Why I cannot get a chlorine reading in my pool

What are the reasons for not being able to get a chlorine reading in your pool water?

The salt chlorinator appears to be working as the lights and monitor says so! What is wrong?

Is the chlorinator really working?

These are common questions that pool owners ask quite often.

Here are some questions you need to firstly ask yourself...

  • How is your pool looking, is there any algae present? 
  • Is there any brown dust in the water that when you brush the pool surface it clouds the water? 
  • What is the salt level of the water?
  • Are you running an Eco pump or is it a standard single speed pump?
  • Have you added stabiliser to the water?
  • Do you use a pool cover?

If you test your pool water and can’t get a chlorine level reading at all it may be due to a very high chlorine demand.

A high chlorine demand is the inability to keep an adequate level of chlorine in the pool water even though the water appears to be balanced and properly maintained.

That means that oxidation levels have increased to a point where chlorine is being consumed faster than it can be put into the water by either shocking or normal applications such as a salt chlorinator.

Contamination, low pH or low chlorine stabiliser levels could cause this situation.

The water might appear cloudy, the pool walls be slimy or the pool may look relatively OK.
 

What might have caused that high chlorine demand in your pool?

There are quite a few possible causes for your pool to develop an excessive chlorine demand.

You might have an infestation of algae, fungus or bacteria that can deplete normal chlorine levels and it is possible for this to occur without many visible signs.

Your pool may appear to have a dusty look on the pool bottom. If you brush it and it clouds the water, then it is most likely a Mustard Algae.

Another possibility is nitrogen contamination, which would produce a high level of chloramines, which then require far larger than normal amounts of chlorine for oxidation.

If the high chlorine demand occurs when you are opening the pool after winter, it may be that a combination of debris, contaminants and algae have developed in the pool over the closed season and that this is the cause.

In the summer months it’s possible that a high bather load, perhaps over a couple of days has caused the situation.

Biofilm is another possible cause. 

What should I do if I can’t get a chlorine reading?

Determine if your pool has a High Demand For Chlorine

The quickest way to determine if your pool is experiencing a high demand for chlorine is to perform a test for free and total chlorine.

Free chlorine shows the level of disinfecting chlorine available to sanitise your pool. Free chlorine isn’t interacting with contaminants, yet. Total chlorine is the amount of chlorine, used or not, in your water.

In the test, if your free chlorine reading matches your total chlorine reading, your pool is NOT experiencing a high demand for chlorine. This is a normal reading.

However, if your free chlorine reading is different than your total chlorine reading, then there’s a problem. You shouldn’t have a free chlorine reading of 3 and a total chlorine reading of 7.

 

Breaking Your Chlorine Lock

While there are many ways to solve this issue, we will only be covering a few of them. Please select the option you are most comfortable with.

Partially Draining Your Pool
One of the simplest methods 
to breaking chlorine demand is by partially draining your pool. The severity of the chlorine lock determines how long this method takes. Unfortunately, there’s no exact science to this. Simply drain your pool little by little, refill it, test it, and repeat if necessary.

Shock Your Pool
Another method of breaking chlorine lock is shocking your pool. Bring your chlorine levels to 20ppm or three times higher than the current levels. We recommend using a non-chlorine oxidizing shock until your free and total chlorine reads the same.

We typically see more pools with a high demand for chlorine during spring opening season. A lot of the times, pools sit for months accumulating a ton of different contaminants. This is one of the reasons we always recommend balancing your pool before you close it. You don’t want to compile pool issues or push them to the side. Although high chlorine demand is more common than you might suspect, it is something pool owners can handle themselves.

If your pool water test kit can’t give you a chlorine reading, the best thing to do is to get your pool shop to conduct a chlorine demand test.

One of the causes of a high chlorine demand is an excessive buildup of algae and phosphates. Although you’re adding chlorine to your water, bacteria or algae are overpowering the chemicals causing it not to show up on tests strips or in water kits.

Keep in mind, organic materials like algae, leaves, sunscreen, lotions, pee, poop, etc., consume chlorine. As chlorine does its job, it is depleted in the process. To prevent the demand for chlorine from happening, help remove the organic material from your pool water by brushing the algae from the pool walls, cleaning your filter, and removing leaves and debris from the water.

Other reasons for no chlorine readings...

Very little or zero stabilizer also creates a demand for chlorine. Cyanuric acid, in a sense, acts like sunscreen for the pool. If you’ve ever worn sunscreen in the hot sun, you know that you have to consistently re-apply. Our pools are the same way. If your CYA levels are really low, the sun can burn through the chlorine in your pool rather quickly.

If you have a saltwater pool, ensure your salt levels are in the correct range, typically around the 4000ppm is okay. Starting a pool after the Winter break or after heavy rain is when your pool will be down on salt.

Low salt = low chlorine production.

Multi speed Eco pumps are now very common. If you have one, ensure you are running the pump for the correct amount of hours for your size pool. Lower flow means less chlorine going into your pool when using a salt chlorinator. Your local pool shop will be able to advise the best running times for your particular setup.

On the Flip side...

In some circumstances, it’s possible that the chlorine level in your pool is so high that it is bleaching the reagent in your test kit. You may "think" that you don't have a chlorine reading, where in fact it's off the scale? So having a professional test the water is really the most sensible option and that will tell you which course of action to take.

Also, this extra high chlorine reading may have been caused by a pool cover too. Keep a check on chlorine levels in a pool using a cover. Excessive chlorine levels can make the water very corrosive and damage plastics such as pool cleaners and the cover itself.

We hope that this article has been of use to you.

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