SpaGuard Spa Sentry (Phosphate buffer)

[NathanT]

Just got a new spa a couple weeks ago, and Spa Sentry came in the starter pack. We have very soft water here already (test strip showed CH of tap water as indistinguishable from 0), and the spa store guy suggested using this instead of bringing the CH up. Apparently it's supposed to protect the heater etc. in an alternative way, so increased hardness isn't necessary. It also stabilizes the PH.

Just wondering if anyone has any knowledge or opinions about this. Will it indeed protect the spa's components from the soft water? If so, I guess there's no harm; the only reason to not use it (and instead raise the CH) on the next fill would be cost? In either case I'll be adding borates (currently with SpaGuard Optimizer Plus, which also came with the starter pack; in the future probably with boric acid and/or borax).

 

[JoyfulNoise]

Phosphates are another way to buffer water from pH changes. Carbonates have their highest buffer capacity at low pH (pH ~6.0) while borates have their peak buffering capacity at high pH (pH ~ 9.0). Cyanurates (from cyanuric acid) also add buffering capacity to water and their optimal pH for buffering is around 6.5. The phosphate buffer system has multiple acid dissociation constants but there is a peak in the buffering capacity at a pH of ~ 7.0. So phosphates make for a good water buffering chemistry. Calcium really has nothing to do with unless you add calcium carbonate to your water (which you would not because it dissolves too slowly) which adds carbonates to your water. Honestly speaking, I'm not sure the added cost of the phosphate buffers is really worth it as the chemical additions needed to keep spa water pH in a comfortable range is pretty easy to achieve.

What is your TA and pH of the fill water? Alkalinity is the chemical parameter that you need to care about. Pool components are not made of plaster, they are mostly plastic and metal. So you need pH buffers in your water to keep the pH stable. Again, calcium is meaningless from a metal corrosion standpoint.

 

[NathanT]

Everything I'd read said that too low calcium hardness can cause corrosion, but I see from some more tfp reading that that's outdated info. So I'll probably use up this spa sentry on the next fill, then see how the pH acts without it next time.

As I recall, the Ta was very low initially, and I brought it up to about 120. pH ended up being about 7.2 and hasn't required adjusting. We'll see if it's as compliant without the buffer.

Thanks for pointing me in the right direction!

 

[JoyfulNoise]

Everything I'd read said that too low calcium hardness can cause corrosion, but I see from some more tfp reading that that's outdated info. So I'll probably use up this spa sentry on the next fill, then see how the pH acts without it next time.

As I recall, the Ta was very low initially, and I brought it up to about 120. pH ended up being about 7.2 and hasn't required adjusting. We'll see if it's as compliant without the buffer.

Thanks for pointing me in the right direction!

Yes, that idea was borrowed form the water-boiler industry where you actually want an over-saturation of calcium and carbonate so that a thin layer of calcium scale can form on your piping network. It's particularly important in old water delivery systems where cast iron and lead pipes or soldier joints might be present. You can think it like an in-situ generation of a plaster-like coating onto the pipe - it's purpose is to slow down the leaching of the metal pipe into a closed-loop, high-pressure/high-temperature water supply. It has no bearing on a residential swimming pool which is a low-pressure, open water system where there is significant contact between the water and the atmosphere. Corrosion is entirely dominated by pH and the amount of dissolved oxygen present (sulfates and chloride can matter too but those really determine which corrosion mechanism might be dominant and not affect the initiation of corrosion).

So, in a spa, your biggest driver of pH change (mostly rising pH) will be the outgassing of CO2 from heat and aeration. This is why we suggest keeping the TA low (around 50ppm) and adding borates to act as a high pH buffer. Phosphates will certainly work as an additional buffer mechanism but they are not necessary (and they add expense). One has to be careful with phosphate buffers because if you have any appreciable calcium hardness in the water (or metals, like iron), the high phosphate levels will cause calcium to precipitate out of solution (as calcium phosphate) and create cloudy water. Many phosphate buffer products for spas will indicate that clouding is possible in the first 24 hours and suggest simply running the filter with an additional polymeric clarifier to remove the cloudiness. So, in effect, adding a phosphate buffer can reduce CH in a spa simply through the process of calcium phosphate scaling.

In the future, if you stop using the phosphate buffer product, you may actually want to bump up the calcium hardness (using calcium chloride) in order to avoid foaming in the spa because really soft water can easily develop foams from body oils, etc.

 

[outdoor]

Yes, that idea was borrowed form the water-boiler industry where you actually want an over-saturation of calcium and carbonate so that a thin layer of calcium scale can form on your piping network. It's particularly important in old water delivery systems where cast iron and lead pipes or soldier joints might be present. You can think it like an in-situ generation of a plaster-like coating onto the pipe - it's purpose is to slow down the leaching of the metal pipe into a closed-loop, high-pressure/high-temperature water supply. It has no bearing on a residential swimming pool which is a low-pressure, open water system where there is significant contact between the water and the atmosphere. Corrosion is entirely dominated by pH and the amount of dissolved oxygen present (sulfates and chloride can matter too but those really determine which corrosion mechanism might be dominant and not affect the initiation of corrosion).

So, in a spa, your biggest driver of pH change (mostly rising pH) will be the outgassing of CO2 from heat and aeration. This is why we suggest keeping the TA low (around 50ppm) and adding borates to act as a high pH buffer. Phosphates will certainly work as an additional buffer mechanism but they are not necessary (and they add expense). One has to be careful with phosphate buffers because if you have any appreciable calcium hardness in the water (or metals, like iron), the high phosphate levels will cause calcium to precipitate out of solution (as calcium phosphate) and create cloudy water. Many phosphate buffer products for spas will indicate that clouding is possible in the first 24 hours and suggest simply running the filter with an additional polymeric clarifier to remove the cloudiness. So, in effect, adding a phosphate buffer can reduce CH in a spa simply through the process of calcium phosphate scaling.

In the future, if you stop using the phosphate buffer product, you may actually want to bump up the calcium hardness (using calcium chloride) in order to avoid foaming in the spa because really soft water can easily develop foams from body oils, etc.

So ..... Are you saying here you don't need any Calcium Hardness? Because I'm told by Pool Heater Dealer and Hot Tub Dealer if I don't have the recommended 150-250 I will void my warranties

 

[JoyfulNoise]

So ..... Are you saying here you don't need any Calcium Hardness? Because I'm told by Pool Heater Dealer and Hot Tub Dealer if I don't have the recommended 150-250 I will void my warranties

No, I'm not saying that. I'm saying that the pool & spa industry completely misunderstands to role of calcium in water. CH can be beneficial in a spa because very low calcium hardness levels in water can easily cause foaming from residual detergents left in swim clothes and/or body oils. CH is also important for fiberglass surfaces as gel coats can be affected by low CH. CH is important to maintain for plaster surfaces so that calcium is not leached from plaster surfaces. TFP recommends a minimal level of CH (50-100ppm) in spas & hot tubs as well as in vinyl pools for this reason. We also recommend the appropriate levels of CH for other surfaces (fiberglass and plaster). TFP's recommendations are based on the very best scientific information available.

Dealers base their warranty restrictions on generally accepted industry standards....even when those industry "standards" make no scientific sense. The warranty is a legal document, not a scientific document. So if you sign a warranty that says you must never allow bunny rabbits to swim in your spa, you are required by the legal force of that document to keep bunny rabbits out of your spa even when the science tells us that bunny rabbits pose very little danger to a spa.