[Note, this was based on answer to a user working in the Arctic.]

We recommend that the “Dark Correct and Tare Depth” routine in µProfile should be run at the start of your intense field work. It should be done when the instrument is at, or near, the temperature where it will be recording data, AND WITH THE DARK CAPS ON. The instrument should have been on for longer than 5 min to allow internal temperatures to equilibrate.  This will set the optical channels and depth sensor to zero.

After you run this dark correction, with the dark caps till on, record a short “Cast” data file (or examine the live time series displayed in the software).  If you look at the dark file (or screen display of live data), you should see in the data that the optical channels are randomly varying about zero.

We also strongly recommend performing the “Record Data At Each Gain” function WITH THE DARK CAPS ON in µProfile after performing the Dark Correct and Tare Depth and before beginning data collection, and also at the conclusion of data collection for the station or day, depending on logistics and time lapse between stations. The “Record Data At Each Gain” results in data files including the letters “CAP” (for Capped Data), and the data can be useful in the event that a dark offset has drifted during the day, or if there appears to be an unexplained issue with data. This dark data at each gain can also be used as the field dark offset, and enables the extraction of maximum dynamic range during postprocessing for science goals with extremely stringent uncertainty requirements.  Conversely, simply recording and averaging dark data as a ”Cast” file and subtracting this from each measurement is an effective technique, especially for extracting dynamic range for low intensity signals (microradiometers in high gain), but performing this for larger signals (microradiometers in medium or low gain) is less effective.  This is because the subtracted offset was recorded with the high gain instead of the actual gain stage in which the measurement was recorded.  Utilizing the “CAP” files in conjunction with intelligent post-processing can minimize this source of uncertainty. The uncertainty from not performing this step is typically small, but for Cal/Val quality data for certain science goals the improvement in data products from these details can be significant.

Optical channel dark current drifts the most over the 25–50°C range, which is often not an issue for Arctic field work. In the case of typical conditions on an Arctic cruise, we anticipate that you will not see significant variation at the colder temperatures at which you are working.  This should remain true as long as the instrument has been allowed to equilibrate as described above, and is not being stored in a warm location and then quickly being deployed into cold waters.