Knowing how much i shocked with drop test kit

[jbequer]

How do i know how much Dichlor to give pre and post hot tub party. My kids like to bring their soccer friends over and want to make sure the water is at its best before and after. (Teaspoons or Tablespoons and how many ?). I was thinking of shocking about 6 hours before and again just after the party. I have 375 gallons and use Dichlor.

 What is safe, as far as hours post shocking and going in after ? Is 6 hours ok ?

I use the reagent type testing, how do i know that i have shocked enough? I have read that going to 10 ppm is correct, but how do i know if the tester range only goes to 3.0 on the color gauge ? I have been using 3 tbs pre and post, the water seems to look good, but I’m not sure what the real reading is.
If i want 10 ppm, how do i know when i have that? I have been going by the CL color being very yellow, is this correct? Is there an exact way of knowing?

[chem geek]

Handling the bather load for a hot tub party is no different than for a normal bather load.  You should ALWAYS add sufficient chlorine to be able to oxidize bather waste and thereafter maintain a residual FC (i.e. not get to zero).  There is a rough rule-of-thumb for the amount of oxidizer needed when soaking in a hot (104ºF) tub with no ozonator.  Every person-hour of soaking requires roughly 3-1/2 teaspoons of Dichlor or 3-1/2 teaspoons of 8.25% bleach or 7 teaspoons of non-chlorine shock (43% MPS).  With an ozonator, these amounts are usually cut at least in half.  If one soaks at lower temperatures as is usually the case when soaking longer (since longer than 20 minutes at 104ºF is not recommended), then the amount of sweat is reduced, usually by one-third but can be more.  Of course, bather cleanliness (or lack thereof) can significantly affect the required dosing.

The above handles the bather load after the bathers have left (if one doses after the soak) and assumes no significant chlorine during the soak itself.  In practice, most people like to start their soak with minimal chlorine, usually 1-2 ppm FC, so that they do not notice significant chlorine or chloramine smell.  Such an approach does not provide fast disinfection for preventing person-to-person transmission of disease so is not appropriate in commercial/public pools where instead the FC level is maintained at all times even during higher bather loads (i.e. more chlorine is added to the water dynamically in response to chlorine demand).  Bacteria take 15-60 minutes to double in population under ideal conditions so for an hour or so soak with a few related healthy people, this is not a big deal.  The chlorine added right after a soak will kill off pathogens in addition to oxidizing bather waste and the short timeframe prevents biofilm formation since that requires significant numbers of generations (doublings of population) to become resistant to chlorine disinfection.  Also, in practice, starting with a lower FC level typically leads to some monochloramine formation that is not all oxidized during the soak and that provides some slower disinfection that helps prevent bacterial growth though is not a fast kill to prevent person-to-person transmission of disease.

So the question for you then becomes how much disinfection you want to start out with and/or maintain while the kids are in the spa.  Since these are people from different families not normally in close contact with each other and since one or more of them may be ill with pathogens and not show symptoms, the safer approach would be to use a sufficiently high level of FC to not run out during their use of the spa.  This either means starting out with an FC high enough to not run out or it means adding chlorine during their soak (which obviously either has to be done very slowly to dilute it or they need to get out briefly then have chlorine added and mixed by circulation and even faster with jets which should only take just a few minutes in a spa).

As for how much would be needed to add before they soak, you could estimate the bather-load requirement, but that may end up with too high an FC level.  So you may need to do a hybrid approach of starting out with a higher FC but then periodically during their soaking have them briefly get out, add some more chlorine, then after a few minutes have the get back in.  As for the FC level you can start out with, this depends on your CYA level.  For every 10 ppm Free Chlorine (FC) added by Dichlor, it also increases CYA by 9 ppm.  It sounds like your test kit is an OTO chlorine test and that is not very accurate and probably does not have a CYA test with it.  This is one reason why I recommend using the Taylor K-2006 test kit since its FAS-DPD chlorine test lets you accurately test high FC levels and it also tests for pH, Total Alkalinity (TA), Calcium Hardness (CH), and Cyanuric Acid (CYA).

Note that there is no need for you to shock prior to them entering and waiting for the chlorine to drop.  If you are to elevate the FC level before the soak, you do that just prior to their soak.  Any "shocking" to an FC level higher than you'd normally use being in the water is done only after their soak.  The FC before their soak and during their soak should not be too high or else the active chlorine level will noticeably smell and be harsh on their skin and swimsuits and on spa equipment.

