Wet Labs ac-s User manual

ac-s User’s Guide (ac-s) Revision F 9 Feb. 2006
Spectral Absorption and
Attenuation Meter
ac-s
User’s Guide
WET Labs, Inc.
P.O. Box 518
Philomath, OR 97370
541 929-5650
www.wetlabs.com

ac-s User’s Guide (ac-s) Revision F 9 Feb. 2006
Attention!
Return Policy for Instruments with Anti-fouling Treatment
WET Labs cannot accept instruments for servicing or repair that
are treated with anti-fouling compound(s). This includes but is not
limited to tri-butyl tin (TBT), marine anti-fouling paint, ablative
coatings, etc.
Please ensure any anti-fouling treatment has been removed prior to
returning instruments to WET Labs for service or repair.

ac-s User’s Guide (ac-s) Revision F 9 Feb. 2006
ac-s Warranty
This unit is guaranteed against defects in materials and workmanship for one year from
the original date of purchase. Warranty is void if the factory determines the unit was
subjected to abuse or neglect beyond the normal wear and tear of field deployment, or in
the event the pressure housing has been opened by the customer.
To return the instrument, contact WET Labs for a Return Merchandise Authorization
(RMA) and ship in the original container. WET Labs is not responsible for damage to
instruments during the return shipment to the factory. WET Labs will supply all
replacement parts and labor and pay for return via 3rd day air shipping in honoring this
warranty.
Shipping Requirements for Warranty and Out-of-warranty Instruments
1. Please retain the original shipping material. We design the shipping container to
meet stringent shipping and insurance requirements, and to keep your meter
functional.
2. To avoid additional repackaging charges, use the original box (or WET Labs-
approved container) with its custom-cut packing foam and anti-static bag to return
the instrument.
• If using alternative container, use at least 2 in. of foam (NOT bubble wrap or
Styrofoam “peanuts”) to fully surround the instrument.
• Minimum repacking charge for ac meters: $240.00.
3. Clearly mark the RMA number on the outside of your shipping container and on
all packing lists.
4. Return instruments using 3rd day air shipping or better: do not ship via ground.

ac-s User’s Guide (ac-s) Revision F 9 Feb. 2006 i
Table of Contents
1. Instrument Setup and Testing.........................................................................1
1.1 Installing WETView ................................................................................................ 1
1.2 Bench Testing .......................................................................................................... 2
1.3 Air Tracking............................................................................................................. 3
1.4 Cleaning ...................................................................................................................3
1.5 System Requirements............................................................................................... 5
2. Operation...........................................................................................................6
2.1 Basic Power On........................................................................................................ 6
2.2 Mounting and Deploying the Meter......................................................................... 6
2.3 Data Collection ........................................................................................................ 9
2.4 Care and Maintenance............................................................................................ 10
3. Data Processing..............................................................................................11
3.1 Analog Acquisition ................................................................................................ 11
3.2 Analog-to-Digital Conversion ............................................................................... 11
3.3 Digital Processing and Data Output....................................................................... 11
3.4 Data Format ........................................................................................................... 11
3.5 Post Processing ...................................................................................................... 11
3.6 Temperature-dependent Absorption Correction .................................................... 13
3.7 Scattering Correction ............................................................................................. 14
4. Calibration and Characterization...................................................................15
4.1 Temperature Correction......................................................................................... 15
4.2 Precision................................................................................................................. 15
4.3 Absolute Calibration .............................................................................................. 16
4.4 Linearity and Dynamic Range ............................................................................... 16
4.5 Air Calibration and Tracking................................................................................. 16
5. Specifications .................................................................................................19
5.1 Connectors ............................................................................................................. 19
5.2 Power Requirements .............................................................................................. 21
6. Technical Reference.......................................................................................22
6.1 Mechanical............................................................................................................. 22
6.2 Optics ..................................................................................................................... 22
6.3 Electronics.............................................................................................................. 25
6.4 Signal Processing................................................................................................... 25
Appendix A: Data Record Format......................................................................28
Primary Processing ............................................................................................................. 30
Appendix B: Pressure Sensor Use and Care....................................................33
Appendix C: Common Mode A/C Induced Noise .............................................34

