Decontamination and FC loss over 3 days

[Matilda]

Hey there.
So we did Nitros decontamination on Saturday and followed to the letter.

On the fresh refill our baseline readings are:
CH - 70
FC - 1.5 (residual from pipes and the shock on the previous clean fill we think )
CC's - 0
TA - 40
PH 7.7
CYA - 0

We added 50 ppm calcium increaser and this took CH up to 120ppm
We added TA + and this took TA up to 50 ppm
We added enough chlorine according to the bottle instructions to take the ppm up to 10 on the new refill to dichlor shock it.
We ended up with 13.5 ppm somehow, so thinking the chlorine tub chart isn't accurate or maybe the testing ? We're testing with the Taylor 2006 kit.

Readings above were taken late Saturday night.

Reading early Sun morn:
FC -13 ( a loss of 0.5)
CC - 0
TA - 50
CH- 120
PH 7.6

We added nothing to these readings as thought all ok ?

Mon Morning readings: No one had used the tub
FC - 9 ( loss of 4ppm since Sun morning)
CC - 0 - 0.5 ? (we had a slight pink hue - not quite the same colour as when we have 0.5 cc's but there nevertheless )
PH 7.5
TA 50

Nothing added

Mon Eve: No one had used the tub
FC - 6.5
CC's 0 - 0.5 ?
PH 7.6
Nothing added

Tues AM: no one had used the tub
FC 4.5
TA 50
PH 7.6
CC 0 - 0.5
Nothing Added

Then after these readings we had 2 people soak for 40 mins in swimsuits, no make up body lotions etc.
Results after soak were:

FC - 0
CC's 0.5
PH 7
TA 50

Do these reading seem normal ? No one used the tub until this morning (Tues) and it seemed the decontamination worked but then the following nights we saw big losses of over 1ppm FC and some cc's started to show up without use ( I read that if it's more than 1ppm on the overnight FC loss test that it needs shocking ? Or am I misunderstanding this and driving myself round the twist with information)
Are we doing something wrong here because I don't know how many times we've deep cleaned/shocked this tub and then things seem to creep right back up very quickly ! Is this rate of FC loss normal after a large dose of dichlor on refill ?

If someone could please offer some insight I'd be really grateful because I'm about to give up with this tub ! :shaking:

 

[AftonJeeper]

Have you read this? How do I use Chlorine in my Spa (or pool)?)

Chlorine gets used up and needs to be replaced. Many on here add daily and then dose again after using the spa.

 

[Matilda]

Yes I have read it and many others, perhaps this is why I'm getting confused. I understand chlorine gets used up and needs replacing but I asked if it's normal to lose so much when the tub hasn't even been used and after using the decontamination method ?
The FC was never below 4.5 until the tub was used days after refill.

 

[YippeeSkippy]

Do you have ozone in the spa? It will degrade FC over time. Basically chlorine is a consumable item in a spa...it will not remain there long.

Maddie

 

[Matilda]

No Maddie, no ozone or anything else. We've just been using dichlor, I just wanted to check these readings seem ok ? I know the FC will degrade over time I'm just wanting to be sure the decontamination will have worked as we lost 4 ppm the following night with no one having used the tub.

 

[Alwayswantedapool]

The chlorine usage doesn't seem high to me. I always have to add chlorine after use to keep the level up. What temp do you have the tub at? Mine is at 101 and it just eats the chlorine and cya. Plus, you never know how things get into the tub, like the bathing suits, if they are not rinsed really well, you will get soap in the tub which will use up chlorine for days.
Note, once your cya is where you want it, just use straight chlorine.

 

[Matilda]

Thank you, it's at 38.5/100 at the moment. It just seems weird to only lose 0.5 the first night and then 4 the next when no one had been in the tub !
We can only get 14/15% sodium hypochlorite here as all the store bleach seems to be less then 5%, I'm a bit worried about the strength being unstable in high temps ?