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post #1 of 9 (permalink) Old 11-02-2002, 03:09 PM Thread Starter
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OK, on some post I remember there was a formula to calculate ppm of CO2. It was 3.6*KH (in ppm?)*10^(7-pH). Well, my KH took 12 drops. Really, I added 2 after the 11th, for a total of 13, but if I assume it changed on the 13th, then I could be over-estimating my KH, hence overestimating my CO2. I converted it to ppm by multiplying it by 17.9.

Well, my pH came out as around either 7.9 or 8.1, so again I will say 8.1 since I do not want to over-estimate my CO2.

So, now for the calculations. I took out my graphing calculator and put in 3.6*214.8*10^(7-8.1), and I got 61.42.... So it says I have 61 ppm of CO2, but then the fish would be dead! I even did it again with a pH of 8.4, just to see if my pH tester if off, and it still came up at 30.7... which is a LOT!

In the formula, is it supposed to be 0.36 instead of 3.6, since then the results would make sence (6.1 ppm or 3.07 ppm seem more realistic). Can someone help me here? Is the formula accurate or am I making a (stupid) mistake?

-Tim
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post #2 of 9 (permalink) Old 11-02-2002, 04:12 PM
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Well I can see 2 things you have done incorrectly, Tim! :sad:

The first thing you did wrong was put the KH into ppm rather than dKH. Secondly you must have put the equation in wrong on your calculator. Try putting in the equation exaclty like this:
3.6*12*(10^(7-7.8))
If you put it in your calculator like this it comes out to around 6.846 ppm.

This sounds correct and checks out with all those charts out there on the internet.

Kyle

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post #3 of 9 (permalink) Old 11-02-2002, 05:14 PM Thread Starter
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OK, that was my question, whether or not the KH should be in degrees or ppm. Also, you don't really have to put the parentheses around the 10^(7-pH), since calculators will automatically do the order of operations taking the exponent after the parentheses (for the (7-pH)). Thanks for answering my question!

-Tim
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post #4 of 9 (permalink) Old 11-02-2002, 05:39 PM
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Yes some of the more advanced graphing calculators will however simple "mickey-mouse" calculators as my old math teacher used to call them will not and the answer will come out wrong.

Kyle

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post #5 of 9 (permalink) Old 11-02-2002, 07:35 PM Thread Starter
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Yeah, I even was able to make up an equation for the ppm of CO2 relative to the pH level when the dKH is held constant. I just put in y=46*10^(7-X). The 46 comes from 3.6*12. Then I just used the trace function to find when the CO2 would equal 15 ppm, which is when my pH is 7.5, and it is 20 ppm when the pH is 7.3. Now I know what pH to aim for when injecting CO2! Thanks for the help!

-Tim
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post #6 of 9 (permalink) Old 11-02-2002, 08:35 PM
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You wouldn't happen to be using a TI-83 + are you SNPIC???? Those controls sound very similar.

KYle

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post #7 of 9 (permalink) Old 11-03-2002, 02:26 PM Thread Starter
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Yep, in fact I was using a TI-83+. My ninth grade advanced algebra course actually required us to buy one of these! I thought I would find very little uses for it outside of math class, and I proved myself wrong in about 2 months!

-Tim
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post #8 of 9 (permalink) Old 11-04-2002, 02:56 PM
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I owe my Bachelor's Degree in Mechanical Engineering to my TI-85. I seriously couldn't have done it without it. I remember not-so-fondly putting equations into the program function into the wee hours of the morning to use in an exam the next day. I programmed this CO2 function into a saved program and can calculate CO2 by just inputting pH and KH.

- Sam P -
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post #9 of 9 (permalink) Old 02-08-2003, 05:15 PM
 
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