ac-s User’s Guide (ac-s) Revision F 9 Feb. 2006 1
1. Instrument Setup and Testing
This section provides an introduction to the ac-s and describes how to set up and perform air
tracking and bench testing prior to deploying the ac-s.
The ac-s is delivered in a sturdy wooden shipping container with the following components:
• ac-s, with one absorption flow tube and one attenuation flow tube
• four flow sleeves
• four protective black plastic caps
• dummy plug with lock collar
• this manual
• WETView User’s Guide
• additional reference: ac-9 Protocol Document
• WETView software and instrument-specific calibration information
• Ten-foot test cable
• double “Y” de-bubbler tubing
• stainless steel intake screens
Familiarize yourself with the ac-s: remove the black plastic flow tubes by grasping the
flow tube sleeves and sliding them away from the ends of the flow tube (toward the
middle of the flow tube). You only have to slide the collars about ½ in. to unlock the flow
tube from its fixed position. The flow tube will lift out, exposing the transmitter and
detector windows on the lower and upper flanges respectively. Observe the flow tubes.
The attenuation tube is different from the absorption tube. Its flow chamber is plastic and
the two sleeves on the tube are identical. This tube installs on the “c” side of the
instrument (the side with the identical looking windows). The “c” tube has no “up or
down” orientation. The absorption tube is lined with a quartz tube and one of the two
sleeves is flat on top (the lip present on all the other sleeves is missing). This tube installs
on the “a” side of the instrument, which can be identified by the “a” detector on the upper
flange and is the only window, which is clearly different from the other three. The flat
flow tube sleeve goes on this detector.
You may want to mark the tubes and their orientation with tape or marking pen before
using the instrument at sea so there is no confusion upon reinstalling the tubes after
cleaning the optics. Reinstall the flow tubes before bench testing your instrument. If you
have removed the plastic caps from the stainless nozzles, replace them at this time.
1.1 Installing WETView
WETView displays data produced by WET Labs ac meters. It runs on PC-compatible
computers with at least 16 Mb of memory and 3 Mb free hard disk space running
Windows-95 or later operating system.
1. Insert the WETView CD in the host computer.
2. Double-click on the SETUP.EXE icon. Setup will guide you through the rest of the
installation process.

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When installed, the following files should be on the hard disk: WETVIEW.EXE,
WETVIEW.UIR, ACSXXX.DEV, AIRXXYYY.CAL. The .DEV and .CAL files are the
calibration files for your particular instrument. They are not installed by setup and will
need to be copied to the hard drive manually.
Caution
If you have old device files from previous calibrations, you should rename them or
archive them in a different directory so that they will not be overwritten.
1.2 Bench Testing
The following items are needed to bench test the ac-s:
• A clean, solid lab table or work bench
• The ac-s with 10-foot test cable (or sea cable)
• A 10–35 volt power supply (the ac-s requires 10–35 VDC)
• A PC with WETView installed.
1. Connect the test cable to COMM1–COMM4 of the computer. Connect the power
leads to the power supply. The black lead is the V+ lead. Before connecting the cable
to the instrument, use a multimeter to check the input power.
2. Connect the ground probe to pin 1 on the pigtail connector (the center pin as you are
looking into the connector). Connect the hot probe to pin 4 (Figure 4). You should
measure the power supply voltage across these two pins.
3. Turn the power supply off.
4. Connect the pigtail to the instrument. Push the connector straight on to avoid
damaging the pins. Apply power to the instrument and allow it to begin warming up.
5. Run WETView. When the interface is displayed, you will need to provide a .DEV file
name. Click on the <O> button in the center top of the screen or choose “Open
Device File” from the File Menu at the top left of the screen. The program will ask
you to choose the proper COMM port. Select COMM1 through COMM4 as
appropriate. Note that WETView supports COMM1 through COMM4 only.
6. Click on the center button or <F1> to start displaying data. After 5–10 seconds,
tabular data should be displayed on the right side of the screen. A real time graph will
begin to develop, depending on the graph parameters set at the time. Refer to the
WETView User’s Guide for details of running the WETView software.
7. After a short time, again click on the center button that will stop the data collection
and ask for a file name to apply to the data if you should want to archive it. Press ESC
if you do not want to save the data. To quit the program, choose QUIT from the File
menu. At this point you have successfully completed a bench test of the instrument.

