Variations in TA & pH Readings & Difficult To Increase pH.

by Lindon
(Canada)

I have an outside in ground liner rectangular pool with typical liner sloping inward sides giving a'hopper' shaped 7'deep end. Length 30'& width 15'.

It's sand filtered, multi port valved, Laars gas heated to 28C in season & fitted with the American 'FROG'mineral cycler which is renewed every season.

I use Fi-Clor range of products & keep the cycler topped up with 2 large type multi functional stabilised tablets.

I have no problems keeping the chlorine level around 1.5 ppm & CNA at around 30ppm & never had green water.

I manually vacuum the pool twice a week. I'm surround by trees & need to regularly backwash. It stays crystal!

Unfortunately I'm useless at colour comparison testing checks so I use an AquaChek digital analyser & their 3 way True Test stips. I use their 4 way strips just to keep an eye on the CNA which I DO find easy to compare!

When I need to dose the pool to adjust TA or PH (up or down) I always carefully follow your 'pump on or off' & dosage technique recommendations regardless of what it sez on the bottle!!

I also try to keep the TA in the 80 to 100 as you suggest rather than the 80 to 120 usually given. The change I seem to have to look out for is high TA.

I take action when it goes above 130 & do it entirely in line with your recommendations.

Here's the problem:

When I(eventually)get the TA down to under a 100 the PH comes down to 7 (or lower) & I seem to have enormous difficulty in getting the PH back to above 7.3.

In fact I've on occasion added 500g of sodah ash & seen the PH actually fall from 7 to 6.8 & the TA increase at the deep end and rise from 7 to 7.7 at the shallow end & the TA fall!! that's AFTER 4 to 6 hours 'pump on' time.

Also the readings of both TA & PH are miles out & TOTALLY different between the ends of the pool EVEN after 24 hours of continuous re cycling... I still get different readings at deep & shallow ends.

A typical reading after continuous 'turn over'is:

Deep end... CL at 1.7, PH at 7.5, TA at 113. Shallow end... Cl at 1.3, PH at 7.1, TA at 098.

I have tried & over long periods of time to get things into balance but either PH goes too low at the shallow end or the TA goes to high at the deep end.

I'd be VERY greatfull if you can suggest a way out of my dilemma. I'm actually not sure if I'm worrying TOO much!!

If you even everything out it probably works out about right!!!:)I should mention that the other problem I have is knowing my volume of water... difficult to work out with a hopper shape!

I think its between 13,000 & 14,000 gallons but I very rarely supershock, the pool is only very lightly used & I give it good blast up to around 15ppm when I do.

Hope you can help.

Thanks

Lindon

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Thanks for the question Lindon

I had to read this a couple of times to really understand what you're saying because the question is unusually long.

From what I can gather, recently you've had an issue of having irregular pH and alkalinity readings, a reading in the shallow end is different than in the deep end.

This is what I can gather from the question.

My first question would be about the flow rate of the pool. Has it decreased over the last few weeks or months? This might be the reason for the different readings.

Your gallons per minute, or GPM, may have diminished over time. It could be the pump motor is going bad, or something internal with the motor like a clogged impeller or warped diffuser.

There may be a clog in the suction side of the system.

An easy way to find that is to turn off the system and close the main drain and one of the skimmer valves if you have more than one).

If you lose pressure, then that's where the suction problem is.

If you don't find it, open the closed valve and close the open valve. Same thing with the bottom drain.

You only want to have one valve open at a time to do this kind of troubleshooting.

To have the relationship between Head and GPM, you would need to know the system curve.

This can be somewhat determined by knowing the diameter and length of pipe.

The calculations can get a bit messy depending on elbows and types of outlets.

The best and easiest way that I know to check the GPM is to have a GPM meter installed right into the line.

You can't check it by the filter pressure gauge alone.

Low GPM might be the problem. This will cause the water to not circulate well, nor fast enough, for proper filtration and may give you different readings at different parts of the pool.

There are some massive calculations involved in calculating GPM such as Total Head and the total resistance to flow is measured in "feet of head".

