OBS-5+ System Revision: 11/13 C o p y r i g h t © 2 0 0 8 - 2 0 1 3 C a m p b e l l S c i e n t i f i c , I n c .
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Table of Contents PDF viewers: These page numbers refer to the printed version of this document. Use the PDF reader bookmarks tab for links to specific sections. 1. Introduction .................................................................1 2. Cautionary Statements...............................................1 3. Initial Inspection .........................................................1 3.1 Ships With............................................................................................1 4.
Table of Contents 7. Calibration .................................................................20 7.1 Sediment and NTU Calibration ......................................................... 20 7.1.1 General Guidance....................................................................... 21 7.1.2 Equipment and Materials............................................................ 24 7.1.3 Procedure for Sediment .............................................................. 24 7.
Table of Contents 8-1. 10-1. 10-2. 10-3. Internal components ...........................................................................31 Effects of sediment size .....................................................................35 Near-infrared reflectivity of minerals.................................................36 Effects of disaggregation methods .....................................................37 6-1. 7-1. 7-2. 9-1. 10-1. Working and Maximum Depths ........................................
Table of Contents iv
OBS-5+ System 1. Introduction The manual describes the features of the OBS-5+®, as well as its use for surveys and battery-powered, internal-storage operations. Using backscatter from a 780 nm laser diode and a patented dual-detection system (U.S. Patent Number 5,796,481), a calibrated OBS-5+ measures suspended sediment concentrations, suspended solids concentration (SSC), as large as 200 g L–1, which is about 10 times higher than standard OBS technology. Before installing the OBS-5+, please study: • • 2.
OBS-5+ System 4. Overview FIGURE 4-1 shows the dimensions of the OBS-5+, the sensors in the sensor endcap, and underwater connection. Detailed specifications are provided in Section 5, Specifications. The OBS-5+ can be operated in Survey or Cyclic Modes. In Survey Mode, the unit sends data via RS-232 or RS-485 to a PC at two hertz, and in Cyclic Mode, it logs as many as 200,000 scans of time, date, depth, and g L–1 in flash memory (one sample per hour for 23 years).
OBS-5+ System FIGURE 4-2. Schematic of optical system 4.2 SSC-Measurement Principle In FIGURE 4-2, suspended particles scatter light from the NIR beam onto the near and far detectors, and suspended solids concentration (SSC) is estimated with signals counts from these detectors by a microcontroller, using a set of logic rules and lookup tables derived from calibration data. Counts are the digital equivalents of the ND and FD signals and are proportional to backscatter intensity.
OBS-5+ System FIGURE 4-3. Sample calibration curves (fine and bold lines), lookup tables I, II, and III (bold curves), and sediment concentrations pf ND (open arrow) and FD (solid arrow) peaks. The concentrations associated with the ND and FD peaks depend on sediment characteristics as shown by the variety of response curves and the colored arrowheads on FIGURE 4-4.
OBS-5+ System FIGURE 4-4. Calibration curves for four different sediments and SSC values for near-detector peaks (colored arrowheads). 5. Specifications Features: 5.
OBS-5+ System 5.2 5.3 5.4 5.5 Accuracy Mud: 2.0% of reading Sand: 4.0% of reading Pressure: 0.5% of full scale Turbidity: 1.
OBS-5+ System 6. Operation 6.1 Instrument Setup 6.1.1 Mounting Suggestions CAUTION Maximum depth for the OBS-5+ is limited by the installed pressure sensor. If the maximum depths are exceeded, the pressure sensor will rupture and the housing will flood. The depth limits are listed in TABLE 6-1. TABLE 6-1.
OBS-5+ System 8 6 7 5 4 6 – 18 V d.c. (Red) Power GND (Black) DB-9 3 8 4, 1, 5, 6, 9 DB-9 3 2 6 2 3 5 (RS485) (A) (B) (GND) (RS232) (RD) (TD) (GND) FIGURE 6-1. Electrical connections 6.1.3 Logging Data In applications where a survey cable is impractical or when the OBS-5+ must be attached to an instrument frame, it can be powered by the internal batteries and the data can be logged by the data flash memory. See instructions for Cyclic Mode sampling in Section 6.2.11, Cyclic Sampling. 6.1.
OBS-5+ System For extended deployment time, lithium batteries are a good alternative to alkaline batteries. Campbell Scientific sells a C-cell-sized battery spacer (pn 21905) that allows lithium C-cell batteries to be used with the OBS-5+. Lithium C-cell batteries have a higher voltage than their alkaline counterparts, necessitating the spacer. Campbell Scientific does not sell lithium C-cell batteries. FIGURE 6-2.
