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Maximum Safe Chlorine Ppm

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Assuming I have 30ppm CYA in my spa, would the maximum safe chlorine level be ~24ppm? My calculations show that this would be about the same as having 4ppm non-stabilized chlorine in the tub, which is generally the highest recommended level for drinking water and pool water.

Is this correct? Can I really use a spa with CYA 30ppm and FC 24ppm?

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24 ppm FC with 30 ppm CYA at hot spa temps of 104F is equivalent in active chlorine (hypochlorous acid) concentration to around 5.1 ppm FC with no CYA (you need to turn on the temperature dependence flag at line 225 of my spreadsheet if that is what you are using; without that flag set, it's around 3.3 ppm FC with no CYA so I'm not sure where you got 4 ppm FC with no CYA).

That's a very high level of active chlorine and is not something you would normally want exposed to your skin and would probably smell strongly of chlorine as it would tend to outgas faster. As for the EPA regulation of 4 ppm FC, that is based on drinking water. If you were to drink spa water (which you shouldn't) then it is the 24 ppm FC that is relevant since that is the measure of the total chlorine capacity or reserve, all of which will eventually react with organics in your body. Though CYA would somewhat slow down the reactions, that's not really relevant.

Why are you asking this sort of question? It seems strange to me.

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24 ppm FC with 30 ppm CYA at hot spa temps of 104F is equivalent in active chlorine (hypochlorous acid) concentration to around 5.1 ppm FC with no CYA (you need to turn on the temperature dependence flag at line 225 of my spreadsheet if that is what you are using; without that flag set, it's around 3.3 ppm FC with no CYA so I'm not sure where you got 4 ppm FC with no CYA).

That's a very high level of active chlorine and is not something you would normally want exposed to your skin and would probably smell strongly of chlorine as it would tend to outgas faster. As for the EPA regulation of 4 ppm FC, that is based on drinking water. If you were to drink spa water (which you shouldn't) then it is the 24 ppm FC that is relevant since that is the measure of the total chlorine capacity or reserve, all of which will eventually react with organics in your body. Though CYA would somewhat slow down the reactions, that's not really relevant.

Why are you asking this sort of question? It seems strange to me.

I don't have your spreadsheet, where can I find it? I am using 5*FC/CYA.

I'm asking because I would like to set the chlorine to the max safe level before 5 people enter the spa, in order to keep active chlorine in the water as long as possible. With 5 people burning 7ppm/hr each, if I get the FC up to 24ppm before entering, we should be able to soak for 30 mins easily without losing all the FC.

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The spreadsheet is here. The 5*FC/CYA rule is approximate and doesn't hold well when the FC is higher than 20% of the CYA level.

The chlorine level of 24 ppm FC with 30 ppm CYA is way too high, even accounting for the CYA moderating effect. People are going to notice the chlorine smell. It will be more like a commercial/public spa that is at the high end of the normal range allowed (max. FC for spas is usually 5 ppm and usually they have no CYA). Also, as people sweat, monochloramine will get produced and might build up during the soak enough to smell.

You can try this out on yourself first to see what the experience would be like. I don't think it will be very pleasant. It's not going to burn your skin or anything like that, but most people don't like to smell much chlorine when they soak. Also note that the CYA test is only approximate and that when the FC gets close to the CYA level the amount of active chlorine jumps up quite quickly so you could be underestimating or overestimating the active chlorine amount. This just doesn't sound like a good idea to me.

I understand your concern for having enough chlorine to satisfy all the bather load and to be able to prevent person-to-person transmission of disease, but overchlorinating doesn't sound like a good approach.

Last, but not least, how did you ever figure on 24 ppm FC? If you've got 5 people in the spa, I'm guessing that the spa is larger than 350 gallons. If that is the case, then you can't just multiply the person-hour by 7 ppm since you also have to account for the larger spa size. It is the absolute chlorine amount that is proportional to bather load, not the FC concentration. So if you spa is 700 gallons, then it's around 3.5 ppm FC per person-hour.

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The spreadsheet is here. The 5*FC/CYA rule is approximate and doesn't hold well when the FC is higher than 20% of the CYA level.

The chlorine level of 24 ppm FC with 30 ppm CYA is way too high, even accounting for the CYA moderating effect. People are going to notice the chlorine smell. It will be more like a commercial/public spa that is at the high end of the normal range allowed (max. FC for spas is usually 5 ppm and usually they have no CYA). Also, as people sweat, monochloramine will get produced and might build up during the soak enough to smell.

You can try this out on yourself first to see what the experience would be like. I don't think it will be very pleasant. It's not going to burn your skin or anything like that, but most people don't like to smell much chlorine when they soak. Also note that the CYA test is only approximate and that when the FC gets close to the CYA level the amount of active chlorine jumps up quite quickly so you could be underestimating or overestimating the active chlorine amount. This just doesn't sound like a good idea to me.

I understand your concern for having enough chlorine to satisfy all the bather load and to be able to prevent person-to-person transmission of disease, but overchlorinating doesn't sound like a good approach.

Last, but not least, how did you ever figure on 24 ppm FC? If you've got 5 people in the spa, I'm guessing that the spa is larger than 350 gallons. If that is the case, then you can't just multiply the person-hour by 7 ppm since you also have to account for the larger spa size. It is the absolute chlorine amount that is proportional to bather load, not the FC concentration. So if you spa is 700 gallons, then it's around 3.5 ppm FC per person-hour.

My spa is rated to hold five people but is only 280 gallons, and with five people it holds less than that... so I'm fairly concerned with keeping the chlorine level at a safe level....

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You've got a catch-22 situation. You either use low chlorine which will run out and then have the low risk of person-to-person transmission (it's a low risk assuming no one is particularly sick in the spa) or you use a high level that people will likely object to. If you used Nature2 or similar metal ion system, then you would get some slow sanitation without it getting too consumed (because it doesn't oxidize bather waste), but there's not much you can do regarding fast-acting sanitation since all of these are also oxidizers that would react with bather waste.

Since most people add chlorine after the soak and since the soak isn't long enough for bacteria to multiply, the risk is low. It would take fecal matter going from one person splashing into another's mouth/nose and in sufficient quantity to cause illness.

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I've thought more about this and think you could use non-chlorine shock (MPS) to supplement the chlorine for oxidation of bather waste. That way, you could probably have just 4 ppm FC or so of chlorine and the rest be MPS to handle the bather load. The chlorine might last through the soak, but even if it doesn't the MPS will provide some sanitation (ableit somewhat slower than chlorine) at hot spa temps. The main downside to this approach is that MPS can be irritating to some people, especially if there aren't silver ions (say, from Nature2) in the water that catalyze the breakdown of the most irritating component in MPS.

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