Taylor Watergram SI vs TFP Pool Calculator CSI.

[lalittle]

I've noticed a significant difference between the SI (Saturation Index) given by the Taylor Watergram wheel and the CSI (Calcite Saturation Index) given by the TFP Pool Calculator (or the "poolcalculator.com" calculator.) These are supposed to give you the same indication of how "balanced" your water is, so it's troubling that they're far enough apart to indicate both "balanaced" and "unbalanced" water depending on which one I look at.

For example, today my readings are:

FC - 9
pH - 7.6
TA - 70.2 (corrected for 60ppm CYA at pH 7.6)
CH - 825
CYA - 60
Salt - 1800
Borate - 0
Temp - 90

On the Taylor Watergram wheel, this gives me an SI of about +0.54, which is outside of the "balanced" range of -0.5 to +0.5. The Pool Calculator, however, gives me a CSI of +0.26, which is WELL within the "balanced" range of -0.6 to +0.6. My instinct would be to trust the Pool Calculator's CSI number because is utilizes more parameters, but I can't help questioning the numbers given that they don't agree by a significant margin. Note also that this margin can be even larger under the right conditions.

Do I just disregard the Taylor Watergram wheel?

Thanks,

Larry

PS. As of this writing, the "poolcalculator.com" site has stopped working. The issue started with none of the "About..." links on the site working, then the whole site stopped. I just keep getting a message that says "This webpage is not available.

 

[JasonLion]

In both PoolMath and generally anywhere on this site, you should always report the TA result directly from the test kit. Never do the adjustment unless calculating the LSI or using the Taylor Watergram.

Because you used adjusted TA instead of TA directly from the test kit, your PoolMath result is incorrect. PoolMath actually reports a CSI around 0.41 (depending on the exact original TA). That greatly lowers the difference you are seeing. A difference of 0.1 is fairly common, so a difference of 0.13 is not really unusual.

 

[chem geek]

How in the world are you getting an SI of +5.4 from the Taylor Watergram and +2.6 from The Pool Calculator? When using the Taylor Watergram, if I align the Calcium Hardness of 825 against the adjusted Total Alkalinity (adjusted) of 70 and then read across from the temp of 90ºF, I get a "pH of Saturation" of 7.05 so the saturation index is 7.6 - 7.05 = +0.55 so I presume your "+5.4" was really "+0.54".

If I put into PoolMath a pH of 7.6, TA of 90 (since PoolMath accounts for CYA when you enter that), CH of 825, CYA of 60, Salt of 1800, and Temp of 90ºF, it gives +0.41. However, note that if you put in 0 for Salt (which calculates a salt implied by the TA and CH of roughly 1000) one gets +0.47. The Taylor Watergram does not account for TDS (salt) so the CSI is within 0.1 of the Taylor Watergram. You should just use PoolMath since it automatically adjusts TA for the CYA and accounts for TDS (salt). As Jason noted, you do not put the adjusted TA into PoolMath (nor the older The Pool Calculator) since we do the CYA adjustment automatically.

 

[lalittle]

In both PoolMath and generally anywhere on this site, you should always report the TA result directly from the test kit. Never do the adjustment unless calculating the LSI or using the Taylor Watergram.

Because you used adjusted TA instead of TA directly from the test kit, your PoolMath result is incorrect. PoolMath actually reports a CSI around 0.41 (depending on the exact original TA). That greatly lowers the difference you are seeing. A difference of 0.1 is fairly common, so a difference of 0.13 is not really unusual.

Ah -- I didn't understand that the calculator took the CYA into account when calculating TA and that the actual TA as measured should be used. As you noted, the new difference of only 0.13 is not nearly as significant. Thanks for clearing that up.

How in the world are you getting an SI of +5.4 from the Taylor Watergram and +2.6 from The Pool Calculator? When using the Taylor Watergram, if I align the Calcium Hardness of 825 against the adjusted Total Alkalinity (adjusted) of 70 and then read across from the temp of 90ºF, I get a "pH of Saturation" of 7.05 so the saturation index is 7.6 - 7.05 = +0.55 so I presume your "+5.4" was really "+0.54".

You got it -- I accidentally wrote "+5.4" instead of "+0.54" and "+2.6" instead of "+0.26." Thanks for catching that. I edited my original post and corrected this for both calculations.

As noted above, I now realize that the pool calculator needs the NON-ADJUSTED TA value as read directly by the test, which brings the results much closer together.

note that if you put in 0 for Salt (which calculates a salt implied by the TA and CH of roughly 1000) one gets +0.47

I'm not sure what you're saying here. I actually measured my salt level using the Taylor K-1766 kit and got 1800. Shouldn't I use this value?

Thanks,

Larry

 

[chem geek]

I'm not sure what you're saying here. I actually measured my salt level using the Taylor K-1766 kit and got 1800. Shouldn't I use this value?

So you measured your salt level, but where on the Taylor Watergram are you able to account for this salt level? It doesn't account for TDS (salt), but PoolMath does. If you do an apples-to-apples comparison putting in 0 for salt in PoolMath (which will be the same as using the minimum implied with the CH and TA if added separately as chemicals) you get +0.47 instead of +0.41 so it's only 0.07 different, not 0.13 different.

 

[Richard320]

A man with two watches never knows exactly what time it is...

 

[lalittle]

So you measured your salt level, but where on the Taylor Watergram are you able to account for this salt level? It doesn't account for TDS (salt), but PoolMath does. If you do an apples-to-apples comparison putting in 0 for salt in PoolMath (which will be the same as using the minimum implied with the CH and TA if added separately as chemicals) you get +0.47 instead of +0.41 so it's only 0.07 different, not 0.13 different.

Ok -- I see what you were saying now.

Thanks for clarifying.

Larry

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A man with two watches never knows exactly what time it is...

But he can estimate better than a man with NO watches.