Different Kh readings; day time/nighttime? - The Planted Tank Forum
 
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post #1 of 4 (permalink) Old 11-15-2006, 04:44 AM Thread Starter
 
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Different Kh readings; day time/nighttime?

If I test when the lights come on, at 10:00am, I get a reading of 0. When I test at night, 8:00pm all lights of at 9:00pm, I get a reading of 190 ppm. Is this posible, or is my kit bad. I show 150ppm Gh. My Ph is 6.4
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post #2 of 4 (permalink) Old 11-15-2006, 05:55 AM
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I'd check your test kit.


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post #3 of 4 (permalink) Old 11-15-2006, 01:27 PM
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Agreed - check the test kit first. However, it is possible to have slightly different morning/evening readings of kH. If you are not using a pH controller, but injecting CO2, you will have a changing CO2 reading during the day. For instance, if your CO2 level is at its highest in the morning then your pH is at its lowest. Since a kH titration uses an acid to lower the pH, it will take less when there is more CO2 in the water - an apparent lower kH. If at the end of the photoperiod much of the CO2 is exhausted (and a higher pH) then the apparent kH will be higher.

Bottom line: if your starting pH of the sample is different due to CO2 levels, then the kH reading could also be different. Not by 190ppm, but the lower the kH the more significant the error. For low kH samples, it is best to let the sample outgas for a while (up to 24 hours) before taking a kH reading.

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post #4 of 4 (permalink) Old 11-15-2006, 02:46 PM Thread Starter
 
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Quote:
Originally Posted by KevinC View Post
Agreed - check the test kit first. However, it is possible to have slightly different morning/evening readings of kH. If you are not using a pH controller, but injecting CO2, you will have a changing CO2 reading during the day. For instance, if your CO2 level is at its highest in the morning then your pH is at its lowest. Since a kH titration uses an acid to lower the pH, it will take less when there is more CO2 in the water - an apparent lower kH. If at the end of the photoperiod much of the CO2 is exhausted (and a higher pH) then the apparent kH will be higher.

Bottom line: if your starting pH of the sample is different due to CO2 levels, then the kH reading could also be different. Not by 190ppm, but the lower the kH the more significant the error. For low kH samples, it is best to let the sample outgas for a while (up to 24 hours) before taking a kH reading.
I don't have CO2 and I'm new at this, so that is a little hard to understand, it's probibly is the test kit.
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