FC usage and air/water temp

[JJ_Tex]

As we are wrapping up the dog days of summer, I've noticed an uptick in posts where people have dropped below their minimum FC level or had some algae issue and they all throw in "because it's hot". Does water temp or air temp have any bearing on your FC usage?

My assumption is FC usage is not directly impacted by late summer water temps, but there may be some indirect impacts on your FC, including:
- Your pool is likely getting more sun when it is hot outside, therefore using more FC
- Hot temps = more swimmers and their cooties that end up using FC
- If your FC drops below the minimum, the algae and bacteria can reproduce faster in the warm water and therefore eat up more FC

Are those assumptions correct, or is there a direct correlation between water temps and FC usage?

 

[mgtfp]

The highest losses due to UV I would actually expect in the northern hemisphere around June 21st, when the sun is highest and when the days are longest. But then you also need to consider cloud coverage. Not sure about the weather patterns in the US, but quite often it is towards late summer that "summer" weather is more stable with crystal blue skies without any clouds, which might overcompensate the fact for a while that days are actually starting to get shorter and the sun doesn't get as high any more (i.e the solar radiation comes down on an angle and therefore less watts per surface area on the pool).

Another factor might be that folks start preparing for winter and are not topping up their CYA losses anymore.

Algae spores might have accumulated over summer, more FC gets consumed each day to prevent a bloom.

The only direct temperature influence that I see, is FC oxidizing CYA (which is one of the main drivers for normal overnight FC losses). I think it is actually mainly OCl^- that is mainly responsible for oxidizing CYA. At higher temperatures, more chlorine is not being bound to CYA, but freely available as HOCl and OCl^- (and more OCl^- at higher pH). This helps us in general, because more HOCl at higher temperatures helps us to deal with faster algae reproduction at higher temperatures (that's why we simplify a bit and generally ignore the temperature dependency in the FC-CYA equilibrium equations). But I assume that the higher OCl^- concentration at higher temperatures will actually oxidize CYA faster. That means you will lose more FC in the CYA oxidization process itself, and (as a secondary effect) the faster decreasing CYA level will provide less UV protection. And because there is more FC not being bound to CYA at higher temperatures, it is also less protected from UV in the first place.

 

[JoyfulNoise]

All first order reaction rates are temperature sensitive and can be explained, in part, by the microscopic theory of collisions. This temperature dependence is mathematically described through an Arrhenius equation -

k_i = A ・e ^-E_a/(R・T)

k_i : First-order reaction rate constant
E_a : Activation energy of the reaction
R : the gas constant
T : reaction temperature

Roughly speaking, for every 10 degree Fahrenheit change in temperature, there is a doubling of reaction rates through changes in the rate constant. This is most easily seen in a hot tub where water temperatures are kept at a much higher temperature than a swimming pool. FC lose rates in a hot tub are quite substantial thus requiring almost daily dosing to keep chlorine in the tub water. As noted above, CYA oxidation by chlorine increases with increasing temperatures - I rarely see any change in CYA during the winter months but, as soon as the June/July/August heat turns on, I'll lose 15-20ppm CYA per month.

 

[red-beard]

The highest losses due to UV I would actually expect in the northern hemisphere around June 21st, when the sun is highest and when the days are longest. But then you also need to consider cloud coverage. Not sure about the weather patterns in the US, but quite often it is towards late summer that "summer" weather is more stable with crystal blue skies without any clouds, which might overcompensate the fact for a while that days are actually starting to get shorter and the sun doesn't get as high any more (i.e the solar radiation comes down on an angle and therefore less watts per surface area on the pool).

As a solar professional, I can tell you for a fact that is the case. In Houston, take a look at the solar radiation over the year.



You can run the same calculations. Put in your zipcode, change the solar size to 1 kW and zero degrees angle. Nothing else will matter. This takes into account the average cloud cover, length of day and solar intensity.

PVWatts

Estimates the energy production and cost of energy of grid-connected photovoltaic (PV) energy systems throughout the world. It allows homeowners, small building owners, installers and manufacturers to easily develop estimates of the performance of potential PV installations

pvwatts.nrel.gov

 

[Holydoc]

According to my graphs over the last few years, at:

• 90 to 95 water temp - SWCG is set to 35%
• 80 to 90 - 25% output
• Below 80 - 20% output

I cannot say whether the water temp is the cause of FC loss or just the indicator of more UV exposure. However it is a good indication to me if I need to adjust my SWG or not.

 

[red-beard]

According to my graphs over the last few years, at:

• 90 to 95 water temp - SWCG is set to 35%
• 80 to 90 - 25% output
• Below 80 - 20% output

I cannot say whether the water temp is the cause of FC loss or just the indicator of more UV exposure. However it is a good indication to me if I need to adjust my SWG or not.

I expect temperature alone is probably a minor contributor. All gasses (and Cl is basically a dissolved gas) have a negative solubility curve with temperature. Think of CO2 in a bottle of soda. Warm, it fizzes and foams away. Cold and the CO2 stays in. Although we are nowhere near the solubility limits at the levels we run in pools.

But you are also correct that the higher temp is mostly due to sun exposure and UV light.

