How to calculate percentages when overflow-style water changing?

[Bill Smith]

Hi:

I am considering setting up some water changes with overflow drainage. But I don't think the math is as simple anymore when I'm trying to figure out the percentages of water changed.

For example:

If I know that I can fill the tank in 20 minutes with a certain flow rate, I normally would assume a 50% water change would take 10 minutes.

But since I am not draining the water first (but instead letting the excess overflow) the new water I'm adding will mix with what's there before it overflows.

So how do I get to 50% in this model? How long will I run the water to make a 50% water change?

Is there a calculation for this?

Thanks!

Bill

[Daniel]

You could measure something like nitrate and see how much water changed it takes for the nitrate to drop 50% and that would give you the amount of water to change. Or some other proxy variable.

[DaveSamsell]

37 minutes ago, Bill Smith said:

Hi:

I am considering setting up some water changes with overflow drainage. But I don't think the math is as simple anymore when I'm trying to figure out the percentages of water changed.

For example:

If I know that I can fill the tank in 20 minutes with a certain flow rate, I normally would assume a 50% water change would take 10 minutes.

But since I am not draining the water first (but instead letting the excess overflow) the new water I'm adding will mix with what's there before it overflows.

So how do I get to 50% in this model? How long will I run the water to make a 50% water change?

Is there a calculation for this?

Thanks!

Bill

Bill, 

Am not sure.  One would have to know how well the new water is actually being diluted & redistributed in the water column.  Many variables there.  Perhaps one way might be is to keep measuring your Nitrate levels until you see a 50% change.  That way, you can then estimate the volume of water that was physically exchanged.

[DaveSamsell]

3 minutes ago, Daniel said:

You could measure something like nitrate and see how much water changed it takes for the nitrate to drop 50% and that would give you the amount of water to change. Or some other proxy variable.

Daniel,

I think we were on the same wavelength there.  You were typing when I was. Same idea.  😂

 

[Daniel]

Just now, DaveSamsell said:

Daniel,

I think we were on the same wavelength there.  You were typing when I was. Same idea.  😂

 

My first thought was that it would involve calculus, but I don't think I remember any☺️.

[DaveSamsell]

1 minute ago, Daniel said:

My first thought was that it would involve calculus, but I don't think I remember any☺️.

I had two years of integrated, differential & statistics.  Way too many numbers.  😅

[Bill Smith]

LOL I came to the right place!

Also found this calculator:

https://www.hamzasreef.com/Contents/Calculators/EffectiveWaterChange.php

I will also try the nitrate check as a way to check the calculator. I have the API test kit, which is a little more precise, and if I do it across 3 different tanks, that should give me a close enough figure.

Thank you!