First attempt at water

No such thing as a silly question. The GH measurement is a bit suspect as there shouldn’t be any calcium or magnesium in there… but it depends on what the kit is actually measuring (e.g. it may interpret the Na^+^ ions as ‘half’ of Ca^++^)

Penance TDS 198

hmmmm…. This doesn’t seem right. I measure the TDS sometimes just to check I haven’t messed up the concentrate or maybe added twice the amount to the same bottle while leaving the other one without anything… :-)

For the concentration I use, approx. 0.1g of sodium bicarbonate (or potassium bicarbonate) for one litre, the TDS comes up around 50ppm using my el cheapo TDS metre.

TDS 198 seem quite a lot. For you to have an idea, when I flush my service boiler, on a bi-monthly cycle, the TDS is approx. 175ppm, and that’s expected.

So you are effectively using a concentration of 0.064g of sodium bicarbonate per litre. That’s almost half of what I use - which is fine. But something is not adding up.

Are you sure you didn’t add 3.2g of sodium bicarbonate?

There’s a good chance you’re not using the test kit properly based on your comments. You’re best off using a 20ml sample. You can put it in a small cup e.g an espresso cup.

Shake the bottles for testing KH and GH. Completely invert the bottles to make sure the drops are the same each time, if you hold the bottles at an angle you’ll get inconsistent results as you add each drop. Stir the sample after each drop (I just stir continuously and add the drops every 5 seconds or so). You probably won’t get too much colour at all in the samples with the amount of reagent you’ll be adding. You don’t keep adding until you get a “bright” colour, you stop at the first sign of colour change.

It’s also good to understand what the kit is measuring. Take your GH for example. You have a set of instructions that tell you 1 drop = 17.9mg/l (as in the colour just changes immediately). With the 5ml sample you know that you have anywhere from 0-17.9mg/l Hardness. If it takes two drops you know you have between 18mg/l - 35.8mg/l etc. With a 20ml sample the accuracy of the test increases x 4 e.g. 4 drops = 17.9 mg/l so the range you are measuring is reduced to +/-4.475mg/l compared to +/- 17.9mg/l.

If you’ve added 0.36g Sodium Bicarb to 5l water you should have an alkalinity of 43.5mg/l

0.1g in 1 litre is just about 60mg/l………you have measured your alkalinity to be 53.8mg/l - 71.6mg/l

Your second attempt was obviously more successful.

GH is hardness and you should have almost none with zero water.

Penance Sounds much more in line with expectations - I would have expected a KH of ₆₀, but the coffee will be fine at 70-80 too; it looks as if the TDS meter is incorrectly calibrated (perhaps for a higher temp? Usually they are calibrated at 25 C, so if you are testing in a house at 18 C, it’s normal to get a lower read)

Penance Could be - but then again, you are within 1 drop of each other (as in: +1 on the “low KH” one, and -1 on the “high KH” are the same count), so they aren’t too far out!