ac-s User’s Guide (ac-s) Revision F 9 Feb. 2006 3
1.3 Air Tracking
WET Labs provides an air calibration (.CAL) file similar to the device (.DEV) file and
can be applied in WETView in the same manner. The DEV file provides the clean water
offsets so that when measuring clean, fresh water, the instrument’s output should be very
nearly 0.0 for all channels. The .CAL file provides the offsets that provide 0.0 values
when the instrument is clean and dry and measuring air values. This is a useful tracking
tool for catching instrument drift, filter aging and improper cleaning.
When you install WETView, the latest air .CAL file should be transferred to your
WETVIEW directory. The name of the file takes the form AIRXXYYY.CAL where XX
is the calibration number; YYY is the instrument serial number.
When a .DEV file is opened in WETView, it will display *.DEV in the dialog box, which
will list all the .DEV files on the default drive. If you change the *.DEV to *.CAL, the
available .CAL files will be displayed. Select the latest .CAL file and start collecting
data. Make sure the black plastic caps are installed on the flow tube nozzles so no
ambient light can enter the flow tubes. If the instrument is clean and dry, the values
displayed in WETView should be very close to 0.0 (within 0.005–0.01). If the values are
within this range, the instrument is clean and ready to deploy. If the values are outside
this range, the first step is to re-clean the instrument and then reapply the .CAL file
offsets.
1.4 Cleaning
1. Remove flow tubes and all O-rings. Remove the sleeves from the flow tubes. Wash
the windows with a mild detergent diluted with distilled, reverse osmosis-filtered
(RO) or de-ionized (DI) water and rinse the flow tubes. Use Kimwipes or other lint-
free tissues to wash the windows. Rinse the meter off completely with water to ensure
no soap residue is left inside the flow tubes or on the windows.
2. Dry the meter. Place the instrument in a protected area where it can dry completely.
Using a small heater to blow warm air over the meter may help speed the process.
Using dry nitrogen to blow-dry the meter and remove water from the small grooves
around the windows will also help speed the process. It is suggested that the
instrument be left overnight to dry out completely. Reassemble the meter. Carefully
replace O-rings and slide collars back on to the flow tubes. Replace O-rings around
the windows.
3. Clean windows using a Kimwipe or lens paper. Place a couple of drops of methanol
or ethanol on the Kimwipe. With firm pressure, gently wipe the windows with
methanol. This should remove any visible streaks on the windows. If necessary,
follow with a dry wipe in one direction across the window face. Blow off any lint or
dust with a dry air source.
4. Clean the flow tubes by putting a few drops of methanol on a Kimwipe and, using a
wooden dowel rod, carefully slide the Kimwipe through the flow tube. Repeat this

4 ac-s User’s Guide (ac-s) Revision F 9 Feb. 2006
procedure with both flow tubes. Examine each flow tube when you are through to
ensure there are no streaks or small pieces of lint left on the inside of the flow tube.
5. Dry the windows. Since small amounts of moisture can affect the air readings, it is
important to ensure the meter is completely dry. Using nitrogen to blow dry the
windows immediately before replacing the flow tubes works very effectively. This
will remove any water or methanol trapped in the small grooves around the window.
6. Replace the flow tubes. Carefully slide the flow tubes into place without sliding dirt
across the windows. Slide the sleeves up around the windows and over the O-rings,
making certain they are firmly in place and aligned correctly. Use small black caps, or
black electrical tape, over each of the nozzles on the flow tube to provide a dark
environment and to keep the meter clean and free of moisture while obtaining data.
7. Turn the meter on and allow it to warm up for at least 15 minutes. When the meter is
stable you should be able to collect 10 minutes worth of data and the values should
not vary more than 0.005 m-1 over the 10-minute time period.
8. Collect data. Record a one- to two-minute file and save data. Repeat steps 4–6 until
you can collect three data files, cleaning the windows after each file, such that the
average values for each channel vary by no more than 0.005 m-1.
The meter is calibrated over a specific range of internal temperatures (refer to your
calibration sheet). If the meter’s internal temperature is higher than the maximum
calibration range, it may be operating out of spec. Setting the meter in a shallow pan of
water (immerse most of the lower can) can help keep the instrument cool. This process
should be repeated until the air values are within specification. This may take three or
more iterations but is very important to be done carefully before taking your instrument
to sea. The air calibration procedure should be done again at the end of a cruise or data
collection period to track the instrument’s performance over time.
Note
The ac-s time averages its dark offsets. If the flow tubes are removed, incoming light
corrupts the dark readings. Following flow tube replacement while the meter is still
running, either wait three minutes for the dark offsets to stabilize, or cycle the power to
the instrument, resetting the dark offsets.

