Water Exchange - Doing the Math


Active member
Aug 21, 2018
Medford, OR

I'm conducting a water exchange on my pool.

CH = 850 (! I was a bit shocked by this number, but we haven't replaced/exchanged water for 2 1/5 years, so I figure it is just a product of that)
Salt = 2000


TA = 110
CYA = 50
PH = 7.6
FC = 1
CC = 0
PWT = 67

FWT = 50
FW Salt = 0
FW CH = 75 (note: this test came up blue-ish purple at 50, full blue at 75, so I am using the higher number in my calculations)

A few questions regarding the exchange math (at the bottom):

Referring to the Draining - Further Reading page,
"Replacing water in-place process depends on several factors:[3]

  • Pool Water TDS (PWTDS)
  • Pool Water Temperature (PWT)
  • Fill Water TDS (FWTDS)
  • Fill Water Temperature (FWT)
DeltaT = (PWT - FWTDS * 0.006) - (FWT - FWTDS * 0.006)

If DeltaT is positive, then you want to fill at the lowest point and extract at the surface and at the greatest distance from the lowest point.

If DeltaT is negative, then you want to extract at the lowest point and fill at the surface and at the greatest distance from the lowest point. <-Preferred as it is easier to do with a drain pump and you can fill inside one of the skimmers.

If |DeltaT| < 5, then there will be a lot of mixing."

1) Is FWTDS simply adding FW Salt + FW CH? So in my case, FWTDS = 75?

2) I'm wondering if the order of operations is correct in this calculation because when I calculate this out as written,
(67 - 75 * .006) - (50 - 75 * .006) = 17

OR is it supposed to be:
((67 - 75) * .006) - ((50 - 75) * .006) = .102

Either way it's positive, so my exchange method is fresh in the deep end, extract from the surface.

3) Can you explain what that number 17 or .102 represents as the delta?



TFP Expert
LifeTime Supporter
May 3, 2007
Pleasanton, CA
Pool Size
Salt Water Generator
Is FWTDS simply adding FW Salt + FW CH?
It depends on how you measure salt. The electronic methods will include CH, a drop kit will not.

The first is correct: (67 - 75 * .006) - (50 - 75 * .006) = 17

0.006 is a conversion factor of TDS ppm to degrees F so it involved only TDS. This is a rough approximation but good enough for this type of calculation.

17 is the equivalent temperature difference in Fahrenheit between the pool water and fill water at an equivalent TDS level. So a value of 17 indicates the pool water will "float" above fill water.


Active member
Aug 21, 2018
Medford, OR
Thank you for the explaination! I appreciate you unpacking that DeltaT number for me -- the calculation makes a lot more sense now.

Side note: I use a drop test for Salt and CH separately.
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