Seems to be a common problem that people have to work and that not all pools or spas have an in house "pool boy" to do daily testing and add chlorine. So if anyone out there is interested, this is what I'm going to experiment with for the next few weeks....based on the calculation to determine compound interest, I think I can estimate how much to bump my chlorine to hold my spa over while I'm gone.

The basic compound interest formula is as follows:

future value = present value multiplied by [(1+interest rate) raised to the nthnumber of time intervals]

For this purpose, I juggle the equation to give this:

present value = future value divided by [(1+interest rate) raised to the nthnumber of time intervals]

example:

present valueis thegoal ppm for the night prior to leaving town

future value:high end of FC range you want to come hometo..... for instance, you might pick 4 ppm from the 2-4 ppm range associated with a CYA of 30 or 3 if you have a CYA of 20 (FC range 2-3).

theinterest ratein this case isyour pool or spa's current baseline 24 hr chlorine loss..... for some spas, that may be as much as 25% and is as good a place to start as any. Because it is a loss and not a gain, the rate is subtracted from 1.

thenumber of time intervalsmay be thenumber of days you will be out of town....for instance, from Sunday to Wednesday night, the time interval is three (3) 24 hr time periods, so that is 3.

So, if tonight is Sunday and I want to come home to a spa with a FC of 4 ppms on Wednesday, and my spa's current rate of loss is ~30% (don't ask, look up my other posts ), then the goal ppm I want to get to tonight is:

goal ppm= 4 / (1-30%) raised to the 3rd

goal ppm = 4 / (0.7x0.7x0.7)

goal ppm = 11.67 or roughly 12 ppm

So, if my current ppm is say 3 ppm, then using PM, for my 390 gallon spa, I would add 5.3 oz of 8.25% bleach to bump to ~12 ppm. Then if my 24 hr chlorine loss holds true to ~30%, when I get home Wednesday night, my FC pre-use should be about 4 ppm.

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Likewise, this formula can also be used to determine your loss rate as follows:

loss rate=1 minus the nth root of (future value divided by present value)

multiple the loss rate by 100 to give your percentile.

nthroots can be calculated here: http://www.calculatorsoup.com/calcul...ra/radical.php

example:

Sunday aka present night - 6 ppm

Thursday aka future night - 2.5 ppm

number of nights or 24 hr intervals - 4

loss rate = {1- [4th root of (2.5/6)]} x 100

loss rate = {1-[4th root of 0.417]} x 100

loss rate = {1-0.8} x 100

loss rate = 20%

Comments or suggestions welcome.