ac-s User’s Guide (ac-s) Revision F 9 Feb. 2006 5
1.5 System Requirements
This section explains the basic system components necessary to operate the ac-s. Note
that components will vary depending on whether the ac-s is used for profiling, on a
mooring, or underway flow.
Instrument
The ac-s and its flow tube assembly form the basic optical sensor.
Pump
The flow-through system will typically require an ancillary pump to assure proper
operation. Pump requirements depend upon desired flow rates, required depth of
operation, power availability, and existing hardware.
Cabling
The ac-s requires a minimum of four conductors for power and RS-485 output. (Three
conductors for RS-232 output).
Power Supply
The ac-s requires a 10–35 VDC supply, capable of providing a minimum of 10 watts
continuous output. Additional wattage will be required to also drive a pump from the
same supply. If a longer cable is used, power losses must be accounted for in determining
the power supply voltage and power requirements.
Serial Communications
Data from the ac-s is transmitted via both RS-232 and RS-485 depending on which
sockets are connected to the instrument’s bulkhead connector. Regardless of the mode of
transmission you use, you must have a serial interface on your computer or data logger
capable of accepting data from the instrument at a rate of 115,200 baud. If you have an
instrument with RS-485 output and are using a computer with standard RS-232
communication ports or USB ports, optional protocol converters can be readily obtained.
Host/Data Logger
The ac-s can be interfaced to any computer or data logger capable of supporting a
115,200 baud rate serial interface. If you plan to use our WETView software acquisition
package you will need a computer running Windows 95 or newer.
Software
The WETView host software package interfaces directly to the ac-s via your computer
serial port. WETView allows real-time graphical visualization of the data in any of
several ways such as a function of time, depth, or wavelength. WETView automatically
applies calibration constants, temperature corrections and generates a tab-delimited
ASCII text file that can be imported into programs such as Excel or MatLab for post-
processing or manipulation.

6 ac-s User’s Guide (ac-s) Revision F 9 Feb. 2006
2. Operation
2.1 Basic Power On
1. Check cable wiring. Make sure the “+” lead of the test cable is connected to the “+”
terminal of your power supply. Make sure the “–” lead of the test cable is connected
to the “–” terminal of your power supply. Verify that your power supply is providing
between 10 and 35 VDC, and is rated for at least 2 amps.
WARNING!
Power input on this unit is diode-protected from reverse polarity power-up, but
this is not 100 percent insurance against damaging the meter, nor will it
protect it from over-voltage.
2. Cable connections. The surface (dry) end of the cable should have terminations for
an RS-232 connection into the computer and leads that connect to a power supply.
Attach the power leads to a stable power source that supplies 10–35 VDC to the ac-s.
Make sure the polarity is correct before switching on the power supply. Connect the
RS-232 connector to the desired serial port of the data collection computer. If your
instrument is sending data in the RS-485 format, an RS-485–to–RS-232 or RS-485–
to–USB converter is required to allow proper operation. Plug the submerged (wet)
end of the cable into the ac-s. Applying a small amount of silicone grease or
equivalent to the base of the instrument bulkhead makes the plug insertion easier and
provides greater assurance of a good seal. Use a connector lock ring if one is
available.
3. Turn on power supply. To verify basic operation when not hooked directly to a
computer, remove one of the flow tubes and, in a darkened environment, place a
white piece of paper into the beam path. You should be able to see the beam image on
the piece of paper. You should be able to hear a faint whirring of the filter wheel
motor if you place your ear directly against the lower can. If you neither hear the
motor nor see the beam, the unit is not working. In this case, check your connections
and your power supply. If the instrument still does not run, you may want to seek
technical assistance from the factory.
4. For optimum stability allow the instrument to warm up for 3–5 minutes before
collecting data. While this is not an absolute requirement, the instrument’s electro-
optics are subject to an initial stabilization period.
2.2 Mounting and Deploying the Meter
ac-s operation is optimized for a vertical to forty-five degrees off-vertical orientation with
the bulkhead connector facing upward (Figure 1).If your application requires a
horizontal mounting, take care to provide proper pump priming and to avoid trapping air

