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Understanding The Term 'shock'


robquick
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Hi,

I'm a little confused with the term 'shock', rather than writting a two page essay I thought it would simpler to ask the following specific questions.

Is 'shock' a process or a chemical ?

If its a process, why is there a chemical called non-chlorine shock?

Assuming its a process, how do I know when the tub needs shocking, is it when my combined chlorine exceeds 0.5ppm (or am I totally on the wrong track!) ?

Can I shock the tub by adding a greater amount of Dichlor, if so what ppm should I be aiming for ?

Any help or clarity appreciated.

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"shock" is a verb so refers to a process of elevating the chlorine level higher than normal in order to accelerate chlorine reactions, either to kill algae faster in pools or to oxidize bather waste faster in spas. Unfortunately, the pool/spa industry also uses the term as a noun as if there is a product called "shock", but such products are the same as regular chlorine products, but sometimes priced higher. The term non-chlorine shock is better called a non-chlorine oxidizer since that is what it is and it can be used in spas with silver ions as a regular disinfectant/oxidizer and not at elevated levels (so not with a shock process).

There usually is no need to shock if one properly maintains the proper chlorine level and adds sufficient amounts of chlorine after using the spa to oxidize the bather waste. If you measure Combined Chlorine (CC), you should do so BEFORE your soak and if it is high (> 0.5 ppm) then yes this may indicate a need to shock your spa. If you use Dichlor, then it will increase the Cyanuric Acid (CYA) level. The following are chemical facts independent of concentration of product or of pool/spa size:

For every 10 ppm Free Chlorine (FC) added by Trichlor, it also increases Cyanuric Acid (CYA) by 6 ppm.

For every 10 ppm FC added by Dichlor, it also increases CYA by 9 ppm.

For every 10 ppm FC added by Cal-Hypo, it also increases Calcium Hardness (CH) by at least 7 ppm.

As for how much chlorine you should be adding, it should be enough so that you have at least a small amount (1-2 ppm or more if you don't mind chlorine smell) at the start of your soak and that it doesn't get towards zero ever in between your soaks. If you don't have an ozonator, then every person-hour of soaking in a hot (104ºF) spa usually requires around 3-1/2 teaspoons of Dichlor or 5 fluid ounces of 6% bleach or 7 teaspoon of non-chlorine shock (43% MPS) to oxidize the bather waste. If you have an ozonator, then you may only need to use half as much or perhaps even a little less than that. In between your soaks if you don't soak every day the chlorine loss rate is usually around 25% of the FC per day (24 hours) without an ozonator and 50% or more with an ozonator. So you can see that having an ozonator makes a lot of sense if you use the tub every day or two, but not if you use it only once or twice a week, say on weekends.

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Chem Geek,

Many thanks for your helpful reply.

Your numbers around chlorine demand match my demand 100% however, I don't understand why CD increases with ozone, can you explain so I fully understand the logic? I assumed ozone would reduce CD, as there would be less work for the chlorine to do, clearly I'm wrong.

If my combined chlorine is less than 0.5ppm is there no need to 'shock'? I see frequent posts suggesting that you should 'shock' every week, which is the best theory to follow ?

Thanks.

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If there is regular bather load, say every day or two, then ozone will oxidize some of it so that chlorine doesn't have to in which case the chlorine demand will be lower. However, ozone and chlorine react with each other so if there is no bather load then one sees this effect as an increase in chlorine demand (i.e. loss rate in between soaks). This is why an ozonator is helpful if you soak every day or two, but is not very helpful if you only soak once or twice a week, say on weekends. The ozone will still oxidize some of the bather waste when you soak, but the longer periods of time in between soaks will use up chlorine faster increasing the risk of it getting too low or requiring you to add chlorine more frequently or targeting a higher level.

