SWG salinity sensor reading vs. actual salinity, is there an allowable tolerance?

[jason8251]

I would like to say thanks in advance for the help, I stumbled on this site this morning and learned more in an hour than I have with countless calls, emails, and face-to-face with my builder's "warranty department".

Since my pool was put into service the salinity reading vs. actual salinity has been 500-600ppm high. I started digging in and found that the SWG stops making chlorine at 4000-ish so I made them come out and investigate. My actual test results were verified and we re-calibrated the sensor down to the actual salinity of 3600ppm. About 4 days later the reading was back up to 4100ppm and throwing a 145 code (high salinity)I did let the pH get a little out of hand and had some minor calcium build up so I pulled the sensor to make sure there wasn't any build up causing erroneous readings... Here's where I stopped listening to this guy and decided to reach out to y'all.

I was told to drop the water level 8" and refill with fresh water to bring the salinity reading down within limits.
I replied "the salinity level is 3600, the sensor is what's reading high shutting down the SWG's chlorine production".
He says "At the end of the day, what dictates your system working is what the system says. There is no calcium build up, so it is not affecting the system reading. Lets just get it to work with what the system says, and we should be ok…."This is my first pool and I in no way claim to be an expert but as an engineer (and a person with at least average mechanical common sense) I find this train of thought in regards to solving this problem totally unacceptable.

Against my better judgement and everything I've learned and experienced in the past 25 years I followed his advice and drained the pool some and refilled because the pool is under warranty and they have been very responsive to all my questions.

Am I off base here, is there an allowable limit to the variance between the sensor and the actual salinity posted somewhere that I can reference?



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[tim5055]

Welcome to TFP!!

First, I do not have a SWCG....

We generally tell folks to go with what the SWCG sensor says. If it's happy and producing chlorine then all is good with the world.

I know as an engineer this answer is not what you wanted to hear. You wanted me to say the sensors are all of by a factor a 12.25% and you can make this adjustment. Sorry, some report they read low, others report they read high some say they are right on the money. I think the problem is that we are trying to create a very accurate sensor on a budget. Accuracy & low budget don't go together. There are some more accurate sensors out there, but they are too expensive for residential use and require constant cleaning and calibration.

 

[Philo]

The same situation can happen in reverse. When many SWG's begin their eventual decline, they start reading "low salt". The pool owner keeps chasing the "low salt" reading by adding salt and the SWG continues to read lower and lower.....eventually the SWG completely stops and the new replacement SWG detects massive salt levels.
(You end up draining the pool. The same situation you were trying to avoid when you graduated from pucks because the CYA was too high....)

I'll add salt to a "low salt" SWG reading up to the point of 3600 by a Taylor K-1766. If the SWG reads "low" still, I'm thinking new SWG cell.

P.S. If my SWG read 4100, the actual reading was 3600 and no "high salt" light were on and chlorine was being made, I'd leave it alone.

 

[Griswald]

As an engineer, remember the word "tolerance". The SWG has a tolerance of...lets just say, because I don't know which one you have, of 2000 ppm. That means anywhere in that 2000 ppm, it will produce chlorine at its design rate. Out of that zone and it shuts down.

So, at 2300 ppm the SWG works at 2000, it won't. But that also means it works at 4300 and shuts off above that. Unless you are right up against the top or bottom, 500 ppm isn't anything to be concerned with.

Worry about the big stuff and don't sweat the small stuff.

 

[Jimrahbe]

Jason,

Welcome to TFP... A Great resource for all pool owners with salty problems...

I don't have your SWCG but my manual says the salt sensor reading is +/- 500 ppm... Kind of like hand grenades, close is close enough.

Within reason your warranty tech guy is correct.. making the salt cell happy will keep it up and running. As long as this does not put you way out of range that is the easiest solution.

In my case, my cell has no user adjustments, but I had to have the temperature sensor replaced under warranty because the cell salt reading was way way off from what the actual salt readings were.

Thanks for posting,

Jim R.

 

[jason8251]

Thanks a lot fellas, I really appreciate it. I just need to get used to "probably, close enough, it should be fine, etc"... Love the site and will continue to visit as well as contribute.

 

[tim5055]

I just need to get used to "probably, close enough, it should be fine, etc"...

Pool stores love to have you chasing "perfect" numbers. Things like, your TA is too low, add this and your pH is rising too fast, add that.

Very few things are disastrous in a pool overnight. Take it slow and your pool will find it's happy place.

 

[JamesW]

How are you getting the actual salinity level?

I recommend the Taylor K-1766 salinity test kit.

When you're sure about the actual salinity, you can calibrate the system.

 

[jason8251]

Not sure on the model# but it's a Taylor system I bought in addition to my K2006. It's a chromate indicator and then silver nitrate reagent. The pool guy used some high-falootin digital tester and we both got the same reading so I'm pretty comfortable with it.

 

[JamesW]

If you're confident in the salinity level, I would recalibrate the box to that.

