FlowTracker User`s Manual (English)
Contents
1. ble from the CastAway CTD e The density of water which can be calculated based on temperature and salinity e The location of the measurement specifically latitude and altitude This data is recorded from the GPS onboard the CastAway CTD With the above data we can convert pressure to depth with considerable accuracy e Gravity is calculated using the WGS84 ellipsoidal gravity formula using local latitude and al titude measured by the GPS o If there is no information from the GPS a default latitude of 30 degrees and a default alti tude of zero are used These defaults can be changed in the software 5 5 1 o These calculations are taken from the WGS84 Ellipsoidal Gravity Formula Department of Defense World Geodetic System 1984 e The depth is calculated from equation 25 of Millard amp Fofonoff 1983 and includes a full integration of the geopotential anomaly over the entire cast to account for changes in water density 74 CastAway CTD User s Manual July 2010 YSI A 7 Best Measurement Practices Making measurement with the CastAway CTD is quite simple the basic steps are described in detail in the Users Manual Section 2 and Section 3 There are a few key items that should be kept in mind to ensure the best quality data e At the start of the cast hold the system underwater near the surface for 5 10 seconds This allows the temperature and conductivity sensors to adjust from air to water conditions and avoids prob2. 5 1 The depth is calculated from equation 25 of Millard amp Fofonoff 1983 and includes a full integration of the geopotential anomaly over the entire cast to account for changes in wa ter density CastAway CTD User s Manual July 2010 81 YSI A 10 Data Processing for Point Measurements Previous sections A 8 and A 9 describe data processing for a CTD cast in detail Some of the same methods apply to a CTD point measurement although the processing is simpler since only a single point is being measured A summary of processing for a CastAway CTD point measurement is below For a point measurement only two data types are available selecting the Down Up Cast data shows the same data as selecting Processed data e Raw Data o Raw data is exactly that the raw samples of conductivity temperature and pressure data versus time No derived parameters specific conductivity salinity sound speed density or depth are available o Pressure data has been corrected for atmospheric pressure using data collected in the air at the start and end of the measurement This sets pressure at the water surface to zero o Raw data is used only for specialized applications In most cases it is better to use the Processed data o Raw data includes all data during the entire measurement in air at the beginning in the water and in the air at the end e Processed same as Down and Up Cast o Pressure data has been corrected for
3. O O 0O 0O 0 0 When it starts the down cast When it reaches the bottom maximum pressure When it starts the up cast When it reaches the surface and leaves the water The downcast is defined as measurements made before reaching the maximum depth The up cast is defined as measurements made after reaching the maximum depth There is only one down cast and one up cast for each CTD cast If multiple up and down motions are contained in a sample period the up and down casts are separated by the point of maximum pressure The maximum pressure is the maximum pressure measured while the fall velocity is greater than 0 05 decibars second This sets the maximum pressure to the measurement taken just as the system hits the bottom or the end of the casting line e Based on the maximum measured pressure O O O O An array of pressure bins is created from the surface downward Each bin has a width of 0 3 decibars The center of the first bin is located at 0 15 decibars the second at 0 45 decibars the third at 0 75 decibars and so on The reported pressure is always the center of the bin except for the last deepest bin The reported pressure in the last bin is set to the maximum measured pressure e The temperature and conductivity from down cast samples and up cast samples are averaged separately into 0 3 decibar bins O 80 If the absolute value of the fall velocity of the system is less than 0 025 decibars second during
4. water so that they can precisely calibrate the ranges of their echo sounding equipment Hydrologists employ CTDs to detect salt water intrusion and other contaminants in rivers lakes wells and reservoirs Biologists in the aquaculture and fisheries industries regularly collect CTD data to ensure the health and well being of the fish they monitor Coastal engineers use CTDs in rivers inlets and estuaries to study local tides Even fisherman use CTDs to locate parcels of water that are attractive to certain species of fish CastAway CTD Users Manual July 2010 69 YSI A 4 Water Layers Temperature Salinity and Density The water in almost every lake river estuary and ocean is stratified into different layers The formation of these layers is usually driver by temperature salinity and or and density In gener al cold water is denser than warm water while salt water is denser than fresh water Denser wa ter usually tends to settle near the bottom while less dense water tends to float towards the sur face although there are exceptions to this Figure 37 is a graphic representation of intersecting layers of water where a freshwater river is flowing into the ocean Because the fresh water is less dense it floats near the surface as it enters the ocean Fresh water wedges can extend out into the ocean for several kilometers and salt wa ter wedges can migrate similar distances up rivers during high tides Figure 37 Bou
