OBS-3A Turbidity and Temperature Monitoring System Revision: 11/11 C o p y r i g h t © 2 0 0 7 - 2 0 1 1 C a m p b e l l S c i e n t i f i c , I n c .
WARRANTY AND ASSISTANCE This equipment is warranted by CAMPBELL SCIENTIFIC (CANADA) CORP. (“CSC”) to be free from defects in materials and workmanship under normal use and service for twelve (12) months from date of shipment unless specified otherwise. ***** Batteries are not warranted. ***** CSC's obligation under this warranty is limited to repairing or replacing (at CSC's option) defective products. The customer shall assume all costs of removing, reinstalling, and shipping defective products to CSC.
PLEASE READ FIRST About this manual Please note that this manual was originally produced by Campbell Scientific Inc. (CSI) primarily for the US market. Some spellings, weights and measures may reflect this origin. Some useful conversion factors: Area: 1 in2 (square inch) = 645 mm2 Length: 1 in. (inch) = 25.4 mm 1 ft (foot) = 304.8 mm 1 yard = 0.914 m 1 mile = 1.609 km Mass: 1 oz. (ounce) = 28.35 g 1 lb (pound weight) = 0.454 kg Pressure: 1 psi (lb/in2) = 68.95 mb Volume: 1 US gallon = 3.
OBS-3A 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...............................................................1-1 1.1 1.2 OBS Sensor........................................................................................ 1-2 Temperature and Optional Sensors .................................................... 1-2 2. Instrument Setup .....................................
OBS-3A Table of Contents 6.1.3 OBS-3A Utility Software Steps................................................ 6-2 6.1.4 Making Turbidity Standards ..................................................... 6-3 6.2 Sediment............................................................................................. 6-4 6.2.1 Equipment and Materials.......................................................... 6-4 6.2.2 Sediment Preparation................................................................ 6-5 6.
Section 1. Introduction The heart of the OBS-3A monitor is an OBS sensor for measuring turbidity and suspended solids concentrations by detecting near infrared (NIR) radiation scattered from suspended particles. With a unique optical design, OBS sensors perform better than most in situ turbidity sensors in the following ways: 1. Small size and sample volume 2. Linear response and wide dynamic range 3. Insensitivity to bubbles and organic matter 4.
Section 1. Introduction Depending on the number of sensors and the statistics selected, the OBS-3A can log as many as 200,000 lines of data (one per hour for 23 years) including: time, date, depth, NTUs, oC, and salinity. When sampling with a full suite of sensors, the unit will run about 300 hours. When using the instrument for surveys, the data are captured by a PC running the OBS-3A Utility in the log file created at initialization. 1.
Section 2. Instrument Setup 2.1 Mounting Suggestions CAUTION Maximum depth for the OBS-3A housing is 300 meters. Working depths for individual instruments are limited by the installed pressure sensor. If exceeded, the pressure sensor will rupture and the housing will flood. TABLE 2.1-1. Working and Maximum Depths Pressure Sensor 0.
Section 2. Instrument Setup Surveys The OBS-3A will usually be towed from a cable harness for surveys. The serial cable supplied with the unit is strong enough to tow the OBS-3A and a 5-kg depressor weight however; the towing forces must be transmitted to the pressure housing and not to the connector. To provide strain relief for the connector, attach a cable grip about 30 cm above the SUBCONN® connector (FIGURE 1-2) and attach a short length of 1/8" (3 mm) wire rope to the cable grip.
Section 2. Instrument Setup For extended deployment time, lithium batteries are a good alternative to alkaline batteries. Campbell Scientific sells a D-cell-sized battery spacer (pn 21906) that allows lithium D-cell batteries to be used with the OBS-3A. Lithium D-cell batteries have a higher voltage than their alkaline counterparts, necessitating the spacer. Campbell Scientific does not sell lithium D-cell batteries.
Section 2.
