How Spa Shock Process Works

[Yuckypuck]

My first post on the site. Would like some input on how the chemistry works during the shocking process.

My test results are:
FAC - 5.8
CC - .5
TA - 100
CH - 340
CYA - 50
pH - 7.8
Temp - 98°

I add calcium hypochlorite (0.36 oz.) to increase the FAC by 5 ppm (.5 x 10). Why & how does the additional 5 ppm of new CL breakdown the existing .5 of CC?

 

[Mdragger88]

Hi & welcome to tfp!
Congrats on having a good kit to test your water.
Why are you using cal hypo? Your ch is already quite high for a standalone spa.
I suggest u switch to liquid chlorine & shock according to the FC/CYA Levels
To eliminate cc’s. Although.5 cc or less is acceptable.
Shovk level would be 20 ppm for a cya of 50.
Your target level fc (always in the water before any bathers) should be 6-8 ppm.
To be able to maintain a lower fc level you should exchange some of your water (about 40%) for fresh as this would lower your cya & also ch.
Use PoolMath for calculating amounts.

Here is the guide for using for using chlorine in your spa.

How do I use Chlorine in my Spa (or pool)?

If you have a question about any information within this Sticky, please start a new thread for yourself where you will receive very quick responses to your specific questions. Thank you! Introduction There are two main things to consider when maintaining your hot tub water, Water Balance and...

www.troublefreepool.com

It is also conveniently pinned to the top of this forum.

 

[Yuckypuck]

Target for pH is 6 - 7.
My makeup (fill water) is @ about a CH of 90 so the cal - hypo is use to balance the spa to a target CH of 350 (low end of ideal range for CH from PoolMath). After a drain & refill (every 4 months) calcium chloride is used to balance the refill close to the CH target. Liquid chlorine or cal - hypo are used to maintain FC and selected based on the current water chemistry.
The CC level is addressed when above .4 ppm using the formula CC Level x 10 and the Taylor table from the K2006C to determine quantities.
MPS is used to shock after each spa use and the tub is shocked once a week, per the Chlorine / CYA Chart with chlorine.
Testing & water level adjustment are daily, in early morning. One to two people use the spa 4 to 5 times per week.

Would like to understand how the chemistry works in the shocking process?

 

[jseyfert3]

Target for pH is 6 - 7.

pH normal range is 7-8, do you mean FC?

My makeup (fill water) is @ about a CH of 90 so the cal - hypo is use to balance the spa to a target CH of 350 (low end of ideal range for CH from PoolMath). After a drain & refill (every 4 months) calcium chloride is used to balance the refill close to the CH target. Liquid chlorine or cal - hypo are used to maintain FC and selected based on the current water chemistry.

Is your spa fiberglass or acrylic? My understanding is nearly all spas are acrylic, and in that case you should select the "vinyl" option in PoolMath or on the recommended levels page. In this case you will see that 0 ppm is acceptable and 50 ppm is ideal. For standalone hot tubs generally only enough CH is needed to prevent excessive foaming, anything else is not needed.

MPS is used to shock after each spa use and the tub is shocked once a week, per the Chlorine / CYA Chart with chlorine.

Note MPS can throw off the FAS-DPD test, unless you have the MPS interference reagent.

I add calcium hypochlorite (0.36 oz.) to increase the FAC by 5 ppm (.5 x 10). Why & how does the additional 5 ppm of new CL breakdown the existing .5 of CC?

The CC level is addressed when above .4 ppm using the formula CC Level x 10 and the Taylor table from the K2006C to determine quantities.

Would like to understand how the chemistry works in the shocking process?

I'm not a chemist, but the x10 is often quoted wrong and there's nothing special about adding a specific amount of chlorine. That's why we teach SLAM, or Shock Level And Maintain. It's better to bring the level up to shock level and hold it there, rather than trying to dump some pre-determined amount of chlorine here. If you want to dive into the actual chemistry of CC breakdown, start here: Chloramines and FC/CYA

If that doesn't scare you off, he's got a lot of other chlorine chemistry stuff here: Pool Water Chemistry

 

[Yuckypuck]

You are correct, my 6 - 7 target is for my FC not pH.

Most above ground, non-wood spas, are manufactured from an acrylic sheet backed with either resin sprayed over layers of chopped fiberglass or ABS plastic.

A vinyl liner used in above or in ground pool is a flexible membrane made with colored polyvinyl chloride (PVC), rolled into thin sheets or films as they are called in the trade, and printed with dyes to produce patterns then the rolls of PVC material are cut into sections for the wall and floor, and heat welded together with a lap seam.

A non-porous and algae resistant gelcoat is generally the outer shell of a fiberglass pool.

