Here’s the thing with the test. 17.848mg/l is 1 degree. So with a 10ml sample each drop will count in 17.848mg/l increments. With 20ml each drop will count in 8.914mg/l increments.
So to reframe your results:
Tap water: GH >44.8 - <53.714; KH >17.8 - <26.714
In other words, based on your water report for alkalinity, it appears you haven’t added any sodium bicarb at all.
Onto the Ashbeck:
Ashbeck GH 41.88 KH 20.5
With 200mg/l Sodium bicarb: GH 41.88 KH 45.
Your test: GH >62 - <70.914; KH >36.6 - <45.514.
So your test results for KH in the case of Ashbeck appear somewhat accurate, but not for GH…..this is assuming you added 200mg/l Sodium bicarb to Ashbeck and it is within its spec to start with.
As I asked before, are you adding drops until the first sign of colour change or are you adding until you get a “bright” colour? If you’re waiting for the colour to change to a bright/solid colour then that would explain why your result is so out for GH. If you did the same thing for the KH sample then that will also be out by a similar % (e.g. just of 50% overestimated).
Basically, it looks like you haven’t weighed the sodium bicarb correctly and have not stopped adding reagent at the first colour change in the case of Ashbeck. In the case of your tap water it looks like you haven’t added any Sodium bicarb at all, as 200mg/l Sodium bicarb in distilled water will give 25mg/l alkalinity (just within your measurement). For your test to be accurate for tap water you would have to have almost 0 alkalinity to start with.
So all in all it looks like the GH (and maybe the KH) test was not correctly conducted for Ashbeck and Sodium bicarb was not accurately measured for your tap water.
Measuring 200mg means you’re probably going to be out by up to 100mg….maybe more depending on the response time of the scales. The spec sheet of the scales will usually tell you how accurate they are e.g. accurate to 100mg, respond to 50mg etc. Usually, they have a minimum threshold below which they don’t work properly too, so they might say accurate to 100mg from 500mg etc. This is why making a concentrate is recommended….though logically volume should be accurate assuming you can find a scoop of the right size.
To conduct the test accurately, shake the reagents and hold them vertically over the sample to release drops of the same size (holding at an angle will give you different sized drops). Also, worth checking the expiration date. Stop adding drops the moment the colour changes from whatever to Green and Red. You obviously have to mix after every drop, I do it in a cup and just stir as I’m adding drops and wait a moment between each drop.