5. A higher flow of water through the cell provides the best measurements o The measured temperature will increase slightly with time when the water or the CTD is stationary o Thus when combining measurements we weight the measured data based on the vertical speed of the system which is determined by monitoring the change in pressure For near surface data the up cast is weighted more heavily than the down cast This is be cause the up cast will have been moving at a steady rate for some time while the down cast has just started moving providing more reliable data For near bottom data the down cast is weighted more heavily than the up cast This is be cause the down cast will have been moving at a steady rate for some time while the up cast has just started moving Below is a step by step description of the data processing steps used to convert Raw CTD data in to the Processed profile Pressure is corrected for the ambient atmospheric pressure o Pressure data is collected at the start and end of the cast while the system is in the air o This air pressure is subtracted from the raw pressure data to get a measure of water pres sure only If there is a difference in the start and end air pressure a linear correction is applied over the course of the cast Next we calculate how pressure changes with time o We calculate the rate of change of pressure versus time for each sample in the data file The rate of change in pressu
6. YSI Appendix A CastAway CTD Principles of Operation suas CastAway amp CTD Most people probably think water is simply water But if we dive a little deeper into the details water is a complex fluid It is arguably the most important molecule on this planet and quite pos sibly the most interesting It exists in a variety of states from ice to steam and clouds to rain It can dissolve rocks and minerals You can walk on it when it s frozen swim in it when it s liq uid and even drink it when you re thirsty It sustains just about every life form on earth and it is an ecosystem unto itself We ve built spacecraft to look for it and submarines to explore it Itis no wonder that oceanographers and hydrologists dedicate so much of their time to studying wa ter The CastAway CTD will open new doors in the science of water The small handheld size makes collecting data easy enough for a single person to do just about anywhere The integrated LCD screen displays a simple yet comprehensive user interface along with instantaneous data feedback You can view graphs and statistics from your casts without the need for a separate computer The integrated GPS for precise location data completes the picture of a functional and field friendly solution for capturing high quality data with a minimum of time and effort Coastal and inland water profiling has never been easier A 1 Purpose The CastAway CTD Principles of Operation provides both e
7. a down cast this sample is not used for the bin average If the absolute value of the upward velocity of the system is less than 0 025 deci bars second during an up cast this sample is not used for the bin average All samples in the up and down portions of the cast that take place within a given 0 3 de cibar slice of the water column are averaged together using the fall velocity as a weight ing function Faster velocities have more weight in the average If there should be no measurements in a bin after filtering based on fall velocity a linear interpolation is applied to fill in missing samples in both the up and down casts using the adjacent bins that contain data If the surface bin or the last bin near the bottom is empty the next nearest value from the cast is used to fill that bin This is applied to the up and the down cast data separately The conductivity and temperature from the deepest two bins in the up cast are replaced with the measurements from the deepest two bins from the down cast to remove the ef fects of thermistor heating while resting on the bottom CastAway CTD Users Manual July 2010 YSI e The down cast samples and up cast samples are combined to get the final Processed data O O For bins with pressure greater than two decibars and less than 0 9 Maximum Pressure the up and down cast temperatures and conductivities are averaged to get the final Processed data For bins with pressures less than two deciba
