App Issues with "Effects of Adding"

[sbcpool]

If I use the PoolMath App to calculate the effects of adding bleach, weird stuff starts happening. Here is a table summarizing the results I see for a 20,000 gallon pool:

Bleach Concentrations	Amount Added	Raise FC by ...
2%	64 oz	1.4
3%	64 oz	1.4
4%	64 oz	1.4
5%	64 oz	1.4
6%	64 oz	1.5
7%	64 oz	2.1
8%	64 oz	2.1
9%	64 oz	2.5
10%	64 oz	2.5

The correct answers should be:

Bleach Concentrations	Amount Added	Raise FC by ...
2%	64 oz	0.5
3%	64 oz	0.75
4%	64 oz	1.0
5%	64 oz	1.25
6%	64 oz	1.5
7%	64 oz	1.75
8%	64 oz	2.0
9%	64 oz	2.25
10%	64 oz	2.5

Does anyone else see this? Any ideas what's happening here?

 

[CTFocusST]

I get same result for 2-4% showing 1.4ppm. I even get 1.4ppm with a concentration of 0.0

 

[mknauss]

@Leebo @redth

 

[sbcpool]

The math should be (using US measurements):

(a x 29574 x b) / (c x 3.7854) = d

a = Amount of bleach added in fluid ounces
b = Bleach concentration
c = Pool size in gallons
d = Increase in FC

Bleach concentrations are usually weight to volume (w/v), so 10% bleach is 100 grams per liter.

 

[JoyfulNoise]

The math should be (using US measurements):

(a x 29574 x b) / (c x 3.7854) = d

a = Amount of bleach added in fluid ounces
b = Bleach concentration
c = Pool size in gallons
d = Increase in FC

Bleach concentrations are usually weight to volume (w/v), so 10% bleach is 100 grams per liter.

Not quite right. You need to modify your formula based on trade percent. Most commercial bleach (like pool chlorinating liquid) is sold as Trade Percent (which is the volume % of available chlorine). Laundry bleaches, typically 8.25% and lower, are sold as weight %.

I believe what is done in the App is the bleach strength is first converted to % Av. Cl2 and then the ppm change is calculated from that. Any input for 10% LC or stronger is assumed to be Trade % while anything below that is Wt. %. You’ll need to know the specific gravity of sodium hypochlorite solutions to make the conversion.

 

[magiteck]

I’m just guessing here, but I’ve noticed this as well and assumed that the app was just rounding up to the nearest commonly found concentration.

For example, 2/3/4/5 in the above table are all giving the result for common 5.25% bleach.

6 is giving actual 6%.

7/8 are giving the result for 8.25% bleach.

and 9/10 are giving the result for 10%.

Not sure of the reason this would be the case, but if the actual math is as complex as JoyfulNoise states it may have been programmed just for known concentrations.

Hopefully someone with knowledge of the code can clarify!

 

[sbcpool]

I believe what is done in the App is the bleach strength is first converted to % Av. Cl2 and then the ppm change is calculated from that. Any input for 10% LC or stronger is assumed to be Trade % while anything below that is Wt. %. You’ll need to know the specific gravity of sodium hypochlorite solutions to make the conversion.

So 8.25% 'bleach' is a 1.1M NaClO solution, but a hypothetical 8.25% pool trade chlorine would be almost perfectly a 1M solution? Interesting.

 

[JoyfulNoise]

So 8.25% 'bleach' is a 1.1M NaClO solution, but a hypothetical 8.25% pool trade chlorine would be almost perfectly a 1M solution? Interesting.

Pool chlorinating liquid is almost always sold as 10% and 12.5% (sold as "Trade %"). Laundry bleaches nowadays are mostly 6% and you very rarely find 8.25% anymore (these are sold as "weight %"). "Disinfecting" or "cleaning" bleach is often sold around 3% concentration but packaging labels can vary widely and some of these types of products aren't required to list the actual percentage.

It is almost never the case that you will find "bleaches" (aka, sodium hypochlorite solutions) in concentrations other than those. The reason being is that the manufacturing process is simply not that precise and process-point controls almost never use hypochlorite concentration but rely on proxy detection methods like specific gravity to determine when an endpoint is reached. Offline statistical process control may use lab detection methods for measuring hypochlorite concentrations (starch-iodine method) but that would only inform the manufacturing process when equipment or chemical levels need to be tweaked. Hypochlorite solutions are not stable and degrade over time due to metal contamination and inherent chemical reactions. Therefore the manufacturing process can not be easily controlled by using the % hypochlorite concentration.

For example, it is very often the case that 12.5% commercial bleach (used by laundromats and commercial cleaning companies) is manufactured and packaged at 15% concentration but is assumed that it will degrade down to 12.5% by the time the end-point supplier sells it or the end-user uses it.

If hypochlorite strength were really important to an individual, then Taylor does sell a version of a chlorine testing kit that is designed for commercial bleach measurements. It gives fairly precise results of the wt % hypochlorite to a precision of 0.5% (maybe even better, I haven't looked in a while).

 

[sbcpool]

It is almost never the case that you will find "bleaches" (aka, sodium hypochlorite solutions) in concentrations other than those.

For example, it is very often the case that 12.5% commercial bleach (used by laundromats and commercial cleaning companies) is manufactured at 15% concentration but is assumed that it will degrade down to 12.5% by the time the end-point supplier sells it or the end-user uses it.

The intermediate concentrations only matter when trying to estimate liquid chlorine requirements over time. I may start out with 12.5% bleach, but whether stored in a Stenner tank or just waiting in my garage to be used, it's not going to be 12.5% by the time I get to the last of it.

