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                                                                     Precision Measurement Engineering, Inc.


Discussing Calibration

FLUOROMETER

The fluorometer is supplied uncalibrated. Calibration must be completed by the customer. Quantitative calibration of the fluorometer is very difficult since the fluorescent response of features in the water column varies in response to environmental conditions that are difficult to predict or model in the laboratory. The best calibration method is to capture bottle samples at several depths, assay these chemically, then match SCAMP fluorometer response with the results. If no quantitative calibration is available, then the fluorometer will still provide a qualitative indication of the distribution of chlorophyll in the water column.

The fluorometer must have its full scale adjusted to match the maximum fluorescence expected. It may be difficult to estimate this in advance. It is most likely that the fluorometer full scale will need to be adjusted on-site after viewing initial profiles. Alternatively, the elbows can be removed, one side of the fluorometer taped shut with black electrical tape, a sample poured in from the other side, and the second side taped shut. Tape is required to contain the sample and also to prevent ambient light from entering the measurement volume.

PME supplies the proper C0..C3 coefficients to record the output of the fluorometer in Volts. Because of internal circuit conditions, the fluorometer only has linear output to 2 volts. It will produce voltages that exceed this value but these are not linearly related to the fluorescence intensity. The fluorometer should, therefore, be adjusted such that 2 volts maxim is observed when it is measuring the maximum fluorescence.

The fluorometer sensitivity adjustment is accomplished by adjusting the gain potentiometer (R13) located on the 5550 fluorometer circuit. This potentiometer is accessed by opening the SCAMP, removing the long electronics cover, and locating the 5550 circuit card. Clockwise adjustment increases the gain (sensitivity) while counter-clockwise adjustment decreases it. In addition, PME supplies scaling resistors that, when installed on the proper circuit connector (located over R8 on the 5550 circuit), multiply the fluorometer sensitivity by factors that are less than 1 (make the fluorometer less sensitive).  Resistor connectors are marked with the appropriate factor.

(Important:  Some SCAMPs have ribbon wiring harnesses that cover the scaling resistor connector.  The resistor can be installed but the ribbon wiring harness must be temporarily removed to do so.  The wiring harness is fragile.   Please see the Cautions section for information on removing the harness.  After installing the scaling resistor the harness must be re-connected).

Note that a quick test to determine if the fluorometer is operational is to record a profile while inserting/removing a blade of grass from the fluorometer measurement tube. This can be compared to the effects from inserting/removing a bit of paper. Take care for preventing the entry of ambient light while measuring. Ambient light won't damage the fluorometer, but it ruins the measurement.