 

[Analogbytes]

The Taylor k-1766 is a great kit, but remember, it's accuracy is +/-200. Add that to the accuracy level of you SWG, and you could really be somewhere in the middle. Not really saying that it is comforting from one engineer to another, but your SWG is within ~12% of the measurement by the k-1766 and ~7% off from the high end of the tolerance of the Taylor kit.

All that being said, I don't have the same concerns as my SWG reports within 100 ppm of my Taylor results, so I don't have the same things rolling around in my head.

Hmm, just thought about something, are you sure you are looking at your instant salt reading versus your average reading?

 

[JamesW]

Aquapure only has instant salinity.

 

[jason8251]

Aquapure only has instant salinity.

Thank goodness, just when I though I was getting a handle on this thing someone's gotta throw in instant, or average salinity readings.

One more question, on this thread anyway, I know the TFP chemistry numbers are different from manufacturers published numbers in some cases but in the Zodiac manual for my SWG it references chlorine numbers above 3.0 may corrode/damage metal fixtures (I'm paraphrasing here) and the recommended chlorine level is .5-1.0 while the TFP target is 5.0 for my CYA level (70). I've drank the TFP kool-aide, I'm all in but what's up with such a large discrepancy?

 

[tim5055]

I've drank the TFP kool-aide, I'm all in but what's up with such a large discrepancy?

No one is willing to read published peer reviewd work to understand that the recommended numbers (theirs) do not take CYA into consideration.

Once you add CYA to the water the effects of chlorine a vastly reduced.

I cut this out of a much larger post, you can see the entire thing here: Certified Pool Operator (CPO) training -- What is not taught

Chlorine/CYA Relationship
The courses (and handbook) do not teach about the true effects of CYA on chlorine, even though this has been known since at least 1974 definitively determined in this paper. This is the technical foundation for the chlorine/CYA chart though this chart was originally developed more from observational experience by Ben Powell (more on that later below) based on the minimum FC/CYA levels for preventing algae growth. At a pH near 7.5, the equivalent Free Chlorine (FC) level with no CYA in terms of the same active chlorine (hypochlorous acid) concentration is roughly close to the FC/CYA ratio as derived in this post. So pools with an FC that is roughly 10% of the CYA level are equivalent to pools with an FC of around 0.1 ppm and no CYA. This is at cooler temperatures (closer to 77F so OK with pools in the 80-85 range; spas at 100-104F have a higher effective FC with no CYA).

The FC test does not measure active chlorine (hypochlorous acid) alone, but also measures hypochlorite ion and, most importantly measures all of the chlorine bound to CYA (i.e. chlorinated isocyanurates) since this gets released quickly in the time of the test (see this paper). Looking at FC alone without consideration of CYA gives a false sense of security since it does not measure the chlorine level responsible for most of the sanitation and oxidation. The reaction rates for chemical reactions are based on the instantaneous concentration, not on whether a chemical gets replenished by conversion from another form (i.e. chlorine bound to CYA). One should stop thinking of FC as meaning anything in terms of chlorine strength when there is CYA in the water. FC is simply a measurement of the total reservoir of chlorine available and NOT a measure of its active strength. The FC/CYA ratio (or scaled up version for spas) is the relevant number for chlorine's true strength for sanitation and oxidation.

I and others who are technically oriented on this forum have pored through the scientific literature and found that the chlorine/CYA relationship holds extremely well in almost every case, be it for killing of bacteria (here, here, here, here, here), inactivation of viruses (here, here), protozoan oocysts (here, here, here, here), inhibition of algae growth (this paper claimed no correlation, but real pools say otherwise; Sommerfeld never wrote back to me when I questioned this), and oxidation of ammonia and organics (here) as well as correlation with ORP (see this post, this paper and this paper). I've also gone through field study data where the industry makes claims that only Free Chlorine (FC) matters in "real pools" yet I saw that bacteria are killed so easily that you can't even draw that conclusion from such studies and that hypochlorous acid (HOCl) is at least as good a predictor though they never looked at that correlation nor the FC/CYA ratio as a proxy (see this thread).
Richard

 

[jason8251]

"One should stop thinking of FC as meaning anything in terms of chlorine strength when there is CYA in the water. FC is simply a measurement of the total reservoir of chlorine available and NOT a measure of its active strength."

This answers all of my questions pertaining to chlorine in a swimming pool, thank you very much.

 

[tim5055]

"One should stop thinking of FC as meaning anything in terms of chlorine strength when there is CYA in the water. FC is simply a measurement of the total reservoir of chlorine available and NOT a measure of its active strength."

This answers all of my questions pertaining to chlorine in a swimming pool, thank you very much.

See, you got it in a few minutes where as the entire pool industry has been ignoring it for over 40 years. But, they have a vested interest in keeping your pool just slightly out of wack.

 

[Analogbytes]

Thank goodness, just when I though I was getting a handle on this thing someone's gotta throw in instant, or average salinity readings.

Sorry about the confusion as my compupool SWG shows the average by default. You have to go through the menu to get the instant reading. I should have looked to see if yours had the same prior to posting.

 

[jason8251]

I was totally joking, no worries at all.