Yokogawa EXA SC202 2-Wire Conductivity Transmitter/Analyzer - Manual (Page 69)

IM 12D08B02-01E
Maintenance 7-1
7. MAINTENANCE
7-1. Periodic maintenance for the EXA 202 transmitter
The EXA202 transmitter requires very little periodic maintenance. The housing is sealed to IP65 (NEMA
4X) standards, and remains closed in normal operation. Users are required only to make sure the front
window is kept clean in order to permit a clear view of the display and allow proper operation of the
pushbuttons. If the window becomes soiled, clean it using a soft damp cloth or soft tissue. To deal with
more stubborn stains, a neutral detergent may be used.
NOTE :
Never used harsh chemicals or solvents. In the event that the window becomes heavily stained or
scratched, refer to the parts list (Chapter 10) for replacement part numbers.
When you must open the front cover and/or glands, make sure that the seals are clean and correctly fit-
ted when the unit is reassembled in order to maintain the housing’s weatherproof integrity against water
and water vapour. The measurement otherwise may be prone to problems caused by exposure of the
circuitry to condensation.
The EXA instrument contains a lithium cell to support the clock function when the power is switched
off. This cell needs to be replaced at 5 yearly intervals (or when discharged). Contact your nearest
Yokogawa service centre for spare parts and instructions.
7-2. Periodic maintenance of the sensor
NOTE :
Maintenance advice listed here is intentionally general in nature. Sensor maintenance is highly
application specific.
In general conductivity/resistivity measurements do not need much periodic maintenance. If the EXA
indicates an error in the measurement or in the calibration, some action may be needed (ref. chapter
8 troubleshooting). In case the sensor has become fouled an insulating layer may be formed on the
surface of the electrodes and consequently, an apparent increase in cell constant may occur, giving a
measuring error. This error is:
2 x x 100 %
where:
Rv = the resistance of the fouling layer
Rcel = the cell resistance
NOTE :
Resistance due to fouling or to polarization does not effect the accuracy and operation of a 4-
electrode conductivity measuring system. If an apparent increase in cell constant occurs cleaning
the cell will restore accurate measurement.
Cleaning methods
1. For normal applications hot water with domestic washing-up liquid added will be effective.
2. For lime, hydroxides, etc., a 5 ...10% solution of hydrochloric acid is recommended.
3. Organic foulings (oils, fats, etc.) can be easily removed with acetone.
4. For algae, bacteria or moulds, use a solution of domestic bleach (hypochlorite).
* Never use hydrochloric acid and bleaching liquid simultaneously. The very poisonous chlorine gas will
result.
Rv
Rcel