Section 3. Operations 3.1 Software Installation Insert the CD and select “Install OBS-3A Utility”. Follow the installation wizard to install the software. This utility is your interface with the OBS-3A. As part of the installation, a system-maintenance program is included. Communication drivers exist on the CD. The main purpose of this section is to explain how to program and operate the OBS-3A with the OBS-3A Utility.
Section 3. Operations Data received from OBS-3A while it is connected to the PC will be stored in this file (see FIGURE 3.2-3). FIGURE 3.2-3. Data Window (above) and OBS-3A Utility’s Toolbar 3. Connect the OBS-3A to a PC with the test cable (FIGURE 3.2-4). FIGURE 3.2-4.
Section 3. Operations 4. Click Connect/Disconnect to get a green light and synchronize the OBS-3A clock with your PC by clicking . 3.3 Pull-Down Menus OBS-3A Utility has four pull-down menus for Files, OBS, View, and Help (see FIGURE 3.3-1). FIGURE 3.3-1. OBS-3A Utility Pull-Down Menus The Files menu allows you to select the location and formatting for OBS files. Files can be opened as plots or ASCII text that can be brought into spreadsheet programs or text editors.
Section 3. Operations box to select ONLY change host computer port. Then click Apply and the button. If you get the OBS-3A 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. FIGURE 3.4-1. Dialog Box for Changing Baud Rate 3.5 Testing Sensors 1. Before daily operations and deployments, verify the instrument works by clicking Open Plot, and then clicking installed sensors and click Start Survey. 2. Survey.
Section 3. Operations FIGURE 3.5-1. Test Data Sample FIGURE 3.5-2.
Section 3. Operations 3.6 Water-Density and Barometric Corrections Since depths are estimated from pressure measurements, it is important to set the water temperature and salinity so the OBS-3A can correct for water density and calculate depth in meters or feet (this will not affect temperature or salinity measurements). Also, the sensor measures absolute pressure so another correction must be made for barometric pressure. Be sure to do this while the OBS-3A is at the surface.
Section 3. Operations length or the 2048 / rate. Note: the product of the rate and the duration cannot exceed 2048. Rate: Rate is the frequency of sampling for the duration of measurements. All sensors are sampled at the same rate, typically 2, 5, 10, or 25 times per second (Hz). For example, a rate of 25 Hz for a 60-second duration will produce a sample with 1500 measurements for each sensor. When wave statistics are chosen, the rate must be selected in the Wave Setup box.
Section 3. Operations interval that will show the changes in turbidity and water depth that you wish to investigate. Rate sets the number measurements per second, in Hz, taken during a sample. 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 the period of time for measurements and how long sensor outputs will be averaged.
Section 3. Operations 3.11 Surveying If you have a pressure sensor, click the OBS menu and select Barometric Correction (do not do this when the OBS-3A is submerged). The OBS-3A takes about five seconds to measure the surface pressure and compute a barometric correction. 1. Connect OBS-3A to PC with survey cable. 2. Use to select: sensors, lines per minute, depth units (Meters or Feet), water Temperature, and Salinity. Selection of temperature and salinity only affects the depth calculation.
Section 3. Operations 4. A file for logging data was created when you started the OBS-3A Utility. Open and import the log file You can review data at any time with directly into an Excel spreadsheet for post-survey processing and plotting (see Section 3.18—Excel Spreadsheets) 3.12 Cyclic Sampling This mode is for logging data at regular time intervals such as 1, 10, 15, 30, etc. minutes for example. 1. Request Barometric Correction from the OBS menu. Be sure to do this while the OBS-3A is at the surface.
Section 3. Operations 3.13 Scheduled Sampling Use this mode when you want the OBS-3A to sample at specific times, scheduled by hours and minutes, on a 24-hour clock. 1. Request Barometric Correction from the OBS menu. Be sure to do this while the OBS-3A is at the surface. Doing so when the instrument is submerged will result in large depth errors. 2. Click 3. Use the Start Times block to schedule sampling. 4. Click Start Sampling to record data.
