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post #1 of 6 (permalink) Old 11-03-2015, 10:35 PM Thread Starter
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Tds

Hey guys,

can anyone explain why my TDS changes from 180 to say 150 when the lights are on and off.

Is it just the ebay tds pen i got maybe faulty?

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post #2 of 6 (permalink) Old 11-03-2015, 10:55 PM
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Temperature affect the reading. You can get a more accurate pen with temperature control.


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post #3 of 6 (permalink) Old 11-03-2015, 11:08 PM
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My pen is temperature controlled and is very accurate. My TDS always rises slowly between water changes which is caused by fish and plant waste dissolving in the water as well as fertilizers. It only goes down when water is changed.
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post #4 of 6 (permalink) Old 11-04-2015, 01:44 AM Thread Starter
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Thanks guys for confirming

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post #5 of 6 (permalink) Old 11-04-2015, 02:41 AM
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The same thing happens to me in all my tanks. TDS will drop when the lights are out by about 15 or so. My pen is accurate. It's calibrated and has almost exact same readings as my TDS apex lab grade probe. The probe doesn't move during the night, but the pen does. I'm not sure why.
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post #6 of 6 (permalink) Old 11-05-2015, 09:57 PM
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As mistergreen already wrote, many cheap TDS meters don't have temperature compensation, so they provide correct measurement only at one temperature (at which calibration was performed). It may be very accurate - but only at this temperature. Your lights go off, your lab grade probe doesn't care (it compensates its measurements for temperature) but your TDS meter doesn't. TDS meter really measures conductivity and conductivity rises with temperature (https://en.wikipedia.org/wiki/Conduc...us_Temperature), that's why your meter shows less TDS with lights off (and cooler temperature).
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