Regardless of what anyone tells you, if you live in North America your pool should not be losing more than about an eighth of an inch of water (3 mm) each day in the summer. If it is losing more that that…don’t ignore it and don’t let people tell you that the larger amount of water loss is to be expected. An inch of pool water is a huge amount and the western world is just beginning to understand that water is our most precious resource. Don’t waste it like I did!
My family has had an in-ground swimming pool since the kids were little. Generally, we have only had to fill up the pool in the early spring and we are more-or-less good for the rest of the season. There is a bit of evaporation, but it is usually replaced by rainfall. This year, unfortunately, was “off the charts” as far as water consumption goes and that is an environmental disaster.Our household water consumption over time
Our little part of the world experienced a class 2 drought this July and August complete with dead lawns and local brush fires. So, when my pool needed frequent top-ups, I put it down to the dryer than normal weather and the kids splashing when cooling off. I was surprised by the amount of water that the pool needed, about an inch and a half (3.6cm per day), but the pool store clerk told me that lots of people were telling her that they were using a lot more water than normal this year, so I didn’t stress about it too much.
Well, the lack of stress lasted for a few weeks, but then the weather turned cooler and the pool continued to soak up the same amount of water each day. This lead me to wonder in-earnest what was going on. I started to look around the internet to find out how much evaporation is normal on a daily basis and I found that there didn’t seem to be a very good understanding of what to expect. I read blogs by people who swore that an inch or more of water loss per day was just fine, and other sites that stated that an eighth to a quarter of an inch a day was closer to the norm. After a review of the internet resources, I finally had a diver come in to check my pool for leaks. What I found out is the subject of this post.
Factors that affect evaporation.
From what I have read, there are a number of factors that affect the rate of evaporation from a swimming pool. For the most part, I have relied on sites like ask.com and Wikipedia, but I have also read a number of papers from various universities on the subject and it seems to me that the four factors that are the most important are heat, pressure, surface area, nature of the liquid (in our case salt or chlorinated pool water).
Heat effects include the ambient air temperature, the use of pool heaters (we have a solar hot water heater), the amount of sunlight falling on the pool, the colour of the pool liner, etc. Generally, the hotter the system is, the higher the level of evaporation and the summer was really hot and the air was very dry.
The pressure effect is the net effect of evaporation and condensation (two opposing processes). Generally, higher pressure means lower levels of evaporation (the boiling point of a liquid is lower at higher altitudes due to the lower ambient air pressure). Pressure would not be a significant problem in Ottawa.
Surface area is directly related to evaporation. Generally, the higher the surface area of the liquid, the higher the rate of evaporation. Surface area is affected by pool size, the amount of splashing that goes on (increases the effective surface area), but it is also affected by the amount of wind that passes over the pool. More wind effectively increases the surface area of the pool because it moves water vapour that arises from the pool away from the pool and replaces it with drier air. We did have a lot of wind, so that may have affected our evaporation rate.
The nature of the liquid also affects the evaporation rate, with the presence of large heavy molecules/atoms reducing the rate and the absence of those large heavy molecules/atoms increasing the rate. This can have an effect if you are getting lots of rain which will dilute the chemicals in the water, but I am largely going to ignore it in this post.
Diagnosing the problem with my pool
As I mentioned earlier, once the temperatures moderated and my pool kept losing so much water, I had to find out whether I really had a problem or not. The internet research produced contradictory results so it seemed to me that the best bet was to measure the evaporation rate myself.
A good method for finding out what the base evaporation rate is
To do this, I took the advice of a number of pool professionals , including the diver that eventually fixed my problem. In my case, I took a clear plastic bucket, weighed it down and placed it on the stairs of my pool. Then I filled it water to the exact level of the water in the pool – using water from the pool. This meant that the pool water and the water in the bucket had the same composition, and were subject to the same pressure, and temperature. I waited a day and measured the drop in water level in the bucket relative to the water in the pool. I found that practically none of the water in the bucket had evaporated while the pool lost another inch and a half during the same period – a clear sign that the pool had a leak.