OBS-5+ System data with a PC or uploading data from the OBS-5+, 5) importing data into a spreadsheet, and 6) plotting data with OBS-5+ Utility. 6.2.2 Running the OBS-5+ Utility 1) Set PC to the time standard for your project. 2) Select the OBS-5+ program to start the OBS-5+ Utility and open the Data Window and Toolbar with the View pull-down menu. 3) The OBS-5+ Utility will create a new data log file and prompt you to accept the name (see FIGURE 6-3).
OBS-5+ System 6.2.3 Pull-Down Menus The OBS-5+ Utility has four pull-down menus for File, OBS-5+, View, and Help. The File menu allows you to select the location and formatting for OBS-5+ files. Files can be opened as plots or ASCII text that can be brought into spreadsheet programs or text editors. Plot files display OBS-5+ data graphically in the main GUI window.
OBS-5+ System adjustment takes two seconds. If your PC is set to the wrong rate for some reason, use the check box to select ONLY change host computer port. Then click Apply and the OBS Settings button. If you get the OBS-5+ information box, the baud rate of the unit is synchronized with your PC. If you don’t get an information box, repeat the above procedure until communication is established. 6.2.
OBS-5+ System 2) Wave your hand in front of the OBS-5+ sensor; the turbidity and SSC levels on the top plot will fluctuate as data scrolls across the plot. 3) Blow into the pressure sensor or press your thumb on it to compress air on the diaphragm (FIGURE 4-1). A small elevation in the pressure signal will occur (bottom plot). Stop and then OBS Settings to view time, serial numbers, 4) Click depth corrections, and software versions. 6.2.
OBS-5+ System 1) Measures of central tendency, including the mean and median. 2) Measures of variation or spread in sample values, including the standard deviation (σ) and cumulative percentages, such as X25 and X75 (where X is the depth, SSC, or NTU values). The mean is the arithmetic average of the values (∑ x / n), where ∑ x is the sum of the sample values (x) and n is the number of values (sample size).
OBS-5+ System recorded. Select the longest interval that will show the changes in turbidity and water depth that you wish to investigate. Rate sets measurement frequency. The quicker turbidity and depth change, the higher the sampling rate should be to get a stable average value for a sample. Finally, Duration sets how long sensor outputs will be averaged. For example, with an interval of 30 seconds and a duration of five seconds, the OBS-5+ will make measurements for five seconds starting every 30 s.
OBS-5+ System lookup table icon and select a calibration table for your 3) Click the survey. The last active table will be used otherwise. Survey to select: sensors, lines per minute, and depth units 4) Click (Meters or Feet). Set temperature and salinity for the survey area. 5) Click Start Survey and check the data flow in the Data Window. 6) A file for logging data was created when you started the OBS-5+ Utility.
OBS-5+ System 6.2.11 Cyclic Sampling This mode is for logging data at regular intervals such as 1, 10, 15, 30, etc. minutes, for example. 1) Request Barometric Correction from the OBS-5+ menu. Be sure to do this while the OBS-5+ is at the surface (see Section 6.2.10, Surveying ). 2) Open the Data Window with the View pull-down menu. 3) Activate the lookup table for your survey area with activate buttons (Step 3, Section 6.2.10, Surveying).
OBS-5+ System 6.2.12 Data Retrieval 1) Remove dummy plug and connect OBS-5+ to PC with test cable (FIGURE 6-1). 2) Run OBS-5+ Utility. 3) Open the Data Window to verify that the instrument is transmitting data. 4) Click a file. to end data collection and use Offload Data to save data in 5) Highlight the data with the start and end times you want. 6) Click Browse, select a destination file and click OK. 7) Wait for the progress bar to disappear and examine data as a plot or text file (see Section 6.2.
OBS-5+ System 2) Click Open and select a file to view. Print will print a graph when Open As Plot is selected. To print a text file, select Open As Text, and use the Word Pad file print functions. For spreadsheet operations, see the next section. The your plot looks. Plot and Port Settings is used for setting up 3) Use the Min and Max and Sample Range (End and Start) values to bracket the data you need on the graph. Plot Width allows the graph to be sized to fit a PC screen.
OBS-5+ System 7. Calibration 7.1 Sediment and NTU Calibration In addition to the concentration, the size, shape, and reflectivity of suspended sediment particles vary from one location to another and will influence OBS5+ measurements. When these sediment characteristics change, they will produce apparent changes in SSC by themselves unless sediment calibrations are performed. All sediments produce a unique set of calibration curves like the examples shown in FIGURE 4-4.