How's the pressure on the filter? Is

it high or low?

Soda ash is used to raise the pH without much happening to the alkalinity, Our tap water is 8.0pH and 30ppm alkalinity.

This is because the city dumps tons of soda ash into the water system.

I would first take a look at the GPM and work your way from there. The readings are not too far off, but I understand you want consistency and accuracy.

Hope this helps and let me know how it turns out for you.

Robert

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Comment

By: Lindon
Date: July 12, 2011

Totally my fault for not giving you sufficient info!! I apologise.

My letter rambled on so long that I stopped writing coz I didn't want to bore you!! Your thinking that my problem is due to lack of flow rate is entirely understandable & I'm certainly going to go through the procedure you have suggested, but I don't THINK its the cause.

But that's only because the water pressure & flow out of the inlet jets SEEMS to be pretty strong. I should of told you that! The streams certainly reach & hit off the opposite wall.

I don't SUSPECT a filter problem because the pressure gauge acts quite normally... rises slightly after Iv'e backwashed a particularly dirty pool & falls back to normal afterwards.

The normal PSI hasn't changed since I first recorded what normal actually is when I installed the pool.

I live in a hard water area... lime-scale is always a problem on electrical elements... so one thing I have just done due to your comment about your tap water... is checked what mine is: TA is 138 and PH is 7.1

Not a brilliant starting point! But the pool is 4 years old since initial fill up. I've now given the pump another 24 hours continuous run without any additional chemicals OR water (from heaven OR tap!)since the readings I gave you... this is what I have now: SHALLOW end... CL = 1.8, PH = 7.7, TA = 086. Fabulous! But DEEP end? CL = 1.3, PH = 7.4, TA = 138!!! I CAN live with that if I have to, I just don't like it!

Could it be a peculiarity of an angular 'hopper'shaped deep end?

I normal circumstances I run the circulation for 4 hours in the morning & 5 hours in the evening is that ok or should the circulation period be continuous?

I keep the water level a third of the way up the skimmer & I also make sensible & judicious use of a solar cover to keep evaporation & heat loss to a minimum but obviously need to top up sometimes as normal, but being entirely surrounded by high conifer type trees I also DO need to vacuum regularly & in turn backwash the filter regularly.

I very often top up to near the top of the skimmer to provide sufficient quantity of water to give a thorough backwash... I now know I'm putting in water which is 'out of balance'.

I've got outside taps on either side of the pool & use 2 hoses hung one over each side with the pump running & inject into the circulation streams so I THINK I get a reasonable mix with the existing pool water.

Could 2/3 inches of fairly regular top up water (say once a month) make that much difference?

Seems suspicious that my kitchen tap TA readings are exactly the same as my pool deep end TA readings!

Could it be a combination of hopper shape & adding high TA water?

I AM going to check the the main drain & skimmer valves just as an exercise in positive elimination. I'll let ya know if that's it!

Thanks again Robert... the way you tell em' is brilliant, the best & easiest to understand explanations on the planet.

Kind regards, Lindon

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Comment

By: Robert
Date: July 13, 2011

Thanks for the follow up Lindon

It could be the hopper shaped pool, but I've never heard of a situation like yours because of the shape of the pool.

You want to keep the filter running continuously for 8 - 10 hrs. per day. Try to not break it up into 4 - 5 hour segments.

I did some research online and on other "pool forms" and they basically said the same thing I did.

It is perplexing that you can get 2 different readings when you have good circulation, the proper GPM, and filtration.

With the alkalinity of 86 and 138 your average is 112. This is very good. Most pool owners would love to have that figure.

The pH seems just a bit off, but if it's between 7.4 - 7.8 at both end, I wouldn't worry about it too much.

The chlorine is holding well also. That's always a good sign.

With this question, I'm at a loss if it's not improper circulation, some kind of filtration issue, or a loss of suction or return.

I can see with an extremely large pool that you may have different readings in the opposite ends of the pool, but not a normal residential pool.

Anyway, if you do find out exactly what the issue is, please come back to this post and let everyone know.

Thanks and have a great rest of the Summer

Robert