chem targets for covered pebble sheen outdoor salt ORP pool

[MattM]

I couldn't find an exact match for the chem targets in pool school and/or reference posts for our situation:
- salt water pool
- ORP managed w/ muriatic acid pump
- Covered 90% of the time
- Outdoor
- Pebble Sheen Plaster
- Area with likely high CH and possible need for occasional R/O service
- Pool season Mid-Feb to Mid-Nov, but not closed in Dec/Jan (we use solar heat, so cost should be minimal to keep running, winter air temp only drops to high 30's for a few hours/night during peak winter weeks, no real worry about pipes freezing ). Day air temps 50-100 all year long, and we'll use heat pump to ensure temp doesn't fall to far during the winter.

Based on what I've read here, the general recommendations for salt and BBB pools, and using the LSI tool at pentair to ensure water is neither too corrosive or scaling, the most reasonable chem numbers to maintain seem to be:

CYA 30-40
FC 3.0-4.0
PH 7.5-7.6
CH 270-370
TA 90-100
SALT 3000 - 3800
BORATES 30-40
TEMP 65-90

Does this look correct?

My justification: I wanted some CYA for the occasional times pool is exposed to the sun, including rare all day pool parties. 30-40 seemed to be the minimum amount that would be effective. FC 3-4 seems like it would be a reasonable level and easy to maintain(should it be lower, e.g. 2-3?), especially as we won't lose much chlorine to sunlight. PH 7.5-7.6...assuming the ORP is set to maintain a ph of 7.5 and it might lag some and let ph rise temporarily to 7.6. CH of 270-370 seems to be the right numbers for pebble sheen while also not impacting LSI too much. Salt 3400 is the ideal number for the intellichlor and 400 seemed to be a reasonable +/- range. Borates 30-40 is also within the levels recommended here, but not as high because I do have a dog that might try to sneak drinks from the pool. Temp 65-90 is roughly what I expect to maintain all year long. TA was the hardest to determine....I tried to use low numbers (60-80) but this always threw the LSI calculation into warning territory when temp was low. Similarly, a TA of 110 threw the LSI into warning when the temp approached 90...so, 90-100 seems to be the desired numbers, but I'm not sure if that is too narrow and be a pain to maintain. I'm guessing I'll have to come up with a way to aerate the water. We don't have any water features (unless you count the tiny bubbles the intellichlor might produce).

 

[DavidJW]

Re: chem targets for covered pebble sheen outdoor salt ORP

I think you've picked up some of the issues yourself - you are aiming at very narrow bands that, in practice, you likely cannot test for reliably, let alone maintain.


I use a Palintest colorimeter based test system aimed at operators of commercial swimming pools (Pooltest 25 Professional Plus) because I have a colour vision defect that means I cannot reliably read many of the indicators used in high quality kits such as the TF-100. The colorimeters aimed at the consumer market aren't that reliable in my experience. Even with my equipment, I can only read total alkalinity to the nearest 5 ppm and pH to the nearest 0.05. In any event, there is a degree of error that is greater than the given resolution of the test - if the true total alkalinity is actually 72 ppm, the system could very easily read the tube as 70 ppm or 75 ppm - indeed, I could read the same tube four times consecutively and get some readings of 70 and some of 75.

There is also a degree of tolerance in each batch of reagents and increased uncertainty over time in the colorimeter's calibration (which can be controlled with a fresh factory calibration or by running a set of certified check standards). This is a tablet reagent system where each strip of tablets is marked with an expiry date and the storage conditions aren't especially critical.


With the TF-100 test kit, the resolution of the tests you carry out will be lower. You can read chlorine to a resolution of 0.5 ppm, and both total alkalinity and calcium hardness to a resolution of 10 ppm - in each case, the true value may be up to the resolution lower than the measured value, as you don't know how much of the last drop was needed to effect the colour change. There's also some variability possible in drop sizes - unless directed otherwise by the instructions, hold the bottles vertically when dispensing drops and wipe away any matter on the tip of the bottle with a clean cloth. You must stir thoroughly between drops - a magnetic stirrer such as the SpeedStir makes this much easier.

pH is read visually with an indicator - the visual scale you have in your kit only has five different colour blocks between pH 6.8 and 8.2, so you can only test to the nearest 0.3. That is about the limit of visually reading a phenol red pH test in any case.

Consumer salt tests can be particularly troublesome. The brands I have seen are expensive and do no better than around 400 ppm resolution. I believe I read somewhere on the forums that different brands can yield dramatically different results from the same sample. My Palintest system reads salt levels to 100 ppm resolution via a fairly awkward test procedure involving a 200:1 dilution (take a clean 100 ml measuring cylinder, add 0.5 ml of pool water using a measuring syringe, top up to 100 ml using deionised water) before generating turbidity using the Palintest chloride chemistry that is read using the colorimeter and light-excluding cap. Repeatability is only to about 200 ppm; this accuracy of this technique obviously depends on the quality of the deionised water used and on washing the cylinder and syringe with deionised water after use.

