7. Post-recovery operations and calibrations

7.1. General considerations

The most essential action after a recovery is to safely archive the collected data following the Standard Operating Procedures for glider mission control and data handling.

The internal clock of the nitrate measuring instrument should be checked and any difference against GPS or a time server should be noted.

To be able to evaluate the performance of biogeochemical (and other) sensors pictures should be taken for a biofuiling assessment.

7.2. Post-deployment field calibration

7.2.1. UV Spectrometers

After recovery of the instrument similar field calibration procedures can be used as before the deployment. The best option is a Spectrometer-on-CTD cast before the instrument gets cleaned so as to have the effects of bio-fouling still present. During that cast discrete water samples have to be taken and analyzed for reference nitrate values. The second best option is to take discrete water samples when and where the last nitrate measurements were performed during the glider deployment.

7.3. Post-deployment lab operations

7.3.1. UV Spectrometers

After recovery, two lamp reference calibrations are strongly recommended.

Immediately after recovery, while the instrument is still wet, a clean water lamp reference calibration should be performed. This must be done before any cleaning of the instrument.

The purpose of this calibration measurement is to record the lamp strength when any possible bio-fouling is still present. After that the instrument should be cleaned according to the manufacturer’s instructions and we recommend a second lamp reference to be measured.

The difference between these two reference measurements should be a measure of the bio-fouling.

It remains to be determined which of the two lamp references should be used for the post-processing. It should theoretically be the first (bio-fouled) one as this is the end point of the effective lamp changes (aging plus bio-fouling) during the deployment.

After the post-deployment lab calibration all data from the UV spectrometers should be downloaded and backed up. This also includes the reference spectra and lamp usage files which are accessible differently than data (see Deep SUNA user manual).

For Deep SUNAs the lamp life time should be noted and, if necessary, a lamp replacement be organized (only possible at Sea-Bird).

7.3.2. Lab-on-Chip Instruments

It is required to check the state of reagent bags and their connecting tubes, to look for leaks, blockages or damage. Reagent and standard bags must be measured to check whether the expected volume has been correctly dispensed in relation to the deployment duration. Check the presence of any precipitation of reagents in the bags and on tubing walls.

For the lab-on-chip analysers, sub samples of blank and standard solutions should ideally be analysed post deployment, this requires clean handling (i.e. follow GO-SHIP recommendations; [Becker et al., 2020]). In cases where the blank and standard solutions cannot be sub sampled (e.g. recovered by partners without such capability), it should be noted that blank and standard solutions have been shown to be stable for 12 months (they are preserved with chloroform).

It is recommended that the sensors are serviced periodically by ClearWater (with the frequency depending on use) and that fluidic channels should be cleaned after use, for example with weak (2%) decon 90 solution.

The raw data is stored on a flashcard on the LoC analyser and can be retrieved via USB. Visibly inspect the filter integrity, there has not been any reports of filter biofouling issues to date.

After recovery of the instrument similar field calibration procedures could be used as before the deployment. To get a meaningful calibration that could possibly be applied to deployment data any Spectrometer-on-CTD casts would have to be done before the instrument gets cleaned so as to have the effects of bio-fouling still present.

7.4. Cleaning and storage

7.4.1. UV Spectrometers

After the post-deployment lab calibrations the instrument should be cleaned and stored according to the manufacturers’ recommendations.

Regular monitoring of the UV lamp usage is important for Deep SUNA instruments. The advertised lifetime of 900 h translates to more than 5 weeks of continuous operation at the fastest sampling rate of 1 Hz or nearly 30 weeks of operation at a reduced sampling rate of 5.5 seconds. At GEOMAR one Deep SUNA was in use for 7 two to three week long deployments before a lamp replacement and servicing was initiated.

7.4.2. Lab-on-Chip Instruments

For the LoC devices, dispose of waste reagents in accordance with local regulations. If the analyser is not intended to be used for several weeks, the cadmium tube should be removed and stored as per manufacturer instructions. If this is not done, there is a risk of the tube drying out and becoming blocked.

Similarly, the LoC analysers have limited storage requirements (room temperature, clean and dry is sufficient). However, the chemical part must be subject to special controls. Reagents have also been shown to be remarkably stable, year long deployments have been conducted in the marine environment and replacement reagents can be purchased. For long term storage (several weeks and more), it is recommended to remove the cadmium reduction tube (accessible outside the housing) and store as per manufacturer instructions (so that it cannot dry out and risk becoming blocked). Al the components of the hydraulic circuit must be inspected (lamp, tubing, reagent and standards bags, cadmium reduction column, syringe filter) and changed if necessary (keep ready supply of spare parts).