Section 3. Operations 4. Select setpoint values for transitions to fast sampling (SLOW>>>FAST) and slow (FAST>>>SLOW) rates. 5. Select one of the five logic criteria with the radio buttons. 6. Click Start Sampling to record data. Monitor the data window to verify that data are being logged. 7. Switch the COM port off (red) with the 8. Unplug test cable; install dummy plug and locking sleeve. The instrument is ready for deployment. icon 3.15 Data Retrieval 3-12 1.
Section 3. Operations 7. Wait for the progress bar to disappear and examine data as a plot or test file (Section 3.17—Graphing and Printing). 3.16 Shutdown From the OBS menu (see Section 3.11—Surveying), select Sleep. See menus shown in the following section. 3.17 Graphing and Printing 1. Use File menu to select how data file will be opened. 2. and select a file to view. Print will print a graph when Click data file is Open As Plot.
Section 3. Operations 3.18 Excel Spreadsheets To make an Excel spreadsheet from OBS-3A data, start Excel and set file type to All. Open a data file and select Delimited in Step 1 of 3 of the Text Import Wizard. Click Next > and select the delimiter Space; Treat consecutive delimiters as one; and {none} for Text qualifier. In Step 3 of 3, select the General Column data format and click Finish.
Section 3. Operations 3.19 Erasing Data Memory To erase the flash data memory, do the following: 1) Click on the Terminal Mode Icon. 2) At the OBS> prompt, type ‘sl 543210’ to unlock the system. 3) Type ‘ef 33’ plus Enter, then ‘ef 55’ plus Enter. 4) The erased-block-interval counter will be displayed every 100 blocks. There are 8192 blocks and the process takes ~ 1/2 hour. 5) When done, type ‘fw’ to reset the file pointer.
Section 3.
Section 4. Troubleshooting This section will help you isolate problems that can be easily fixed such as cable-continuity, processor reset, and battery replacement from serious ones such as sensor, computer and electronic malfunctions, and damaged mechanical parts that will require our help. The problem symptoms are shown with underlined, bold text. FIGURE 3.19-1. Component locations Unit does not communicate with PC. There are several possible causes for this symptom. 1.
Section 4. Troubleshooting 4. The OBS-3A and PC are not set to the same baud rate or communication protocol (e.g. RS-232, USB, RS-485). x and check port settings on the serial port tab. The default Click baud rate is 115.2 kb. If the PC is not set to this speed, follow the steps in Section 3.4 to set it. x If the OBS-3A still fails to respond, try changing PC speeds and until communication is established (e.g. 57.6, 38.4, 19.6, clicking 9.6 kb, etc.). If this fails, switch the PC back to 115.
Section 4. Troubleshooting Bright sun near the surface ( < 2 meters) 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. Changing the water temperature in the setup dialog box does not change the temperature measurement. This is normal. Temperature inputs only change the water density correction used to convert pressure to depth.
Section 4.
Section 5. Maintenance 5.1 OBS Sensor The OBS sensor must be kept clean to measure sediment concentration or turbidity accurately. A gradual decline in sensitivity over a period of time indicates fouling with mud, oil, or biological material. Regular cleaning with a water jet, mild detergent and warm water, or a Scotch-bite abrasive pad will remove most contaminants encountered in the field.
Section 5. Maintenance during surveys or after each deployment. A sensor that has been stored dry should be soaked in water for 15 minutes prior to use. If the sensor becomes fouled with sediment, oil, or biological material, conductivity will decline over a period of time indicating cleaning is necessary. If a water jet fails to remove contaminants, the sensor can be flushed with hot soapy water or warm alcohol. Do not use solvents.
Section 5. Maintenance carefully inspected every six months and serviced before all deployments longer than one month. 1. Disassemble O-ring seals and inspect mating surfaces for pits and scratches. 2. Inspect O-rings for cuts and nicks; replace if necessary using spares provided. 3. Clean O-rings and mating surfaces with a cotton swab and alcohol. Remove fibers from groove and mating surfaces then grease O-rings with DOW Compound 55 and reassemble. 5.
Section 5.
