Is the Alkalinity dKH scale linear?

dangros · Mar 11, 2016

My alkalinity ran a bit high at 12.02, so I turned off the CO2 for a couple days and measured again. It then measured exactly 2pts lower at 10.o2. Assuming my alk consumption rate is the same, will it be likely be 8 dKH in 2 more days?
Just curious.

JimWelsh · Mar 11, 2016

To answer the question in the thread's title: Yes, the scale is linear. And assuming your alk consumption remains constant, then in two more days your alk can be expected to read 8.02. That assumption may not be valid, though, since various things can affect the consumption rate (such as alk swings).

dangros · Mar 11, 2016

should I not force such a quick swing down? I just dont want it to be that hight

JimWelsh · Mar 11, 2016

I didn't mean to imply that. I think just letting it drop to where you want it is fine, as long as you don't overshoot. I just meant that the consumption might possibly change over the next two days in response to recent events.

dangros · Mar 12, 2016

Gotcha! I'll check again tomorrow and prob keep it around 9. I made some changes to my reactor and didn't test enough afterwards. It got away from me a little. Thank you!

Randy Holmes-Farley · Mar 12, 2016

One complication to figuring depletion rates is that it is typically faster at higher alk. So at 11 dKH you'll get one rate of depletion, and at 5 dKH it will be much slower or even close to zero.

That is true because both main ways of alk depletion (calcification by organisms and abiotic precipitation on pumps and heaters) slow down at lower alkalinity.

ckozma · Mar 12, 2016

So not linear in other words...

GatorGuys · Mar 12, 2016

The scale is linear - but the consumption in the tank is not. Since you are trying to determine the amount to dose base on your consumption, it will seem to be different. The higher your alk - the faster your corals will consume it. As your alk drops, the corals will consume it at a slower rate.

Randy Holmes-Farley · Mar 12, 2016

ckozma said:
So not linear in other words...

As noted, the scale is definitely linear.

Consumption rate vs dKH in a reef tank is not.

ckozma · Mar 12, 2016

Got it, and thanks for explaining. When I have seen you explain it before, it always seemed like it was not linear, that it would drop faster if kH was higher. Seemed that way this time also, and Tue explanation helped a lot.