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 support_at_rocklandscientific.com