For example, if I buy roughly a 30-day supply of 12.5%, in my area it's going to be somewhere around 9% by the end of that month - a 33% difference in dosing requirements. If I prefer convenience over efficiency I can dose based on the expected end concentration, but that requires calculating based on 'odd' concentrations.

 

[JoyfulNoise]

The intermediate concentrations only matter when trying to estimate liquid chlorine requirements over time. I may start out with 12.5% bleach, but whether stored in a Stenner tank or just waiting in my garage to be used, it's not going to be 12.5% by the time I get to the last of it.

For example, if I buy roughly a 30-day supply of 12.5%, in my area it's going to be somewhere around 9% by the end of that month - a 33% difference in dosing requirements. If I prefer convenience over efficiency I can dose based on the expected end concentration, but that requires calculating based on 'odd' concentrations.

I get what you are trying to do, and it certainly is more "accurate and precise" to think of it that way, but most people simply don't try to base dosages like that. Most will simply assume a set concentration, set a dosing rate based on that concentration, and then let their daily/weekly testing inform their pool care. If the FC seems a bit low, then they'll just increase their dosing time, if it seems a little high, then lower it for a few days. Given all of the variables that a pool is exposed to which can affect FC consumption and the fact that chlorinating liquid has a somewhat variable concentration, it's not really worthwhile to try to over-engineer the problem. There will be a natural, cyclical variation of FC throughout the day, the week, and the months, and it's simply less complicated to let testing dictate dosing. Does that require a pool owner to be more involved in their pool? Yep, but that's kind of what is taught here...less "set it & forget it" and more "you can't control what you don't measure".

 

[sbcpool]

I get it. I just know how reality is compared to theory. In theory I'd test my pool every couple of days, make the kids clean their rooms and do their homework, get all the 'honey-do's' done, and return those extra bolts back to Home Depot. The practice is that more often than not unplanned life events get in the way and before I know it it's been a week since I've tested.

I'd rather use double the chlorine than is necessary than to test my pool every day or look out the window a week later and see mustard algae and have to do the whole SLAM thing.

 

[jseyfert3]

Interesting. That said, I never use the Effects of Adding for calculating my chlorine additions. Doing so is guess and check, and pointless when it has a section for calculating the special amount of chlorine to add. And this seems to work as expected for any concentration of bleach.

For example, to raise 2.5 ppm with 6% requires 104 oz.


If I change to 3% it's 214 oz, not quite but very very close to 2x as much, and 12% calculates out to 64 oz.

If you type these values into effects of adding, only the 6% calculates out to 2.5 ppm. The 12% numbers work out to 2.1, and the 3% say 4.5 ppm. So it appears that the Chlorine addition section works linearly with %, but not the effects of adding.

Annoyingly, I've sometimes noticed a bug in the chlorine additional section, where it will sometimes give an incorrect value for the amount to add. Usually I spot this when using in real life, and it's rare. It happened a LOT more trying to write this post with these demo numbers, and I tried, and failed, to tack down a specific, repeatable sequence of events that caused it, so it could get fixed. When it happened instead of telling me 104 oz of 6% I'd get something like 124 oz, but if I would change a value and change it back it would then say 104 oz again. I've seen this enough there's an actual bug, but until I (or someone) can nail down the exact sequence it'll be tough to fix.

 

[HermanTX]

I noticed similar output on Effects of Adding between 10%, 10.5% and 12.5% bleach.
Got a question for @JoyfulNoise I recently purchased bleach that they state is replenished weekly. The label states 10.5% sodium hypochlorite and 89.5% other ingredients. Then it states Provides 10% available chlorine.
Know there is not much difference between on 0.5% but other bottles I purchase from different stores only list the % NaOCl and nothing about % available chlorine. Can you explain? is this Trade % vs. weight %?

 

[JoyfulNoise]

Treat it as 10% no matter what their label says ... it’s kind of a gimmick for them to advertise it that way as I’m sure it’s delivered to them as 10% +/- 0.5%.

Available chlorine is just a way of stating the percentage of the chlorine in solution as “chlorine gas” (Cl2) which is what your FC is measured as by the Taylor kit. So if the bleach you are using is 6% by weight sodium hypochlorite (NaOCl), the it is 5.7% available chlorine (Cl2) by weight. One can convert weight percent to trade percent using the specific gravity of the solution which then allows you to calculate ppm’s FC by straightforward volume dilution.

You can also calculate the “available chlorine” of trichlor pucks or cal hypo and then you’ll have a direct way of comparing solid chlorine sources to liquid chlorine sources. Typically solid chlorine sources have much higher available chlorine content (50% or more as compared to liquids which rarely have more than 12%).

 

[HermanTX]

Treat it as 10% no matter what their label says ... it’s kind of a gimmick for them to advertise it that way as I’m sure it’s delivered to them as 10% +/- 0.5%.

Available chlorine is just a way of stating the percentage of the chlorine in solution as “chlorine gas” (Cl2) which is what your FC is measured as by the Taylor kit. So if the bleach you are using is 6% by weight sodium hypochlorite (NaOCl), the it is 5.7% available chlorine (Cl2) by weight. One can convert weight percent to trade percent using the specific gravity of the solution which then allows you to calculate ppm’s FC by straightforward volume dilution.

You can also calculate the “available chlorine” of trichlor pucks or cal hypo and then you’ll have a direct way of comparing solid chlorine sources to liquid chlorine sources. Typically solid chlorine sources have much higher available chlorine content (50% or more as compared to liquids which rarely have more than 12%).

thanks very much. Very helpful.