Chlorine and acid drum feeders

[chem geek]

The green line in those charts was just the HOCl level for the example I gave of 3 ppm FC with 30 ppm CYA. That's higher than needed to prevent green algae growth. Ben Powell came up with the initial estimates in this chart based on his experience, but this also has some leeway since it's for manually dosed pools. Roughly speaking, it's around a minimum FC that is 7.5% of the CYA level. We've found that SWG pools can have a minimum FC that is 5% of the CYA level. Much of this is described at the Pool School.

CYA should probably be used in small amounts (around 20 ppm) in indoor pools, but the industry mantra is not to do this because "CYA doesn't matter; only FC matters" and "CYA only protects chlorine from sunlight; it shouldn't be used in indoor pools". Both of those statements completely ignore the actual chemistry that is involved. It may even turn out that part of the reason for respiratory and ocular illnesses that are all reported in indoor pools may be not only from poor air circulation and lack of sunlight, but from the 20+ times greater active (hypochlorous acid) chlorine concentration that produces 20+ times the amount of nitrogen trichloride at a 20+ times faster rate. This is shown in all the breakpoint chlorination models of ammonia by chlorine -- the only open question is what really happens with urea.

Richard

[max]

I'll aim for the 5% minimum for SWG which for 50ppm CYA is 2.5ppm FC. With my current SWG settings, FC sometimes gets to 1ppm so I will have to lengthen the chlorination time a little to ensure a minimum of 2.5ppm is maintained. In doing so, there will be occasions when it exceeds 5ppm or 10% but the HOCl level won't be so bad. I'm still amazed at how much the chemistry changes with CYA.

The green line I was looking at was from the chart "HOCl vs FC at different levels of CYA". I'll keep reading!

Thanks,
Max.

[chem geek]

You might also consider raising the CYA level to 70-80 ppm and having a minimum target of 4 ppm FC. That way the change in FC will be a smaller percentage change in HOCl "active" chlorine. In fact, the recommendation for most pools with an SWG (those with a lot of exposure to sun during the day) is to have the higher CYA level since it protects the chlorine from sunlight more. This is another area where the traditional industry advice is wrong. They were probably looking at commercial pools where the bather loads are high and overwhelm the loss of chlorine from sunlight so it seems like anything above 30 ppm doesn't do any good. However, careful experiments by a forum user showed that the higher CYA levels protect chlorine in a non-linear way probably related to the CYA shielding effect of UV from lower depths.

So you might end up going from 5.5 to 4 in a day or even less such as 5 ppm FC to 4 ppm FC, but at 70-80 ppm CYA that's not very much of a change in HOCl. Remember that the FC number by itself is meaningless. The amount of active chlorine is proportional to the FC/CYA ratio.

Richard

[max]

I see what you mean. First I'll experiment with the SWG and see if I get less variance at a higher level, in percentage terms especially. Eg it is usually around 3ppm but occasionally 1ppm and 5ppm which in percentage terms are major swings. The same 2ppm variance with 5ppm average would be better because as a percentage against itself and the CYA the variance is less. It will take a number of months to assess properly - I record test results to a spreadsheet so I'll be able to make a reasonably accurate comparison.

Because it is FC I am measuring and not HOCl, I gather that the level of CYA has no bearing on the FC readings at all, so the FC range I observe at 50ppm CYA will be the same range I can expect at 70-80ppm - is that correct? (Sorry if this is covered in the long Pool Water Chemistry sticky - I'll have more time to read it properly on the weekend!)

Thanks,
Max.

[chem geek]

I'm not sure what you mean by the FC range you would observe. If you are talking about the expected amount of FC drop per day, that is a function of several different sources of losses, some of which are affected by the CYA level while others are not.

For example, the amount of FC drop from sunlight is a proportion or percentage of the FC, BUT this percentage drop is lower at higher CYA levels. Somewhere between 40 and 70 ppm CYA, this percentage drop gets low quickly such that even keeping a higher FC at a higher CYA to keep the FC/CYA ratio constant, there is less absolute FC loss (not just percentage) at the higher CYA level.

