[Coral-List] Odyssey PAR Loggers

[Richard Dunne]

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?
IN AIR
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.

UNDERWATER
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.

SPECTRAL RESPONSE
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 
turbid).

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