ac-s User’s Guide (ac-s) Revision F 9 Feb. 2006 7
bubbles in the flow tubes. This can be accomplished by taking the system to a depth of 20
meters and allowing the required in-water warm up period to occur at depth. This helps
the pump to prime properly and compresses small air bubbles, allowing them to be
expelled from the ac-s.
In addition, operating the instrument in the horizontal position could potentially cause
small variations in the meter’s response due to the different orientation of the filter
wheel’s rotational plane. Therefore, we recommend that both air and water calibrations
be done in the orientation in which the meter will be deployed to obtain the best results.
Figure 1. ac-s mounting positions
Specific mounting instructions will depend upon implementation of the sensor. To assure
long term instrument integrity and optimum operation, observe the following basic
procedures:
1. Do not make direct contact between the ac-s pressure housing and a metal frame
or hose clamp. The ac-s is available in one of two pressure-housing materials.
Aluminum is used for full ocean depth rated units and an acetal copolymer plastic is
used for the standard units rated to 500 meters. The aluminum housing is hard
anodized with a special plastic impregnation to assure minimum corrosion damage
and to provide electrical isolation from the aluminum housing and its surrounding
environment. However, metal-to-metal contact with the housing can damage this
coating and result in possible corrosion of the pressure case. We recommend a
neoprene spacer between the unit and its frame or clamp. At the very least, any
contact area should be taped carefully to assure mechanical and electrical isolation.
2. Do not apply torsional stress to the instrument housing. The optical path is
encased in a rigid housing, but is still subject to distortion if the unit is subjected to
undue stress. The instrument has a delicate optical path that is subject to

8 ac-s User’s Guide (ac-s) Revision F 9 Feb. 2006
misalignment if stress is applied unevenly to the upper and lower cans. Make sure the
unit is mounted on at least two points and that neither point is a stress point.
3. Make sure you have provided for an unobstructed upward flow through the flow
chambers and the pump (Figure 2).
Figure 2. Proper connection of ac-s flow tube
The following suggestions will help you obtain the highest quality data from your ac-s:
• The instrument is extremely sensitive and should be handled carefully.
• A sturdy shipping/transport container should be used to transport your instrument to
the field. The instruments are sturdy, but the optics can be jarred out of alignment
when subjected to shock.
• The data will be adversely influenced by bubbles, dirt or grease in the flow path or
misalignment of the flow tubes. Make sure that the flow tubes, pump tubing and
screens are free of dirt and grease. Clean with ethanol or warm soapy water. Rinse
with clean tap or distilled water. Do not allow water to dry on the windows as this
will leave a residue that may be hard to remove.
• Secure the ac-s to a sturdy cage or lowering frame that will protect the instrument from
striking the deck, ship’s side, or sea bed. Use a dielectric isolator (rubber sheeting or
thick tape) to isolate the instrument’s aluminum case from the steel cage.
• When clamping the instrument to the cage, make certain no torsional stresses are
applied. Even slight wracking of the instrument can alter the beam alignment. This
will severely degrade the data quality, especially on the ‘c’ side, which requires an
extremely tight alignment tolerance.

