accidental chemist

[accidental chemist]

Well I am new to the site but not new to pools. After paying a guy to care for my pool, his boss took over and screwed it up. I canned the idiot, purchased a Taylor complete test kit, started to learn about pool chemistry.

Being inherently lazy, I purchased a Polaris Watermatic pH and orp controller and integrated it with a Goldline Controls SWCG. To minimize the wear on my Polaris 380, I installed a solenoid valve that energizes with the booster pump. Finally swapping the guts on an Intermatic mechanical timer with a PE153 digital timer, I ended up with what is basically the ugly cousin of the Pool Pilot Total Control Digital for chorine and pH control.

So I empty the skimmer, clean the polaris bag once or twice a week, check chemistry once a month or so, and spend the rest of my time looking at crystal clear water.

I need to get a salt test kit though as for the first time in 7 years I was burned by a failing salt cell that developed a nasty habit of lying about the salt level before finally purchasing an agricultural enterprise. To quote one of the greatest Presidents of the 20th Century, I learned to "Trust but verify".

I hope to learn a whole lot more here and perhaps share some of what I have learned over the years.

 

[chem geek]

Welcome to TFP!

By "Taylor Complete", do you mean the DPD Complete Chlorine Kit? If so, then you should consider getting the FAS-DPD Chlorine kit. This would have come with the Taylor K-2006 or the TFTestkits TF-100. If you got the DPD chlorine in your test kit, it will only test 0-5 ppm, will bleach out at higher chlorine levels, and is not as accurate.

 

[accidental chemist]

Taylor Complete High DPD Chlorine 0.5 -5.0, pH, Acid & Base demand, TA, Calcium Hardness, and CA. I typically only test TA, harness and stabilizer since the watermatic does the rest. I clean the probes and standardize the pH probe usually once a year.

 

[dmanb2b]

Welcome to TFP AC

Does the watermatic test FC and CC?

 

[accidental chemist]

It is looking at CC. The pH being stable and in-range is critical for this system to control properly. It is basically measuring the presence of the components needed to oxidize contaminants. Here is a quote lifted from a cut sheet on an industrial ORP sensor
"Finally, ORP is measured, in some instances, for the
control of biological growth. The principle behind
these applications is that a minimum ORP value will
successfully destroy microorganisms. This approach
has been used in the chlorination of swimming pools
and cooling towers. It should be noted that both of
these applications also include pH control."

 

[chem geek]

ORP as an absolute measure is nearly pointless. The same water measured by two different ORP sensors (from different manfuacturers, typically) will measure differently. As described in this post, 23% of pools with ORP measured by two different sensors had differences exceeding 100 mV.

It sounds like the Watermatic is an ORP sensor that only guesses at the FC level, if it even reports that (does it?). It is not looking at CC -- I don't know why you say that. The Polaris Watermatic is an ORP and pH sensor and controller for feeding chlorine and acid. You can use ORP as a process control device where you accurately measure your FC level and get it to what you want and then SET the ORP setpoint to the ORP level that is measured on the Watermatic. That is, the absolute ORP is pretty much irrelevant. You just get the FC/CYA ratios right and then set the ORP to keep it there. You really should get a FAS-DPD chlorine kit so you can measure your chlorine accurately since you will be setting your FC level near the "edge" to prevent algae growth.

The problem is that in saltwater chlorine generator (SWG) pools, the hydrogen gas that is generated can interfere with the ORP readings to make them inconsistent so that they read lower ORP when the SWG is on and may overshoot the FC levels as a result.

So read Water Balance for SWGs since that is what you will follows. The ORP will just be used for automation control. You will still want your TA to be lower so that you won't be using so much acid, even if it is automatically dosed.

 

[accidental chemist]

I don't disagree with the variability in ORP probes. The same can be said for the pH probes. ORP probes measure the potential available which will be primarily associated with FC. But what is actually being measured is the potential to transfer electrons. The molecules associated with CC will also impact that reading albeit to a lesser degree. One article I found suggests that ORP is a more accurate indicator of "bactericidal properties of chlorine than PPM Free Chlorine values".

Regardless of the method, we all are trying to achieve the same end result, clean, clear pool water. I am please with the results of the system I cobbled together. If I could manage my lawn as easy as the pool water life would be less stressful. LOL

 

[chem geek]

I think you missed my point. The SAME WATER was being measured with two different probes at the SAME TIME and got differences of over 100 mV. That means that you can't rely on ORP as an absolute measurement for water quality except at extremes. You can, however, use it as a relative measure for process control.

As you could see from the graphs in the link I gave, the ORP tracks the hypochlorous acid concentration, not the FC level, so yes it roughly measures the active chlorine level. However, we can calculate that knowing the FC and CYA levels since there is a chemical equilibrium relationship between these that has been known definitively since at least 1974.

As for the CC, it is usually near zero and it does not affect the ORP reading (that is, when you have both FC and CC, the FC/CYA ratio defines the ORP reading independent of the presence of CC). The active chlorine level, roughly proportional to the FC/CYA ratio, is what affects the ORP level and as I said things like hydrogen gas can interfere as well yet have absolutely nothing to do with disinfection or oxidation rates.

I simply cannot emphasize enough how ORP as any absolute measure of water quality is terribly misleading except in the grossest of rough terms. Use it as a process control measurement, but use actual FC and CYA reading from a good test kit, such as the Taylor K-2006 or the TFTestkits TF-100 to determine the active chlorine level or more simply to know that your FC is high enough for your CYA level.

All the studies showing ORP as a better measure for disinfection and oxidation rates compared ORP against FC, but that was a false comparison since everyone knows that it is hypochlorous acid that disinfects, not the chlorine bound to CYA nor hypochlorite ion. So the proper comparison would be against the calculated hypochorous acid concentration, but no ORP study has done that comparison. However, there are numerous studies showing the disinfection and oxidation rates both with and without CYA and they do show that it is mostly the hypochlorous acid concentration that matters and some of these studies even calculated that and found the direct correlation. Read more this in the "Chlorine / CYA Relationship" section in this post.

 

[accidental chemist]

Yea I will look that over but in the mean time. Its clear, its clean and its easy.