With your current test kit and given the wide variability of their spa usage, cleanliness, etc., you could just take a simpler approach and raise the FC to something like 5 ppm for the start of their soak and then test after 15 or 30 minutes and if the chlorine is low (say < 2 ppm) then add more to get it back up to 5 ppm by having them briefly exit the spa while you add more chlorine.  5 ppm FC in 375 gallons should last through roughly 45 person-minutes of soaking so 3 people for 15 minutes, BUT all bather waste doesn't get oxidized that fast so the FC may not drop that quickly.  You might consider using bleach instead of Dichlor if you've used Dichlor for a while in this spa's water (i.e. if you haven't recently changed the water).  That will maintain a consistent active chlorine level whereas using Dichlor would increase CYA and reduce chlorine's effectiveness.  If you maintain the FC level in this way, then keep track and just add some extra after their soak based on cumulative bather load less the chlorine you've already added.

[jbequer]

Thanks, that's a great way to explain it all. I like the part where you mention the 5ppm before and during, this is duable since I only have the oto kit.

[Sam]

You are an amazing asset to this community chem geek.  Thank you so much for all of your information.  I have learned a lot from you 

[Vinny]

The one thing to be aware of is that x number of sweaty kids will use more chlorine and quicker then the same number of just showered kids. Your water will need more frequent changing as well.

Something that is ingrained into my head is knowing how much 1 tablespoon full of chlorine yields in chlorine; I know that by throwing in 2 tablespoons it brings free chlorine to 3 PPM in my tub. This shouldn't change much with an empty tub so this should be something that is an "automatic" thing. After use (depending on people) I know that I can start with 2 tablespoons and if need be (a lot of people) I can throw in 2 more for an initial value of 6 PPM. Shock can be 6 or more.

But dichlor comes in different chlorine strengths so if you pick up a different brand it could take more or less than the previous brand.

[jbequer]

Yeah, 3 Tbs for me brings it up to over 6ppm. At least by looking at my tester, it looks pretty yellow, much more than 3 ppm . This is why I was asking the question about how one knows how much cl there is.

[Vinny]

Chem Geek talked about the Taylor kit - that's the most accurate kit around,  it can count PPM way up there accurately.

If you are using an indicator kit that shows yellow for chlorine it is somewhat inferior to the Taylor DPD kits. The K-2005 and the K-2006 can differentiate between free chlorine and combined chlorine where the yellow drop kits can't.

If you use non chlorine shock then the yellow drop test kit will be very off for days as that shows up as combined chlorine and will be very yellow.

As far as using any kit - once you have the measurements down pat, 1 tablespoon give 2 PPM free chlorine then you know that 2 tablespoons = 4 PPM free chlorine, 3 = 6 PPM and you don't need to measure that often IMO. You will need to measure if you are having a party and I have done as Chem Geek has said - add chlorine during a lull in the usage back up to 3 or 4 PPM to combat the bather load. One of the things that's nice about bromine (I use chlorine) is that both free bromine and combined bromine actually fight bacteria so it's almost a no brainer that way unfortunately combined chlorine is very ineffective in killing bacteria.

[jbequer]

Is a good shock dose 8 to 10 ppm ? Or is this excessive ? Are 4 tbs Dichlor shock for 375 gallons too much?

[Vinny]

The rule of thumb is if you want to break the combined chlorine then you add 10x the amount that you measured. So if you measured 0.5 PPM then you need to add at least 5, for 1 PPM then it would be 10.

The way you know how much combined chlorine you have is to use a drop indicator and with the free chlorine close to or at 0, the other chlorine is the combined chlorine. With the Taylor kits since you can separate the free chlorine then once you test for the combined chlorine you just put in 10 x that amount. For me, I have a hard time differentiating between 3 or 5 PPM (or more) so I wait until my free chlorine is at 0 then I run the test for combined chlorine - any color indicated that I do have combined chlorine and if it's below 0.5 PPM (an amount on the indicator vial) then I add enough to get it above 5 PPM which for me is 4 tablespoons.

It is a good idea to every so often "shock" the tub with a little extra chlorine to give a little extra killing power to the chlorine if there's anything in the tub that 3 PPM doesn't immediately kill.

[jbequer]

Thank you for the info, I'm pretty new to all of this and am still doing a lot of reading about water care. Getting to know my spa is the trick , i am noticing that everyone uses a different amount for their individual needs.

[Vinny]

Chem Geek is really up on all of this, I too learned from what he knows.

We've all been there, I owned a pool before owning a hot tub and thought I had it down pat - nope - totally different.

As far as water care - every tub has different gallons, different bather load and what works for me may not work for you.

My tub is 400 gallons, usually I am soaking (when I do soak) alone and I am usually using 3 PPM free chlorine. When my wife soaks I add twice as much chlorine. When my sons were younger and their friends would go in - all bets were off! I have had a lot of foam, cloudy tubs and a lot of changed water. I once had new water in my tub, my son's friends came over and I had to change it again ...

It gets easier!