Happy new season everyone (for the UK anyway).
I've just filled my 1000 litre hot tub with fresh tap water. I live in an area of hard water (limescale).
I've taken a reading and have a CaCO3 level (TA?) of 267 and a ph of 7.1. CYA is virtually non existent and no chlorine.
Before I add chlorine I think I need to reduce my TA down to the 70-80 mark.

this is going to take a fair bit of acid I guess. I'm just going to add a heap of dry acid and then aerate with the blowers. And I guess I get my TA level down before I add stabilized chlorine?

Can someone confirm please?

CaCO3 is what we normally refer to as CH. 267 is only a little high for CH, probably not high enough to worry about. The only way to lower CH is to replace water. You use acid to lower TA, one of the major elements of which is NaHCO3, a different chemical entirely.

I thought I was getting confused
So I have a test kit that measures KH Alkalinity. I added 15 drops of something (Sodium Hydroxide?) and it turned from blue to yellow.
The box shows a diagram of the procedure and a calculation at the bottom. The calculation on the side says 1 drop = 1DH 17.8ppm (CaCO3)
But inside the box there's an instruction that says one drop required equates to 50ppm of calcium.

I'm thinking the KH value is 15, but then that is really low? HELP!!!

thanks

Hum, this is more complex than I originally thought. As best I can make out you can convert ppm KH to TA by dividing by 2.8, so it appears that your TA is actually around 95.

KH is carbonate hardness, one of the elements of total alkalinity and nothing to do with calcium. However, nearly all KH test kits actually measure total alkalinity, not carbonate hardness, except they do it in units of calcium carbonate for obscure historical reasons. Just to make things more complicated some people measure KH in grains of hardness, which is what the 15 number is.