With liquid reagents like many of those in the TF kits, you ideally should discard them after a year and are dependent on them having been stored in appropriate conditions prior to purchase (there should be no problem with TF, who know about the problems and likely turn over their inventory relatively quickly - the same cannot necessarily be said about reagents purchased from a pool store). In practice, some of the reagents in the TF kit are relatively safe to continue using for a longer period - there have been previous threads about this in the forums.


Another issue is sampling technique. I use a sampling pole and bottle, so I routinely sample from the same spot (by the skimmer - you always sample near an outlet, never near an inlet) and at the same depth. Even so, the degree of mixing of the water is not always the same. In essence, you are only measuring the parameters at a single spot within the pool; even when sampling with the pump on, you cannot guarantee that your results are valid for anywhere other than that spot.


Your automated pH control is via an electrode, which is probably not as accurate as an indicator test, even if you follow the manufacturer's recommendations for maintenance and regularly carry out a two point (or, if possible, three point) calibration using in-date calibrated buffer solutions. Your automated free chlorine control is, I believe, controlled using ORP rather than directly reading free chlorine in an amperometric cell. ORP is not directly related to the free chlorine level, so you may not get as tight a control on free chlorine levels as you hope.


A further issue is whether you want to keep messing around with the pool's chemistry with routine chemical additions, especially as changing one parameter will upset some of the others. My strategy is to intervene to correct a parameter only when it is some way from the ideal - that way, I'm not constantly testing every parameter and throwing chemicals into the pool to deal with the consequences of my last chemical addition.

You have to remember that every chemical you add reacts with the pool, but until water is replaced (including via rain dilution and from top up after splash-out) the products of that reaction stay in the water. For example, if you use dry acid (sodium bisulfate) to lower pH, this adds sulfates to the pool. Excess sulfates can damage plaster and other cement based products. Other parameters can only easily be adjusted one way - you can easily raise calcium hardness or cyanuric acid levels, but the only ways to reduce these parameters is swapping some or all of the water for water with lower levels or to use reverse osmosis. Raising total alkalinity is much easier than lowering it.

This, of course, brings me to the single biggest determinant of your goal parameters - you have to start from the water you have available. It's no good wanting calcium hardness of 250 ppm if your water supply has calcium hardness of 400 ppm. In that case, your only options are to import water or use reverse osmosis. I'm pretty sure the calcium hardness of a new gunite pool tends to increase quite significantly over the first few months anyway, even if the water isn't particularly corrosive.


A much better approach is, wherever possible, to seek a place where the pool chemistry is relatively stable, then correct what you have to. This is particularly the case with total alkalinity, as this parameter affects the stability of the pool pH (which, in turn, affects the pool's chlorine chemistry). If the pH is most stable at a particular total alkalinity and the value isn't too far from the 'ideal' for your pool, I'd stick with it.


There is ongoing debate about the best scaling index to use for pools - many on this forum prefer CSI to LSI, but whichever one you choose, you have a relatively wide band within which the situation is acceptable, even for surfaces like gunite where either extreme is troublesome. Temperature has a relatively small effect on these indices, but seeking one set of values that is OK over such a wide temperature range is probably unrealistic. You may have to allow excursions some way out of the ideal, particularly when the pool is colder and any reactions with the water inevitably slower as a result.


My advice, therefore, is to start by testing your tap water with your test kit, assuming that that is what you will fill the pool with. Once you have those figures, you can set your goals - but don't obsess about holding your water within such tight parameters as your post implies, as that is more likely to make your pool infuriating than trouble free!

 

[MattM]

Re: chem targets for covered pebble sheen outdoor salt ORP

DavidJW -- you've given me something to think about, thank you....and wow, I did a google search on the price of the colorimeter...ouch!

 

[duraleigh]

Re: chem targets for covered pebble sheen outdoor salt ORP

Does this look correct?

Hi, Matt,

Essentially, yes. I am not a big fan of either LSI or CSI but your parameters are generally within the guidelines that work well for thousands of users here on the forum.

I would probably run FC a little lower (2-4ppm) and CYA a little lower (20-30) due to the sunlight protection you provide with the cover. That said, a pool covered as often as you suggest will need to be monitored pretty carefully to watch for combined chloramines and, in my opinion, to keep an eye on the auto pH pump which can cause havoc should it fail in any way.

None of the above is any cause for alarm but just sort of a heads up that a pool covered as much as yours will be requires just a little more diligence for a while.

Like the other David, I agree you are perhaps trying to get your parameters into pretty tight ranges that are not mandatory (example, pH 7.2-7.8 will work in almost any pool with normal measurements for TA and CH) and, to an extent, close to the margin of error in the TF-100 kit. Establishing the numbers you posted as goals should work well but don't spend too much time forcing your pool to comply with every one of them exactly. Again, like David says, you want a troublefree pool, not a PITA.