8. atmospheric pressure using data collected in the air at the start and end of the cast This sets pressure at the water surface to zero o We calculate the rate of change of pressure versus time which is the vertical speed of the system through the water We use this to eliminate data collected in the air o We then look for the point where the change of pressure is greater than 0 15 decibars from the first recorded point This typically occurs when the system is first submerged into the water o We wait 2 seconds and take one sample of conductivity temperature and pressure This single sample is used as the point measurement o After the Processed temperature conductivity and pressure sample is selected The derived parameters of specific conductivity salinity sound speed density and depth are calculated 82 CastAway CTD User s Manual July 2010 YSI A 11 References International Temperature Scales of 1948 1968 and 1990 http www ices dk ocean procedures its htm 4 28 2010 7 24 44 PM Millard amp Fofonoff Algorithms for computation of fundamental properties of seawater UN ESCO technical papers in marine science 44 1983 WGS84 Ellipsoidal Gravity Formula Department of Defense World Geodetic System 1984 NIMA TR8350 2 3 edition amendment 1 January 2000 Technical Report pages 3 5 to 4 2 It is worth noting that a new standard for CTD calculations has been proposed to the scientific community the Thermo
9. dynamic Equation of State of Seawater TEOS 10 However the me thods shown in the UNESCO 1983 publication are still the most commonly used calculations and are the only ones supported by the CastAway CTD Users interested in applying the TEOS 10 equations can export raw data from the CastAway CTD to perform these calculations CastAway CTD User s Manual July 2010 83
10. e system through the water We use this to eliminate data collected in the air or while sta tionary at the surface or bottom as well as to separate the down and up casts i o We then combine the raw data from the down and up casts in 4 to vertical bins based on pressure All samples within a bin size of 0 3 decibars are averaged into a single value giving 12 profile data with regular spacing for easier post processing 13 o When looking at data from the down or up cast only the bin averaged values from the desired cast are shown Data when out of the water waiting at the surface or waiting at the bottom have been removed o The figure to the right shows temperature data from the down and up cast for the same file shown in Raw data The difference between the two casts are likely caused by internal waves propogating along the different z temperature and density layers 23 When you first view the data difference between the up and down cast in this data set it may appear that there is a problem with the sensor However there are many factors that can influence the distri 4 1 6 n 1 bution of measured parameters in the water column Internal waves are certainly one phenome non to be aware of but more often the differences will be from the CTD moving through differ ent columns of water Even though you may be stationary during your measurements the water could be moving Even enclosed bodies of water have thermally driven cur
11. equal to 35 grams of salt per litre of water Ocean salinity general ranges from 31 to 39 PSS 78 CastAway CTD User s Manual July 2010 73 YSI Using the EOS 80 equations salinity can be directly calculated from temperature conductivity and pressure measurements Millard amp Fofonoff 1983 The CastAway CTD records these pa rameters and calculates salinity onboard after each cast Sound Speed The speed of sound in water is a critical value for any application that makes use of underwater sound These applications include bathymetric surveys done in harbors estuaries rivers and coastal areas Detailed knowledge of the sound speed profile is needed to convert the acoustic data to accurate water depth information Sound speed can be calculated from temperature salinity and pressure Millard amp Fofonoff 1983 Since the CastAway CTD measures and calculates these parameters with each cast this data can be directly used for a variety of applications using underwater acoustics Density Density is the mass of a given volume of water It is important for converting pressure data to depth and also can be used to predict density driven currents Density can be calculated from temperature and salinity Millard amp Fofonoff 1983 Depth A CTD does not directly measure depth it measures pressure To accurately determine depth from pressure data we need several other pieces of information all of which are directly availa
12. ese depths vary slightly with changes in density caused by temperature and sa linity differences The CastAway CTD removes atmospheric pressure from the underwater pressure measurements this is sometimes referred to as gauge pressure The atmospheric pressure is measured before and after each deployment During post processing the change in atmospheric pressure over time is removed from the entire sample using a linear fit 0 0 10 10 20 20 30 30 T 40 40 oO a 2 2 s50 50 5 D 7 2 A 60 60 70 70 80 80 90 90 kar 12 14 16 18 1097 38 39 4 41 42 43 44 a Temperature C Conductivity uS cm x10 Figure 39 Example Temperature and Conductivity Profiles Figure 39 gives an example of the raw conductivity temperature and pressure data collected by the CastAway CTD this data was collected in the Pacific Ocean near San Diego California In this data a temperature gradient of about seven degrees exists in the upper layer of the ocean The thermocline is located at a pressure of about 30 decibar depth of about 30 meters Below this the temperature is much more stable The influence of temperature on conductivity is seen in the nearly identical vertical structure of the two measurements Since the salinity of the ocean is fairly constant in this area the change in temperature is mostly responsible for the change in conductivity To
