I am trying to do the overnight chlorine test with my TFT100 but sometimes have a 0.5 - 1.0 variation each time I do the test. I've been doing it three times and averaging the results but I was wondering if there is some better more accurate way to do this test. The little blue scoop for the DPD powder does not seem like the most accurate way to measure the powder. Also I am using a small measuring 50 ml measuring cup for the fluid measurement which might also be a source of error as it has a relatively wide cross-section.
What are best testing practices?