You generally do not need to shock regularly, every week or otherwise, if you properly maintain chlorine levels in between soaks and use sufficient chlorine to oxidize bather waste. The exceptions to this are when using MPS as the primary oxidizer, such as when using the Nature2 or other silver ion system, since MPS doesn't oxidize the same chemicals as chlorine so chlorine once a week helps keep the water clear. The same is true for bromine though shocking with chlorine every so often also helps to reduce bromamines that can smell (they are still disinfectants, unlike chloramines which are not). One other time where shocking with chlorine may be needed is if you use only stabilized chlorine, such as Dichlor, in which case the build-up of Cyanuric Acid (CYA) will less chlorine's effectiveness to the point that you may need to add a boost of a higher chlorine level to oxidize bather waste faster. If you use the Dichlor-then-bleach method, then you avoid this problem completely and usually have the water last at least twice as long between soaks.

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0.5 ppm if tested using a 10 ml sample so is the lowest resolution is OK. If you use a 25 ml sample then 0.2 or 0.4 is OK, but 0.6 might be something to consider shocking. It also partly comes down to whether you notice a smell. Some of the CC is chlorourea which is relatively innocuous. It's the inorganic chloramines, especially dichloramine and nitrogen trichloride, that are a bigger issue and they smell more (the latter is particularly irritating).

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  • 1 month later...

This is why I installed an on/off switch on my ozonator. My tub is at a ski cabin that averages three or four days/ week usage. I use Borax, di-chlor and muriatic acid with a fresh fill and then liquid bleach and muriatic acid to maintain, thanks to advice from experts here. But my ozonator was consuming too much chlorine during my absence so now I turn it off a few hours after the last soak. Works great!

The bather load is about 5 30 minute soaks/week and my water has just passed the one year mark and is better than ever. I keep the pH in perfect range by adjusting the air valve slightly more or less with acid rarely required. About once per week I add 12 Oz. Of Clorox 6% bleach to the 385 gal. tub when I leave and about 2-4 Oz. every day it's used. Water smells good and remains crystal clear, my skin does not prune. Once every two or three weeks I rub the sides of the tub with my hands as I soak (at the waterline) to prevent a ring from forming. I never dreamed a tub would be this easy to maintain! I think one reason for this is guests are instructed to bathe with soap and hot water before entering and I do not use lotions or body oils.

Anyway, thanks to all the useful advice I've received here.

If there is regular bather load, say every day or two, then ozone will oxidize some of it so that chlorine doesn't have to in which case the chlorine demand will be lower. However, ozone and chlorine react with each other so if there is no bather load then one sees this effect as an increase in chlorine demand (i.e. loss rate in between soaks). This is why an ozonator is helpful if you soak every day or two, but is not very helpful if you only soak once or twice a week, say on weekends. The ozone will still oxidize some of the bather waste when you soak, but the longer periods of time in between soaks will use up chlorine faster increasing the risk of it getting too low or requiring you to add chlorine more frequently or targeting a higher level.

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Spasome, thanks for that info. In spite of you and numerous others demonstrating how ozone increases chlorine demand when there is no bather load and in spite of this being described technically in scientific literature, the ozone manufacturers still don't believe it. They think the contact time is too short to deplete chlorine, but they neglect to account for the fact that the ozone is not removed in a degassing tank so stays in the hot tub water and is therefore around until it breaks down or reacts with chlorine (it does both) and that this process occurs for the long periods of time that an ozonator is turned on. Over a day (24 hours), this results in a measurable increase in chlorine demand. If it's from 25% to 50% with a 2 ppm FC, then that's an increase from 0.5 ppm to 1.0 ppm FC of loss and though it doesn't sound like a lot, it adds up quickly over several days and since it's a percentage loss, raising the FC level initially only helps to some degree.

Over 7 days, a 25% loss per day is a loss of 87% by the end of the week. A 50% loss per day is a loss of 99% by the end of the week. So starting with 10 ppm FC, with a 25% daily loss one ends up with 1.3 ppm FC, but with a 50% daily loss one ends up with 0.1 ppm FC so too low.

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