13. ivity R is the measured resistance in ohms and K is the cell constant P A Lis the distance between the electrodes in centimeters and A is the cross sectional area of the vo lume of water enclosed by the cell in square centimeters The CastAway CTD measures resis tance in ohms and this is converted to conductivity reported in units of microsiemens per centi meter uS cm Temperature is measured using a thermistor Thermistors have a well known relationship be tween resistance and temperature resistance of a thermistor changes as the temperature changes The thermistor in the CastAway CTD is calibrated to the ITS 90 standard following the recom mendations of the Joint Panel on Oceanographic Tables and Standards and Saunders 1990 The CastAway CTD like all CTDs uses a pressure sensor to determine depth It is important to note that depth is not directly measured pressure is measured and depth is derived from this data The calculation of depth is described later A 6 but it is important to understand that al though depth and pressure are related they are not necessarily equal CastAway CTD User s Manual July 2010 71 YSI Scientists often use units of decibars for pressure because one decibar is approximately equal to a depth of 1 meter This approximation gets worse the deeper you go At a pressure of 100 deci bars the depth in the ocean is closer to 100 5 meters while at 10 000 dbars the depth is about 9 700 meters Th
14. lems in the first part of the down cast e For the down cast allow the system to free fall to the bottom o The size shape and weight of the CastAway CTD has been specifically designed to free fall at approximately 1 m s 3 3 ft s o Allowing a clean free fall ensures a steady flow of water through the flow through cell and past the sensors e For the up cast retrieve the system at a steady rate of about 1 m s 3 3 ft s o A steady rate ensures a steady flow of water past the sensors for the best quality data o Do not let the system rest on the bottom Begin the up cast as quickly as possible e Do not pause during either the down or up cast CastAway CTD User s Manual July 2010 75 YSI A 8 Overview of Available Data The user can view several types of data from the CastAway CTD Processed Down Cast Up Cast or Raw For most applications we recommend using the processed data without additional modification This section provides an overview of the different types of data available from the CastAway CTD Raw Data O 76 Raw data is exactly that the raw samples of conductivity temperature and pressure data versus time No derived parameters specific conductivity salinity sound speed density or depth are available Pressure data has been corrected for atmospheric pressure us ing data collected in the air at the start and end of the cast This sets pressure at the water surface to zero Raw data i
15. ndary Layers for a Fresh Water River Flowing into the Ocean Temperature conditions in one layer may be notably different from conditions in another layer The boundary between two layers with different temperature is called a thermocline Water of different salinities will often be stratified in layers especially where fresh and salt waters come together The boundary where there is a notable change in salinity is called a halocline Like wise a change in density is called a pycnocline In coastal waters the pycnocline usually coin cides with the halocline because water density is directly related to temperature and salinity Underwater acoustics are widely used to map the depth of rivers lakes and the ocean as well as to measure physical properties of the water and its motion Thermoclines and haloclines are areas of interest in the field of underwater acoustics because these boundaries have unique physi cal properties They are capable of reflecting sound from acoustic instruments as well as chang ing the direction or path of an acoustic beam through refraction The local change in water densi ty creates this unique boundary Thus measurements of different water layers can be essential for many studies using underwater acoustics 70 CastAway CTD Users Manual July 2010 YSI A 5 CTD Measured Data Flow Through Cell The CastAway CTD uses three sensors to profile condi tions in the water conductivity temperature and pressure The co
16. nductivity and temperature sensors are located in the flow through channel along the back of the CastAway CTD housing while the pressure sensor port passes through the housing at the top of the battery cap Figure 38 The flow through channel is designed to ensure a steady flow of water past the sensors when the system is Temperature Sensor Conductivity descending and ascending through the water column Electrodes Conductivity refers to the electrical properties of water or Pressure specifically how much electricity the water conducts Pure Sensor water has low conductivity while seawater has high con Port ductivity The conductivity of water is proportional to the concentration of ions in solution which carry the electrical current The most common ion in seawater is chlorine Figure 38 CTD Sensors primarily from dissolved salts To measure conductivity the CastAway CTD has six electrodes in its flow through channel Al though only two electrodes are required for a basic conductivity measurement using six elec trodes within the enclosed channel improves the accuracy of the conductivity measurement while increasing its immunity to calibration errors that plague many conductivity measurements Two electrodes generate an electrical current and the remaining four are used to measure the resistivity of the water contained within the cell Resistivity and conductivity are related by the following equation c R C is conduct
