Pool math csi calc question

[ryang6]

I recently test a new pool which had the following readings:
FC 15
pH 7.2
Alk 80
Cal hard 400
Cya 300
Temp 85

Pool math is showing 0.00, levels look good but when I bring Alk up 10 it goes to -1.24. This water obviously needs work and draining but why would it show 0, looks good?

 

[proavia]

First, with FC greater than 10, your pH reading is invalid.

With a CYA of 300, maintaing adequate sanitation is almost impossible.
CYA needs to be 40 or 50 for a non-SWG pool in Florida.
Chances are you need to do a drain/refill or water exchange to lower your CYA into an acceptable range.

How are you chlorinating?

Fill out your signature (or provide this info separately if this isn't your personal pool) with pool, pool equipment (including manufacturers and model numbers) and test kit info.
Be specific and include automation if you have it.

https://www.troublefreepool.com/account/signature

 

[JamesW]

Pool Store Tests often Report Adjusted Alkalinity.​

Pool store TA tests often report lower then what you get when testing with the TF-100 or Taylor K-2006 TA tests. They subtract about .35 x CYA from the measured TA to get your adjusted TA.

When using TFPC and Pool Math you do not need to calculate adjusted alkalinity. Input the Total Alkalinity from the TF-100 or K-2006C test into Pool Math and it will make the necessary adjustments.

The adjusted alkalinity is the TA - (CYA x cf).[7]

cf is the correction factor, which is based on the pH.

pH	CF
7.0	0.22
7.1	0.24
7.2	0.26
7.3	0.28
7.4	0.30
7.5	0.32
7.6	0.33
7.7	0.34
7.8	0.35
7.9	0.36

For example, if the pH = 7.6, TA = 90 and CYA = 70, the adjusted alkalinity is 90 - (70 x 0.33) = 67.

TA - Further Reading

www.troublefreepool.com

 

[JamesW]

FC 15
pH 7.2
Alk 80
Cal hard 400
Cya 300
Temp 85

If the pH = 7.2, TA = 80 and CYA = 300, the adjusted alkalinity is 80 - (300 x 0.26) = 2.

Due to the high CYA, the carbonate alkalinity is basically 0 and that is causing the calculator to give inaccurate information.

You need to get the CYA to 80 ppm or lower.

 

[JamesW]

PoolMath shows an error with the given numbers.

 

[ryang6]

thanks for the responses. I figured it had something to do with adjusted alk. I’m surprised programmers have this scenario as perfect water vs null/invalid on the app

 

[JamesW]

I’m surprised programmers have this scenario as perfect water vs null/invalid on the app

Can you show the result that you are getting?

What app shows perfect water?

 

[ryang6]

It’s on the iOS app

 

[HermanTX]

I have found that when using Pool Math and CYA is above 250 and the CSi gets close to -2.0 then it starts to default to ZERO.
It seems not to calculate a CSI lower then -2.0. Maybe @Leebo has some insight.

Also, using the recommended test kits, it is very difficult to actually measure CYA above 200 with any accuracy. Pool stores and other electronic test kits may try to calculate high CYA values. .

With such a high CYA, the “acid” component has a huge negative effect on CIS calculations.

 

[Leebo]

It seems not to calculate a CSI lower then -2.0. Maybe @Leebo has some insight.

At this point the math is rather unreliable and not trustworthy. I’m not sure why it’s reverting to zero as a default when it’s basically throwing its hands up in error, but I’ll make a note to check for next update.

 

[JamesW]

With such a high CYA, the “acid” component has a huge negative effect on CIS calculations.

It is the base component that is affecting the CSI calculation.

The cyanurate alkalinity has to be subtracted from the total alkalinity to get the adjusted carbonate alkalinity.

If you subtract about 1/3 of the CYA from the TA, a CYA of 300 means that you are subtracting 100 from the TA, which give a bad result for TA levels of 100 or lower because the carbonate levels are 0 or lower.

 

[JamesW]

The calculators are designed to be optimized for water parameters that are in certain ranges.

Once you go outside the range, it’s pointless to try to manage the water using the calculator.

A CYA at 300 is out of range and it is pointless to try to manage the water with the CYA at 300.

It would also be pointless to try to manage the water if the pH was 3 or 12 or if the Calcium was 5,000 ppm etc.

Once your chemistry levels are that far out of range, you better really know what you are doing or just replace the water and start over.

As soon as you see a result of 300 CYA, deal with that before you try to do anything else.

 

[JamesW]

What was the condition of the pool when you first saw it?

Was the old service person fired or did they quit?

With a CYA of 300, the pool will be basically impossible to maintain.

It indicates that there was excessive trichlor tab use.

The TA was probably 0 at some point and it was probably raised once someone saw that it was 0.

You would need either a very high FC of about 23 or a lot of algaecide or something to keep algae under control.

If the TA was zero, that will keep the water clear and it would also make the FC more effective, so the water might be clear even if the FC was not kept above 20 ppm.

In any case, it is obvious that the water chemistry was a disaster and you can only speculate about what it was maintained at because the current readings only give clues about what the past numbers were.

 

[mgtfp]

I remember a similar situation where PM showed CSI 0 with CH 0. It looks like PM doesn't handle log(0) situations properly. Rather than showing an error it shows 0.

 

[JamesW]

It looks like PM doesn't handle log(0) situations properly.

I like natural logs because they are made of wood.