Calcium for metal

[RavenNS]

"Total Hardness (calcium)" was how it was written from the Pool Store test...
so I guess that was a calcium only test.

My test strip for T.H. showed it at over 350 but under 500 ( that, I would guess would include the remaining metals)

My TDS was a bit over 800 ( I had them check that, out of curiousity, because they didn't test for metals)

to date we have added over 100KG of calcium this year which seems really high to me,
( but what do I know about it? newbie...lol), for a 140000 Liters ;
this is not fresh water either, it hasn't been "changed" in at least 8 years ( possibly a few years longer).

thanks for your help :-)

[DavidD]

The short answer is no, you do not need calcium. Chances are your pool store is just following protocol. You know, the selling you more stuff protocol...

I wouldn't add any more. And you should ditch the "guess" strips.......

Dave

[RavenNS]

Thanks for the help guys

& yes, i would ditch the "guess" strips, but Ben doesn't sell to Canada...lol

Actually there were two different reagent type kits in the "pool house" of this place that we bought this last winter; unfortunately, I'm apparently too "stupid" to figure out how to use them.
The directions didn't make any sense to me. Even watching the Pool store people do these types of tests still had me confused about what i was really looking for in each test... ( I mean yeah if it's just colour changing, I can do that... but if it's watching how the "dots" spread out or not & how many drops needed before..? whatever?..... I don't get it...lol)

So I have actually 5 different types of strips... we have two hot tubs as well, so I have to keep on it.
And I also have an electronic CL & PH meter. I know those aren't supposed to be that acurate either, but i do comparrisons with all the strips & so far everything seems to be matching up.
I also do weekly water tests through our Hot Tub store ( rather, the store that we use; it's not "my" store, although I'd love to work there)... they're computerized for testing.
I also am doing monthly water tests at another pool store, just to compare test between the two stores.

My conclusion is that nothing is 100% acurate, but I can get a pretty good picture between all of these different tests

[chem geek]

Calcium, along with carbonate (alkalinity) and appropriate pH, is needed when you have a plaster pool or other grout in order to prevent them from corroding (dissolving) into the pool water. A vinyl pool does not need calcium for this purpose.

The belief that calcium is needed for heaters or for other metal surfaces is that in certain environments that can be normally corrosive to metal, having a "protective" layer of calcium carbonate scale can help to protect the metal from corroding. However, corrosion of metal is not at all the same thing as corrosion (dissolving) of plaster. The fact that you do not have calcium does not make your metal corrode any faster -- only lower pH, higher temperature, increased dissolved oxygen or oxidizing chlorine (HOCl) levels, and higher conductivity (higher salt or TDS) can cause that. In practice, if the pH stays above 7.0, then corrosion effectively does not occur (in pool water; sea water is another matter, but that's due to higher levels of dissolved oxygen and salt and even then it is iron that normally corrodes well before copper).

It is not clear whether you can realistically maintain pool water in a slightly scaling state (for calcium carbonate) to have this deposit on metal pipe surfaces (typically copper) in your pool heater and to do so as a thin film that is enough to prevent corrosion and yet does not continually build up to the point of blocking water flow. In heat exchangers this balance between corrosion vs. scaling is a serious issue, but in a pool it is much less clear that calcium carbonate water balance is important for anything except plaster/grout pools and even then it appears that you have to stray very far away from ideal balance before you run into problems.

So, bottom line, for your vinyl pool, don't worry about calcium. If you are worried about corrosion of metals, then keep your pool's pH in the normal range (well above 7.0) and don't use Tri-Chlor tablets for your regular chlorine source as these are highly acidic.

Richard

[mbar]

Thanks Richard for clearing this up. I am still hearing different takes on if you should use calcium in a fiberglass pool. Now I can see that it really isn't necessary.

[chem geek]

Thanks Richard for clearing this up. I am still hearing different takes on if you should use calcium in a fiberglass pool. Now I can see that it really isn't necessary.

