LSI, CSI, APPs, Watergram…

JoyfulNoise

TFP Expert
Platinum Supporter
May 23, 2015
14,850
Tucson, AZ
Thank you JoyfulNoise.

I have noticed that the APP to calculate CSI gives the same values regardless of whether I select C or F for temp. I think the C values are correct. Any idea?
Which app are you talking about specifically?
 

JoyfulNoise

TFP Expert
Platinum Supporter
May 23, 2015
14,850
Tucson, AZ
Yep. I see what you’re saying. The developer will take a look at it.

Most people setup their pools with a specific set of units (English/F or Metric/C) and then don’t bother changing the temp units on the fly. It calculates the CSI appropriately when the correct units are used. It should be a simple fix to allow for either entry method.

We will log this is the bug-fix list. Thanks for highlighting the issue.
 

CFB

Member
Jan 7, 2018
13
Victoria
JoyfulNoise,

A quick question: the alkalinity of my pool drops from around 90 to 70 over 4 weeks, over which time no noticeable amount of acid is consumed. Can this be explained by outgassing of CO2? All other parameters are very stable.

Pool chemistry:
pH 7.8
Cl 5.0
CH 250
CYA 45
ALK 90 down to 70 (uncorrected for CYA) over 4 weeks
SALT 3800

To bring the ALK to around 90, making CSI closer to 0, requires 2 kg of bicarbonate every month for my pool. Obviously would be good to work out how to avoid this requirement.

Thanks.
 

JoyfulNoise

TFP Expert
Platinum Supporter
May 23, 2015
14,850
Tucson, AZ
Outgassing of CO2 increases pH, not decreases. You’d have to outgas a huge amount of CO2 to see TA decrease that much. If you are losing TA, then something acidic is consuming it OR you are adding water to the pool that is acidic and low in TA.

What’s the pH and TA of your fill water?
 

CFB

Member
Jan 7, 2018
13
Victoria
Thanks! TA of fill water is about 20 and pH around 7.6. Can understand the low TA of fill water would be a contributing factor but evaporation and hence tap filling has been minimal, I would say only a few percent of the entire volume of water over the month. Virtually no acid consumption over the month - the acid is diluted 1:10 with water so very, very little acid if any used to neutralize the bicarbonate.
 

JoyfulNoise

TFP Expert
Platinum Supporter
May 23, 2015
14,850
Tucson, AZ
Well, a look at some of the advanced calculations for this stuff indicates that you would have indeed seen a huge rise in pH had you outgassed enough CO2 to reduce your TA by 20ppm. The math indicates that if you started at a pH of 7.4, your ending pH would be well over 8.2 for only half that amount of TA reduction. So I don’t see how outgassing could have happened without your pH going through the roof.

Do you add water through an auto fill or by hose? Have you used any acidic forms of chlorine like trichlor pucks?
 

CFB

Member
Jan 7, 2018
13
Victoria
Interesting re calculations. I have a SWG for chlorine generation - no other chemicals added. In fact I find I only need to add bicarbonate. I checked my Poolmath app logs and can confirm only bicarbonate addition of around 2 kg per month to get TA around 90 from around 70. Acid has been at a stable level and no other chemical additions apart from fill water.

Re fill water, I have an auto fill that I manually switch on when I wish to fill any water lost through evaporation or backwashing, and manually switch off. Hence I have an idea of how much is filled each time.

Again, the amount of fill water added would be only a few percent for both evaporation and backwashing.

An indication of no significant dilution is that other readings (CYA, CH) have remained stable over the past 2 months. (I realize CYA may not be a good indicator of dilution.)
 

JoyfulNoise

TFP Expert
Platinum Supporter
May 23, 2015
14,850
Tucson, AZ
Well, I would keep a close watch on it and see if happens again. Your situation is unusual as few pool owners need to intentionally add bicarbonate on a regular basis. Most fill water has enough alkalinity in it to not require it.

Do you get much rain or snow melt where you are? Sometimes people measure low alkalinity from pools that receive a lot of rain water or snow melt but that usually happens as a result of testing water on the surface of the pool before the pump has had time to adequately mix the water.
 

JoyfulNoise

TFP Expert
Platinum Supporter
May 23, 2015
14,850
Tucson, AZ
Just as an update to put some numbers to what I was saying - if you start with pool water at a pH of 7.5 and you assume your above parameters, then it takes 1.47lbs of CO2 to outgas to be an equivalent reduction in TA of 20ppm. However, when the CO2 outgasses, the equilibrium reaction consumes one mole of hydrogen ions (H+) for every mole of CO2 liberated and thus the pH will increase. Losing 20ppm equivalent TA to CO2 outgassing would raise the pH up to ~9.2.

So, unless you saw a very large increase in pH, then I doubt the TA drop was from CO2 outgassing.
 

CFB

Member
Jan 7, 2018
13
Victoria
JoyfulNoise, thanks. Will keep monitoring the TA with time as well as any acid consumption and amount of fill water.

We have had very little rain over the last 3 months. We do not get snow here, so no ice melt.
 
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