Teledyne 6020 - Photometric analyzer - Manual (Page 68)

1-6 Part II
1 Operational Theory Model 6020
Teledyne Analytical Instruments
The procedure to set up the optical bench, the signal processing front-
end amplifiers, the standardization of outputs, and alarm systems are described
in separate sections of the manual.
1.4 Automatic Zero System
To compensate for zero drift, which may occur during sampling, the
analyzer zero reading is updated by the Auto-Cal function of the controller. An
electronics timing circuit provides a timing cycle that is user programmable.
The Auto-Zero system is turned off (see chapter 3 section 5). You have
the option of setting the analyzer for one six minute zero cycle during hourly
intervals of time from one to 23 hours, and daily from one to 30 days.
The Auto Zero system compares the present zero reading of the zero
fluid with the zero reading of the zero fluid as it was in the last zero calibration.
When there is a difference, the electronic zero circuit sets the zero reading to
what it was in the last scheduled zero calibration. This zero reading is set at zero.
The Auto Zero circuit is a digital circuit, which employs a DAC (Digital to
Analog Converter) that can go out of range.
When the threshold cannot be found (oscillation persists), this means
that measuring and reference peak signals as viewed on the oscilloscope at the
output of the second amplifier in the detector module are too far out of balance
on zero fluid. When this occurs, you must initiate optical balancing of the
optical filters for equal light transmission on zero fluid. Measuring and
reference peaks must be within one volt with zero fluid in the cell.
Zero drift may occur in the following cases:
1. The output source changes or chemical or solid deposits form
on the cell windows, but the application is such that interfering chemicals
(sample background changes) are not a problem. The zero fluid in this case
may be the major component of the sample, void of the component of interest.
2. The sample may contain chemicals that are not of interest, but
absorb UV energy at the measuring wavelength used for analysis of the
component of interest (for example, oil in water applications). These chemicals
produce a signal that adds to the signal of the component of interest and makes
it inaccurate. The Auto Zero system discriminates the two signals and drives the
interfering signal of the background chemicals below zero on an hourly basis.
The zero fluid in this case is the sample of which the component of interest is