Section 6. Calibration 6.1 Turbidity This section briefly describes the materials and equipment you will need and the basic procedures for calibrating OBS sensors with AMCO Clear and sediment. All sensors are factory calibrated with AMCO Clear and include a calibration certificate expressed in nephelometric turbidity units (NTU). AMCO Clear is available from GFS Chemicals Inc. (800-858-9682; www.gfschemicals.com).
Section 6. Calibration 6.1.2 Preparation 1. Experience has shown that only three calibration points are needed to get sub-1% accuracy. It is recommended to do one for clean filtered water, a midrange value (e.g. 125, 250, 500 or 1000 NTU) and one at the high end of the desired measurement range (e.g. 250, 500, 1000, or 2000 NTU). 2. Scrub the sensor, container, spoon, and glassware with detergent and water and rinse everything twice with filtered water. 3.
Section 6. Calibration WARNING There is no way to cancel a calibration after the Compute button is clicked. Review the quality assurance checks discussed later in this section. Look at the plot of residuals, which show the differences between the standard and computed NTU values. The average residual magnitude should be less than 1% of the calibration range. For example, a calibration range of 2000 NTU (maximum minus minimum NTU values) should produce an average residual less than 20 NTU.
Section 6. Calibration Where: Tstd = Turbidity of the standard solution; Tstk = Turbidity of the stock solution, usually 4000 NTU; Vstk = Cumulative volume of stock solution at each calibration point; Vdw = Initial volume. TABLE 6.1-1. Mixing Volumes for Formazin Standards Formazin Volume Vstk (ml) Solution Turbidity Tstd (NTU) 12.7 50 32.3 125 66.7 250 143 500 333 1000 1000 2000 6.2 Sediment The procedure for sediment calibration is more involved than for turbidity.
Section 6. Calibration x 2 gallons filtered distilled water (purified water from the super market works fine), x Hand-drill motor, x Paint stirrer. 6.2.2 Sediment Preparation Sediment preparation is a critical factor in calibration quality. It is most convenient to use dry material because it can be accurately weighed with an electronic balance. However, this only works well for clean sand because disaggregation produces a sediment size different than existed in the field.
Section 6. Calibration Preparation 1. Clean containers and glassware with detergent and rinse with filtered water. 2. Perform the calibration under fluorescent lighting. 3. Based on the material, select the appropriate sample duration from TABLE 6.2-1. 4. Open the calibration box with values. 5. After each addition of sediment compute mg/l or ppm with the equations given below. 6. Start the OBS-3A Utility and wake the OBS-3A. Click the mg/l or ppm button.
Section 6. Calibration 9. Weigh 5 to l0 equal increments of the sediment so that the total dry weight will produce the maximum concentration expected at the monitoring site. 10. For each sediment standard, repeat Steps 2 through 4 of Section 6.1.3. 11. After all sediment values have been logged, follow Steps 6 through 9 of Section 6.1.3 to complete the calibration. 6.
Section 6.
Section 7. Optics and Turbidity Measurements Turbidity is the cloudy appearance of a liquid produced by light scattered from suspended matter. It is an apparent optical property that depends on the size, color, and shape of scattering particles, and the instrument used to measure it. In accordance with standard method 2130B and ISO 7027, turbidity is usually measured with a 90o-scatterance nephelometer and reported in nephelometric turbidity units (NTUs). Turbidity standards are discussed in Section 6.
Section 7. Optics and Turbidity Measurements Can turbidity be converted to suspended solids concentrations and viseversa? In most situations, conversions between turbidity and suspended solids concentrations will give misleading results because the conversion equates to an apparent optical property, in relative units, with one precisely defined in terms of mass and volume; these are "apples and oranges".
Section 8. Factors Affecting OBS Response This section summarizes some of the important factors that affect OBS-3A measurements and shows how ignoring them can lead to erroneous data. If you are certain that the characteristics of suspended matter will not change during your survey and that your OBS was factory calibrated with sediment from your survey site, you only need to skim this section to confirm that no problems have been over looked. 8.
