Hi fablau,

Wow, yeah I can see how that is all so confusing. I hadn't looked at this calculator for a long time and the code is really confusing too. Hearing you're using it makes me want to rewrite the instructions and the calculator so maybe we can keep adding to it!

The first problem that probably causes confusion is the margin of error input only works with two drop checkers and has no effect on one drop checker's calculations (this is because we use the margin of error in color between multiple drop checkers). I added a note about that but need to spend some time redoing some stuff to make it cleaner looking (I hope!

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"So, with the above input, wet's calculator tells me to add 159.2 mg of baking soda do 1 liter of DI water to have a solution of 5.31 KH. Then add that solution to my drop checker, add 2 drops of Bromithymol, and the checker should turn yellow when Co2 concentration reach around 40ppm.

Ok, now get back to the calculator form again, and change the color option from "yellow" to "green"... Well, you would expect different instructions, instead you still get the exact same dose (159.2 mg of baking soda per 1 liter of DI water to have a solution of 5.31 dKh). Why's that??"

This is actually correct. It's because if you calibrate for "green," the center of that range in the drop checker will be the same as if you calibrate for "yellow."

An example may explain this better. Let's say we're using the 30ppm/4dKH standard most folks use with dropcheckers.

If I calibrate for green, "green" will be 30ppm. If my drop checker turns yellow, I have more than 30ppm. If it turns blue, I have less than 30ppm.

If I calibrate for yellow being 30ppm, it's exactly starting at the same place as if I calibrated for green. It's just that I pick yellow for that state at 4dKH instead of green. So, if the drop checker now turns green to my eyes, I have less than 30ppm CO2. If it turns a pale yellow/etc, I have more than 30ppm CO2.

Another example because even as I type that it's confusing.

Let's say I draw a line across a see-saw that is perfectly level and I intend to balance an object on that line. The object will roll to the left if the see-saw lowers on the left. It will roll to the right if the see-saw lowers on the right.

It does not matter where I draw the line if I know it is level. I could even draw the object all the way at the left or right edge and it will still work.

It's the same idea with starting at yellow or green. The drop checker is the same if you pick between green or yellow for a given CO2 (with one drop checker). The formula for dKH will only change if you change the target CO2 for that starting state, be it yellow or green.

Please let me know if that is all badly explained and still confusing!

"Also, try the "two dropcheckers" option, with the "yellow" option checked, and you'll get a completely different recipe for the first drop checker which should target 45ppm of co2: 45 mg of baking soda per 1 liter of DI water to reach a solution of 1.50 dKH."

This will be even trickier to explain!

If we use the pH chart that comes with most AP tests, green can be anywhere from pH 6.4 to 6.8. Yellow is just 6.0 on that chart.

So in that calculation, that value you're sharing is just for the first drop checker. You'll make an entirely different mix for the second drop checker.

Now, the idea is if both drop checkers (with different dKHs and therefore different pH and CO2 ranges for a given color) agree -- that is they are the *same* color -- you can have a reasonably high degree of confidence that your tank really is at the CO2 ppm. The margin of error lets you adjust that range but always acknowledges there is some margin of error. (Say +/- 5ppm.)

In other words, you add drop checkers so that you can be more precise, assuming both drop checkers are calibrated correctly. This is a big advantage vs one drop checker, especially if you think the colors to the eye aren't as precise as the pH chart. (This is also why the calculator prints a pH chart -- it helped me visualize it but I can see how it only adds to the confusion!)

"Now, get back to the calculator form, then enter 45 ppm as target, leave the rest as it is, and check "one drop checker".... Well, you'll get a different result than the suggested dosing for the first drop checker with the "two dropcheckers" option just tried above:"

Yeah, I think this confusion is from the margin of error. The reason that 45ppm +/- 5ppm is so different than 40ppm +/- 5ppm is the +/- part doesn't count on single drop checkers.

I hope the new line there helps that.

Does this help? Sorry it's such a weird tool to use without knowing these things. I'll try to cleanup the "How it works" section when I redo it!