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See Calcium Nodules in Pools. It's primarily due to a defect in workmanship where a void was created between the plaster surface and the gunite below -- that is, a bad troweling job in some places. The problem may not show up for years -- in my pool it started to show after around 7 years...

It happens in standard chlorinated water, not just in a pool. If you look even at George Clifford White's "Handbook of Chlorination and Alternative Disinfectants", you will find all kinds of inconsistencies even in pure water with only chlorine added. There is no ammonia or organics added so...

That's an excellent document. While the information on the pH electrode is excellent and indeed pH electrodes follow the Nernst equation such that pH (negative log of hydrogen ion activity) and voltage are linearly related, that is NOT true for real ORP sensors. The RT/nF that they call the...

When plaster is cured it produces calcium hydroxide, but during curing the water is supposed to be saturated (or even somewhat over-saturated) with calcium carbonate and the calcium hydroxide as a result should get converted to calcium carbonate in place. That "seals" the plaster. Marcite is...

So 26 gallons of acid for a 17,000 gallon pool since January is 2.4 gallons per month or a little more than 1/2 gallon per week. It's understandable to add a lot of acid soon after a replaster with most being added in the first month but then should noticeably trail off over the next months...

Proper plaster curing does NOT result in any CH increase. Instead, the calcium stays in place in the plaster and bicarbonate in the water is exchanged with hydroxide. If CH is increasing due to the plaster, that is bad and means that your plaster is dissolving calcium carbonate into the water...

Technically, the issue is the low pH and its effects on a copper heat exchanger. The pH isn't actually terribly low so the advice is conservative. Also, if one has a titanium or cupro-nickel alloy heat exchanger there is less risk. In your case with no heater there should be no problem at all.

It only goes away if there is something in the pool to oxidize. The MPS lasts longer than chlorine if there is nothing to oxidize in the pool. Of course, with reasonable bather load, the MPS should get used up in hours as you say. The problem is that you can never know for sure. However, in...

If the surface was composed of some calcium carbonate, then if that were dissolving into the water it would raise pH, TA, and CH. If you add acid to compensate for the pH, then you still have some TA rise and of course the CH remains higher. For an initial dissolving that increases CH by 10...

Yes, you are right. I was thinking the pellets partially stayed intact and fell into the water, but it sounds like that's not the case.

As I noted in my post in the dry ice thread, "Adding one pound of dry ice to 10,000 gallons would drop the pH from 7.5 to 7.0 if the TA is 100 and if all of the gas dissolved in water." When I added leftover dry ice in my pool, I registered a pH drop consistent with about half the amount I...

Well, I converted prices to price per weight and then compared by chlorine content by weight. I could have done the same thing by volume instead but most products are sold by weight. The exception is the chlorinating liquid or bleach which are sold by volume (i.e. $/gallon). Ultimately, you...

Dry ice in the pool water will lower the pH with no change in TA. Not all of the CO2 will outgas. Some will dissolve in the water. See the thread 60 Pounds of Dry Ice and a Swimming Pool!.

Those prices are pretty old, but their relative relationship shouldn't be too far off. The 12.5% is Trade % which is the VOLUME % of available chlorine. For the comparisons I used the normal definition for % Available Chlorine which is a WEIGHT %. The difference is due to the 1.16 density of...

Use the directed amount on the MPS bottle or instructions. It may or may not help get rid of existing CC. It all depends on what the CC is. Unfortunately, CC can be many different types of chlorinated compounds. If it were monochloramine, then that is most easily removed by chlorine so...

In theory, yes, but in practice not necessarily. It all depends on the source of the CC. MPS oxidizes some chemicals before chlorine gets a chance to form CC, but that's not the case for ammonia and probably not for urea either. So using MPS can be hit or miss. It can certainly be tried...

See this post for chemicals names for each reagent. Matt's description of the iodide reaction is not correct. Iodide is easily oxidized even by chloramines so the free chlorine and the chloramines react with iodide to form iodine. So both free and combined chlorine are consumed (the combined...

Draining before is more effective because you initially remove high CYA water. If you don't drain before and the water level rises but does not overflow, then subsequent evaporation will return things to exactly as they were before. Even if there is rain overflow, you will remove very little...

Bulk density can vary by product. Calcium chloride density is going to vary depending on the granular size of the product and also on whether it is anhydrous or dihydrate. Powder will be less dense than pellets. This MSDS for Dowflake Xtra says 51-61 pounds per cubic foot which is the same as...

Chlorine slowly breaks down metal sequestrants. When that happens, the metal that was bound to the sequestrant gets released back into the water and can potentially stain. This is why one must maintain a maintenance dose of metal sequestrant. If you run at a higher FC/CYA level then you'll...

All Trichlor tabs will increase CYA as indicated. If you saw a difference between last year and this year, then perhaps you used more chlorine this year so increased CYA faster or had less dilution of water this year which have CYA rise faster (i.e. not get diluted down as quickly).

Is the anode the wire coil and is it just sitting on top of the pump strainer or is it going through the top of the strainer so that some metal is exposed to pool water? Anodes are most effective if they are buried in moist soil. If they are just loose in the air, they won't work as well...

You don't need to drain. If there isn't too much oil, you can use a Scum-Ball. If you have a lot of oil, you can use a PIG® Oil-Only Absorbent Mat Pad.

Well, that's the point. ORP sensors aren't following the Nernst equation in any rational way except that they somewhat follow HOCl concentration but with a scale that doesn't make sense (much larger mV per doubling than expected for 2 electron transfer; Chemtrol behaves as if it's a 0.8...

To get -0.7 CSI in PoolMath with pH 7.4, TA 60, CYA 70 and an assumed salt level of 3000, the temp has to be around 64ºF. Is that what you have been using? I've been assuming 0 ppm Borates. I think BuckeyeChris neglected to put in the 3000 ppm for salt and used a higher temperature in the...

SIMPLIFIED ANSWER The variation of ORP with pH has a lot to do with the change in pH in addition to the HOCl vs. OCl- concentration. The following shows ORP at various pH with 1 ppm FC and no CYA (using Chemtrol ORP) in the first section and 1 ppm HOCl in the second section and constant pH in...

Your acid demand will be lower not only at lower TA but also at higher pH. See this chart that shows the degree of over-carbonation in a pool at various TA and pH where you can see that at lower TA and higher pH there is less over-carbonation so there will be less carbon dioxide outgassing and...

So the thing to do is to have an opaque preferably white or reflective cover you can put over the rolled-up solar cover so that it blocks the UV from sunlight and also does not get too hot. Both heat and UV are damaging to the plastic in bubble-type solar covers. On the water, it's much more...

I'm surprised that they set a target ORP of below 650 mV, but then different ORP sensors have different readings of the same water.

Yes, please do that experiment. It does sound like perhaps chlorine does oxidize the inhibitor. We know it does for chemicals like EDTA that are not that dissimilar to ATMP except that the latter has phosphate groups while the former does not. You normally don't need to use stain and scale...