 

[chem geek]

Re: chem targets for covered pebble sheen outdoor salt ORP

With the TF-100 test kit, the resolution of the tests you carry out will be lower. You can read chlorine to a resolution of 0.5 ppm, and both total alkalinity and calcium hardness to a resolution of 10 ppm - in each case, the true value may be up to the resolution lower than the measured value, as you don't know how much of the last drop was needed to effect the colour change. There's also some variability possible in drop sizes - unless directed otherwise by the instructions, hold the bottles vertically when dispensing drops and wipe away any matter on the tip of the bottle with a clean cloth. You must stir thoroughly between drops - a magnetic stirrer such as the SpeedStir makes this much easier.

pH is read visually with an indicator - the visual scale you have in your kit only has five different colour blocks between pH 6.8 and 8.2, so you can only test to the nearest 0.3. That is about the limit of visually reading a phenol red pH test in any case.

Consumer salt tests can be particularly troublesome. The brands I have seen are expensive and do no better than around 400 ppm resolution. I believe I read somewhere on the forums that different brands can yield dramatically different results from the same sample. My Palintest system reads salt levels to 100 ppm resolution via a fairly awkward test procedure involving a 200:1 dilution (take a clean 100 ml measuring cylinder, add 0.5 ml of pool water using a measuring syringe, top up to 100 ml using deionised water) before generating turbidity using the Palintest chloride chemistry that is read using the colorimeter and light-excluding cap. Repeatability is only to about 200 ppm; this accuracy of this technique obviously depends on the quality of the deionised water used and on washing the cylinder and syringe with deionised water after use.

The FAS-DPD chlorine test has a resolution of 0.2 ppm if you use a 25 ml sample size. Such resolution isn't normally needed, but is available. Accuracy is +/- 1 drop or 10%, whichever is greater. So accuracy is +/- 0.2 ppm up to 2 ppm and is then 10% for measurements above 2 ppm.

As for the variability in drop sizes, Taylor specs the bottle tips to 24 drops/ml +/- 1 drop so an error of around 4% from the drop size. The rest of the error to 10% is mostly from sample size measurement and some from reagent concentration variability.

pH can usually be read to with 0.2 and with practice even better than that, though it does take getting used to. However, accuracy is another matter since there is a delicate balance to neutralizing the chlorine without affecting the pH in the phenol red test though Taylor seems to do a decent job with this up to around 10 ppm FC or thereabouts.

The K-1766 Taylor Salt Test has a resolution of +/- 200 ppm if you use the 10 ml sample size, but if you use the 25 ml sample size then you can get +/- 80 ppm though the endpoint is not quite as distinct.

 

[JasonLion]

Re: chem targets for covered pebble sheen outdoor salt ORP

If you have a SWG, which it sounds like you do, you may need to lower TA a bit. Keeping the pool covered most of the time will greatly reduce how much of an issue this is, so it might not come up. Still, if the PH tends to rise, you will want to lower the TA. Lowering TA will reduce the rate at which PH rises, or eliminate the increase completely. As I said, this may not be an issue. Still, if CH goes up, you will likely need to lower TA for other reasons anyway.

 

[MattM]

Re: chem targets for covered pebble sheen outdoor salt ORP

Thanks for the great info all. I'll see if I can widen my ranges somewhat and focus more on maintaining proper CSI/LSI rather than exact numbers for each metric. For CYA, I'll try 20-30 to start but might have to move it up to 30-40 if FC/swg usage gets out of hand - especially during the summer.

On another note, we did the plaster today and I reviewed the warranty for our pebble sheen finish. The plaster company requires that TA stay between 90-100, PH between 7.4-7.6, and free chlorine between 1 - 3 over a ten year period for us to be "covered' by the warranty. I'm guessing these numbers are as strict as they are because many pool owners will use test strips/etc with poor accuracy. Otherwise, this would seem to be rather hard for anyone to meet.

 

[JasonLion]

Re: chem targets for covered pebble sheen outdoor salt ORP

The numbers are strict because they don't want to actually pay anything on warranty coverage. It would take a very unusual level of dedication to maintain those numbers, and if you slip up even a little or fail to document your levels then they don't have to pay you anything on your warranty.

 

[MattM]

Re: chem targets for covered pebble sheen outdoor salt ORP

Pool finally filled and took first chem checks today using the TF100 test kit:

PH: 8.2!
FC: 0.5 - 1.0
CH: 150
TA: 100
Temp: 59

Strangely enough, because of the low temp (heater not on yet) the CSI is apparently only 0.35

Pool builder will be out in an hour to start the pump and I think they want to manage some of the chemistry. I'll let them put in muriatic acid, but probably not trichlor or other pucks right? We don't want to raise TA...
Not sure how much to expect the CH to rise over the next few weeks.