ac-s User’s Guide (ac-s) Revision F 9 Feb. 2006 9
• Once every couple of days, take a data file in air to track any instrument drift. The
instrument must be very clean and completely dry to achieve an accurate air
calibration measurement. Using compressed dry nitrogen or oil-free dry air to blow
the instrument’s flow tubes and windows dry will speed the drying process.
• Upon deployment, the instrument should be lowered to just below the water’s surface.
Turn on the instrument and pump and check to ensure that the pump has primed and
is operating properly. Lower the package to a depth of 10–20 meters. Run the
instrument for 3–5 minutes to allow the motor controller to stabilize, the flow tubes to
clear and the instrument to begin to equilibrate with the water temperature.
• After the warm up period, raise the package to just below the surface and begin data
collection. The initial depth will be dependent on the natural surface conditions and
the amount of bubbles that the ship itself is generating. Steadily lower the cage
through the water column.
• The upcast can proceed immediately after the downcast. It is your choice whether to
open a new data file for the upcast or include the down and upcast data in a single file.
• Once the cage is back to the just below the surface, stop the data collection and turn
off the pump. Carefully bring the cage on deck and lash it down. Give the cage and
instrument a fresh water wash down after every cast. If this is not practical, wash the
instrument at the end of each data collection day. Holding a hose (at low pressure)
over the pump discharge port will flush the tubing and the flow tubes. Rinse the
flanges and connectors. If leaving the instrument on deck for more than a few
minutes, cover the cage with a tarp to avoid over-heating the instrument due to solar
insolation. At the end of each data collection day, remove the tubes and carefully
clean and dry both the flow tubes and windows. Failure to flush the instrument with
fresh water may cause corrosion damage over time.
2.3 Data Collection
WET Labs offers several output configuration options to provide flexible interfacing to
different systems. These various output protocols are discussed at length in the Data
Processing Section of the manual. Unless supplied with a custom output protocol the
instrument powers up in a free run mode. This means that when turned on the unit
automatically begins acquiring data and outputting that data in its appropriate format.
Typically the instrument comes supplied with RS-485 and RS-232 output operating at
115,200 baud.
RS-485: For longer cable lengths and maximum data integrity, RS-485 protocol is the
preferred method of data transfer. Data is transmitted from the instrument in a binary
format. To view this data, you must have a program capable of reading binary data. If
you are using our WETView software package, the binary read is done automatically. If

10 ac-s User’s Guide (ac-s) Revision F 9 Feb. 2006
you do not plan to use WETView or a WET Labs data logger, consult the Data
Processing section of this manual for a detailed description of the binary data format.
RS-232: Operating across an RS-232 cable, you can obtain binary data from the
instrument. If you are using WETView, read the operational instructions contained in the
software manual.
2.4 Care and Maintenance
Built for field deployment, the ac-s requires minimal maintenance. However, following
these simple recommendations will assure optimum data integrity as well as longer
instrument life.
After a field deployment of the ac-s you should clean the instrument prior to storage.
Refer to Section 1.4 for detailed cleaning procedures. The following steps will help
prolong the life of the instrument:
1. Rinse the pressure housing. If a dummy plug for the connector is available, install it
on the main bulkhead connector before flushing the instrument. Begin with a
thorough rinsing of the unit and its flow tubes with fresh water. After rinsing, towel-
dry the pressure housing and remove the flow tubes.
WARNING!
Do not use acetone on the windows. It will damage the window holders.
2. Wash the windows. The windows should be cleaned with dilute soapy water,
followed by ethyl alcohol, and should receive a final rinse with distilled or reverse
osmosis-filtered water. This will remove any fingerprint oil, grease or other
contaminants from the windows. Use lint free lens cleaning paper to avoid scratching
the windows or detectors.
3. Wash the flow tubes. To clean the absorption path’s reflective tube, carefully plunge
an alcohol-soaked tissue through the tube and rinse thoroughly with distilled water.
Whenever plunging a tissue through the tube, use a wooden or plastic dowel to
prevent scratching the sides of the tube. After rinsing, dry the tube either by blowing
dry nitrogen through it or by plunging a soft tissue. The attenuation path flow tube is
virtually maintenance free, except for occasional cleaning. Follow the same basic
procedures supplied for cleaning the absorption path tube. Remove the flow tube
sleeves when drying the flow tubes.
4. Store the meter. Store and transport the ac-s in a shock-protected environment.
Typically, units are shipped in a sturdy wooden crate. Using the crate will assure that
you can safely transport the instrument, providing it is handled in a reasonably careful
fashion.

