What is the algorithm used for daily chlorine doseage?

[rock]

Just order that test separately then if you think that is all you need.

Ah! That is GREAT advice!

My dilemma had been that the recommendation for the test kit didn't make logical sense given the situation I outlined.
But, I fully agree (belated as it is) that I need precision in a range that is way above the puny 5 to 10 ppm that the basic kit provides.

You solved the dilemma!
I think all I really need is CYA (accurate and repeatable in the range of at least 30 to 90 ppm) and Chlorine (accurate in the range up to at least 30 ppm and repeatably precise enough to discern a drop of 1 ppm overnight).

If I can simply buy those two tests, I'm all set.

EDIT: Leslies reads the CYA inside the store with store lighting; however, I had "assumed" that, much like most of us don't do camber and toe wheel alignment even though they're simple length and angle measurements, that Leslie's had better equipment than I can possibly hope to have. Certainly they use a bunch of machines when they read my water - but I had never analyzed what those machines did.

 

[harleysilo]

You can just buy those two very tests. Link in my sig.

 

[techguy]

I would equate going into a pool store like going into Frys for computer or O'Reillys for auto parts. It would seem normal to make the assumption that the guy or gal at these stores would know more about pools/computers/auto parts than you do. They are the professional... right? They get paid to know about these things. Problem is many of these people only experiences with pools/computers/autos are as retailers. They know how to sell your "something". It may not be the correct something but they sold you something, they were successful.

Learning how to use the test kit ensures you know more about YOUR pool than anyone and provides you the knowledge of how to manage your pool. You could choose to mange your pool in the way the store tells you to but you know if you do, you will have sky high CYA and a green monster. This means more chemicals in the recommendations from the store.

 

[linen]

My dilemma had been that the recommendation for the test kit didn't make logical sense given the situation I outlined.
But, I fully agreed that I needed precision in the range that was way above the puny 5 to 10 ppm that the basic kit provides.

You solved the dilemma!
I think all I really need is CYA (accurate and repeatable in the range of at least 30 to 90 ppm) and Chlorine (accurate in the range up to at least 30 ppm and repeatably precise enough to discern a drop of 1 ppm overnight).

If I can simply buy those two tests, I'm all set.

To be blunt...my dilemma is that you seem to want to risk your big and expensive pool on pool store testing that may or may not be consistent or accurate. I can think of two obvious reasons you should be testing yourself (all the tests FC, CC, ph, TA, CH and cya):
1. Algae outbreaks are time consuming and can get expensive, especially for a pool your size.
2. Since you have a plaster pool, scaling (or pitting) can ruin your pools surface.

 

[techguy]

Have we spent $100 worth of time avoiding buying the TF100 for $68. The kits gives YOU control of the pool, not the kid at the pool store that has nothing invested in bad suggestions. All he or she knows is that you walked in the door and what he or she has been told by you and the people that taught them how to do the tests.

You have a $100K pool, spend the $68 and save your self several $100 this summer alone. This stuff works. You will have the best looking pool in town.

 

[rock]

They know how to sell your "something"

Agreed.
In fact, they tried to sell me on chlorine and phosphate reducers on this last trip.

To be blunt...

It's ok to be blunt. I'm trying to understand. I pity you guys, who already understand, who are trying to help us (who don't yet understand), understand what is really going on.

The kits gives YOU control of the pool

Understood. And so does this web site! Thanks. I'm picking up a kit, and, the SLAM method is in full play at the moment!

EDIT: I called Leslies and they told me the following:
- They do not use any test strips other than a single phosphate test strip
- They use chemicals and a "water test station"
- The water test station has a light so they can see colors
- They also use a magnetic stirrer

 

[chem geek]

They use a colorimeter for measurements and that's OK when done properly, but has its limitations. The DPD chlorine test will bleach out at higher FC so becomes less accurate and then no good at all at some point. The CH test has problems at a higher CH level. The CYA test often gives bad results though I'm not sure why since technically it should be OK.

The main problem is the user of the test. If it's a temporary summer-only high school kid, they may not have been properly trained or may rush the test or not do it properly.

I'm not saying that pool stores never get it right, but rather that they often get it wrong. Also, it's far better to make test measurements soon after taking a water sample since the chemistry continues to change over time. The pH drifts up as carbon dioxide is outgassed unless you have your sample in a sealed container with little or no headspace (air gap). The chlorine continues to get used up, though that's mostly a risk if you have the sample bottle exposed to sunlight. The TA, CH and CYA values should be pretty stable over time.

