Chlorine Functionality Testing

[Pool-newb]

Since I've become a pool owner I've been oddly fascinated with the hows and whys of the ways pool chemicals work.

We all measure free chlorine, but chlorine is more or less effective depending on cya and pH, as well as other things in the water. A more precise way of measuring the disinfecting quality of the water is to measure the Oxidation-Reduction Potential. This seems to give an accurate measure of what the water is doing right now, and includes the effects of pH and cya.

Here is one of the articles explaining ORP.

Having learned this I searched for a cheap ORP meter 4and got lucky with this one.

I really like this device. I've been using it since last year, constantly comparing it with the cl/cya levels and it really seems to be accurate. My goal is to get over 650mv though usually I see something lower than that except just after I've added bleach to the pool.

I found it for a little more than $100 plus it needed calibrating fluids, and I can measure pH, ORP and temperature in a few seconds without any fiddling with chemicals.

Comments from anyone out there with real chemistry knowledge? Could an ORP/pH meter [mostly] replace chemical cl and pH testing?

[chem geek]

Though you could use ORP calibrated to regular drop-test measurements as a way of doing automated chemical dosing control, it's not really useful as an "absolute" measurement. Different ORP sensors from different manufacturers will give different ORP readings of the same water. Not only at one point, which can be calibrated away, but the "slope" of ORP mV change per doubling of chlorine is even different with different probes. The ORP will roughly track hypochlorous acid concentration (once you figure out the probe's "slope" and calibrate it to a single point), but pH has affects on it that aren't related to hypochlorous acid concentration which affects kill times.

[Pool-newb]

Oh. Dang. :-/ ...

Are there no calibration standards for ORP sensors? Hanna claims ± 2mv accuracy. Is this not enough? Recommendations I've seen around the web for minimum ORP vary from 600mv to 740mv. Last year I rarely got much over 600 except immediately after adding bleach and after shocking.

I have compared results with my 5ppm cl test kit and it seem to be acceptable. When I have zero cl the meter gives me between 200 & 300, when I have a little but not enough I can see the ORP rise, and when I get over 600mv, the 5 ppm test kit overshoots.

I just ordered the TF test kit because the one I have only goes up to 5 ppm cl, and with ~60 ppm CYA I need 7 ppm cl.

When it arrives I'll coordinate the cl levels with the ORP readings of my meter and post the results.

Until the kit gets here, can I add 50% distilled water to my pool sample to get a multiply-by-2 result for an accurate cl reading using my 5 ppm limited kit?

I have investigated controlling a peristaltic pump based on an ORP sensor. Programmable, built-in meters are available that can be used to control a pump but my superficial investigation tells me that this route would be more expensive than a SWclG and I'd still have to lug bleach. Of course the advantage to this would be a constant, accurate cl level at all times.

Btw, I just saw the Hanna meter on E-Season gear here for $89.10. I really like this method of measuring the water because - if it works - you don't need an accurate CYA count and you don't need to fuss with chemicals to get the info you want. You just stick the meter in a sample of water, wait a few minutes and you have sanitizing levels, pH and temperature.

[chem geek]

I write about the ORP problems in this post. The most disturbing thing is the variation in the "slope" of the various sensors. That is, the mV per doubling of chlorine concentration, all else equal. In addition to the sensors in the chart, I also have data for an Oakton sensor that has 28.7 mV per doubling of FC (compared with 22.7 for Chemtrol and 45.7 for Aquarius).

The industry is in total denial about this, but their own measurements show this and I've talked to people who have had multiple sensors and they say the same thing. It's not just the "absolute" number that is wrong, as that can be calibrated, but the slope and there is no adjustment any of them have for that. It's also not just that the sensors vary in technology for the reference either -- sensors using silver/silver halide show this problem as much as those using other standards.

Like I said, you should be able to use the sensor for process control to keep the FC stable IF you also make sure the pH and temperature are consistent as these parameters affect the sensors in ways that do not necessarily reflect actual sanitation levels.

[chem geek]

Read this thread for some info on digital/electronic pH meters.

[Pool-newb]

Yep - I zipped over and read much of the ORP post and I think I just sprained my brane.

I am still using the Hanna ORP/pH meter though. While I don't use it for absolute testing, I can dunk it in to the pool and get a good idea of any problems.

It has a two point calibration for the pH but it only requests 2 test liquids. I have all three liquids though and I use the third one to confirm, and it is generally ok.

I also have the problem with the colors of the liquid tests and I really dislike that the scale of the color chart on the testing container; it doesn't show many different shades.

The resolution of the Hanna meter is .01 pH and claimed accuracy is ± 0.02 while the color tests are ± 0.2 (?). While my meter may drift over time I never dose the pool based on a single reading of the electronic meter, rather I look for relative values.

The best example of this is that I've had a floating erosion feeder in the pool with two tabs for a few days now trying to raise CYA levels. Measuring twice per day I was watching the pH go from about 2.2 to about 2.7. I also watched the ORP, cl and cc levels and added enough cl to bring the level up to 13.5 ppm. I also saw the pH go up to 7.39.

So the electronic meter may not be accurate, but I can "calculibrate" it so I know very quickly whats happening without having to fiddle with the chemicals.

I've been having problems with the cl/cc levels because I left it without cl for a while but once I get it cleaned up I intend to use the meter to check daily levels and if I think I need to change the amount bleach I dose or the pH I'll certainly whip out the FAS-DPD.

My take away from all of this is that I shouldn't rely on the "minimum 650mv" from my meter, nor should I use it to dose cl or adjust pH, but after testing and comparing it with other test methods, I can use it as a quick, easy check for appearing problems. Great as a daily or even before and after swimming test.

Thanks everyone for the input.