APPLICATION NOTE NO. 77
Entering Calibration Coefficients for the Seapoint Ultraviolet Fluorometer
Revised February 2010

Application note in pdf format

The Seapoint fluorometer is used for in-situ measurements of chromophoric dissolved organic matter (CDOM). This fluorometer uses modulated ultraviolet LED lamps and excitation filter to excite CDOM present in water. The fluorescent light emitted by the CDOM passes through a blue emission filter and is detected by a silicon photodiode. The low level signal is then processed, generating an output voltage proportional to CDOM concentration.

Sensitivity of this fluorometer is determined by two control lines that allow the user to change the range and sensitivity as required for a particular application. Jumper cables may be purchased from Sea-Bird to allow the sensor range to be changed by inserting the jumper cable in line with the original cable purchased.

When setting up the configuration (.con or .xmlcon) file in our SEASOFT V2 software package, the Seapoint Ultraviolet Fluorometer is not available in the list of voltage sensors. However, our software includes a user-polynomial function that allows you to define an equation to relate the sensor output voltage to calculated engineering units:

User-Polynomial in Sea-Bird software:
Value = a0 + (a1 * V) + (a2 * V2) + (a3 * V3)

where:
Value = output in desired engineering units
V = voltage from sensor (Volts)
a0, a1, a2, and a3 = user-defined sensor polynomial coefficients

For this fluorometer:
Value (μg/l) = (Voltage * Range / 5) + Offset

Setting this equal to the user-polynomial equation and using consistent notation (Voltage = V):
(V * Range / 5) + Offset = a0 + (a1 * V) + (a2 * V2) + (a3 * V3)

Left side of equation has no V2 or V3 terms, so a2 and a3 are 0; rearranging:
Offset + (Range / 5) V = a0 + (a1 * V)

a0 = Offset                  a1 = (Range / 5)                a2 = a3 = 0

NOTES:

• In our SEASOFT V2 suite of programs, edit the CTD configuration (.con or .xmlcon) file using the Configure Inputs menu in Seasave V7 (real-time data acquisition software) or the Configure menu in SBE Data Processing (data processing software). The software allows you to enter the sensor name, which will appear in the data file header, as well as the coefficients.

• The fluorometer is adjusted at the factory for a nominal range and sensitivity for a given gain setting. Factors such as fouling, scratches, or lamp degradation will reduce the sensitivity of the sensor and/or create the need for an offset (a0) to account for discrepancies in the equation. When greater accuracy is desired, calibrate the sensor prior to deployment. Consult the fluorometer operating manual or Seapoint Sensors, Inc. for maintenance and calibration procedures.

Last updated: 10 Mar 2010

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