[Coral-List] Odyssey PAR Loggers
RichardPDunne at aol.com
Fri Sep 10 11:41:26 EDT 2010
ODYSSEY PAR LOGGERS
Three years ago there were a few posts about low cost submersible PAR
(Photosynthetically Active Radiation) loggers. One, the Odyssey
Photosynthetic Irradiance Logger manufactured by Dataflow Systems of New
Zealand was mentioned and recommended by a reader.
The Odyssey is sold as a low cost (approx US dollars 220) 2 phi PAR
sensor for use in air or underwater. Because it is self contained and
waterproof to 20m and can be deployed for extended periods, it is
attractive for use in scientific studies. Since 2004 its has been used
in at least 14 published studies.
I recently had the opportunity to calibrate 3 of these Odyssey PAR
loggers using a LiCor 192 PAR sensor and a Macam PAR sensor. Coral
Listers might find my observations informative.
THE ODYSSEY AS A PAR SENSOR
Although it is sold by Dataflow Systems as a PAR sensor, the Odyssey
produces its output as a milliVolt signal which is integrated over a
user selected time period. The shortest sampling interval is 10 secs. It
is not capable of recording instantaneous values. The logger is not
‘calibrated’ when supplied. In order to convert the milliVolt signal
into PAR units (micromole/ sq m/ sec) the user must calibrate it against
another PAR sensor. Typically this might be done against a LiCor 192.
Calibration against a 4 phi sensor such as a LiCor 193 is not
appropriate because the geometry and response is different. There is
also no information available as to the immersion effect correction
required when the Odyssey is used underwater. This requires a
calibration to be run in both air and in water.
None of this information is easily available to the potential purchaser
since the company’s website contains minimal information.
Of the 14 scientific publications, 9 made no mention of calibration, and
the remaining 4 did not say whether the calibration was in air or water.
None contained any details of the magnitude of calibration errors, and
therefore the errors in the data presented.
WHAT DID THE CALIBRATIONS SHOW?
Firstly, the measurement errors for the Odyssey were very large at low
sun angles. Although I was unable to conduct a detailed examination of
the cosine response, this result suggested that the cosine errors are
much larger than for the LiCor 192 or Macam. These latter instruments
have errors better than 5% out to 75 degrees when used in air.
Underwater, the cosine errors are about double the air values.
In their handbook, Dataflow state that “The sensor features a cosine
response and is based on a design evaluated by the University of Western
Australia. The design was published by the Freshwater Biological
Association in the UK as a simple and inexpensive equal energy response
photosynthetic irradiance sensor….” I was unable to find any details of
either study, and the company did not respond to my requests for the
information. In these circumstances the detailed cosine errors remain
unknown, but indications are that readings taken when sun angles are
lower than 20 degrees result in measurement errors in excess of 10%,
rising to 24% at 13 degrees, and considerably higher thereafter. This
makes the sensor generally unreliable for use during winter months at
high latitudes, and for data collected during the first and last 1.5 hrs
of each day in the tropics.
Excluding data from sun angles below 20 degrees, the Odyssey error was
approximately +/- 4.5% from about 100 – 2500 micromoles/ sq m/sec (95%
prediction interval +/- 25%). This is about double the comparable error
using the LiCor 192 sensors.
Because of the geometry of the underwater calibration setup, cosine
errors could not be examined. For the range 200 – 2000 micromoles/ sq m/
sec errors were of the order of +/- 8 to 9% (95% prediction interval +/-
25%). Again, the errors in the LiCor 192 sensors were about half this.
A computation of the Immersion Effect Correction for the Odyssey gave
values of between 1.54 and 1.76 depending on the sensor. This indicates
that between 54% and 76% of light is reflected back out of the sensor
when it is used underwater. This is unusually large for a 2 phi sensor
design. The comparable LiCor 192 has a correction of 1.32.
COMPANY CALIBRATION RECOMMENDATIONS
Dataflow produce an Excel spreadsheet for customers giving an ‘example’
calibration of an Odyssey. Worryingly, this is incorrect since not only
does it regress the Odyssey and calibrating sensor the wrong way round,
but it also uses only the slope and ignores any offset. As a result a
customer using the ‘Dataflow method’ will obtain a calibration where the
Odyssey systematically over-reads the true irradiance by about 2% at
1500 micromoles/ sq m/ sec, rising to 5% at 500 micromoles, and above
15% below 100 micromoles. Despite drawing this to Dataflow’s attention
they remain in denial and continue to inform customers of their
incorrect calibration method.
When a PAR sensor is used underwater there are additional errors which
arise because the spectral response of a given sensor cannot be tailored
to the ideal quantum response. Each sensor manufacturer uses their own
combination of filter glasses to achieve the best response possible and
normally makes details of the spectral response available to customers.
For both the LiCor 192 and the Macam sensors the respective responses
keep additional errors to within about 5%. This error is in addition to
the calibration errors above, resulting in a systematic shift in the
readings depending on the water depth and water type (clear oceanic or
No spectral response is available for the Odyssey and Dataflow did not
respond to a request for this information. It is not possible therefore
to estimate the additional errors for the Odyssey logger.
IS THE ODYSSEY A RESEARCH GRADE INSTRUMENT?
Certainly it is not of a comparable quality to other established PAR
sensors such as those sold by LiCor or Macam. Even when used
appropriately its errors are x2 greater compared to these instruments.
At low irradiances and when the sun is low in the sky, the errors
associated with the Odyssey rise somewhat alarmingly. However, LiCor and
Macam sensors are about x10 more expensive and additionally are ‘wired’
sensors which can only be deployed by a user above the sea surface.
Dataflow must therefore be commended for having produced a true
submersible sensor in the Odyssey. What is regrettable is their failure
to release relevant information about the sensor, or to respond to
information requests. Neither of these things are to be expected from a
responsible manufacturer of scientific instrumentation.
The Odyssey PAR logger has a useful role to play in underwater
scientific research but its use requires caution, proper calibration and
a sufficient assessment of errors in relation to the data being
examined. It behoves on authors to include these details in any
publication, otherwise we should remain sceptical of the data quality.
Finally, if any Coral Lister has access to an optical bench and the time
to examine the spectral and cosine response then we would also be much
wiser. Better still if Dataflow were to do this and also offer customers
an optional pre-purchase calibration service.
Richard P Dunne
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