Yokogawa DO402 Dissolved Oxygen Analyzer - Manual (Page 11)

IM 12J05D02-01E
Introduction 1-3
1-3. General information
Flexibility, reliability and low maintenance are among the benefits provided by the EXA DO402G dis-
solved oxygen analyzer. Designed to meet the exacting requirements of measuring dissolved oxygen in
the modern industrial environment, it contains many features to ensure the best precision whatever the
application.
This 4-wire converter is housed in a robust IP65 field mountable case. Two mA outputs, four relays, dig-
ital communication and a clear LCD make the DO402G a truly comprehensive package.
The DO402G features PI control on both the auxiliary mA output and the pulse proportional relay out-
puts, thus avoiding the need for a separate controller.
The famous EXA sensor diagnostics are now expanded with a logbook facility in combination with the
RS485 two wire communication software option. This can be used to record events like calibration and
diagnostic messages, and to update configuration of the converter remotely.
The DO402G accepts inputs from galvanic, polarographic and optical sensors. Percent saturation, mg
oxygen/l water, and ppm DO can be displayed and transmitted. Compensation for atmospheric pressure
altitude, salinity and temperature are included for the best accuracy of measurement.
Features
• Simple 3 Ievel operation
• Display mg/l, ppm, % saturation
• Air calibration or saturated water calibration
• Chloride concentration compensation can be programmed
• Automatic temperature compensation
• Built-in barometric air pressure compensation
• “Hold” function enables fixed output signal during maintenance
• Two separate mA output signals
• Wide flexibility incorporating free programmable range settings, selection of output signals and alarm
functions
• Analog output with adjustable damping time
• Password protection for each programming level
• Built-in wash timer with remote start possibility
• IP65 weather protection
• Universal mounting possibilities
• Sensor diagnostics
• RS 485 bi-directional communication
• Logbook via RS 485 link
• PI control on mA and pulsed contact controls
Method of operation
The EXA DO30 operates on a galvanic cell principle.
The sensor contains a measuring cell consisting of a silver cathode and a lead anode. The voltage gen-
erated by this electrode pair is sufficient to generate a spontaneous reduction of oxygen at the cathode,
so no external voltage source for this reaction is required. The electrodes are immersed in an electrolyte
containing potassium hydroxide. Oxygen molecules can pass through a permeable membrane into the
cell. The output current of the cell is directly related to the partial pressure of oxygen at the sample side.
In the sensor a thermistor is integrated to be able to correct for temperature changes. Both signals are
used in the converter to ensure an automatically compensated value for dissolved oxygen. Calibration
is worked out by simple air calibration to make sure that the performance of the instrument will be main-
tained.
The DO70G sensor operates based on the principle of optical (fluorescence) measurement.
The sensor comprises a fluorescent membrane, light-emitting part, light-receiving part, and internal cir-
cuit. An external power source is required. Oxygen molecules pass through the permeable membrane to
a fluorescent substance and shift the phase of fluorescent emission. The degree of this shift is inversely
proportional to the partial pressure of oxygen on the sample side, resulting in the output of a current
equivalent to the polarographic current adjusted by the internal circuit.
A temperature sensor is integrated in both sensors, and the temperature signals are used to automati-
cally compensate the measurements of dissolved oxygen.
The combination of air-saturated water calibration and zero calibration helps maintain the performance of
the instrument.