to begin this procedure , we ensure that all scuva components have sufficient battery power , recording tape ( for the high - definition or hd video camera ) , and function properly . depending on the flows to be measured , select video camera resolution and frame rates that yield best results for digital particle image velocimetry ( dpiv ) .
prepare the laser and camera housings for use by cleaning the o - ring grooves and o - rings with a clean towel or wipe .
spread manufacturer provided o - ring grease evenly on the o - rings and replace them in the housing grooves .
in addition , clean the laser and camera housing apertures to prevent laser sheet deformation and marks on the camera housing lens . check the o - ring seals by placing both empty housings in a tub full of water .
weighted objects will need to be placed on top of the housings to submerge them since the housings float when empty .
after 5 to 10 minutes , remove the housings from the tub and towel dry the outside .
check whether there is any moisture inside the housings . also consider using disposable , paper moisture strips during the pressure test to indicate whether there is moisture in the housings after the test .
after the housings pass the pressure test , place scuva components inside the housings .  
ensure that the lights are oriented in such a way that they illuminate the area directly ahead of the camera and operator , and do not interfere with maintaining grip on handles and operation of camera controls . in a low light environment , ensure that the laser beam is properly aligned relative to the optical lens installed in the laser housing .
when properly aligned , the laser / lens combination will create a vertical sheet of light that is oriented perpendicular to the camera housing . for safety , use a temperature - sensitive sheet of paper to determine laser sheet orientation .
using scuva attachments and the rigid , extendable arm , connect the laser housing and the camera housing to each other .
ensure that the housings are firmly attached and that the housings can not rotate with respect to each other .
it is critical that the laser sheet remains oriented perpendicular to the camera s field of view throughout the measurement . due to the current capabilities of scuva
, measurement dives can only be conducted in low - light locations or at nighttime to prevent natural light interference with the laser sheet .
the camera housing has a built - in electronic moisture sensor that provides visual warnings ( flashing led lights ) in case of moisture in the camera housing .
once attached to the apparatus , release scuva to determine the buoyancy characteristics of the device .
depending on the buoyancy characteristics , attach buoyancy foam or lead weights to one or both housings to ensure neutral buoyancy and prevent rotation of the apparatus in water .
position the laser using the extendable arm sufficiently far from the diver to minimize measurement of diver - induced flows .
any measurements of diver - induced flows near the target introduce error and are not used for subsequent analysis .
adjust the camera zoom until the field of view frames the target and surrounding fluid . while keeping the apparatus stationary , focus the video camera on the laser sheet until particles appear sharp and in focus .
once the laser sheet plane is in focus , switch the camera to the manual focus mode .
this will prevent the camera from refocusing on any objects that appear in the field of view during measurement , resulting in blurred particles in the laser sheet .
to calibrate scuva , place an object with known dimensions in the laser sheet within the video camera 's field of view .
after the dive , an image will be extracted from this video sequence to determine a calibration constant that converts the field of view size from units of pixels to cm .
if at any time the operator adjusts the field of view size be re - position the extendable arm or changing the camera zoom during the dive , steps 12 and 13 will need to be repeated .
the environmental bulk flow properties need to be determined . if present , the current direction will dictate apparatus and diver positioning relative to the target during measurements .
the direction of bulk flow surrounding the target can be inferred by observing bubbles exhaled from the diver and noting their lateral motion .
in addition to bubbles , a small quantity of fluorescent dye ( i.e. , fluorescein ) can be released to determine the current direction .
since diver - generated flow can be a source of dpiv measurement error , the diver should not be located upstream of the target .
in addition , the laser sheet should be positioned parallel to the direction of current so as to maximize particle residence time within the laser sheet , thereby minimizing dpiv errors . however , if no current or bulk flow is present , diver and scuva positioning relative to the target are unrestricted .  
if attempting to record the flow surrounding a moving target first predict the location of the target , and then position scuva to the predicted location while remaining motionless . as the target moves through the camera s field of view ,
if the target is motionless , frame the target and surrounding fluid in the video camera s field of view and begin recording while remaining motionless .
the operator should refrain from rotational and out - of - plane motions during video recording since these motions result in erroneous dpiv results .
therefore , measurements collected during rotational and out - of - plane diver motions will not be used for further data analysis . once video collection is complete , turn off all components of scuva and restore the laser arm to its retracted position .
remove scuva from the water and detach the camera and laser housings from the arm .
rinse or soak the apparatus in fresh water before drying to prevent rusting of the apparatus . once the housings are dried , remove components from the housings , and recharge and replace batteries if needed for another dive .  
connect the video camera to a computer and extract video from the hd tape by using a hd video software package ( i.e. , adobe premiere pro or imovie ) .
after the video is extracted , determine the range of video to be converted into a series of images for dpiv analysis .
ensure that the pixel aspect ratios and extracted image sizes match the hd video settings .  
these images are imported to a dpiv processing program ( i.e. , davis or matpiv ) .
after proper selection of calibration constant and image capture parameters , which are prompted from the dpiv software package , velocity fields can be generated from consecutive particle images .
additional post - processing steps , depending on the quality and types of measurements , can also be applied . when the protocol is done correctly
, the particle images surrounding the target will be sharp and easy to distinguish . using the particle fields captured in situ by scuva s video camera ( figure 1a ) and a dpiv processing software package , velocity fields of flow surrounding the target ( figure 1b )
vectors in the velocity field indicate magnitude and direction of the local flow velocity . if sufficient video is collected to provide a time series of images , a time series of velocity fields can also be determined .
