Chlorophyll-a values from CLTU-VMP-250 Nose-Mounted Fluorometer/Optical Backscatter Sensor

The VMP-250 features an integrated compound fluorometer and optical backscatter (OBS) sensor to measure chlorophyll and turbidity.  This CLTU-VMP-250 sensor is co-located on the nose of the profiler with the microstructure sensors and designed for Rockland for vertical profiling applications. 


The CLTU-VMP-250 compound chlorophyll/turbidity sensor uses a light emitting diode for fluorescence and backscattering light.  The CLTU-VMP-250 is factory calibrated with Uranine for Fluoresence and Formazine for Turbidity.  For calibrated characteristics of sensitivity and sensor specifications, see the figure and table below.

Units of Fluorescence:

The short answer is that the unit for the fluorescence intensity is [ppb], i.e. parts per billion, of the uranine concentration. The value in [ppb] is what is reported after running quick_look or convert_odas in the software provided with the VMP-250 instrument.

The extended explanation is that this fluorescence intensity is determined from the raw counts (N) by a linear equation
ppb = A+B*N

where A and B are calibration co-efficients that are in your setup.cfg file. The values of A and B were determined by JFE Advantech Co. Ltd. (the manufacturer of the fluorometer sensor) by comparing the fluorescence signal to a known standard concentration of uranine.  You do not need to worry about implementing this equation because it is already handled by the VMP-250 software.  However, you should verify that the coefficients in your setup file match those in your calibration report.

Converting Fluorescence to Chlorophyll-a:
In order for you to convert to chl-a concentration, you would need to either:

1.  Conduct a literature search to determine how the typical chlorophyl signature of the phytoplankton community at their deployment site relates to concentration of uranine, OR
2.  Collect/filter/analyze for [chl-a] whole water sample at time of deployment and relate to ppb value.

If you have further questions, please email