# Thread: Testing Shock levels of FC

1. ## Testing Shock levels of FC

Testing Shock levels of FC (FAS-DPD) uses a lot of Reagent. Even if you are testing 10mL 0.5 ppm, that's 40 or more drops for 20+ ppm. I am wondering if my math below is correct and if this should work. Going to smaller sample sizes is more intuitive half the sample size = twice the ppm. I wanted to check that against the 2 know values we had, and came up with this formula ppm = 0.5ppm/(x mL/10ml). Plugging in 25mL for x does get you 0.2ppm, so it looks like the formula works. I shocked my Spa to what I thought should be 30ppm and tested using 2.5mL and got 16 drops (32ppm) so it might not be super accurate but it will save you some reagent when testing shock levels. I don't want to waste 64 drops of reagent to check my findings. Maybe I will order an extra bottle of reagent just to test this.
 mL's of water to test Drop = x ppm 2.5 mL 2 5 mL 1 10 mL 0.5 15 mL 0.33 20 mL 0.25 25 mL 0.2 30 mL 0.17 40 ml 0.13 50 mL 0.1

2. ## Re: Testing Shock levels of FC

While this will certainly give you a ball park number, keep in mind that the error in a 10 mL sample is =/- 0.5 ppm. You'd be at +/- 2 ppm with that small of a sample, I'd say even higher unless you're using a narrower mixing vial with more accurate volumetric graduations or a 5 mL syringe to measure the volume. Reagent is really inexpensive and can be gotten quickly from tftestkits.net so I don't see a lot of value in reducing sample volume to save \$ or time.

Also, why are you having to shock a spa a that high of an FC? For normal spa maintenance you shouldn't need more than 30 ppm CYA, which would be a shock (SLAM) FC of 12.

3. ## Re: Testing Shock levels of FC

JVTrain,

Yes I was using a syringe that came with kids liquid medication to measure, and only about a half a scoop of the DPD powder. I guess I need to order more reagent, and be more patient, counting to 60 drops is tedious. The high shock is part of decontamination. This is a in-ground SPA, that when we bought the house we thought was a water garden. It had 8ft tall cat tails growing out of it. It has not been used in over 10 years. I really had no hope for it, but after cleaning out about a 500lb floating cat tail root ball (had to back my truck in the back yard to pull it out), I was able to get everything to work except the air pump. (I hope no one comes after me for destroying wetlands). I have had to fill and drain 3 times just to get the mud out. It's looking crystal clear now, but at 32 ppm of FC it better.

4. ## Re: Testing Shock levels of FC

Ahh. I see. Makes sense now.

Wow... congrats on diving in with both feet and taking back the spa! Just don't tell the local DNR or you're likely to get a nasty letter and fine for removing habitat! I hope no Swamp Things or Loch Ness Monsters were harmed in the process.

Also... PLEASE TELL ME YOU HAVE PICTURES OF THE PROCESS!

If so... Posting Photos Tutorial

5. ## Re: Testing Shock levels of FC

I could kick myself for not taking before pictures, but I really did not think I would ever get it back in useable condition. I have taken pictures of the pool that is in even worse shape, I may tackle that project next summer. All of the great info here on TFP has helped a bunch, and had given me hope for the pool too.

6. ## Re: Testing Shock levels of FC

Awesome! Sounds like you've got a great start and since I didn't say it before... WELCOME TO TFP!

7. ## Re: Testing Shock levels of FC

So for a decontamination, one usually adds the required amount of chlorine but usually doesn't test that water presuming the chlorine is at the strength it's supposed to be. A decontamination isn't something where one tries to maintain the high FC level. It's more of a blast that is just done and then after some time the water is replaced.

Also, if you are doing a decontamination, I'd also suggest using Ahh-Some as that helps to remove biofilms, greases, and oils that may be stuck to surfaces including pipes. Even chlorine doesn't do a great job with such items if sticky and stuck to such surfaces.

8. ## Re: Testing Shock levels of FC

I don't know the exact number of gallons for the SPA, so I was checking that I had gotten the FC high enough (reason for testing). I was really only shooting for about 25ppm, so now I know its less gallons than I thought. Yes I wish I had done a Ahh-Some run, and will order some for the next water change. After 10+ years of mud and plants growing in it I don't know if Biofilm would still be there, but will still do another Decontamination just to make sure. Found an old MLS listing picture. You can see the Spa to the far left of the picture, with cat tails growing. It was a lot worse as that picture was from 2011 so there was 3 more years of growth.
PW7662043_5_1.jpg

9. ## Re: Testing Shock levels of FC

Wow. Amazing that you got that spa back in working order. It looks like a planter!

Any hope for the pool? I actually really like the look of the brick coping and the decking looks like it's in great shape too.

#### Posting Permissions

• You may not post new threads
• You may not post replies
• You may not post attachments
• You may not edit your posts
•