Deepwater in-situ fluorometer

Claim 1

An improved in-situ fluorometer for underwater use, the fluorometer being of the type comprising an underwater projector unit operative for emitting pulses of fluorescence-exciting radiation, a cooperating underwater receiver unit operative for receiving the resultant fluorescent radiation and generating a meterable output signal, an onboard power supply and metering unit, and an overboard connecting cable electrically connecting the onboard power supply and metering unit to the underwater units for transmission of operating power to the underwater units and for transmission of the meterable output signal to the onboard power supply and metering unit, (a) the receiver unit comprising receiver circuit means, the projector unit comprising light-pulse-generating circuit means, the onboard power supply and metering unit comprising a constant-current source supplying operating power to the receiver circuit means and to the light-pulse-generating circuit means in the form of a constant current transmitted through said cable; (b) the onboard power supply and metering units comprising a meter, the receiver circuit means generating in response to detected fluorescent radiation an output signal in the form of a current and transmitting the output signal current to the meter through said cable, whereby, due to the energization of the circuit means to the underwater units by an onboard constant-current source, and due to the generation by the receiver unit of a cable-transmitted signal in the form of a current, the length of the overboard cable is made uncritical, (c) the receiver circuit means comprising: a photodiode exposed to the fluorescent radiation resulting from the pulsed radiation from the projector unit, integrating means integrating the photodiode current to form an integrated photodiode-current signal, peak-detecting means operative for determining and holding the peak value of the integrated photodiode-current signal, to form a held integrated photodiode-current peak-value persisting in the time beyond the peak value of the integrated photodiode-current signal itself, gating means, connected to the signal-transmission path intermediate the photodiode and the peak-detecting means for permitting and preventing signal transmission, synchronizing means operative for rendering the gating means signal-transmissive for a brief time interval concurrent with the emission of a light pulse by the light-pulse-generating circuit means, logarithmic amplifier means receiving the held integrated photodiode-current peak-value signal and deriving therefrom a logarithmized integrated photodiode-current peak-value signal, sample-and-hold means operative for sampling and the holding the value of the logarithmized integrated photodiode-current peak-value signal, the sampling occurring subsequent to the signal-transmission interval of the gating means and subsequent to the peak-value determination by an interval sufficient to assure that the logarithmic amplifier means has fully responded to the held integrated photodiode-current peak-value signal, the signal held by the sample-and-hold means being in the form of a held voltage, voltage-to-current converting means receiving a voltage derived from that held by the sample-and-hold means and producing a corresponding current constituting the output signal transmitted from the receiver unit through said cable to the onboard meter, and automatic offset drift control means serving, during the time interval between the start of signal transmission by the gating means and the sampling by the sample-and-hold means, to counteract any offset and/or drift developed in the peak-detecting means and/or in the logarithmic amplifier means, by applying a balancing signal to the input of the peak-detecting means, this automatic offset drift control means comprising means operative, intermediate successive pulse-measurement operations, for deriving the balancing signal by feeding back to the input of the peak-detecting means a signal derived from the output signal of the logarithmic amplifier means.