Section 8. Factors Affecting OBS Response 8.2 Suspensions with Mud and Sand As mentioned earlier, backscattering from particles is inversely related to particle size on a mass concentration basis (see FIGURE 8.2-1). This can lead to serious difficulties in flow regimes where particle size varies with time. For example, when sandy mud goes through a cycle of suspension and deposition during a storm, the ratio of sand to mud in suspension will change.
Section 8. Factors Affecting OBS Response 5 A Signal (V) 4 3 2 1 0 0 5 10 15 Sediment Concentration (g/l) FIGURE 8.3-1. Response at High Sediment Concentrations 8.4 Sediment Color Sediment color, after particle size, has a major effect on OBS sensitivity, and if it changes, it can degrade the accuracy of measurements. Although OBS sensors are “color blind”, “whiteness”, color, and IR reflectivity (measured by an OBS sensor) are well correlated.
Section 8. Factors Affecting OBS Response 1.0 Calcite Infrared Reflectance 0.8 Bytownite 0.6 Actinolite 0.4 0.2 Magnetite 0.0 0 2 4 6 8 10 Munsell Value (Black = 0) FIGURE 8.4-1. IR Reflectance of Minerals 8.5 Water Color Several OBS users have been concerned that color from dissolved substances in water samples (not colored particles discussed in the previous section) produces erroneously low turbidity measurements.
Section 8. Factors Affecting OBS Response in this region. OBS sensors are also more sensitive to mineral particles than either bubbles or particulate organic matter by factors of four to six. In most environments, interference from these materials can therefore be ignored. One notable exception is where biological productivity is high and sediment production from rivers and resuspension is low. In such an environment, OBS signals can come predominately from plankton.
Section 8.
Section 9. References See www.campbellsci.com/obs for a complete list of references. 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 and W.E. Asher. 1997. The Effects of Colored Water and Bubbles on the Sensitivity of OBS Sensors. American Geophysical Union, Fall Meeting, San Francisco, CA. Downing, John and Reginald A. Beach.
Section 9. References U.S. Geological Survey. 2003. National Field Manual of the Collection of Water-Quality Data. Book 9, Handbooks for Water-Resources Investigations. Zaneveld, J.R.V., R.W. Spinrad, and R. Bartz. 1979. Optical Properties of Turbidity Standards. SPIE Volume 208 Ocean Optics VI. Bellingham, Washington. pp. 159-158.
Section 10. Specifications MEASUREMENT RANGE Turbidity (AMCO Clear) ............................... 0.4 to 4,000 NTU1 Mud (D50=20μm) .......................................... 0.4 to 5,000 mg/l Sand (D50=250μm) ........................................ 2 to 100,000 mg/l Pressure2 ........................................................ 0 to 10, 20, 50, 100, or 200 m Temperature................................................... 0o to 35oC Conductivity (salinity) ...................................
Section 10. Specifications CSI # 21135 End Cap Screws, 8-32 x 3/8”, socket (5/64”) CSI # 21120 Dummy Plug CSI # 21122 Plug Locking Sleeve, Subconn® MCDLSF CSI # 425 Alkaline D-Cells Batteries CSI # 21136 Screws, #4-40 x 1/4”, socket CSI # 20792 OBS-3A Test Cable, 2 m (6.5 ft) CSI # 21149 Hex Driver, 5/64” 10-2 1 0-100, 0-250, 0-500, 0-1000, 0-2000, and 0-4000 NTU ranges are available. 2 Range depends on pressure sensor option chosen.
Campbell Scientific Companies Campbell Scientific, Inc. (CSI) 815 West 1800 North Logan, Utah 84321 UNITED STATES www.campbellsci.com • info@campbellsci.com Campbell Scientific Africa Pty. Ltd. (CSAf) PO Box 2450 Somerset West 7129 SOUTH AFRICA www.csafrica.co.za • cleroux@csafrica.co.za Campbell Scientific Australia Pty. Ltd. (CSA) PO Box 444 Thuringowa Central QLD 4812 AUSTRALIA www.campbellsci.com.au • info@campbellsci.com.au Campbell Scientific do Brazil Ltda.
