In situ Absorption and
Attenuation Meters
WET Labs
OVERVIEW
Brief introduction to the optical properties
we are trying to measure
Measurement of absorption and
attenuation with an ac9:
– Meter description
– Calibration
– Protocols for use
– Performance
Current developments
Absorption and Attenuation by a
Water Solution
Relationships:
INCIDENT
scattered
scattered
c=a+b
absorbed
ct = cpart + cdiss + cw
particle
at = apart + adiss + aw
absorbed
dissolved
materials
and water
c, attenuation coefficient (m-1)
a, absorption coefficient (m-1)
b, scattering coefficient (m-1)
bt = bpart + bw
Measuring attenuation
In theory
In practice
I (r )
−c r
Tr (r ) =
=e
Io
dI 1
c≡−
dr I (r )
loss by
absorption (a)
dr
finite r
Io
I
I
Io
loss by
scattering (b)
Measure Io and I
Conventional collimated beam
methodology for measuring
attenuation in situ
lens
window
window
lens
beam
splitter
signal
detector
sample
(near) collimated,
unpolarized
source
reference
detector
(near) collimated
detector
Methods of measuring
absorption in situ
• Collimated source, reflective sample cell with
diffuser in front of wide area detector (ac9
method)
• For dissolved fraction, use in situ attenuation
meter with prefilter to remove particles
ac9 description
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•
•
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OVERVIEW
9 wavelengths of absorption
(a) and attenuation (c)
Dual flow cells
10 nm FWHM bandpass filter
Incandescent sources
10 or 25 cm pathlengths
6 Hz sampling rate
500 or 6000 m rating
ac9
c-star
• 1 λ attenuation
• LED source
• 10 or 25 cm path
• 1 Hz sample rate
ac9
cross-section
ac9 description
1mm
c beam optics
a beam optics
6mm 6mm
6mm 1mm
ac9 description
Reflective tube absorption meter design
Forward scattered light
from ~0 to 41.7 degrees is
included in the signal
measured by the detector
beam passing
through sample
quartz tube
thin annular volume of air
plastic flow cell cover
adapted from Zaneveld et al. 1992
ac9 description
Computation of c:
Transmittance =
c − b0−0.7o
I sig
I ref
G=e
⎛ I sig
ln⎜⎜
I ref
⎝
=−
r
−( c −b
0 − 0.7 o
)r
⎞
⎟
⎟ ln (G )
⎠−
r
• where b0-0.7 is a scattering error
• where G is a constant that accounts for the relative gains
of the SIGNAL and REFERENCE detectors
ac9 description
Computation of a:
Transmittance =
a + b42−180o
I sig
I ref
G=e
⎛ I sig
ln⎜⎜
I ref
⎝
=−
r
−( a + b
42 −180o
)r
⎞
⎟
⎟ ln (G )
⎠−
r
• where b42-180 is a scattering error
• where G is a constant that accounts for the relative gains
of the SIGNAL and REFERENCE detectors
ac9 description
Scattering errors:
For c:
~ 0.7o
b0−0.7o = 2π ∫ β(θ)sin (θ)dθ ≈ 0.15b
Rarely corrected
~ 0o
For a:
~180o
b41.7o −180o = 2π
∫ β(θ)sin(θ)dθ ≈ 0.1b
~ 41.7o
Typically corrected
ac9 description
10-cm or 25-cm pathlength?
• Accuracy and uncertainty are a
function of c (or a) and r
• Accuracy is optimized when
cr ≈ 1 and ar ≈ 1
ac9 calibration
Factory calibration for a and c:
1. temperature characterization*
– acquire table of values for correction, ε
2. optically clean water calibration*
– obtain engineering offset, e.g., coff = ln(G)/r
– serves as water blank
⎛I
ln⎜⎜ sig
I ref
⎝
c=−
r
⎞
⎟
⎟
⎠ − ε (T ) − c
c
off
⎛ I sig
ln⎜⎜
I ref
⎝
a + be = −
r
⎞
⎟
⎟
⎠ − ε (T ) − a
a
off
3. air calibration
*data stored in device file and applied by WETView during measurement
ac9 calibration
Field calibration for a and c:
GENERAL CONSIDERATIONS
• Air tracking vs. water calibrations
• Calibration frequency
• Mounting/deployment considerations
ac9 calibration
Water calibration setup:
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Cleaning optics
Plumbing
Obtaining optically clean water
Water temperature measurement
REPLICATION
ac9 protocols for use
Field deployment:
GENERAL CONSIDERATIONS
• Cleaning optics
• Plumbing
• Water temperature measurement
• Warm-up
• Mounting
• Auxiliary equipment
• Fouling
ac9 protocols for use
Modes/Platforms include:
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•
•
•
•
Vertical profiling
Bench top
Towed vehicle, AUV
Moored
Diver
WETView ac9 acquisition software
Simple Vertical Profiler
More Complex Vertical Profiler
Bench top
Flow through
Towed Vehicle
intakes
ac9
pump
Moored
• Vertical orientation preferred
• Allow at least 30s warm-up
• Minimize fouling:
– Copper tubes
– Bromide pack
– Use pump
– Protect from surf zone
• Water calibrations:
– Before
– After deployment, before
cleaning
– After deployment, after
cleaning
Dickey et al.
Diver
ac9 protocols for use
OTHER CONSIDERATIONS
• Use of prefilters or screen
• ac9 combinations to measure suite of IOPs
• Removal of c-side flow cell
• Choice of wavelengths
Post-measurement processing
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Drift, from water calibrations
Water temperature
Salinity
Scattering in absorption channel
Time lags
Derived parameters
ac9 performance
• RAW UNCERTAINTY
~0.0001 m-1 in waters with low attenuation
• ESIMATED UNCERTAINTY
~0.002 m-1 in waters with low attenuation
- due to remaining bias errors after corrections
(improved from ~0.005 m-1 from Twardowski et
al. (1999) due to improved stability and
improved temperature correction algorithm)
Current developments
ac-spectra: “ac-s”
filter wheel
form
factor
ACW AC9 Phaeodactylum Comparison Test (5/29/03)
1.8
1.6
1.4
1/m
1.2
linear
Interference
filters
1
ACW A
AC9 A
0.8
0.6
0.4
0.2
0
390
490
590
wvl (nm)
690
Current developments
“ac-s”
Current developments
Compact attenuation meter, “SAM”
r3
r4
θs
r1
S
r2
D1
D2
SAM geometry
⎛ β m1 ⎞
⎟⎟(l 2 − l1 ) −1
c = ln⎜⎜
⎝ βm2 ⎠
ac9
SAM
patent pending
Current developments
Compact attenuation meter, “SAM”
6
DO1=33
DO2=66
5
Narragansett Bay
y = 9.3655x - 2.3922
2
R = 0.9964
c650
4
3
2
1
MAALOX
0
0.2
0.3
0.4
0.5
0.6
0.7
ln(S1/S2)
Lake Tahoe
0.8
0.9
SAM Data – LEO 15