measure your free chlorine residual as soon as you have achieved thorough mixing. In my case, I feel this occurs approximately 5 to 10 minutes after addition. You can determine this by taking repeated measurements (5, 10, 15 minutes, etc.) and see if there is any change - when it remains steady , you can use that as your time delay for measuring in the future. I don't think you need to go much higher than 2 ppm, but if you can hit 2 - 3 ppm and if it stays there for a few hours before dropping way down you should be fine.

You also should initially measure free chlorine after dosing each day until you get used to your tub and its "chlorine demand" characteristics. Once you see a consistent dosing/concentration pattern, I think an occasional check is fine (I check maybe once a week or ten days now) to make sure that you are in the 2 - 3 ppm range.

If you change your use pattern (e.g. more people, longer tubbin' times, etc.) don't assume your normal dose is OK - you should check the free chlorine after dosing and add more (as needed) to get to the 2 - 3 ppm range of free chlorine.

4) pH is important for the disinfection kinetics of chlorine. I can give you more details if you would like (or search on some of my old posts) but free chlorine is most effective if you keep the pH below 7.5 - the lower the better, but I wouldn't go below the general recommendation of about 7.2 due to corrosion considerations. Very briefly, free chlorine exists in your tub split between two forms - hypochlorous acid and hypochlorite ion. The first form is many times more (approximately 80 - 100 times) efficient in disinfection than is the second form. The first form dominates below a pH of about 7.5 - the reverse is true for a pH above 7.5. Again, if you would like more information on this, let me know but at a pH of 8, the split is approximately 80:20 for the "bad" form - hypochlorite ion; at a pH of 7, the ratio is approximately 80:20 in favor of the "good" hypochlorous acid form - the 50:50 split is at a pH of 7.53. The bottom line is that chlorine will work at higher pH's, but not nearly as well.

5) Shocking - this isn't part of the "daily" dose question you posed, but it is an important part of chlorine disinfection. Again, briefly put, as you add your daily dose of chlorine, you are generally forming chloramines (basically combined chlorine that gets tied up with nitrogen - which comes from sweat, other body waste, environmental contaminants, etc.) and they become nasty smelling and irritating to eyes and skin.

You can counteract this process by "shocking" on a regular basis. Typically, this is weekly, but it doesn't have to be - you can sort of set your own schedule based on the "nasty" smell. The longer you wait, however, then theoretically you have to add more and more chlorine to break down the chloramines.

What you are attempting to do is to add about 7 times the "combined" chlorine level - the "combined" chlorine is the difference between total chlorine and the free chlorine you are dosing to. Many people don't measure both - if you don't, you don't have that exact value. However, it is a pretty safe bet that your combined chlorine is less than your free chlorine (as long as you don't wait too long between shocks) so you can use your free chlorine reading as a guide (conservatively high estimate).

But, this approach involves measuring and dosing, and measuring again after your shock dose, etc. - which is a bit of a deterrent to many. So, an easy way (rule of thumb) would be to take your daily volumetric dose and add 7 times that for your shock. So, if you use 1 teaspoon daily, then add a bit more than 2 tablespoons (3 teaspoons per tablespoon) and you should be fine. You can probably use a bit less and be OK - the only reason to use less would be to enable the chlorine levels to drop a bit faster so that you can use the tub sooner after shocking.

Shocking also gives you a "super disinfection" of your tub on a regular basis - while this isn't the primary purpose of shocking, it is a very definite added benefit.

There are a couple of compounds you can use to neutralize chlorine but you have to be careful not to overdose (then your normal dose of chlorine will be neutralized by the residual chemical) and you also need to watch your pH. It will tend to drop with the neutralizers. Both sodium thiosulfate and sodium sulfite can be used - you can actually use the same thing that you lower pH with (sodium bisulfate), but the pH issue is a real problem then and I wouldn't recommend it. You will also add somewhat to your TDS, but not enough to be a problem.

Dichlor is used by more tub owners than any other disinfectant - and is pretty much the standard for commercial spas. It is simple, tremendously effective (when used properly) on bacteria, viruses and some other pathogens and is inexpensive to dose and to monitor. It provides a disinfectant residual which helps safeguard your tub during periods of non-use. It also is an efficient oxidizer and, for that reason as well as a few others, contributes to the clarity of the water in a tub.

There are other choices - some of which I think can be equally as effective, but the simplicity of chlorine is hard to beat and I think it is a great choice to begin with.

Good luck and post back if you have any questions or problems.


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