OCLT Procedure ?'s

[y_not]

When doing an OCLT, what is the recommended procedure as far as timing goes?

Specifically, do I test just after full sundown, when completely dark, or can I just do it right before bed? I go to bed around midnight.
When I test in the morning, do I test before dawn, so still dark outside, or do I test at dawn, or when the sun is actually becoming visible?

To recap in short order, the following questions.
1. How long of a time span, too long/too-short between the initial reading and the final reading? Night & morning.
2. When does UV hit the point, roughly, where it begins to consume CL in the morning, thus invalidating the test?

Thanks everyone.

 

[duraleigh]

There is no absolute rule but the point of the test is to keep light (UV) from having any effect on chlorine consumption. Direct sunlight on the pool is obviously not good.

Indirect lighting at dusk or morning is likely just fine but it doesn't make sense to allow ANY direct light or extended indirect light onto the pool.

 

[y_not]

OK, sounds like a plan. That's pretty much what I was thinking.

So does any chlorine evaporate or otherwise dissipate over time, such that 10 vs. 6 hours between tests, in the dark, would matter?
IE. CL loss just simply due to sitting, algae or other organics aside.

Thx.

 

[chem geek]

There is a slow loss of chlorine as it oxidizes chemicals in the pool and a little does outgas and these losses are a function of temperature. In my pool with a pool cover that slowly gets oxidized, at a warm 88ºF the loss rate with no sun and no bather load is around 0.7 ppm FC per day so 0.03 ppm FC per hour or less than 0.4 ppm FC over 12 hours. So the absolute loss does depend on time, but when we say "overnight" we are talking about no more than 12 hours and usually less in the summertime since daylight is longer. The <= 1 ppm FC absolute loss is just a rough guide and the real rule is if your overnight loss increases from its baseline significantly, assuming your water temperature is the same.

 

[y_not]

Thanks Chemgeek.

Yeah, I don't know what my baseline OCLT is for my pool, as I have never done that test before. Only just got my FAS-DPD test /w the TF-100, so I'm only now starting to get a solid handle on what my true CL usage is, instead of a really rough and often inaccurate estimate from a bad OTO kit that I had been using up until now.

So even in a perfectly clean pool /w zero organics, one can expect to loose 1/2 a ppm or so of FC overnight, just simply from CL outgassing? Although you did say "oxidizes chemicals", so does the CL get consumed by other chemicals that are in a properly balanced pool, organics and outgassing aside?
Lastly, I presume the outgassing is less at much lower temperatures, say low 60's to low low 70's?

So I'll just shoot for adding my CL a bit before midnight, test 30min afterwards /w FAS-DPD, then go to bed, sun rises at about 6:30am right now, so I'll get up at 6am and do another FAS-DPD test. Sound like a plan?

 

[chem geek]

An outdoor pool usually gets leaves or pollen and if it's ever been used it has leftover organics that are slow-to-oxidize. Also if the filter hasn't been cleaned, then chlorine can continue to oxidize organics caught in the filter (dead skin, suntan lotion, etc.) though possibly slowly depending on the chemicals. So a properly balanced pool still loses chlorine even without sunlight -- it's just slower. If it has a pool cover, then that gets slowly oxidized. So chlorine slowly gets used up in this way. Some also outgasses, but that's probably a somewhat smaller amount. The same process happens in indoor pools as well though those that don't use CYA lose chlorine faster due to faster outgassing if the pools aren't covered. Anyway, most pools that are in good shape only lose around 0.5 ppm FC or less overnight while some may show up to 1 ppm FC overnight. It's when you get over that amount that we consider that to be a red flag.

 

[y_not]

Excellent info ChemGeek, I will keep that in mind when I'm doing my OCLT.
Is it a bad idea to do it that night, after the kiddos have been playing in the pool for hours on end during the day?

I'm not after seeing what they added, just after seeing what was already there and may be living in the water prior to the arrival of my nifty kit.
IE. I want to make sure there isn't a problem already brewing.

 

[chem geek]

You can test whenever you want so long as UV from the sun is not hitting the pool, but if you test the night after a high bather load, you may notice some extra chlorine demand. Usually the chlorine usage from bather load is much smaller than from other sources, but of course that depends on the bather load. If someone urinates in the pool, then that increases chlorine demand by a noticeable amount. In your case, however, with a small 3284 gallon EasySet pool, it won't take much bather load to generate a chlorine demand. Every person-hour in your pool would typically create a chlorine demand of around 0.3 ppm FC so having several kids in the pool for more than an hour could have you see more than 1 ppm FC chlorine demand, but some of that will have been consumed during the day with only some leftover for overnight.

 

[Bjarni]

Every person-hour in your pool would typically create a chlorine demand of around 0.3 ppm

This is good stuff. This will come in handy for being able to roughly figure demand and make proactive adjustments before a party.
Thanks