Instrument related problems with optical backscatter sensors Joan Backers Royal Belgian Institute of Natural Sciences, OD Natural Environment, 3de & 23ste Linieregimentsplein, B-8460 Ostend, Belgium

Introduction 1. Short history of the OD Nature tripod moorings. 2. Calibration procedures. 3. Problems with an OBS-3+ sensor connected to a LISST-100X. 2

1. Short history of the OD Nature tripod moorings. First tripod deployment in 2003 with R/V Belgica.

23 May 2011: Tripod ready for deployment near MOW1 pile.

11 July 2011: Tripod recovered after diver intervention (surface buoy disappeared).

First considerations: For continuous moorings you need a good planning: Man power Vessel availability Sufficient number of instruments Weather conditions play an important role in the recuperation of the tripod What if surface buoy is gone? Need for Navy divers Real-time data transmission Virtual AIS for the protection of the mooring

2. Calibration procedures 2.a. In the lab with AmcoClear Use same data acquisition system as during field measurements. Used for veryfying sensors.

2.b.Watersampling and in-situ measuring A typical configuration for turbidity calibration measurements: 2 Sea-Bird SBE09 data acquisition systems 6 turbidity sensors 6 five litre Niskin bottles Procedure: 13h cycle during all seasons water sampling every 20 minutes (2 bottles) vertical cast every 30 minutes at low speed bottles are emptied at the hour immediate filtration

Configuration table of both SBE09 CTD systems SBE09 SN parameter Instrument Model Serial Number Remarks 792 C1, T1 Sea-Bird N/A -   C2, T2 D 86815 Voltage V0-V1 Campbell Scientific OBS-3 1811 0 – 2000 Voltage V2-V3 OBS-3+ T8730 0 - 500 : 2000 Voltage V4-V5 T8849 0 – 500 : 2000 Voltage V6-V7 T8557 206 04:03 1857:2306 39867 None DataSonics PSA-100 altimeter 1801 Seapoint 0-25 12755 range * 5 (0 – 125)

Results of the comparison of the analysed water samples (SPM) and the turbidity measurements (NTU).

Considerations: Is the filling of the Niskin bottles during calm weather conditions and low current flow fast enough? A test is planned with one OBS-3+ inside a bottle and one OBS-3+ at the outside of the Niskin bottle. The distance of the carousel to the bottom is not constant during a 13 hour cycle. Do we have to take this variation into account? Would it be better to sample with horizontal mounted Niskin bottles? Does a multiple bottle system exist on the market?

3. Problems with a OBS-3+ sensor connected to a LISST-100X Upper graph: OBS-3+ low range (LR) connected to LISST-100X Middle graph: OBS-3+ high range (HR)connected to LISST-100X Lower graph: 2 OBS-3+ connected to a SonTek hydra system

Data values of previous slide sorted from high to low. The values of the OBS-3+ connected to the LISST-100X are much to low.

Analysis of the problem let to a correction factor.

Measuring configuration for the determination of the correction values.

The reason for bad data values of OBS-3+ connected to LISST-100X: LISST power to external instruments is insufficient. 6 Volt DC 1 Volt DC Turbidity = 0 NTU, MA = 10

6 Volt DC 1 Volt DC Turbidity = 2000 NTU, LR analogue channel, MA = 10

6 Volt DC 1 Volt DC Turbidity = 2000 NTU, HR analogue channel, MA = 10

6 Volt DC 1 Volt DC Turbidity = 0 NTU, MA = 50

6 Volt DC 1 Volt DC Turbidity = 2000 NTU, LR analogue channel, MA = 50

6 Volt DC 1 Volt DC Turbidity = 2000 NTU, HR analogue channel, MA = 50

6 Volt DC 1 Volt DC Turbidity = 0 NTU, MA = 150

6 Volt DC 1 Volt DC Turbidity = 2000 NTU, LR analogue channel, MA = 150

How does an OBS3+ react to a power shortage

Considerations: The problem is that neither the LISST-100X neither the OBS-3+ gives a warning for a low voltage. How are other data acquisition systems working? With a Sea-Bird you can set a pre-measurement period (eg. 4 seconds for warming up a sensor)