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Part III: 1-3
Oil in Water Part III
Teledyne Analytical Instruments
The detector output, which is a sequence of pulses which directly reflect
the photo energy transmitted by the measuring and reference filter, is a measure
of the concentration of the component of interest in the sample. The difference
of the energy in the measuring and reference pulse is exponentially related to the
concentration of the component of interest.
The photo detector current output is amplified by a current to voltage (I to
E) converting amplifier, followed by a second amplifier. The gain can be
adjusted to obtain any desired output level.
To obtain electrical signals which are linearly related to the concentration
of the component of interest, the output of the I to E Converting amplifier is fed
to the input of a logarithmic amplifier, which produces a signal that represents
the logarithm of the output signal of the second amplifier. The output of the
logarithmic amplifier is fed to the input of an inverting amplifier, which acts like
a buffer between log amplifier and switch and inverts the input signal for further
processing.
The output of the inverting amplifier is fed to a magnetically activated
SPDT reed switch, synchronized in such a way that all measuring pulses are
collected on one switch contact and all reference pulses on the other.
The pulses pass through diodes which isolate the integrating networks
from each other. The integrators convert the reference and measuring pulse
energy to a DC level representing them. These reference and measuring DC
levels are applied to the subtracting amplifier. The output of the subtractor is a
DC voltage linearly related to the concentration of the component of interest.
From this point on the signal progresses to the A to D converter, where the
signal is digitized for micro controller. The micro controller performs operation
on the signal such as spanning, zeroing, triggering alarms, etc..
The technique of
dual wavelength spectroscopy provides compensation
for such phenomena as turbidity, sediment, algae, cell window coatings,
component aging and other extraneous electro-optical attenuation.