ORP vs. Free Chlorine: Which is more important?

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Pool sanitization is essential

Swimming pools (especially heavily used public pools) can have serious demands put on them. Keeping water sanitized is paramount to people’s health and safety, but it is not always easy to do. Disease outbreaks like cryptosporidium can happen, and people sometimes get sick. Thankfully, chlorine is an excellent sanitizer. And chlorine is easy to measure in water. We can test free chlorine, total chlorine, and from those two, we can calculate combined chlorine. But how do we know if the chlorine we have is effective in our water?

We measure chlorine’s effectiveness in real time using the oxidation reduction potential (ORP).

What is ORP?

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ORP and pH probes

ORP stands for oxidation-reduction potential. ORP is a measure, in millivolts (mV), of a chemical substance’s ability to oxidize or reduce another chemical substance. In the pool industry, chlorine is usually the primary oxidizer, and contaminants like ammonia non-living organics are the target of oxidation.

ORP is measured by a probe in a small sample of flowing water, usually next to your chemical controller. An ORP sensor consists of an ORP electrode, and a reference electrode. Basically a signal is sent between them which determines your oxidation and reduction potential. So let’s quickly discuss what oxidation and reduction mean, because “oxidation-reduction” are not the same thing; they are opposites. We get the following two definitions from this source.

What is Oxidation?

Oxidation is the loss of electrons by an atom, molecule, or ion.” Often, the lost electrons are replaced by oxygen.

What is Reduction?

Reduction is the net gain of electrons by an atom, molecule, or ion.”

This means that electrons transfer from one thing to another. The rate of electron transfer is measured in millivolts as ORP.

Free Chlorine

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Advanced chemical automation controllers like ProMinent’s DCM5 can measure ORP, chlorine levels, pH and more.

Every certified pool operator knows to measure free chlorine. Operators should also measure total chlorine and calculate combined chlorine. Such information gives operators an idea of the sanitizer levels in their water, and can adjust accordingly. Say there’s a swim meet with a much higher-than-normal bather load. It will most likely take more chlorine to accommodate the demand. Even if you’re treating the water with enzymes, chlorine still has contaminants like ammonia to oxidize…so measuring chlorine levels is important.

Nowadays, quality chemical controllers can read chlorine levels and calculate combined chlorine automatically, as well as measure ORP, and control both ORP and chlorine levels as needed.

But without knowing the ORP, we don’t know how effective the chlorine is. That’s why we strongly suggest measuring and documenting both ORP and chlorine levels. We would argue both are important, but of the two, ORP is more important. Your pool can have 1.0 ppm free chlorine and 800 ORP with the use of enzymes, UV/Ozone or HDO. We have seen it happen. We would argue that’s better than 4.0 ppm chlorine and the same 800 ORP. If less chlorine has the same sanitizing and oxidizing power as more chlorine…why would you want more than you need?

Ways to optimize pool sanitization

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You know by now that we manufacture NSF/ANSI Certified enzymes to break down and remove non-living organics. With enzymes, chlorine has less carbon-based bather waste in the pool to oxidize. Less oxidant demand means more residual chlorine to conquer other contaminants, so on and so forth. Enzymes help optimize pool sanitization, water clarity and overall water quality.

Secondary sanitation systems like UV an Ozone also help chlorine. There are secondary oxidation systems like Hyper-dissolved Oxygen (HDO) and Advanced Oxidation Processes (AOP). With the exception of HDO, these systems are point-of-contact systems that only work with the water they touch, which circulates through pipes in the pump room. Enzymes and HDO are out in the pool alongside chlorine the entire time. The bottom line is this: pool sanitization is critical in swimming pool management, and the ORP is the best metric to measure sanitation power. If you can have great ORP with a minimal amount of chlorine, that’s the best of both worlds.

The Undeniable Importance of Calcium

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Last week we got a phone call from a long-time customer of ours. They had just renovated a commercial swimming pool for a Homeowner’s Association (HOA) community.  The plaster was scheduled to be done the next day, and they called to ask us for help with their startup. Their primary goal was to protect the surface for the customer, and at the same time, minimize plaster dust so their people did not have to visit every day to brush.

