Algebraic equation for salt so I can work backwards and figure out the size of my pool.

fabsroman

Member
Apr 10, 2020
13
Winfield, Maryland
I added 120 pounds of salt to my pool and raised the salt reading by 325ppm. I am trying to work backwards from this to get a better feeling for what my pool size is. The previous owner told me that it was 37,000 gallons and I am just trying to confirm this number. I have used the Pentair calculator and worked somewhat backwards, but I get something around 40,000.

I am using a digital salt meter. Pool started out at 2,475 ppm and adding 120 pounds of salt got me to 2,800 ppm. If I try to use the Pentair salt calculator and work backwards from there, then I get something around 40,000+ gallons.

Just wondering if there is an algebraic formula for this calculation. Actually, I know there has to be one that these online calculators use, but my Google fu is failing me and I cannot find the formula. Not only that, but my algebra skills from 30+ years ago are failing me in trying to create the formula on my own under the assumption that 1 pound of salt increases salinity by 3200 ppm in 37.5 gallons of water (got this from my Google fu and cannot translate it into an equation for my needs).
 

JamesW

TFP Expert
Mar 2, 2011
20,387
Pool size comes out to about 44,000 gallons.

You can use poolmath to see the effect of adding chemicals by using the bottom section called "Effects of Adding Chemicals ".

Put the pool size in the box at the top.

 

JoyfulNoise

TFP Expert
Platinum Supporter
May 23, 2015
17,222
Tucson, AZ
Digital TDS meters are rarely that accurate and must be calibrated against known salinity standards in order to determine the calibration factor. You might be tempted to think that it’s just an offset value and so the difference between the initial and final reading is ok but that’s not the case. The linearity of the response also needs calibration. Most TDS meters cant do better than +/-200ppm and that is over a very a narrow range.

The K-1766 titration test is the most accurate way to determine salinity as it directly measures chloride ion concentration. The typical water sample size is 10mL which gives a drop sensitivity of 200ppm/drop of titrant. However, if you use a 25mL sample, that sensitivity drops to 80ppm/drop.

A better chemical method of measuring pool volume is to use TA. See here -

 

fabsroman

Member
Apr 10, 2020
13
Winfield, Maryland
Digital TDS meters are rarely that accurate and must be calibrated against known salinity standards in order to determine the calibration factor. You might be tempted to think that it’s just an offset value and so the difference between the initial and final reading is ok but that’s not the case. The linearity of the response also needs calibration. Most TDS meters cant do better than +/-200ppm and that is over a very a narrow range.

The K-1766 titration test is the most accurate way to determine salinity as it directly measures chloride ion concentration. The typical water sample size is 10mL which gives a drop sensitivity of 200ppm/drop of titrant. However, if you use a 25mL sample, that sensitivity drops to 80ppm/drop.

A better chemical method of measuring pool volume is to use TA. See here -

Thank you very much. Just bought 24 pounds of sodium bicarbonate yesterday. Thing is, I think I am going to end up using sodium carbonate (pot ash) right now because both my ph and alkalinity are low in the pool. Pretty sure I will have the "pleasure" of using the sodium bicarbonate at some point and will determine pool size at that point based upon the method you linked.

My wife actually did the algebraic formula for me and she is getting 44,000 gallons. So, my guess is that there is some error in all of this, and possibly a lot. I dumped in another 120 pounds of salt this morning and will see how much that changes the salinity of the pool. If it ends up changing it another 325 ppm, then I will be scratching my head some more. My wife also pointed out that if I am using the same digital meter each and every time, the inaccuracy should be the same between tests.

All I know is that I am getting a headache.

My gut is telling me that the previous homeowner has it right at 37,000 gallons.
 

tim5055

Mod Squad
TFP Expert
LifeTime Supporter
May 11, 2014
10,998
Franklin, NC
both my ph and alkalinity are low in the pool.
Before you go trying to raise the TA, what is it?

Our opinion and the "industry" opinion of a good TA are two very different numbers.

TA is one parameter that you usually don't have to worry about too much, but get it up to the industry levels and your pH will be rising too quickly.