# Re: [APD] Interesting idea

```Stuart Halliday wrote:
> I believe Thomas Barr wrote this email section below:
>> Scott et al,
>>
>> The good idea from all this is the fact it gets around the KH
>> issue in the tank and variation it can cause.
>>
>> We have folks claiming 50-200 ppm ranges of CO2 based on the
>> pH/KH test readings and the fish are just fine, others have
>> 35-50ppm and dead fish...........we know there's screwy issues
>> with our assumptions about KH.
>>
>> By using a referenced solution for the KH, we eliminate one
>> variable/assumpotion that has been causing issues for sometime,
>> but by using a pH color change to measure, we lose some of the
>> accuracy.
>>
>> ==>>>
>>
>> So this **obviously** leads us to make a referenced KH solution,
>> that we can __insert a pH probe into__ so we can get very
>> accurate deterimination of CO2 ppms.
>>
>> ==>>>
>>
>> That way we have successfully removed the KH interference issues
>> from the tank water and still have saved the accuracy of the pH
>> meter.
>>
>> You simply add the pH meter inside the solution of the drop
>> checker.
>
> So once you know it's in a KH of a known value you could design a ph meter
> that doesn't just display the ph but the CO2 values too?
>
> After all it's just a mathematical formula isn't it? Or does temperature
> come into it?
>
> This would make for an interesting CO2 meter wouldn't it?
>
> Hmm I wonder how long it would take to get a design knocked up?
>
> My company is just about to design a ph meter that measures the ph of the
> flesh of human tissue. Don't ask. ;-)
>
> I'll ask the engineers at work tomorrow.
>
> Anyone got a spreadsheet that calculates the relationship of ph/KH/CO2?

You can use my Web-based calculator which solves for any unknown, but
the formula is CO2 = 3 x [degrees KH] x 10^(7-pH). If you were making a
spreadsheet formula, it would be something like:

=3*A1*(POWER(10,7-B1))

Where A1 was KH, B1 was pH and the formula is in C1.

--
Jerry Baker
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```