OBS-5+ System FIGURE 7-1. Manual (left) and automatic (right) sediment suspenders 7.1.1 General Guidance The OBS-5+ uses response curves from the near and far detectors to create three lookup tables like those shown on FIGURE 4-3 and FIGURE 7-2. The objective of sediment calibration is to create lookup tables from the sediment you will monitor. The OBS-5+ can store lookup tables for as many as 14 sediments. The fifteenth table is reserved for auto-saved archives. button (step 3 in Section 6.2.
OBS-5+ System from the descending limb of the near detector curve. A calibration consists of a set of 15 to 30 calibration points that each includes an SSC value, a near detector count, and a far detector count. FIGURE 7-2. Lookup tables and table limits (a, b, and c) Six to ten calibration points are needed to define each table.
OBS-5+ System TABLE 7-1. Schedule of Concentrations for Sediment Calibrations MUD (D60 < 62 µm) Sand (D60 > 62 µm) Low SSC (0-5 g/l) Midrange (5-20 g/l) High SSC (> 20 g/l) Low SSC (0-10 g/l) Midrange (10-40 g/l) High SSC (> 49 g/l) 0.0 5 25 0.0 12 50 1.0 6 30 2.0 14 60 1.5 7 40 3.0 15 70 2.0 8 50 4.0 20 80 2.5 9 60 5.0 25 90 3.0 10 6.0 30 100 3.5 15 8.0 35 140 4.0 20 10.
OBS-5+ System Sediment concentrations can also be calculated manually with the following equations: M s = Sediment mass (g) g/l= Ms ⎡M ⎤ Vi + ⎢ s ⎥ ⎣ ρs ⎦ Vi = Initial volume (liters) ρ s = Sediment density (usually 2.65 x 103 g/l) 7.1.
OBS-5+ System 3) Click the button to view the list of lookup tables stored in the unit. Select an EMPTY Table number for the sediment calibration. 4) Start the calibration with the button and secure the unit in a big black tub filled with clean tap water (FIGURE 7-3). The sediment-calibration dialog will appear (the initial display will not show the red and green symbols). FIGURE 7-3.
OBS-5+ System 5) Enter 0.001 in the value box and click the Record button to log the clearwater data point. The unit will take 1,200 measurements in 60 seconds. When the process is complete, the data appear in the data table and the ND (red) and FD (green) points will be plotted on the calibration graph. If you are satisfied with the data, mount the unit in the suspender. 6) Position the OBS-5+ so that it produces the minimum FD signal in clear water (FIGURE 7-4). FIGURE 7-4.
OBS-5+ System 8) Withdraw about 10 ml of water from the suspender and add it to the tea cup containing the dry sediment. Stir the water-sediment mixture into a homogeneous slurry, breaking up clumps of sediment as you go. 9) Pour the sediment slurry into the suspender and rinse the cup with suspender water until it is clean. Make sure all the sediment gets from the cup to the suspender.
OBS-5+ System the FD curve where it starts decay exponentially, also 10 on FIGURE 7-2 9. Points ‘b’ and ‘c’ will usually have the same numerical value. 15) Save the table in the EMPTY Table number selected at Step 3. 7.2 Turbidity (NTU) Calibration 7.2.1 Equipment and Materials • Large black, neoprene or polyethylene tub • 100 mm test cylinder (www.deslinc.com, No. TC4) • AMCO Clear turbidity standards (GFS No. 8429, 8430, and 8431, www.gfschemicals.com) 7.2.
OBS-5+ System FIGURE 7-5. OBS-5+ in 100 mm cup 6) Add enough 250-NTU standard to cover the sensor end (FIGURE 4-1) and swipe bubbles off the sapphire windows with your finger. Click the Record button. 7) Repeat Step 6 for the 500 and 1,000 NTU standards. 8) Review the data table and graph, and if they look satisfactory, click the Calculate Fit button. 9) Verify that the fit curve passes through the calibration points and that the residuals are less than 10 NTU. Then click the Done button. 8.
OBS-5+ System corroded surfaces of the contact and track with a scouring pad and reassemble unit. Unit does not communicate with PC. There are several possible causes for this symptom. 1) The batteries are dead. 2) The OBS-5+ will not wake up. 3) The test/umbilical cable is damaged or improperly connected 4) The OBS-5+ and PC are set to different baud rates or communication protocols (for example, RS-232 versus RS-485). • Click Plot and Port Settings and check port settings on the serial port tab.
OBS-5+ System FIGURE 8-1. Internal components OBS-5+ or pressure sensor malfunction. • Open unit and inspect for: 1) broken sensor wires, and 2) loose pressure sensor connector (FIGURE 8-1). • Check sensor power by clicking Survey and selecting all sensors; the green LEDs should illuminate. If they do not, the sensor power circuit may not be working. • If the depth sensor reads high and does not change, it may need to be cleaned (see Section 9.2, Pressure Sensor).
