**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 n

*th*number 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 n

*th*number of time intervals]

example:

**present value**is the

**goal ppm for the night prior to leaving town**

**future value**:

**high end of FC range you want to come home**to..... 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).

the

**interest rate**in this case is

**your 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.

the

**number of time intervals**may be the

**number 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.

==================

**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.

n

*th*roots can be calculated here: http://www.calculatorsoup.com/calculators/algebra/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.