 

[BowserB]

G'day our Aussie friend. You are correct about June 21/22 being the longest sun exposure, but as Redbeard's table shows there must be more to the story than date and clouds. One other factor, unless I'm wrong, is that the earth's orbit has the earth closer to the sun in the northern hemisphere winter than in summer. So while the days get "shorter" and the rays less direct, we're closer to the sun, and that makes a huge difference not just in UV radiation but also in outright heat--as demonstrated by our August electric bills--always much higher than June. This June our house consumed 1869 kwh compared with 2186 kwh this August. Last August was worse, when we used 2360 kwh and at a very high price, as U.S. wind powered generators sat still a lot of the time, and demand for electricity nearly tripled the wholesale price here in southeast Texas. Yes radiation and climate are affected by many, many factors...not just SUVs and cattle's intestinal gas in Montana!

 

[JoyfulNoise]

I expect temperature alone is probably a minor contributor. All gasses (and Cl is basically a dissolved gas) have a negative solubility curve with temperature. Think of CO2 in a bottle of soda. Warm, it fizzes and foams away. Cold and the CO2 stays in. Although we are nowhere near the solubility limits at the levels we run in pools.

But you are also correct that the higher temp is mostly due to sun exposure and UV light.

While it is true that gases have negative solubility, the chlorine in water is not chlorine gas but chlorine bound in a compound phase (either hypochlorous acid, hypochlorite anion or chlorinated cyanurate). The simplest speciation diagram for aqueous chlorine solutions shows clearly that chlorine gas concentrations are negligible above a pH of 3. So chlorine outgassing has almost nothing to do with chlorine loss.

Rate constants for first order chemical reaction, including the breakdown of hypochlorite into oxygen gas and chloride, are defined by an Arrhenius equation with temperature. This means that reaction rates will generally double with every 10 degree F increase in temperature. Loss of chlorine increases with temperature from both oxidation and sanitation reactions as well as self extinction and photolysis. Even in the absence of UV light, chlorine losses can easily be measured in the cleanest of TFP pools when water temperatures are warm. This is especially true in hot tubs that see low bather use; chlorine must be replenished daily.

 

[red-beard]

Yep, I agree, that there are other things that can account for loss when compared to temperature.

 

[red-beard]

G'day our Aussie friend. You are correct about June 21/22 being the longest sun exposure, but as Redbeard's table shows there must be more to the story than date and clouds. One other factor, unless I'm wrong, is that the earth's orbit has the earth closer to the sun in the northern hemisphere winter than in summer. So while the days get "shorter" and the rays less direct, we're closer to the sun, and that makes a huge difference not just in UV radiation but also in outright heat--as demonstrated by our August electric bills--always much higher than June. This June our house consumed 1869 kwh compared with 2186 kwh this August. Last August was worse, when we used 2360 kwh and at a very high price, as U.S. wind powered generators sat still a lot of the time, and demand for electricity nearly tripled the wholesale price here in southeast Texas. Yes radiation and climate are affected by many, many factors...not just SUVs and cattle's intestinal gas in Montana!

The issue with respect to electric bills is that it takes time for the earth to heat up and time to cool off. June & July have the longest days. But the in-coming-solar-radiation, which UV should follow, is effected by clouds. If we were in a desert, like eastern California, no clouds, no rain, the incoming solar radiation will peak in June.

 

[mgtfp]

G'day our Aussie friend. You are correct about June 21/22 being the longest sun exposure, but as Redbeard's table shows there must be more to the story than date and clouds. One other factor, unless I'm wrong, is that the earth's orbit has the earth closer to the sun in the northern hemisphere winter than in summer. So while the days get "shorter" and the rays less direct, we're closer to the sun, and that makes a huge difference not just in UV radiation but also in outright heat--as demonstrated by our August electric bills--always much higher than June. This June our house consumed 1869 kwh compared with 2186 kwh this August. Last August was worse, when we used 2360 kwh and at a very high price, as U.S. wind powered generators sat still a lot of the time, and demand for electricity nearly tripled the wholesale price here in southeast Texas. Yes radiation and climate are affected by many, many factors...not just SUVs and cattle's intestinal gas in Montana!

I was just introducing effects bit by bit. Start with angle of the sun and daylight hours, then introduce weather patterns.

It really depends on many parameters. Distance to the sun has a major influence when comparing solar radiation on Earth with solar radiation on Pluto. Within the seasons on Earth, it accounts for differences of about 7% between closest distance to the sun (in January) and the furthest (in July). But the influence of the sun-angle and length of day is much bigger, especially when you get further away from the equator. And then you have to consider weather patterns and cloud overage.

In the data for Houston that red-beard posted above, you see the highest amount of solar radiation per day in August. A bit further north, that can shift closer to June, look for example at this data for Ottawa:

Solar Radiation - Monthly data for Ottawa (Kanata - Orléans)

Chart and data: Solar Radiation - Monthly data (1 year) for Ottawa (Kanata - Orléans) for Ottawa (Kanata - Orléans), Ontario, Canada

ottawa.weatherstats.ca

Different behaviour due to different latitudes and different weather patterns. That means UV-dependant effects will be very dependant on the actual location.

Then you have to consider temperature effects (that was actually the OP's question). Important for air temperature is the whole system around you. Over summer all the water reservoirs around you will have heated up, oceans, lakes, rivers, etc. Global water currents can change over the seaons. In Australia for example, the East Australian Current (that's the one that Nemo swam in with the turtles) will hit the east coast in summer and bring warm water from the tropics. All of that will result in warmer temperatures in general, and will also help to keep the pool water warmer, it won't cool down as much over night when the air temperatures are higher.

So, temperature dependant effects will usually be highest towards the end of summer, when everything around you has been "soaked" in heat all summer. And this trend is usually less dependant on the exact latitude.