OBS-5+ System Bright sun near the surface (< 2 m) or black-colored sediments cause erroneous OBS readings. Do not survey in shallow water between 10:00 and 14:00 local time and avoid areas with suspended black mud; see Section 10.2, NIR Reflectivity. OBS-5+ indicates different NTU values in the field than other turbidimeters. Not all turbidity meters read the same! OBS-5+ sensors are checked with U.S. EPA-approved AMCO Clear turbidity standards before leaving our factory (see Appendix A).
OBS-5+ System 9.3 Batteries The unit runs on three, C-size, alkaline batteries. Buy the expensive ones with the longest expiration date (“use before May 20XX”). While operating continuously, the OBS-5+ will run 125 hours (15, eight-hour surveys) in Survey Mode and for as long as 3,000 hours in the Cyclic Mode. CAUTION Always put OBS-5+ to sleep for storage to conserve battery capacity (see Section 6.2.13, Shutdown). Refer to FIGURE 6-2 for battery installation.
OBS-5+ System pn 4576 Alkaline C-Cells Batteries pn 20806 OBS-5+ Test Cable, 2 m (6.5 ft) pn 21381 7-piece Allen Wrench Set, 5/64 to 3/16 Ball End pn 21139 SS Hex Socket Screw, #2-56 x .187 10. Interfering Factors Changes in sediment concentration (SSC) are the primary cause for OBS-5+ output fluctuations in the environment. In some monitoring areas, however, factors other than SSC, will cause the OBS-5+ to indicate SSC variation that are invalid and which the user does not wish to measure.
OBS-5+ System FIGURE 10-1. Effects of sediment size 10.2 NIR Reflectivity The output of an OBS-5+ will increase with the NIR reflectivity of suspended sediment independent of SSC. This can degrade accuracy when unknown reflectivity changes occur during a monitoring campaign.
Reactive Sensitivity OBS-5+ System FIGURE 10-2. Near-infrared reflectivity of minerals 10.3 Particle Shape, Flocculation, and Disaggregation Particle shape can be an interfering factor. The sensitivity of an OBS-5+ sensor to plate-shaped particles is about ten times higher than it is to spherical particles. Disaggregation of dry sediment by grinding can cause the sediment to become finer grained than it was in the environment and this will bias a sediment calibration.
OBS-5+ System FIGURE 10-3. Effects of disaggregation methods Finally, flocculation of clay particles in estuaries can affect sensitivity by causing small particles to clump together into larger ones to which the OBS-5+ is less sensitive. For example, when a dredge works into a zone of saline water where flocculation occurs, an OBS-5+ can indicate less than the actual level of SSC. A summary of interference effects on OBS-5+ measurements follows.
OBS-5+ System 11. References Black, K.P., M.A. Rosenberg. 1994. Suspended Sand Measurements in a Turbulent Environment: Field Comparison of Optical and Pump Sampling. Coastal Engineering, 24, pp. 137-150. Conner, C.S. and A.M. De Visser. 1992. A Laboratory Investigation of Particle Size Effects on an Optical Backscatterance Sensor. Marine Geology, 108, pp. 151-159. Downing, John. 2006. 25 Years with OBS Sensors: The Good, the Bad, and the Ugly. Continental Shelf Research. Downing, John. 2005.
Appendix A. Turbidity Standards AMCO Clear, supplied by GFS Chemicals (www.gfschemicals.com), is an approved calibration standard and is the one we use to certify our instruments. It is made from styrene divinylbenzene (SDVB) microspheres. SDVB spheres have a median size and standard deviation of 0.28μm (~1/5 that of formazin particles) and 0.10 μm respectively and a refractive index of 1.56. As shown on the SEM image, they are dimensionally uniform.
Appendix A. Turbidity Standards In the USA, formazin is a primary standard for the calibration of turbidimeters. The median particle size of formazin is 1.5 μm; the standard deviation of size is 0.6 μm (see size distribution graph); and as shown by the SEM images below, formazin particles have many different shapes. The preparation, storage, and handling of formazin will affect its accuracy and stability. Recommended formazin storage times are listed in the accompanying table.
Appendix A. Turbidity Standards We must emphasize that unlike SSC, which has physical units, turbidity values (NTUs, FTUs, etc.) do not. Therefore, if you measure water turbidity to be 100 NTU, you cannot directly infer any physical quantities from it. Turbidity values do not represent particular SSC values, indicate light levels at the bottom of a stream, or quantify biological process’. Moreover, it is often assumed that turbidity standards behave optically like sediment.
Appendix A.
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