LSI, CSI, APPs, Watergram…

CFB

New member
Jan 7, 2018
3
Victoria
#1
I am new to the forum and enjoy reading the useful information. I mostly keep (or at least try to!) my pool water within the ranges given on the forum.

My pool is SWG, values are (from Taylor's Kit):
pH 7.6
CH 230
ALK (total) 70
CYA 45 [I keep this less than 50 ppm because I have an ORP probe]
Temp 70
Salt 3870

Using the PoolMath APP and the above values I get CSI +0.18.

Using the Taylor Watergram I get LSI -0.18. I understand that the Watergram does not take into account salt. The standard constant in the LSI equation goes from 12.10 to 12.41 to consider salt (using the relevant Table) so the correction seems to be -0.3 (negative because the factor is subtracted). This makes LSI even more negative at a value around -0.48.

Using the Orenda APP (available online, takes into account salt and CYA) the same values above give LSI -0.13.

These differences appear to be quite large (assuming I have calculated correctly and inserted values correctly in the APPs/equations) and so it would be good to understand what is different, if this is indeed the case.
 

JoyfulNoise

TFP Expert
Platinum Supporter
May 23, 2015
14,490
Tucson, AZ
#5
CSI and LSI are calculated differently. Honestly, CSI is the better value to use because it uses actual thermodynamic models of calcium carbonate saturation/precipitation while the LSI was derived from a more phenomenological basis with correction factors to make the data fit. There are historical reasons why the LSI is widely used and, sadly, it’s mostly due to technical laziness on the part of the pool industry. Pool water is not the same as boiler water or cooling tower water but, for the purposes of saturation calculations, the pool industry doesn’t care to recognize those “subtle” differences. However, both saturation indices should agree on the degree of scaling or etching. The Taylor Watergram is just the LSI with no ionic strength correction factor. Not sure what Orenda is using but it is very likely the LSI.
 

CFB

New member
Jan 7, 2018
3
Victoria
#7
Thank you JoyfulNoise. I have now crunched the numbers and all is clear. The link was helpful!

My other question is on ORP. It is related to CSI, in that for a CSI near zero my ORP is low.

Here goes...to stop pH rise and to balance the CSI I maintain my pH at 7.8. However, the ORP at 7.8 is around 500 mV, well under the min 650 mV recommended. When the pH was at 7.3 the ORP would be around 625 mV but at this pH I had the zig-zag cycle of acid addition and bicarb (buffer) addition.

ORP does decrease with increasing pH, as noted on other posts in the forum and other literature. So this is not surprising.

Given that my pool is now rather stable wrt pH, ALK etc and the CSI is good, my thoughts are turning to why the ORP is low.

I have tested an independent ORP probe and my pool probe is accurate (or both are off-unlikely in my view).

So the other possibility is high organic content. My combined chlorine is essentially zero, but I am not sure if this means organic content is low.

Question: what is recommended to test organic content and more importantly to remove it? Shocking with chlorine apparently removes some but not all organics, even forming other unknown substances, while MPS also does not remove all organics.

I have read that enzymes can be added to remove organic content. Comments, advice, ideas?
 

JoyfulNoise

TFP Expert
Platinum Supporter
May 23, 2015
14,490
Tucson, AZ
#8
In my opinion, so take it for what it’s worth, ORP is useless. As you can see from all of your testing, the ORP signal is sensitive to lots of different variables (even the presence of sunlight affects the signal). However, there is only one important parameter that matters for water sanitation and oxidation - free chlorine (FC) level (and more precisely, hypochlorous acid concentration). Chlorine is the primary sanitizer and oxidizer.

So the question that needs answering is this - in both the short term (24 hour period) and long term (months), how does the FC level correlate to the ORP reading? While the basic epidemiological literature talks about specific ORP levels being a indicator of sanitary water (pathogen kill rates) in highly controlled lab environments, most manufacturers of these dosing systems try to extend those concepts to something that is not controlled - pool water. With all the external factors that can affect the various chemical levels in pool water, it’s not at all unreasonable to see probe readings that drift around and correlate poorly to actual FC levels.

One specific issue that is an Achilles heel for ORP is the presence of the chlorine stabilizer cyanuric acid. As you have already stated, you must keep your CYA below 50ppm in order for the ORP probe to work correctly. In a lot of pools, that amount of CYA can be too low to protect the chlorine against high rates of UV photolysis. So, in order to make an ORP system work, one needs to use more chlorine than is necessary. CYA will also foul the probe membrane (as do other organic compounds) and so one must periodically clean the probe to keep it working correctly.

So why bother with ORP? It’s a proxy value that requires additional work to keep stable and tells you no more information that what one wouldn’t get from simply measuring FC each time and dosing appropriately. As an automation control point, it’s not very helpful because it’s too sensitive to drift and too influenced by other parameters unless one keeps the water in a condition that is harsh to bathers (high sanitizer levels and low stabilizer levels).

As for organic load, yes, there are dissolved substances in the water that can shift your ORP signal. The “standard” method of analysis would be something called a “total organic carbon” (TOC) Test. Unfortunately, that’s not something that can be easily measured pool side and typically requires a lab analysis. Combined chlorine is only an indicator of nitrogenous waste and the test itself is really only sensitive to a few types of combined chlorine. Chlorine can oxidize some simple organic compounds fully to inorganic carbon and nitrogen but, more often than not, chlorine creates organic chloramine, trihalomethanes (THMs), and disinfection by-products (DBPs). These are persistent pollutants until they outgas from the water or UV light from the sun slowly degrades them further.

I very much doubt the efficacy of any enzyme products made for pools as enzymes are substances that are easily inactivated by chlorine or UV light and they usually require very narrow ranges of pH in order to work. Enzymes usually require high levels of dissolved oxygen to work effectively and pool water is often oxygen deficient. So, in all likelihood, those products would do very little for you and add to the organic load in your pool.