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Water Chemistry Explained

The pool is finally open again! Yay!

The first thing you need to do (after running your filter for 24-48 hours and adding shock and algaecide) is bring a sample of your pool water in for us to analyze to get your pool on the right track to a crystal clear pool. You’ll hear us say things like, “Your TDS is very high, you should consider draining some of your pool water.” Or, “It looks like your pool is in a Chlorine Demand, I’m going to run another test and call you with the results.” Or, “You have a lot of Combined Chlorine. We need to shock the pool to bring that down.”

It can be confusing; we know. That’s why we go to school for it every year. So what does it all mean, exactly?

Let’s go through this one piece at a time, and I’ll explain what your pool chemistry means to us, and what it means to you.

TDS (Total Dissolved Solids)

Imagine you’re in a conference room. You’re by yourself. You should be easily able to quickly get to the other side of the room, right? Now imagine there are 10 other people between you and the other end. It’s still pretty easy to get around them to the other side. Now there are 50 people in the room with you. The time it takes you to get to the other side is going to be a bit longer, and it will be more difficult to navigate through the crowd. Now imagine there are 100 people. Can you still make it to the other side quickly?

That is your TDS. When you put chemicals in your pool, after a while the residuals left behind add up. Rain water, detergents from bathing suits, fertilizer… any contaminants that enter the pool water contribute to this. Once your Total Dissolved Solids reach a certain point (about 2500 ppm above your source water) it makes it very difficult for chlorine to get where it needs to go and do it’s job, causing algae blooms and cloudy water.

Unfortunately, there really is no way to get rid of high TDS, except to drain out some of your water and replace it with fresh water. Some chemicals like Liquid Chlorine will add to your TDS much quicker because of all the fillers the liquid contains. That’s why we only recommend BioGuard chemicals during the season; the best in the industry.

Chlorine Demand

Chlorine Demand is the consistent inability to establish a FREE chlorine residual. If you are adding shock week after week and the chlorine pad on your strip is still white, you may be in a Chlorine Demand. What this basically means is that there are more contaminants in your water than your normal dose of chlorine can handle. There is no test that would allow us (or anyone in the industry) to test how many contaminants you have in your pool, and unfortunately that normally means a lot of guess work on the water testers’ part. However, BioGuard dealers (and only BioGuard dealers) DO have the equipment to accurately determine how much chlorine you will need to satisfy that demand. Sometimes that means 10 pounds of shock, and sometimes that means 100 pounds, but you can rest assured that we will help you find the best solution to your situation, and get you back on track in time for your big pool party next week.

Combined Chlorine

Combined Chlorine and Chlorine Demand are often confused, because they can both require a lot of shock to correct. They’re actually completely different. Combined Chlorine is created when a chlorine molecule bonds with a nitrogen molecule or ammonia molecule. Where does nitrogen come from? It can come from many different sources, including Lawn care products, Acid Rain, Perspiration and Urine or other types of Ammonia. When you smell chlorine but your free chlorine is within range, you’re smelling chloramines. Chloramines are very poor sanitizers and can cause swimmer discomfort. Combined chlorine is the measure of chloramines in your pool water. When the level of chloramines gets high, it can take a lot of shock to burn those off. Fortunately, just like chlorine demands, we have the equipment to accurately measure how much shock it will take to fix the problem. The first time.

Free Chlorine VS. Total Chlorine

When we talk to you about your pool water and when your looking at your analysis print out, you’ll see a reading for both Free Chlorine and Total Chlorine. Free chlorine is active chlorine available in your pool to kill bacteria and algae. Total Chlorine measures both your free chlorine and the amount of Chloramines (or combined chlorine) in the water. Ideally, your free chlorine should always be between 1.5ppm and 3ppm, and your total chlorine should be the same number, or less than 0.5ppm over your free chlorine residual. For example, If you have 3ppm of total chlorine and 1.5ppm of free chlorine, that means you have 1.5ppm of chloramines in your water that need to be oxidized. If it isn’t taken care of quickly, the number of chloramines will continue to grow, causing your little swimmers to complain of burning eyes and stinky water.

To avoid problems with your pool water before they turn your pool cloudy or green, bring a sample of your pool water (at elbow depth, away from the returns) to our convenient location in Hudson for a FREE water analysis by BioGuard Water Care Experts!


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