Isolating the problem
Now I had confirmation that the pool was leaking, but from where? The possibilities included leaks in the pumphouse and lines, leaks in the skimmer or lights, and leaks from the liner. To narrow down the problem, I isolated the pool from the lines and pumphouse by inserting the plug into the exhaust line in the skimmer and I installed the winter gaskets over the input line at the base of the skimmer. I marked a water level skimmer face-plate and waited for yet another day. With the pool totally isolated from the lines, the pool lost the same inch and a half of water in a 24 hour period. This was a good indication that the leak was not likely to be in the lines or the pumphouse…what a relief!
Time to bring in the diver
Now that I knew the leak was in the pool, the skimmer or the light pots, it was time to call in the diver. My pool company recommended a diver with 25 years of experience doing this type of diagnostic and I am glad I did. When he arrived and heard my story, he looked at the pool setup and told me right away that it was likely that the leak was either in the skimmer or behind the underwater lights where the conduit connects to the pot for the lights. After turning off the pump and letting the water calm down, he used a flourescent dye in a syringe to look for currents in the pool around each light, the skimmer and the stairs. He immediately found a fast leak from one of the two pot lights. To rule out a second leak, he also did a thorough in-pool visual inspection of the entire liner, looking for scratches or holes.
Finding no obvious holes in the liner, he removed each light from its pot and re-checked the light-pots with the flourescent dye. As he had suspected, the gasket that goes around the incoming 12 volt electrical line and that makes the seal with the light’s in-pool pot had given up the ghost.
Once the leak was found, the diver used an underwater epoxy to reinforce the gasket and to make a permanent seal between the cable/conduit and the interior of the pot. Even though the second pot was not leaking, we took the opportunity to epoxy that pot too. The diver explained that while the pool liner might last 20 or 25 years, the gaskets in these pots seems to give up the ghost after about 5 or 6 years, so while only one of them was leaking now, the other would likely begin to leak shortly so the epoxy on the second light was preventative rather than curative.
I had to wait for 48 hours for the epoxy to cure (if you don’t, you risk breaking the seal and you are back in the same boat you started in) and during that time watched the water level intensely. During that two-day period the pool level only went down by about 3/16ths of an inch – which was in line with my prior expectations for evaporation. It has been about 3 weeks now since I had the pool repaired, and the water level has risen continuously (we have had a rainy September). So it looks like the problem is solved …. Yeah!!
The results – what was the cost of the leak?
Aside from the $250 that I paid to the diver, I spent roughly $300 I spent replacing chemicals as the water leaked which isn’t too bad all things considered. The real cost of the leak, however, was the cost to the environment.
Over the course of the summer my pool leaked between 62 cubic metres and 360 cubic metres (or between 64 and 400 tons) of water and all of the associated pool chemicals. The chart presented above highlights very clearly the water costs associated with this leak. Had I found the leak earlier in the summer I could have saved literally tons of water. As my pool is a salt-water pool, the chemicals include salt (NaCl), Hydrocholric acid (HCl or pH down), stabilizer, Sodium Bicharbonate (AlkaPlus – NaHC03), and others. All of these chemicals ended up in the soil around my pool). While the economic cost of this leak may have been small, the environmental cost was huge.
What is the lesson to be learned?
The first lesson to be learned is not to be complacent. Your pool should not be losing more than about an eighth of an inch of water (3 mm) each day in cities like Ottawa. If it is losing more that that…don’t ignore it!
The second lesson is not to be put off by nay-sayers. Don’t let other people tell you that a larger amount of water loss is to be expected given the weather conditions. While you may experience a bit more evaporation in hot weather, the total evaporation in our climate zone won’t amount to more than about a quarter of an inch per day…and certainly not an inch or an inch-and-a-half per day. An inch of pool water is a huge amount and we in the west are only now beginning to understand that water is our most precious resource!
Finally, at the first sign of a leak, do the evaporation test I mention above and then isolate the problem (is it in the pool or in the lines and equipment). It is free and it will tell you definitively whether you have a leak or not. Stop leaks early, and enjoy your pool with an easy conscience!