17. re can be thought of as the vertical speed of the system through the water o We set a minimum value for the rate of change of 0 025 decibars second If the rate of change of pressure is less than this we consider the system to be stationary o The sensors and flow through cell are designed for measurements while the system is in motion while processing the data we discard any samples where the system is consi dered to be stationary The conductivity data is de spiked This is to remove erratic measurements near the water surface that can be caused by air bubbles trapped in the conductivity flow cell or measure ments made when the system is only partially submerged o Using a one second moving window the standard deviation of the conductivity measure ments is computed o Ifthe center measurement in the window is more than 3 standard deviations from the mean value of the windowed samples this spike is replaced with the mean of the win dowed measurements o After filtering conductivity samples measured when the pressure is less than 0 15 deci bars are replaced with the nearest conductivity measurement in time taken at a pressure greater than 0 15 decibars This is to preserve as much of the cast as possible before mea surements in air contaminate the data To separate the down and up cast we monitor the change in pressure to determine o When the system first enters the water CastAway CTD User s Manual July 2010 79 YSI 0O
18. rents that you can t see from the surface As a result your down cast and up cast could be through two different vo lumes of water Changes in water temperature and conductivity can be abrupt especially near merging bodies of water like lakes and rivers or rivers and oceans These areas will often pro duce interesting data CastAway CTD Users Manual July 2010 77 YSI e Processed Temperature C o This is the most common form of the data from the CastA 9 way CTD and is what we recommend for the majority of our 1 users o In this form data from the down and up cast have been com bined to give the best possible measurement of the true pro 4 file 5 o The simplest description of CastAway CTD data processing i is that Processed profile is a weighted average of the down and up casts based on fall velocity Up Casts are more heavily weighted near the surface and down casts are more mA heavily weighted near the bottom 1 o Details of how data are processed to produce the final profile 12 are given in section A 9 13 o The figure to the right shows an example plot of processed 14 CastAway CTD data using the same data shown for Raw and Down Up casts 14 15 16 17 1 78 CastAway CTD Users Manual July 2010 YSI A 9 Data Processing Algorithms When processing data from the CastAway CTD we take a number of things into account The sensors work best when they have a steady flow of water through the cell o
19. rs the down cast is weighted by the square of half of the bin pressure before averaging with the up cast This is an exponential decay weighting function on the down cast data near the water surface This weighting function is shown for temperature below the same formula applies to conductivity 2 P 6 i T Down F Top DownCast P lt 2dbar T 7 Processed P 2 1 2 For bins with pressures greater than 0 9 Maximum Pressure only the down cast data is used e After the final Processed temperature and conductivity profiles are computed O O The derived parameters of specific conductance salinity sound speed density and depth are calculated for each pressure bin Algorithms for these derived parameters are taken directly from Millard amp Fofonoff 1983 using the 1980 International Equation of State for Seawater EOS 80 Temperatures are multiplied by 1 00024 to convert from ITS 90 to ITPS 68 before com puting the EOS 80 derived parameters The up down and processed output temperatures are in ITS 90 the conversion to ITPS 68 is only applied for EOS 80 calculations e The final step is to convert the pressure bins to depth bins O Gravity is calculated using the WGS84 ellipsoidal gravity formula using local latitude and altitude measured by the GPS If there is no information from the GPS a default latitude of 30 degrees and a default altitude of zero are used These defaults can be changed in the software 5
