Question about the effects of low TA

[Btd001]

I've read different places that low alkalinity acts like aggressive water and can be corrosive to pool equipment.

I have a few questions about this.

1. Is this true? and What is the process and how does low alkalinity do this?
2. Is this speaking about Total Alkalinity, or Carbonate Alkalinity?
3. What are the minimum levels of TA or carbonate Alkalinity to mitigate this in a pool or spa?

 

[jblizzle]

Welcome to TFP!!!

How low of a TA are we talking about? Really the TA is a buffer to pH changes. When the TA is very low, the pH can swing wildly. When the TA is too high, it is harder to change the pH and it will tend to rise. It is really low pH that can cause the water to be "aggressive".

The TA levels we recommend are dependent on you source of chlorine.

Here is some relevant reading in Pool School:
pool-school/pool_water_chemistry
pool-school/recommended_levels

 

[linen]

Welcome to tfp, Btd001

1. Is this true? and What is the process and how does low alkalinity do this?

Low TA combined with other low parameters can cause damage, but there is no magic number to stay above for TA to protect against this. For example if you have extremely low TA (say <20 ppm) then your ph will be low as well, and it is the low ph that can do damage to metals in the system. Another case is where low TA combined with low ph and low Calcium Hardness (CH) can cause pitting on plaster surfaces. Our recommendation for TA minimum is typically 70 ppm (assuming you do not have a Saltwater Chlorine generator, and are not using Borates). See: http://www.troublefreepool.com/pool-school/recommended_levels

2. Is this speaking about Total Alkalinity, or Carbonate Alkalinity?

On tfp, we use Total Alkalinity (TA). The poolcalculator.com takes that into account when it does its calculations.

3. What are the minimum levels of TA or carbonate Alkalinity to mitigate this in a pool or spa?

There is no right answer to this question as it stands. What is probably best to do is to post a full set of test results. See this pool school article: http://www.troublefreepool.com/pool-school/read_before_you_post

 

[Btd001]

Ty for the replies. So, let me make sure I am understanding everyone correctly. Low TA can be corrosive because it leads to low PH which acts aggressively in a pool? I tend to deal with a lot of pools with high stabilizer levels, and I know that a portion of the TA comes from the stabilizer reading. So If my pool has high stabilizer levels, lets say 200 and the TA is reading 90 or so. Is this still acceptable or do I need to do the subtraction of 1/3 of the stabilizer?

 

[BoDarville]

Really the TA is a buffer to pH changes. When the TA is very low, the pH can swing wildly. When the TA is too high, it is harder to change the pH and it will tend to rise.

This is really the key point regarding TA. When it comes to managing the "aggressiveness" of the water, think of pH as the main tuning parameter and TA as the fine-tuning parameter.

Low pH is the main factor in causing corrosion. The water in a pool with an in-range TA of 80 and a below-range pH of 6.9 will be considered more aggressive than the water in a pool with a below-range TA of 50 and a pH of 7.5. While it is true that lowering TA will tend to lower pH, the effect and degree of this relationship will differ among each pool. A key factor is how much aeration the water gets via fountains, waterfalls, swimming/splashing. The guideline we recommend most often is, if your pH is within range and relatively stable, then don't mess with the TA.

 

[linen]

Low TA can be corrosive because it leads to low PH which acts aggressively in a pool?

Low ph alone can cause corrosion in metals regardless of TA, though a low TA will often aid in letting ph get low (and in general allow ph to move quickly). In the case of plaster pitting, then CH is involved too, and you really need to look at the "Calcite Saturation Index" (csi). See: http://www.troublefreepool.com/pool-school/calcium_scaling

I tend to deal with a lot of pools with high stabilizer levels and I know that a portion of the TA comes from the stabilizer reading. So If my pool has high stabilizer levels, lets say 200 and the TA is reading 90 or so. Is this still acceptable or do I need to do the subtraction of 1/3 of the stabilizer?

The poolcalculator.com takes into account the cyanurate alkalinity portion of TA when doing the csi calculation. All of the recommended levels on tfp are in terms of TA. Here is more discussion about it: http://www.troublefreepool.com/the-1-3-rule-t8508.html

I tend to deal with a lot of pools with high stabilizer levels

It sounds like you are "in the industry"? So it is somewhat understandable that you are having very high cya levels due to only being able to service the pools weekly? Much of the information on here is geared towards pool owners that can monitor and adjust their pools more often. At high cya levels, you will need to maintain a much higher Free Chlorine (FC) level. See: http://www.troublefreepool.com/pool-school/chlorine_cya_chart_shock

 

[Btd001]

TY again for the replies.

This thread was mainly me wanting to make sure that my measured TA with the high amount of cyanurate alkalinity from high stabilizer pools was fine in buffering my ph's and keeping corrosion at bay. The Ph's rarely move btw when I test them, which makes me believe a good deal of buffering is taking place.

 

[linen]

Although higher TA levels do buffer ph more, high levels also cause a steady ph rise. In your case since you are using trichlor pucks on the pools you maintain, the pucks are acidic and will counteract some of this ph rise. We typically recommend a TA of 100-120 ppm for trichlor puck users.