Robert Bartz

A Transmissometer For Profiling And Moored Observations In Water

Numerous applications exist for a beam transmissometer that is low in cost, relatively stable, and consumes very little power. In this paper we present the design and calibration of a beam transmissometer that is constructed with P.V.C. to simplify the design and minimize costs, contains stable temperature compensated electronics, and consumes less than 150 milliwatts of power. Accuracy and stability with proper calibration and careful use will provide data with an error of less than 0.5% transmission.

Reflective-tube absorption meter

The design and calibration of a proposed in situ spectral absorption meter is evaluated using a laboratory prototype. The design includes a silver coated (second-surface) glass tube, a tungsten light source (stabilized by means of optical feedback), a monochromator, and a solid state detector. The device measures the absorption coefficient plus a portion of the volume scattering function. Theoretical analyses and laboratory experiments which explore the magnitude and variation of the errors due to scattering and internal reflections are described. Similar analyses are performed on the Cary 1 18 Spectrophotometer to allow cross calibration. Algorithms to yield the abscrption coefficient and the zenith-sun diffuse attenuation coefficient are presented and evaluated. Simultaneous measurement of the beam attenuation or backscattering coefficient allows use of algoriThms with much narrower error bands. The various methods of obtaining absorption and diffuse attenuation values are compared. Procedures for using reverse osmosis filtration to produce a clean water calibration standard are described. An absorption spectrum for pure water is obtained. Development of the absorption meter is proceeding along two lines: 1) a two-wavelength side-by-side LED is being fabricated to allow an in situ chlorophyll a absorption meter to be constructed, and 2) scientific projects using a shipboard or laboratory flow.-through pumping system are being planned.

A Low Power, High Resolution, In Situ Fluorometer For Profiling And Moored Applications In Water

Sea Tech Inc. has developed an in situ fluorometer to measure chlorophyll a fluorescence in aquatic environments. The instrument has been developed within stringent constraints of size, weight and power consumption. The use of custom-designed components, including the flashlamp, excitation and emission filters, and energy storage capacitor for the light source has permitted optimal mechanical, optical and electrical design of the instrument. This new design results in efficient stimulation and detection of chlorophyll a fluorescence. The instrument is not sensitive to ambient light and has excellent stability over time and temperature. Chlorophyll a concentration is measurable on three selectable ranges of approximately 3, 10 or 30 µg/1 full scale with a minimum detectable signal of <0.02 gg/1. Time constants of 0.1, 1, 3 and 10 seconds are selectable to smooth the output data. Power requirements are nominally 12 VDC at 150 mA, and output signal is 0 to 5 VDC. These power requirements and signal levels make the fluorometer compatible with most oceanographic moored and profiling data acquisition systems. Operating depth for the instrument is rated at 500 meters with a plastic housing or 3000 meters with a stainless steel pressure housing.

Optical Properties Of Turbidity Standards

Measurements of light scattering and light attenuation were made for suspensions of formazin and diatomaceous earth. Light scattering was measured for light of wavelength 632.8 nm at angles from 0.1° to 1.0° and for light of wavelengths 400, 500, 550, 600, 650, and 700 nm at 45°. Light attenuation was measured over a 25 cm pathlength for light of 660 nm. These measurements were made for suspensions which varied from 0 to 40 Jackson Turbidity Units of formazin and 0 to 40 mg/1 of diatomaceous earth. The results indicate the necessity for multiple optical measurements for determinations of turbidity of water. In addition the tables and curves presented may be used in the calibration of light scattering meters and transmissometers which are used for turbidity studies.

ROST and BEAST: Devices for in-situ measurement of particle settling velocity

Abstract The principles and design of two devices are described that measure the particle settling velocity in situ. The Remote Optical Settling Tube (ROST) designed by a group at OSU has been successfully deployed at the HEBBLE site. The Benthic Autonomous Settling Tube (BEAST) has recently been operated at the same site.

Beam Attenuation And Absorption Meters

The calibration of beam attenuation meters by means of a water standard measured with a two-pathlengths device is discussed. It is shown that the particulate scattering to attenuation ratio can be assumed to be constant to the same accuracy that the shape of the volume scattering function can be assumed to be constant in sea water. This observation allows one to correct beam attenuation meters without the need to measure forward scattering. An equation for this correction is derived and a table of values for the correction at a wavelength of 665 nm is given. The principles of construction as well as potential errors and their corrections are shown fora reflecting tube light absorption meter. Preliminary results for such a device are also shown.

Deep water characteristics of the Maui Basin

A profile of conductivity, temperature and optical clarity in the Maui Basin shows several interesting features. A monotonic decrease in transmissivity with depth is explained by the compressibility of sea water, the variation of index of refraction with depth and the suggestion of increasing particle concentration with depth. A 400-m-thick bottom layer exhibits anomalous values of optical transmissivity, salinity and temperature, suggesting that Maui Basin is stagnant below the 400-m sill depth.

Modern spectral transmissometer

We have evaluated a number of spectral attenuation meter designs based on constraints related to power consumption, spectral bandwidth, sampling time, accuracy and stability . Our fmal instrument design employs a unique optical bridge deve1oped1r Sea Tech with ONR support, a tungsten light source and a holographic grating monochromatorThe instrument design is summarized as follows: White light from a 10-Watt tungsten lamp with a 1mm2 filament is collected by a condensing lens and then spatially filtered by a 1mm diameter pinhole which is placed at the entrance port of a monochromator. The monochromator has a 45°, 1200 lines/mm, holographic grating 37 mm in diameter with a 91 mm focal length. The grating is rotated about its vertical axis with a sine arm driven by a stepping motor, allowing wavelength to be selected from 400 to 800 nm. At the exit port of the monochromator we use a 1mm diameter pinhole which spectrally filters the output light, resulting in a spectral bandwidth of 9. 1 nm. This nearly monochromatic light is then measured by a unique reference detector with a 0.5mm diameter pinhole at its center, allowing light to be transmitted through the center of the detector. The transmitted light has a bandwidth of 4.5 nm. The monochromatic light is then collimated by a 50mm focal length achromatic lens and stopped down to a beam 1 cm in diameter. This light then enters the sample chamber. After passing through the sample the light is received by a 61mm focal length achromatic lens and is focused onto a signal detector with a diameter of 1.25mm. Digitized ratios ofreference detector to signal detector voltages allow transmission to be measured with an accuracy of 0.05% and a resolution of 0.01%. By monitoring temperature we were able to temperature compensate the instrument to within 0.05% transmission from 00 C to 25° C. Based on these results it is now possible to construct a spectral attenuation meter with the required sensitivity and accuracy to measure beam attenuation in water as clean as oligotrophic ocean waters.

In-situ active fluorometer to measure cocoid cyanobacteria fluorescence

An in situ fluorometer has been designed to determine the concentration of a bacterial-sized photoautotroph. The concentration may then be used to estimate their effect upon optical properties in the upper ocean. Their fluorescence signal at 575nm enables assessment of their natural abundance fluctuations. The fluorometer is designed to be used in profiling and in moored modes. Such an instrument can provide valuable information necessary for improving our understanding of the dynamics of bacterial-sized microorganisms within the particulate matter in the upper ocean.