Marie,

If anyone has either real-world experience with metal corrosion due to low or no calcium (as opposed to low pH) or if they are aware of scientific reasons why calcium is needed, I'd like to hear about it. Please don't take my word for it just because the science seems to indicate that it isn't a problem. I have been wrong before and will no doubt be wrong again so if real-world experience contradicts my beliefs, I certainly want to know about it. I'm just relating my understanding as it is at this time.

Thanks,
Richard

P.S. My own personal experience with metal corrosion in my own pool (we have bars in the pool just under the water line) is that the only place corrosion occurred was near where the floater with Tri-Chlor tablets had been and that I had the pool pump running 8-10 hours/day (i.e. not 24/7). I also had a thermometer corrode when I put Tri-Chlor in my skimmer. So localized low pH conditions can certainly cause corrosion. Needless to say, I don't use Tri-Chlor anymore and all of this occurred long before I got involved with this forum.

[mbar]

The experience I have had is that when I first had my fiberglass pool, the pool company put in lots of calcium. When I switched from baquacil to chlorine, they put in mega doses of calcium, then shocked it with calhypo. I ended up with lime green water, then milky water, then a pool that was completely stained yellow. That's when I took control of my own pool, I found this forum, and have been happy ever since. Last year I ran my pool with calcium of 25, I had no problems. This year I bumped it up to 125, because of all I read about fiberglass leaching out if there is no calcium - it didn't make sense to me. I know that fiberglass has trouble with plating - but in my own experience, and my opinion, it is high ph along with high chlorine that brings on that situation. So far this year by running my pool with a good sequesterer, ph no higher than 7.4, and alkalinity on the low side - about 80, I have not had any staining problems. I have read on some sites that fiberglass pools will not be warranteed if there is not calcium of 300 in the water. My pool didn't come with that provision. I can understand that calcium is needed in a plaster pool, but I don't know why fiberglass would dissolve unless the water was acidic.

[RavenNS]

ChemGEEK (lol, I love your name),

you mentioned that temperature would have "play" in corrosion...
I heat my pool to 90F... would this indicate that i should have higher calcium levels to prevent corosion of the Heat Pump ( & filter pump)?
( I'm sure most people don"t regularly leave their pools that hot...lol)

[chem geek]

I heat my pool to 90F... would this indicate that i should have higher calcium levels to prevent corosion of the Heat Pump ( & filter pump)?
( I'm sure most people don"t regularly leave their pools that hot...lol)

Actually, we run our pool, which is solar heated most of the time (for 5 months and gas heated about two months a year), at 88F since my wife uses the pool for swim therapy and I've always liked the warmer water temperature (I'm still pretty skinny).

The best research on corrosion that I can find shows a rough doubling in corrosion rates for each 10C (18F) rise in temperature. The temperature inside a gas heater could be, near the metal surface, up to 30F higher so that is the most likely place for a problem (if any) but typically it isn't corrosion that people are worried about (since they keep their pH well above 7.0) but rather scaling since the higher temperature (ironically) makes it more likely to precipitate or scale calcium carbonate. So in plaster/grout pools, some people (Ben, for example) keep their pools on the slightly corrosive side so that the higher gas heater temperatures don't cause scaling. However, I've been educated on this forum by how extreme the water balance must get before either corrosion or scaling of plaster starts to occur (the LSI or equivalent index seems to tolerate even 1.0 without problems rather than the "official" 0.3 or 0.5 limit) so I really don't have a good answer except that we (including myself) probably worry about this too much.

So if I were you, I wouldn't worry about corrosion or scaling and just keep you pH well into the normal range above 7.0. As far as temperature goes, spas are typically at 104F and I don't know if anyone adds calcium to spas nor have I heard of corrosion problems in spas that are not due to low pH. Of course, the relatively low volume of water in a spa compared to the relatively large number of people who can use a spa makes maintaining pH balance a real headache! Again, if anyone has any contradictory experience, I'd like to hear about it.

Richard