How to interpret SWG percentage in PoolMath app?

[Transient]

I was just trying out the PoolMath app and I'm not sure how to intepret the SWG settings and recommendation under FC.

I put in my current readings:

65,800 litre pool
25 CYA
2.5 FC
3.0 Target FC

I have a Hayward TCELL940-CUL, which isn't listed in the app, but according to the Hayward website, this produces 1.45 lbs/day, so I entered that manually.
I also have a variable speed pump that I run 24/7 (mostly at a lower speed), so I entered that as well.

So with all this entered, it recommends setting my SWG Cell at 5%, which sounds really, really low.
Does this actually mean I should increase it by 5%? It's currently at 50%, so maybe increasing from 50 to 55% would bump it up to 3.0 FC?

 

[mknauss]

You have a T15 cell.

You need to set the SWCG for how much chlorine your pool consumes each day. So start with 3 ppm.
If run 24 hours per day, that is a 30% generation percentage.

 

[mknauss]

What is your CYA? A 3 ppm FC target is very low. Your CYA should be at least 60. The Target FC would be 5-6 ppm.

 

[Transient]

CYA is at 25 right now, so I will add more and bring it up to 60. Maybe this is a silly question, but does CYA need to be regularly added?

If I raise CYA to 60, does that allow one to lower the SWG %? Right now it set to 50%.

And I still don't really understand the 5% recommendation in the app, but I'm probably not using it correctly.

 

[mknauss]

You were asking it how to add 0.5ppm FC. The difference between the test result and the target FC based on 25 ppm CYA.

Raise your CYA to 60 ppm. Target 5-6 ppm FC. You should test CYA every month during swim season and if you have significant water exchange with fresh water (typically rain overflow).

With the higher CYA, a % setting of 30 should be pretty good. Test your FC each day and adjust the % to be sure you are always at or above your target FC.

 

[mgtfp]

Poolmath just told you what your SWG needed theoretically be set to to generate 0.5ppm (to increase FC from 2.5 to 3.0) worth of chlorine in your pool volume, assuming that it runs for 24 hours.

This would only work if there were no chlorine losses in this 24 hour period, which is not the case. You will typically loose about 2-4ppm per day due to UV and bather load, assuming there are no organics like algae creating a higher chlorine demand.

You have to let PoolMath know about that.

 

[Transient]

Okay, thank you, that makes sense now.

I'll raise CYA as you suggested and keep a closer eye on it going forward. I appreciate all the advice!

 

[riny]

Personally I found this spreadsheet to be very helpful in getting started. I had no idea how to pick a starting runtime or percentage so I entered all of my info and estimated 2ppm demand. Like you, I run my pump on low speed 24/7, but my low speed is just below the working range of my SWG. I also pump at a higher speed for 6 hours a day for better filtration, so I figured I would run the SWG during this 6-hour window. The spreadsheet recommended an 80% setting for 6 hours so I started with that, and it's only been about 2 weeks but so far it's been dead accurate. I also like that it's not running at 100%, so I have room to adjust upwards if conditions change throughout the season, without changing my automation program.

 

[red-beard]

As a side note, the OP is in Canada, and the sun will not be quite as strong.

To the OP, I find good way to set the SWG is to test FC as soon as the sunlight is off the water in the evening. Then test FC just before sunlight hits the water in the morning. And finally, test FC again at night as soon as the sun is off the water. This is assuming you are running the VSP at a high enough speed to keep the SWG running. Yes, I had this problem and bumped my speed up 150 RPM!
The overnight test verifies how much FC you generate per hour. This also assumes you don't have anything eating the Chlorine. Subtract Morning Reading from the Night reading and divide by the number of hours between the tests. This is your FC generated per hour, call it "FCH"
The Daytime test, multiply the FCH by the number of daylight hours. This is how much FC is generated during the day.
Now, subtract FC level at the end of the day from the morning reading and add in the daytime generated amount. This is the amount you used during the day. Call it "FCU"
You need to generate over 24 hours this FC used during the daylight hours.
New SWG setting = Present SWG setting * FCU / (24 * FCH).
Example:
SWG = 25%
1st Evening reading at 7PM = 5
Morning reading at 6AM = 6
2nd Evening reading at 7PM = 4
FCH = (6-5)/11 = 1/11 = 0.0909 ppm FC/hr
FCU = (6-4)+ 13*FCH = 2+1.1818= 3.1818, call it 3.2 ppm.
The new SWG setting should be
SWG(N) = SWG(O) * 3.2/ (24*0.0909) = 25% * 3.2 / 2.1818 = 36.45% -> 36%

 

[Transient]

This is very helpful and appreciated, thank you. We get about 10-11 hours of sun per day on the pool. I don't really know the science behind it, but we hit anywhere from 10-12 on the UV index daily, which is still pretty high, I think.

I'll use that Excel sheet as a starting point and then adjust using the above formula.

 

[red-beard]

I'm in the solar industry. Here is the yearly average for incoming solar radiation for North America

 

[mgtfp]

I agree that BC will get less UV compared to Florida or Texas, especially on a yearly average. But there will still be very significant UV in summer. These yearly averages get very skewed at higher latitude locations by the large differences in sun angle and daylight hours between summer and winter. The average is a good value to estimate for example how much solar power you can generate over the course of a year, but not very helpful to estimate your maximum values in summer (which may be short).

I don't have the numbers for North America, but I do have numbers for Melbourne (moderate climate ) and Darwin (tropical) in Australia.

Melbourne:

	Summer	Winter
solar noon angle	76°	28°
daylight hours	15 hours	9.5 hours
UV-index	12	1-2

Darwin:

	Summer	Winter
solar noon angle	79°	54°
daylight hours	12.5 hours	11.2 hours
UV-index	15	8

In Melbourne, you get a short, intense summer, where UV-index and solar noon angle are nearly as high as in Darwin (some of the difference will even get compensated by the longer daylight hours). But in winter, the sun is much lower and the UV-index is next to nothing. In my case, for a period of about 2 months, I don't get any direct sunshine on the pool, at that low angle it is very effectively shaded by trees.

Since it stays warm enough to never freeze, my pool stays open all year (even though not being used in winter with water temperatures down to 8°C), and my SWG works all year round. In winter, I need to produce around 0.3ppm of chlorine with my SWG per day to maintain a constant FC, just a few hours at 25%. In summer it is - without bather load - more like 1.5-2 ppm on sunny days, but can go down towards 0.5-1 ppm when overcast.