ac-s User’s Guide (ac-s) Revision F 9 Feb. 2006 11
3. Data Processing
Data from the ac-s is acquired and processed through the following steps.
3.1 Analog Acquisition
Optical radiation at the reference and signal channels is continuously monitored by
operational amplifiers operating in a current to voltage configuration. The amplifiers are
configured for a gain of approximately 107for the reference channels and approximately
2 * 106for the signal channels.
3.2 Analog-to-Digital Conversion
The A/D used in the ac-s supports six input channels that are triggered pair wise. Four
channels are dedicated to the attenuation beam reference and signal channels and the
absorption beam reference and signal channels. Another channel is dedicated to the
output of the optional pressure sensor amplifier. The sixth channel is preceded by an 8
channel analog mux, and is used to sample ancillary analog signals including the internal
and external temperatures via thermistors within the unit. During a single filter wheel
rotation, the A/D concurrently samples all 4 signal and reference detectors for both
beams. Ancillary signals are sampled during the “dark” periods when the beams are not
passing through the filters.
3.3 Digital Processing and Data Output
The CPU takes multiple samples of both signal and reference channels, accumulates them
through the sampling period, and then averages the values at the end of the sampling
period. Once averaged, light and dark values are collected for each channel. The CPU
takes the difference of these values to derive its output value. Once signal data is
accumulated over a given filter wheel rotation its output is transmitted through the RS-
232/RS-485 port. For a more detailed explanation, see Section 6.4, Signal Processing.
3.4 Data Format
The ac-s outputs data in binary format. If you will not be collecting the data using
WETView, your collection system will need to be able to handle binary data. Beware that
some serial port terminal programs such as HyperTerminal are not capable of capturing
true binary data streams. HyperTerminal, for example, fails to capture ASCII NULL
characters (hex 00). WETView is capable of processing collected binary files in addition
to serial data. If you prefer to use your own software to process the ac-s output data,
please refer to Appendix A for a description of the ac-s data packet formatting.
3.5 Post Processing
Once accumulated by a host computer the data must be post processed to yield
meaningful scientific data.
The primary transfer equation for yielding the attenuation coefficient, c, or the absorption
coefficient, a, for a given wavelength is:

12 ac-s User’s Guide (ac-s) Revision F 9 Feb. 2006
Tr = e-cx (1)
where Tr is the transmittance, c is the attenuation coefficient, and x is the pathlength of
the water volume being measured. The following treatment applies to the absorption case
if cis replaced by a. The pathlength of the ac-s is fixed at 25 cm. In the case of the ac-s
the transmittance is computed by taking the ratio of the signal value to the reference
value:
Tr= (Csig/Cref) / N (2)
Substituting equation (2) into equation (1) we get,
(Csig/Cref ) / N = e-cx, (3)
where Csig and Cref are the signal and reference count values from the instrument and N
is an instrument specific calibration constant obtained in the laboratory using clean water.
For more information about how N is derived, see Section 4, Calibration and
Characterization.
We therefore can solve for c (or a) by,
c= - 1/x [ln (Csig/Cref) - ln (N)] (4)
or c= - 1/x (ln(Csig/Cref)) - (- 1/x (ln(N)) (5)
or c= [(ln(N)/x) - (ln(Csig/Cref)/x)] (6)
Since the ac-s pathlength is a fixed constant at 25 cm, we only require the determination
of (ln(N)/x) in order to accurately measure c. This value is derived for each channel and
is supplied with each instrument’s calibration data sheet as the “Water Offset” value and
is referred to as Coff in equation (7).
The temperature correction is applied using the temperature from the reference line and
the channels correction table from the configuration file. The approximate correction
value is linearly interpolated from the table. First, the correct temperature bin is
determined by finding the two bin temperatures, T0and T1, that bracket the current
temperature. Then, using the values,
∆
Tn and
∆
Tn+1, from the table, we obtain
()
()
()
TnTnTnT TT TT ∆−∆
−
−
+∆=∆ +1
01
0*(7)
where,
∆T= compensation constant
T= current temperature