User guide

Chelsea Technologies Group
FAST
tracka
User Guide HB179 Issue 7.0
Page
23
of
47
WARNING: Never look directly into the LED light source of the FAST
tracka
. Although the LED
sources, unlike flashlamp sources, do not emit appreciable amounts of UV, the LEDs are
focussed into a small sample volume and the intensity is correspondingly high. Damage to the
eyes may occur if the beam is viewed directly.
4.4 DETECTOR BOARD
A Hamamatsu R928 side-on photomultiplier tube (PMT) is used to detect the fluorescence from
phytoplankton exposed to excitation light. The emission light, after passing the entrance of the
optical windows and aperture, is directed by a series of prisms to a passive collimator. After
collimation, the emission light is filtered by a CI custom specified fluorescence band pass
interference filters (Corion Coporation, Holliston MA, USA), and then passes through 12mm of
Schott RG-665. After filtering, the emission light is focused onto the PMT photocathode.
The PMT gain is controlled by a programmable PMT high voltage supply. Varying the HV on a
PMT will consequently vary the overall PMT gain characteristics, and the FAST
tracka
controls its
gain in a series of steps, each step being a factor of four greater than the previous. After
amplification, the PMT signal and the reference excitation signal described above are sent to the
interface board for digitisation, manipulation and storage.
An onboard PMT calibration system provides a history of PMT performance over time.
Periodically, a small and known amount of red light is leaked to the PMT from an onboard stable
LED. The observed PMT signal is compared to a factory determined pre-set level, and
differences between the present and the pre-set level indicate the amount of PMT wear.
4.5 POWER BOARD
The instrument takes power from its own battery pack or from the Interface connector, or from
power on the Battery connector. The Battery input is 13.5 -18 VDC, unless the CTD detect line is
enabled, which allows voltages down to 10.5 VDC. Host power is 18 - 72 VDC.
4.6 BATTERY PACK
The battery can contains three tiers of 7 Gates X cells, each at 2V nominal and 5Ah. A vented
charging connector is accessed through a charging plug in the bottom of the can. Power output
is provided through a bulkhead connector and waterproof pigtail in the top of the can. An
intelligent battery charging circuit is designed into the battery can to automate the charging cycle.
4.7 PAR SENSOR
The FAST
tracka
is fitted with a bulkhead connector and external connecting cable which mate
directly to a Chelsea Instruments PAR Irradiance Sensor, which is shipped standard with the
instrument. The PAR sensor is powered by the FAST
tracka
, and the PAR signal is returned to the
FAST
tracka
for measurement. The FAST
tracka
outputs PAR readings in the range 0 to 4095. This
corresponds to an analogue input range of 0 to 4095 mV. The light intensity can then be
calculated from this value using the formula supplied on the PAR sensor calibration sheet.