Another source of FC drop is from oxidizing ammonia/urea and organics in the pool. This is mostly a fixed FC ppm drop per day relatively independent of the FC and CYA levels.

Richard

[max]

That sounds promising. The fixed component remains fixed and the higher CYA with the same FC/CYA ratio results in less loss.

My local pool shop's test which is better than mine showed CYA to be 60ppm a month ago - there has been little change in the water since so that is probably closer to the truth. Given the cover, I think 60ppm will do for which I'll find a SWG setting which results in lows of 3ppm (5%) rather than 1ppm.

Thanks for your help Richard.

Max.

[max]

Hi Richard,

I had another read of the Chemigem liquid feeder manual. You mentioned it may use a "rough proxy such as ORP" - that indeed is what the Chemigem uses. The default setting is 650mv. The manual says "For an ORP controller to work properly the pH, Total Dissolved Solids (TDS), stabiliser (iso cyanuric acid) and calcium hardness must be maintained within the recommended levels" which are:

• pH 7.2 to 7.8
• Total Disolved Solids (TDS) 300 to 1500 ppm or 1500 mg/l
• Stabilizer 30 to 50 ppm or 50 mg/l
• Calcium Hardness 90 to 300 ppm or 300 mg/l

"Chlorine and pool Acid will feed separately for a few seconds approximately every 3 minutes until the required levels are reached".

The comments I have seen regarding ORP in other threads are not favourable. If the feedback system is not that reliable, I don't see the point in them. Have you any comments on ORP?

Thanks,
Max.

[chem geek]

ORP can be useful as a feedback control system for the chlorine level, but one critical factor is that the pH be maintained at a constant level since ORP is very much affected by pH. The other parameters, such as CH and TDS, aren't as critical. Some sensors don't do very well measuring lower ORP levels associated with high CYA and lower FC, but generally you operate at a level that is reasonable for the sensor. So you should first accurately measure your FC and CYA and then note the ORP level and set the "setpoint" for that ORP level. That is, don't try and use the absolute ORP level to determine the amount of chlorine you should have.

ORP doesn't work so well in SWG pools due to the hydrogen gas generated from the SWG that interferes with the ORP reading, but the system you describe isn't for SWG pools as it doses chlorinating liquid or bleach instead.

[Guest]

So it now time to trash my ORP meter and get a test kit that measure free chlorines.

Thank all you guys for the good knowledges you share here

[Me...]

Well first off don't go trashing your ORP meter, they are very useful tools.

Second. Don't compare a SWG with no controller to a liquid feed one with a controller. Your chemical feed pump can be set similar to your SWG. There are dials on the face of the pump that allow you to control feed rates but you can also play with the chemical strength that it feeds, you can't do that with a SWG.

Either way a controller is something I always like to see used. ORP does seem to have an issue in salt pools and is no doubt why controlling the chlorine level appears to be more problematic than a non-salt pool. Some have said it is stray current that can blind the probe and cause false readings. I have put small current collectors on the flowcell lines to see if that helps, and at times it seemed to. Or go to isolated true PPM Sensors.

[in2thegarden]

I work for a botanical center, and we have a very special set-up. We have a mini-Mississippi river, that is essentially a wading pool for kids. I have an underground reservoir that hold about 3,800 gallons. We have a pump that moves the water from the reservoir to top of the river, where it flows through grates in the delta that drain back into the reservoir. Our river flows between perennial beds, and kids often add sticks, mulch, sand, other organic things found in a garden, to our water system.

We have a Chemtrol ORP and pH reader, and use acid and liquid chlorine auto-injectors to sanitize the water. I am having a difficult time balancing a good ORP level with a FC level that is within state standards of 2-4. The mechanicals add CL to clean the water, but then FC levels are too high. What other steps can I take to help the ORP be higher without too high FC levels?

[sammantrao]

hi guys,

i wanted to know if basic garden water timer can be used to dispense chlorine. I wanted to know if the plastic of will withstand liquid chlorine coming out of it and for how long.