 

[rock]

it's far better to make test measurements soon after taking a water sample

Hi Richard,
So that's why the guy once poured my entire sample down the drain when I brought it in warm.
He no longer works there, but, he brusquely told me he couldn't run the sample because it was exposed to the sun, and I had to come back for another test!
Thanks for explaining why!

 

[rock]

A funny thing just happened at the pool.

The sun went low so I started my new nightly routine, scrubbing the green that grew during the day, checking, and then adding chlorine.
But wait! I've never had to have shock chlorine levels of 20 ppm before. The darn basic test kit only goes to "5-10", and I'm well past that.
How much chlorine do I have to add to get to 20 ppm anyway - so I can figure out how much I should add - and so I can tell if it is consumed overnight?

Leslies is closed. So I'm blind.

But you knew this would happen ...

 

[chem geek]

Exactly.

 

[JohnN]

There's reasons we make the recommendations we do... And it's not to drain your wallet (unlike the pool store).

 

[rock]

I'm going to get the recommended test kit; but I have one remaining question, since the ratio of FC:CYA is a critical factor:

Q: What's the algorithm to figure out an optimum target CYA level for the pool?

Note: I have 900 square feet of water surface, exposed to strong sunlight most of the day, and practically zero bather load.

 

[chem geek]

There isn't a handy formula for calculating a reasonable CYA. A higher CYA even with a proportionally higher FC, loses less absolute FC per day (this is shown in the table in this post but of course the absolute amounts will vary depending on specific conditions of temperature, amount of sunlight, bather load, etc.). If that were the only effect, then a high CYA would be the way to go. However, at a higher CYA level if anything goes wrong and you need to SLAM the pool, it takes a lot more chlorine to get to shock level for the higher CYA level.

So the general rule-of-thumb that we use is that somewhere in the 30-50 ppm CYA range is reasonable for most non-SWG pools. Pools with more sunlight can be at the higher end of the range and pools with lots of intense sun may go even higher, but with the risks well understood. Pools with no sunlight, including indoor pools, can go lower to 20 ppm if they want but 20 ppm is the lowest measurable on some CYA tests (yet other CYA tests only measure down to 30 ppm). For SWG pools, 60-80 ppm with 70-80 ppm preferred is the usual recommendation because one wants to minimize SWG on-time to minimize the rate of pH rise and to lengthen SWG cell life. Since the dosing is more continuous and automatic, it is less likely to run into problems with an SWG pool and therefore less likely to need to shock to high FC levels. So the risk of the higher CYA level is a lower risk (i.e. less likely).

With full sunlight on your pool, why don't you start with 40 ppm CYA and see what kind of daily chlorine loss you have. If it's acceptable, you're good to go, if it's too high, then raise your CYA level but understand the risk you are taking the higher you go. There's no problem so long as you properly maintain your pool, and that of course requires a proper test kit. If you go with the supplemental algae prevention at extra cost that you seem to be interested in, then you lower your risk of a problem so a higher CYA would save you money on chlorine due to lower daily chlorine loss. It's a tradeoff between the extra cost for supplemental algae prevention vs. the cost of chlorine. It's not what we normally recommend because it's more complicated and doesn't apply equally to most pools (i.e. which approach to take depends on other factors like phosphate level, cost of products, size of pool, amount of water dilution, whether some products are already being used for other purposes such as borates, etc.), and can be more expensive for some pools (those with very high phosphate levels, for example).

 

[rock]

There isn't a handy formula for calculating a reasonable CYA.

Thanks Richard for the advice. I'm doing the SLAM method as best I can with limited equipment at the moment. I had never needed the overnight-demand precision before, nor the shock-level range, but I also hadn't known what I was doing (and had not had a high CYA level either); but now I'm convinced of the need for a better chemical test kit.

With full sunlight on your pool, why don't you start with 40 ppm CYA and see what kind of daily chlorine loss you have.

Makes sense. Thanks for the starting-point suggestion.

If you go with the supplemental algae prevention ...then ... a higher CYA would save you money on chlorine

I will continue to explore rare-earth salts for a dirt-cheap industrial alternative, which, I am hopeful, exists. I spoke to Duraleigh by phone this morning about the kits, and will order the recommended set - but he doesn't normally sell the stannous chloride, SnCl₂, phosphate test kits (e.g., Taylor K-1106).