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Automatic air calibration
Calibration for a dissolved oxygen instrument is performed by simple air calibration.
Criteria for automatic calibration (stabilization time, DO values) can be set to suit the sensor.
In addition to the air calibration three additional calibration procedures can be used:
1. Span calibration using air saturated water
2. Zero calibration using sulfite saturated water
3. Process calibration using laboratory reference method
Alarm and control functions
The EXA D0402 has four built-in relay contacts. From the factory the first two contacts are pre-defined as a
high or low alarm.
The third contact its pre-defined as a wash contact. This wash contact is driven by a wash-timer with
adjustable wash-time, interval time and relaxation time to control the wash cycle.
The fourth contact is fixed to function as an alarm, indicating that the EXA has found a fault in the
measuring loop. FAIL safe.
Cleaning
ln combination with the appropriate sensor and immersion type fitting the wash timer with remote start
possibility can be used. Interval-time, washing time and relaxation time are free programmable.
Damping time on output signal
Under certain circumstances disturbances can cause high peaks in the output signal. To avoid these
disturbances the EXA DO402 has an electronic filtering which averages the output signal during a
programmable period. This damping time is programmable between 0 and 120 seconds.
Salinity compensation
In order to take the effect of salinity into account for oxygen measurement an average chloride
concentration can be programmed. The chloride concentration value is set manually via the service level.
The EXA D0402 takes account of the effects of salinity and temperature simultaneously. The advantage of
this construction is that the result of the measurement is available immediately. A separate conversion table
is not necessary.
Temperature compensation
The micro-processor makes an accurate temperature compensation possible that performs well over the
entire range of the instrument. No further adjustment tables are required.
Barometric air pressure compensation
Air pressure differences, due to weather conditions or altitude, can cause a variation up to 20 % in the
dissolved oxygen concentration. A built-in air pressure sensor automatically compensates for barometric
influences between 900 to 1100 mbar (90 to 110 kPa).
Sensor diagnostics
The DO sensor is checked for low impedance between the silver electrode and an earth contact in the
liquid, to detect membrane integrity. Temperature sensor connections and sensor connections are checked
for impedance. These faults are signalled by the FAIL contact and can be signalled to the control room by
an output of 0/3.5 mA or 22 mA. The fault is also signalled by a special marker held on the display, a LED
on the front and an error code in the message display.
During calibration of a DO measuring system the slope deviation from nominal value (%) and sensor output
(µA) at 0 mg/l are calculated and checked.
If any of these are outside the limits, an error is signalled.
Logbook
Software record of important events and diagnostic data. Available through RS485, for use with the
Yokogawa PC402 communication software.
Serial Communication
Bi-directional according to the EIA-485 standard using HART-protocol and PC402 software.
1-4 Introduction
IM 12J6B3-E-H
Summary of Contents for EXA DO402G
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