20. s used only for specialized applications In most cases it is better to use either the Processed data or the Down Up Cast data Raw data includes all data during the entire cast in air at the beginning at the surface down cast at the bottom up cast and in the air at the end The figure to the right shows raw temperature data from a sample cast collected in a reservoir in San Diego California Notice the variation in temperature at the top indicating some of that data was collected in air This data is re moved by the CastAway CTD processing Notice the temperature at the bottom When the system first hits the bottom the temperature is accurate As the system sits on the bottom motionless the thermistor be gins heating the water around it This only occurs when the system or the water is not moving This heating is a result of the electrical current that is required to read the temperature sensor This data is removed by the processing ao nm hk ow hf Temperature C CastAway CTD Users Manual July 2010 YSI e Down and Up Cast Temperature C o Converting raw data to down and up casts takes several steps 5 o Pressure data has been corrected for atmospheric pressure us 1 ing data collected in the air at the start and end of the cast This sets pressure at the water surface to zero o We calculate the rate of change of pressure versus time which is the vertical speed of th
21. separate the influence of temperature on conductivity scientists often use a de rived parameter called specific conductivity A 6 72 CastAway CTD User s Manual July 2010 YSI A 6 CTD Derived Parameters In addition to conductivity and temperature a number of valuable parameters can be derived from the data collected by a CTD Using an advanced microprocessor the CastAway CTD pro vides four of the most commonly derived parameters after the completion of each cast specific conductance salinity sound speed and depth In the CastAway CTD Software you can also view density To derive these additional parameters the CastAway CTD uses established internationally rec ognized methods These methods have been compiled into something known as EOS 80 the International Equation of State for Seawater 1980 The equations were published by UNESCO in 1983 A 11 Specific Conductance The CastAway CTD measures the conductivity of the water which is primarily a function of the concentration of ions in the water However conductivity is also affected by water temperature To remove temperature effects we derive a parameter called specific conductance Specific con ductance may be a more useful parameter for certain applications Hydrologist sometimes use specific conductance to identify pollution events in fresh water Be cause pesticides and fertilizers increase the conductivity of water agricultural runoff can often be de
22. tected by analyzing profiles of specific conductance in fresh water basins The calculation of specific conductance from conductivity is a fairly simple formula This is the standard method for applying a temperature correction to any conductivity measurement the slope depends on the material measured The slope of 0 020 used here is for water C SpC 1 0 020 T 25 Where e SpC Specific conductance uS cm e C conductivity uS cm e T temperature C Salinity Prior to the 1970s the salinity of water was measured primarily through the chemical analysis methods of titration and evaporation In the mid 1960s oceanographers began using conductivi ty meters to analyze water samples exploiting the direct correlation between the electrical con ductivity of water and the amount of ions it contained Since Chlorine from salt is the primary ion the amount of ions can be directly related to the amount of salt in the water Historically salinity was defined in terms of parts per thousand ppt A salinity of 35 ppt meant 35 pounds of salt per 1 000 pounds of seawater In 1978 oceanographers redefined salinity in terms of the Practical Salinity Scale PSS 78 based on the conductivity ratio of a sea water sam ple to a standard potassium chloride KCl solution at 15 C and atmospheric pressure Ratios have no units so technically the salinity given by PSS 78 has no units and it is not the same as ppt a salinity of 35 PSS 78 is not
23. xperienced and novice users with a technical description of the CastAway CTD and the data it records It includes basic advice on methods for collecting the best CTD data and how to interpret your data This document is in tended to work in tandem with the CastAway CTD Users Manual which describes the details for operating the system A 2 What is a CTD A CTD measures conductivity and temperature profiles with respect to depth The acronym CTD stands for Conductivity Temperature and Depth For a CTD profiling refers to a series of measurements collected as the instrument itself descends from the surface of the water to the bot tom and back up again To collect profile data a CTD is typically lowered from a boat or dock by hand or with a winch Conductivity temperature and pressure are recorded while descending and ascending This data provides a complete picture of water properties that change with respect to depth Using these three measurements and applying the appropriate equations we can calculate salinity sound speed density and depth to name a few of the more commonly sought parameters 68 CastAway CTD User s Manual July 2010 YSI A 3 Who Uses CTDs The CTD is a versatile tool that is used by many different groups of people Oceanographers studying and modeling the intricate details of seawater consider the CTD to be one of their most essential tools Hydrographic surveyors use CTDs to characterize the speed of sound in
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