CSI and TA and Temperature

[JamesW]

Split off of this topic. JasonLion

Based on your numbers, to get a CSI of +0.12 your water temperature would need to be 58 F, is that correct?

One of the things you need to think about is how things will change as the water begins to warm up. If you use the same numbers at 88 F, then you get a CSI of 0.39. Here are some things that happen as the water warms up.

1) Carbon dioxide off gasses faster at higher water temperatures.
2) Warmer water has a higher CSI than cooler water for the same chemistry.
3) Warmer water means higher agitation and corresponding carbon dioxide loss and pH rise from increased usage

Therefore, while you are not having any trouble with a TA of 130 now, it will become more of an issue as the water temperature increases. Adding acid to keep the pH in the 7.5 to 7.6 range will cause the TA to steadily decline. This will help prevent problems as the water warms up. I think that a target of about 70 ppm for a water temperature of 88 F would do well.

Suggestion

Temp................TA
58...................130
68...................100
78.....................80
88.....................70

For adding borates, I recommend using boric acid rather than Borax and acid (Borax + acid = boric acid). For your 18,000 gallon pool, it will take 43 pounds of boric acid.

http://www.dudadiesel.com/choose_item.php?id=boric55

 

[JasonLion]

Re: First TFT-100 test results

Temp................TA
58...................130
68...................100
78.....................80
88.....................70

Most people shouldn't be thinking about CSI at all in the first place, let alone trying to keep it that tightly controlled. In general, trying to micro manage levels leads to excess chemical usage and really doesn't provide any benefit. And if you were trying to regulate CSI, doing so by changing TA is completely impractical.

 

[chem geek]

Re: First TFT-100 test results

(Jason, feel free to move to The Deep End, though I kept this mostly non-technical).

Agreed. The saturation index is something to consider as a long-term target and one has a reasonable amount of leeway since scaling is not normally seen in pools until the index gets pretty high -- usually +0.7 or higher unless one has a specific localized situation such as a saltwater chlorine generator with one of its plates at high pH or a gas heater at lower flow rate with much higher temperature. On the other side, well-made plaster in water with a negative saturation index doesn't break down very fast unless there are other factors such as unusually low pH. Remember that the saturation index says whether something CAN occur (i.e. scaling or dissolving of plaster), not how quickly it will occur. So having the index in the +/- 0.3 range is certainly very reasonable.

The main reason to target something closer to zero than just +/- 0.6 that The Pool Calculator refers to as suggestive limits is that there are all sorts of measurement errors that can occur so being closer to zero gives you more slack in case one or more of the measurements is off (especially pH, since that's the most influential on the saturation index).

As for the saturation index varying with water temperature, this is most certainly not something to worry about because the pH naturally varies with temperature such that it tends to keep the saturation index from varying very much. As colder water warms up, the pH drops and the saturation index only goes up a small amount. Basically, I wouldn't worry about adjusting for the saturation index until the water gets up to the temperature when one opens their pool at which point one can adjust the water chemistry parameters as needed. When the water is colder over the winter, all chemical reactions slow down so much that having the water chemistry somewhat out of balance isn't a big deal. In my own pool, I use winter rains to dilute the water, but I don't bother to adjust the lost CH and TA until I am ready to use the pool in the spring.

 

[JamesW]

Re: First TFT-100 test results

My idea was that as the water warmed up, he would begin to have problems with the TA being at 130. I was just trying to give some idea of TA targets I thought would work best for expected water temperatures.

For example, if the water got to 88 ^oF, it would cause more pH rise. If the pH got to 7.9, that would put the CSI at 0.69, which could end up causing scale.

The basic premise is that as the water temperature increased, he should decrease his TA accordingly.

As colder water warms up, the pH drops and the saturation index only goes up a small amount.

As the water warms up, carbon dioxide will begin to off gas at a faster rate due to the lower solubility of carbon dioxide in warmer water. Carbon dioxide will also be lost at an increased rate due to usage (kids splashing, using the waterfall etc.). Increased usage of liquid chlorine will also cause pH rise. Wouldn't the overall net effect be a rising pH?

I wouldn't worry about adjusting for the saturation index until the water gets up to the temperature when one opens their pool at which point one can adjust the water chemistry parameters as needed.

Essentially, what I was trying to say. I wasn't saying to lower the TA immediately, but to begin to lower it gradually as the temperature increased.

 

[chem geek]

Re: First TFT-100 test results

As the water warms up, carbon dioxide will begin to off gas at a faster rate due to the lower solubility of carbon dioxide in warmer water. Carbon dioxide will also be lost at an increased rate due to usage (kids splashing, using the waterfall etc.). Increased usage of liquid chlorine will also cause pH rise. Wouldn't the overall net effect be a rising pH?

What you wrote is only true when the TA is higher. When the TA is lower, then the outgasing is so slow that the higher temperature of the water doesn't matter much. The equilibrium of water with the components that determine pH varies with temperature and that's what I was referring to. Anyway, I wouldn't worry about the temperature part so much and instead just focus on whether the pH tends to rise and if it does then one can lower the TA to slow that down.