U26-001 Readings below zero

I am working with some of he DO readings collected with a U26-001 DO Logger. Some of the readings are below zero (ex: -0.02 to -0.04). The spec sheet lists the accuracy of the loggers as 0.2 mg/L up to 8 mg/L. So my question then is should those values be treated as 0s since the negative values are within the accuracy range? Thanks!
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Thanks for your question!
These raw data points could be an indication of sensor drift due to bio-fouling or from an error during the lab calibration (even though they are, as you correctly indicated, within the accuracy spec of the logger). If you have access to the logger you could try running through the lab calibration procedure and checking the zero response using the sodium sulfite solution, which should return a zero value. If this is found to be off, the calibration should be re-done.
This solution can be purchased from Onset's website: http://www.onsetcomp.com/products/accessories/u26-cal-sol

Also, please note that there are some very detailed product training courses now available on our website that you may find interesting as well (one for the U26 is included) .
www.onsetcomp.com/resources/training?page=1

I hope this helps and happy logging!

Thank you for your reply and help! The logger were calibrated before deployment. They were deployed in wetlands so I'm sure that there was bio-fouling at some points in time. The curious thing is that on the same days where we got some negative readings, the sensor also read positive, "normal-looking" readings during the same day. Any more thoughts on whether or not to treat as a zero? Thank you,

Hi and thanks for the additional information.
Yes these slightly negative values should be considered as zero.

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