Measuring FC during mustard shock

[mh0520]

So unfortunately my CYA is around 80 and that requires higher levels of chlorine. I've been battling mustard algae for a while now. At first I thought it was pollen but it would clear up with a SLAM and then come back once levels dropped.

So, I just did another SLAM and now I'm doing a mustard shock before closing up in a week or two. With the CYA being so high, I have to use a lot of reagent to test FC during this process and I'm low with it being the end of the season. Is there any way to dilute the water to get an an accurate measurement with less of the reagent or do I just need to get more reagent?

 

[Dirk]

Well, you definitely don't want to run out of reagent during a SLAM, or you might have to start over.

You can stretch it by reducing your sample to 5ml (don't dilute). That won't be as accurate, and this trick shouldn't be used for normal day to day testing, but it's usually accurate enough for a SLAM. Use one scoop of powder. Measure out the 5ml carefully. Each drop of reagent will equal 1 ppm FC.

So your SLAM level would be 31 drops.
Your mustard shock level would be 46 drops.

I had to look that up here. I'm not very familiar with a mustard shock, so you might know the numbers better than I do.*

Chlorine/CYA Chart

EDIT: You will find a simplified and standardized version of this chart at Pool School, Chlorine / CYA Chart. Chlorine/CYA Chart by Chemgeek FC = Free Chlorine in parts-per-million (ppm) CYA = Cyanuric Acid in parts-per-million (ppm) CYA ........... Min FC3 ..... Target FC ...... Yel/Mstrd...

www.troublefreepool.com

* I've never run a SLAM, let alone a mustard shock. I don't say that to rub it in. You should never have to SLAM a pool. If you're having reoccurring algae outbreaks, you might be missing something or not actually following the TFP protocol correctly. Or you might have to increase your normal target FC amount if conditions at your pool are prone to algae. If you'd like to explore that, we can help...

 

[gregsfc]

I found this answer in a similar post inquiry in 2018...

For testing FC during a SLAM, you can use a 5 ml sample, one scoop of powder (maybe even less), and each drop will count as 1 ppm. That probably won't help out much at this point, but keep it in mind as you move forward.

I was following a post a few weeks ago with a similar instruction, but can't find it now, so I can't confirm if the recommendation above is exactly as the "experts" recommend to conduct the one-for-one drop/ppm method to half the required level of the liquid reagent. I do remember in the thread that I was following that one should not to do this 1/2 method for an OCLT test; it's just for saving reagent during the period that you're trying to maintain shock or mustard shock FC levels, because it is not as accurate, but close enough during the maintaining period.

 

[Dirk]

I found this answer in a similar post inquiry in 2018...


I was following a post a few weeks ago with a similar instruction, but can't find it now, so I can't confirm if the recommendation above is exactly as the "experts" recommend to conduct the one-for-one drop/ppm method to half the required level of the liquid reagent. I do remember in the thread that I was following that one should not to do this 1/2 method for an OCLT test; it's just for saving reagent during the period that you're trying to maintain shock or mustard shock FC levels, because it is not as accurate, but close enough during the maintaining period.

That's correct. Thanks for reminding about the OCLT.

 

[gregsfc]

That's correct. Thanks for reminding about the OCLT.

Sorry for the duplication Dirk; you got your reply in just before mine.

 

[Dirk]

Team work!

 

[mh0520]

Thanks for the info. That's exactly what I needed.

As for the problem starting in the first place, the reason I was having issues is because my CYA numbers we're much higher than I thought they were. When I started the season, I only had a cheap test kit and test strips so I didn't have an accurate way of measuring CYA or chlorine. So, because my CYA levels were higher than I thought, I was not keeping chlorine levels high enough. I'm planning on draining and adding new water to bring the levels down some in the spring but I want to try to clear this up as best as possible before closing for the season.

 

[Dirk]

Gotcha! Sounds like you have a good handle on the cause and the solution! Next season will be a good one!

 

[mh0520]

I hope so!

 

[Rattus Suffocatus]

I have a similar problem right now in reverse. I got really busy and was unable to test for a couple of weeks as the days got a lot shorter apparently. I had the SWCG drift up at about 1.5-2 PPM a day during this time apparently so I have had it off since Friday morning, and I suspect it might get turned back on with a much lower run time duration for the rest of the season tomorrow or Friday morning. I was up to 26.5 PPM FC when I finally tested everything properly after those two weeks! (Which is SLAM level, apparently for the 50 PPM CYA I am running currently)

You do go through it really fast when you are testing daily and have high levels. It might matter more for you, but what you are up against is the error in the tests. I am going to go out on a limb and say that testing 5 mL with one drop = 1 ml is not going to increase your error much. (2x in theory, I bet you'll be there in practice too.) If you are trying to then do CC it might throw stuff off a bit but I bet it'll still be okay.

Since I just effectively did a SLAM for no good reason, I am not worried about CC nor even accuracy beyond a couple of PPM. I've been pulling 10 mL of water and adding 20 mL of RO (in the beginning I did 30 ml RO) to it and then doing a OTO test on that in another vial (x3 the reading). It works but when I compared it against the proper FAS-DPD I was about +/-2 PPM off the limited times I did both. So that should give you the feeling of how error can multiply when you dilute or half the sample or whatever. It's bad, but maybe not bad enough to not be useful.

In my case, however, once I get down to about 6-7 FC, I'll do the proper test, figure out my current chlorine loss per day and adjust down the unit until I shut it down for the season.. so I maybe am not out as much as you would be by losing that accuracy.

So if you just want to keep the level up for several days, I am going to go out on a limb here and say you could probably do that with a 4:1 dilution and OTO.. when you need more accuracy, use the other test. Just don't rely on the number you get that way for calculations or dosing...

But I feel your pain with this. I don't want to run out of R-871 this year until I am below 65F or so on the water and we stop using the pool entirely... because I want to buy the whole refill kit early next year... And I can easily see how you can do this in a SLAM situation with daily tests. My dropper is getting pretty light....

 

[Dirk]

I won't counter Rattus' comments. Just remind that you don't dilute for the FAS-DPD test, you only half the sample. He's using RO water for his dilution MO, but even still, diluting with water can introduce chlorine, so that would skew the results. (City water would have chlorine in it, so you wouldn't want to use tap water for anything.)

 

[mh0520]

Thanks! A lot of info here. I think I'm probably pretty close to 60 degrees at this point. I need to check the temperature tomorrow. What is the significance of the temperature? Will algae stop growing below a certain temperature?

 

[Rattus Suffocatus]

So, even better than RO would be to use distilled water. I get about 15 PPM TDS out of my RO system, while store bought Distilled usually reads 2-3. You can also boil tap water or let it sit for several days to get rid of chlorine, but it might be questionable with chloramines which many systems use. So yeah, if you use dilution I would always suggest Distilled or RO.

Algae will stop growing below a certain temperature.. I would guess about 55F. But I was referring to switching the system from the SWCG to liquid chlorine at that point for the winter. No we don't "close" in Tucson, and our winter cover is the solar cover. Since if you were losing, say 6 PPM a day at the peak, you'll only be losing maybe 1 PPM/day over the winter or maybe even less. I want to have enough R-871 to make it until spring, but I probably will only test every couple of weeks with the FAS-DPD when we are at that point. Right now I am losing less than 3 PPM a day, which is what got me into this mess. Literally the going from near 90F to 79F or so and the shorter days had more than halfed my chlorine demand.. and I let it go for two weeks without realizing this. I'm waiting until FC hits six or so and then I am going the set the timers for half of what they were... I am going to go away on a business trip so I need to resolve this fairly soon though...