figure 1  measured in situ particle fields ( a ) surrounding aurelia labiata . corresponding velocity field ( b ) with yellow vectors indicating flow direction and magnitude .
red arrow in a indicates a region of high reflectivity , which results in saturation of the image , making it difficult to distinguish between particles and the target .
red arrow in b indicates a region of streaking that results when the flow rate is not sampled at a high enough frequency .
when the protocol is done correctly , the particle images surrounding the target will be sharp and easy to distinguish . using the particle fields captured in situ by scuva s video camera ( figure 1a ) and a dpiv processing software package , velocity fields of flow surrounding the target ( figure 1b ) will be revealed .
vectors in the velocity field indicate magnitude and direction of the local flow velocity . if sufficient video is collected to provide a time series of images , a time series of velocity fields can also be determined .
figure 1  measured in situ particle fields ( a ) surrounding aurelia labiata . corresponding velocity field ( b ) with yellow vectors indicating flow direction and magnitude .
red arrow in a indicates a region of high reflectivity , which results in saturation of the image , making it difficult to distinguish between particles and the target .
red arrow in b indicates a region of streaking that results when the flow rate is not sampled at a high enough frequency .
a potential constraint in the field is the need for particles in the flow , which are necessary to implement digital particle image velocimetry ( dpiv ) . in coastal water ,
suspended particulate matter exhibits sizes on the order of 10 m in diameter and concentrations between 0.002 and 10 per mm .
additional studies using a submersible holocamera for particle detection confirm sufficient presence of seeding particles to perform dpiv in ocean water . during open sea and coastal ocean diving
, we have found that particle densities and sizes are not a constraint for conducting in situ dpiv . aside from particle densities and sizes ,
qualitatively , if a region within an interrogation window has greater particle concentrations than another , the velocity magnitude generated by the dpiv analysis will be biased towards the region with higher particle concentrations .
we found thatcle concentrations are relatively constant during particle concentrations are relatively constant during dives where the diver is suspended in the middle of the water column .
however , particle fields in benthic environments have the potential for inhomogeneity due to resuspension of particles by environmental or diver - induced flows near the sea floor .
care must be taken to minimize disruption of particles during measurements in benthic environments . to the authors ' knowledge
, a formal analysis of errors generated by inhomogeneous particle concentration fields has not been conducted in either laboratory or field conditions , and should be a subject for further consideration in a separate publication .
several different issues should be considered when preparing and conducting in situ experiments using the protocol .
while recording , the operator is instructed to remain stationary and refrain from all out - of - plane and rotational motion .
this request is simple in theory but difficult in practice , and these measurements require advanced diving skill to be completed successfully . out - of plane and rotational motions of the operator result in erroneous dpiv data .
however , in - plane motions can be corrected by using in - house software .
it is recommended to the operator to practice buoyancy control for several dives before using scuva to maximize measurement efficiency . besides buoyancy considerations
flows that travel out - of - plane relative to the laser sheet will not yield reliable dpiv results , and the operator should orient scuva to capture these flows most effectively .
in addition , the position of the diver relative to the target must be selected so as to minimize diver - induced flow in the measurements .
diver - induced flow introduces error to the target flow , and measurements that include diver effects should not be used for further analysis . in the event that the target has a highly reflective surface , the fluid region surrounding the target
will be strongly illuminated , making it difficult to distinguish nearby individual particles from surrounding fluid ( region indicated by red arrow , figure 2a ) .
filters or polarizers can be added to the laser or camera housings to reduce the intensity of the laser light captured by the video camera sensor .
if this is not possible due to logistical constraints and limited access to equipment , post - processing of images using in - house software can provide sufficient correction by subtracting from the images the elevated pixel intensities near the target .
another consideration that affects the quality of dpiv data is whether particle streaks are present .
if particle fields have regions of streaking ( indicated by red arrow , figure 2b ) , the video camera is recording at a frame rate too low to resolve these high velocities . by increasing the frame rate ,
, this results in a reduction of light reaching the video camera sensor and makes the particle field look dimmer . if the video camera has the ability to manually set aperture settings , increase the aperture setting to prevent dimming of the particle field . determining the optimal device settings may require multiple dives with scuva before successful data collection .
 the ability to directly measure velocity fields in a fluid environment is necessary to provide empirical data for studies in fields as diverse as oceanography , ecology , biology , and fluid mechanics . 
 field measurements introduce practical challenges such as environmental conditions , animal availability , and the need for field - compatible measurement techniques . to avoid these challenges , scientists typically use controlled laboratory environments to study animal - fluid interactions . however , it is reasonable to question whether one can extrapolate natural behavior ( i.e. , that which occurs in the field ) from laboratory measurements . 
 therefore , in situ quantitative flow measurements are needed to accurately describe animal swimming in their natural environment.we designed a self - contained , portable device that operates independent of any connection to the surface , and can provide quantitative measurements of the flow field surrounding an animal . 
 this apparatus , a self - contained underwater velocimetry apparatus ( scuva ) , can be operated by a single scuba diver in depths up to 40 m. due to the added complexity inherent of field conditions , additional considerations and preparation are required when compared to laboratory measurements . 
 these considerations include , but are not limited to , operator motion , predicting position of swimming targets , available natural suspended particulate , and orientation of scuva relative to the flow of interest . 
 the following protocol is intended to address these common field challenges and to maximize measurement success . 
Protocol
Representative Results:
Discussion
Disclosures