I met with a pool company this morning to talk about installing a dosing system with automatic controller (so we can say good bye to the tablets).
We never got to talking about different kit and equipment, we got stuck on types of chlorine. Not surprisingly, because I've spent way too much time on this forum, I reckon that sodium hypochlorite would be a good way to go. It adds nothing but chlorine and salt, right?
The salesman reckoned that I would have to increase the dilution to compensate for the TDS going up, and that this was a significant drawback. Increasing dilution means the cost of water and, the cost of heating it up. Also, it would need a lot of acid, since we're in a hard water area.
I thought about this a bit and then later went to pool calculator, plugged in some numbers for the amount of chlorine I thought we use in a season, and this is what I got:
200 litres 12.5% liquid chlorine raise FC by 488 and salt by 803
27.306 kg trichlor will raise FC by 488, CYA by 296 and salt by 399
38.769 kg cal hypo 65% will raise FC by 488, CH by 344 and salt by 495
Looking at it this way, the problem with the trichlor is obvious - I would have to drain at least two thirds of my pool just to keep it under 100 (in practice a lot more since the drain and refill is done in stages, not all at once). I've drained and topped nearly 20% earlier this week, and the fresh water needed a lot of heat (the weather is rubbish right now), and messed with my TA, which was stable, but is now up and the pH along with it. So I totally agree that needing lots of dilution is a Bad Thing.
The CH of our fill water is 290, so we would get to over 600 in a season using calcium hypochlorite, which seemed a tad high to me. The salesman seemed a bit unfazed by the fact that the CH was starting at 290, and said we wouldn't have a problem. He did say they were looking for pools to trial a new type of cal hypo feeder, which might be on offer for free, so I suppose that's a consideration.
The extra TDS seemed a bit of a non issue to me, and even if it was, it seems like a lot less water replacement would needed than either of the other two options - and could be dealt with just by increasing the backwash time. I'm sure we would need plenty of acid; everything needs acid with our water until the TA comes down.
So, I think I've pretty much convinced myself that I'd prefer sodium hypochlorite, if we can work out where to store it, where the tanks might go, and the practical stuff like that... (and find some money).
I posted this for a sanity check... am I talking sense? I feel really frustrated when talking to some people in the pool industry, they're really nice people, but when they come up with things that don't make sense, it's hard to challenge them, bearing in mind they have like 40 years experience and I have 18 months.