A vinyl liner may require a specific chemistry to prevent harding and fading of the PVC material which was the reason for choosing fiberglass from the Pool Math dropdown list as the acrylic's properties most closely compare to the gelcoat of a fiberglass pool. PoolMath should consider adding acrylic as a "Type of Pool" choice.

The CH target of 350 has resulted in zero buildup at the waterline on the shell and the water has a "nice feel". The fill water is city which is pumped from the Edwards Aquifer having a CH of 90. With the next drain and fill the CH target will be reduced back to 300, TA to 80, the CYA to 30 with a target pH of 7.5.

A Monopersulfate Interference Remover Reagent Pack (K-2042) was added to the K2006C test kit to address the impact of using MPS.

Using the formula ......Combined Chlorine (CC) level x 10 = ppm of chlorine to add to the water to reduce the CC when the CC are .5 or higher has worked well.

Chloramines and FC/CYA & Pool Water Chemistry were good "deep dive" reads and exactly what I was looking for. Thanks.

 

[jseyfert3]

Most above ground, non-wood spas, are manufactured from an acrylic sheet backed with either resin sprayed over layers of chopped fiberglass or ABS plastic.

A vinyl liner used in above or in ground pool is a flexible membrane made with colored polyvinyl chloride (PVC), rolled into thin sheets or films as they are called in the trade, and printed with dyes to produce patterns then the rolls of PVC material are cut into sections for the wall and floor, and heat welded together with a lap seam.

A non-porous and algae resistant gelcoat is generally the outer shell of a fiberglass pool.

A vinyl liner may require a specific chemistry to prevent harding and fading of the PVC material which was the reason for choosing fiberglass from the Pool Math dropdown list as the acrylic's properties most closely compare to the gelcoat of a fiberglass pool. PoolMath should consider adding acrylic as a "Type of Pool" choice.

Yes, my spa is an acrylic sheet with fiberglass support.

There is nothing special about the recommended levels for a vinyl as far as fading. All fading would be chlorine or UV based (or age), and the TFP recommendations for FC do not change for any type of pool.

The general idea with the recommended levels is to give an easy to follow range of values that should result in a reasonable CSI number, without a user having to specifically track and worry about CSI. For a plaster pool, a negative CSI will cause calcium to leach out of the pool, etching and removing material over time. So the recommendation for CH is higher, such that the CSI should be close to neutral. A fiberglass pool has a gelcoat, as you mentioned. A lot of older gelcoats had calcium based materials in them, and could be damaged by a negative CSI. So you'll note the recommended levels for fiberglass and plaster pools match for this reason.

Vinyl liners have no calcium in them, so a calcium level of 0 is acceptable, but the recommended minimum ideal level is 50 as there is a sub-note that extremely low calcium levels could cause metal damage. Other than that, a negative CSI does not matter as there is no calcium in the vinyl. As far as I am aware, there is no calcium in the acrylic liner in a spa either, hence a negative CSI is fine and the recommended levels for vinyl can be followed.

My point was that if desired you can try skipping adding the CH, as it's not required for acrylic. There's nothing wrong with adding CH if you'd like, but it's just another step and another expense that you may find is not needed.

All that said, I do agree we should have an acrylic selection on that page, for this reason. I've been meaning to suggested it. @Leebo perhaps add a selection that says "Acrylic (spas)" in the Recommended Levels page?

A Monopersulfate Interference Remover Reagent Pack (K-2042) was added to the K2006C test kit to address the impact of using MPS.

Using the formula ......Combined Chlorine (CC) level x 10 = ppm of chlorine to add to the water to reduce the CC when the CC are .5 or higher has worked well.

Again though there is nothing special about that, except that FC will break down CC. I don't know where the exact link is, but as I mentioned before the 10x is based off one particular type of CC I think, but there's a lot of types of CC, which require different amounts of FC to break down. And if there's waste that has yet to get broken down when the FC is added, or something is growing, then a one-time addition of FC will not remove all CCs, as more CCs will be produced as the FC breaks down that waste or whatever is growing. That's why TFP recommends the SLAM process in general, and not a one time addition of chlorine.

Clearly it worked for you, I'm not trying to deny that. And it probably often will. What I just wanted to point out to you (and anybody reading this later) is that there definitely are times when a one-time addition of FC will not eliminate the CCs, and a SLAM will be needed.

Chloramines and FC/CYA & Pool Water Chemistry were good "deep dive" reads and exactly what I was looking for. Thanks.

I spent a lot of time searching for "X" from the user "chemgeek" when I first found TFP. Learned a LOT reading his posts. Unfortunately he's not actively posting anymore, but that's cause he's busy trying to change the pool regulations at the top end. Last I heard it sounded like the official regulation in the US was getting changed to FC levels as a % of CYA levels, the core TFP methodology, but one the pool industry has not been able to wrap their heads around for one reason or another. So he's actually making progress there.