Yokogawa DO402 Dissolved Oxygen Analyzer - Manual (Page 12)

IM 12J05D02-01E
1-4 Introduction
Display functions and ranges
The display continuously gives you all necessary information at a glance. The process values are shown
in easily readable programmable units. Either mg/l. % saturation or ppm can be chosen.
The user-interface is simplified to a basic set of 6 keys accessible through the flexible window cover. It
uses a simple step by step, question and answer style to communicate with the operator by giving mes-
sages on the second line of the display and indicating which keys are to be pressed in the display.
Automatic air calibration
Calibration for a dissolved oxygen instrument is performed by simple air calibration (please refer to 6.3.1
section for the optical sensor).
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 DO402 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 meas-
uring 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 dis-
turbances the EXA DO402 has an electronic filtering which averages the output signal during a program-
mable 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 concen-
tration can be programmed. The chloride concentration value is set manually via the service level. The
EXA DO402 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 con-
trol 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 out-
put (µA) at 0 mg/l are calculated and checked.
If any of these are outside the limits, an error is signalled.