This customer had heard we know how to prevent plaster dust…and we do. Why? Because we have a few core beliefs about water.

Core belief #1: Pool Chemistry should be LSI-Based

If you test for pH, alkalinity and calcium only, you’re missing half the equation for true water balance, according to the Langelier Saturation Index (LSI). When managing pool chemistry, what are you chasing? For most people, it’s what we call “range chemistry”, which is keeping those three values (pH, total alkalinity and calcium) within ‘ranges’ set forth by industry textbooks. But have you noticed how difficult it can be to keep pH and alkalinity consistent? They are always moving targets…and that’s not unusual.

You see, pH is a natural phenomenon. As long as there has been water, there has been pH. The pH of water impacts almost every other aspect of water chemistry. Yet it’s always moving, because it’s an equilibrium that can be affected easily by products like acid or soda ash. So instead of chasing the moving target of pH, we believe you should be chasing a more all-encompassing target: the LSI. The LSI can be driven by your best friend in water balance, calcium hardness. Unlike pH and alkalinity, calcium hardness does not change easily, and it helps keep your LSI more stable.

For more on why we prioritize LSI before range chemistry, read this.

Core belief #2: Water will seek equilibrium

Unlike #1, which is our opinion, this is a fact. Water will seek equilibrium, both physically (think of plumbing and how important gravity is), and chemically. Water wants to be balanced, and will stop at nothing to get there. Because of this, we believe you have a choice: give water the chemistry balance it craves, or it will steal it from anywhere it can.

In swimming pools, we’re primarily talking about calcium hardness first, since the LSI is an index of calcium carbonate saturation. Five other factors affect this saturation:

  1. pH
  2. Carbonate Alkalinity
  3. Water Temperature
  4. Total Dissolved Solids (TDS)
  5. Cyanuric Acid (CYA)

But primarily, calcium is the rock to build your LSI pool chemistry castle upon. Give the pool the calcium it needs, and by and large, it will not have to seek calcium on its own.

Core belief #3: Timing matters

The National Plasterers Council offers startup courses that are very informative. In the startup technician course, you will learn the in-depth chemistry of plaster surfaces and how they behave in the field. One thing that stood out when we took the course was how much time it takes plaster to cure.

The most vulnerable time for plaster is the first 60 days, which is when the bulk of its curing happens. Part of curing cement is a process called hydration, when Calcium Oxide (CaO) in Portland Cement is hydrated (H2O). This reaction yields Calcium Hydroxide, Ca(OH)2. In a perfect world, no calcium leaves the cement. But in a pool with aggressive water filling it? Calcium hydroxide is the first thing to be stolen from the cement in the plaster. It’s being stolen to feed the water the calcium it craves (see Core belief #2 above).

So timing matters. The sooner we can give water the calcium it craves, the less calcium it will steal from the curing plaster. To prevent plaster dust, balance the LSI as soon as possible. Some have proven this can be done using high levels of sodium bicarb on startup, and we have proven it can be done using calcium on startup. Do what works best for you, and you can use our free LSI Calculator App to dose it properly.

The undeniable importance of calcium hardness

Putting our three core beliefs into practice is simply a matter of testing tap water for all the LSI factors. Then, using our LSI Calculator App, figure out how much calcium and sodium bicarbonate you will need for the pool. Add calcium first by pre-dissolving it in buckets (to let the heat off). Get it in the water as soon as you can. Bucket by bucket, continue to add it.  Some people even use large trashcans and siphon calcium-rich water into the bottom of the pool as its filling. Again, find what works for you.

naturally free scale and metal sequest

So we told our customer to get their pool up to 300ppm calcium ASAP, and to use our startup chelant, Naturally FREE. They followed our recommendations and were thrilled to find out they had no need to brush plaster dust…because there wasn’t any.

You can do the same. If you need help with the startup, contact us. Thanks for your time.