I have been using Salifert for my KH/Alk and Ca tests, the problem I noticed with these tests is at step 3. (the part in red)
Put the plastic tip firmly on the 1 ml syringe. And draw into the syringe the KH reagent (ensure that the end of the plastic tip is constantly submersed in the KH reagent) till the lower end of the black part of the piston is exactly at the 1.00 ml mark. There will be some air present just below the piston. This is the air which was present between the end of the plastic tip and the piston. This will not influence the test result.
It seems like an inaccuracy would have to happen because the amount of air present in the syringe is different every time I do a test, sometimes the air gap is as little as 0.01ml and sometimes it's more than 0.15ml. That comes out to a difference of about 2.2 KH.
The KH/Alk test does come with a check solution, so I used that to calibrate the test using the liquid line rather than the syringe's "black piston", that way it takes out the air gap problem. But the Ca test doesn't have the check solution and it seems the less testing solution you have left the more inaccurate the tests get.
So does anyone else using Salifert tests have this problem or am I missing something? And can anybody suggest better tests? Thanks for any help.