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11 MAINTENANCE
Warning.
•
Each unit of this system forms an integral part of a
certified intrinsically safe system. Appropriate safety
precautions must be taken to prevent any incendive
electrical discharges in the hazardous area when
carrying out any of the following tasks.
•
Equipment in this system operates on a.c. mains supply
voltage electricity. Suitable precautions must be taken to
avoid the possibility of electric shock.
•
The maximum pressure and temperature specified for
particular parts of the system must not be exceeded.
The katharometer unit and its associated equipment are
designed for stable and accurate operation over long periods.
This section covers the requirements for fault finding, diagnostic
tests and maintenance tasks.
11.1 General Maintenance
11.1.1 Pressure
The operation of the katharometer units is not affected
significantly by changes in pressure providing that they are within
the pressure limits given in Section 13.
11.1.2 Flow
The katharometer zero balance and sensitivity are independent
of the sample flowrate, as the sample gas sensing system
depends on molecular diffusion. However, the speed of
response is affected by the flowrate. This means that the flow
resistance of the drying chamber is a compromise between
obtaining speed of response, and avoiding a rapid degradation
of the desiccant.
11.1.3 Leaks
There is an inherent safety requirement that there are no leaks
into or out of the sample system. Any leaks could also affect the
correct operation of the katharometer unit.
11.1.4 Vibration
The
katharometer
unit
mechanically induced vibration. Pulsations due to unsteady
sample flow can affect the katharometer filaments and cause
errors due to excessive cooling.
11.1.5 Contamination
Contamination in the sample system can arise from oil or
suspended particles, or from erosion of material from the sample
system upstream of the katharometer unit.
11.1.6 Ambient Temperature
The calibration of the katharometer is not significantly affected
by variations of the ambient temperature. Temperature changes
can affect the sensitivity and reduce accuracy on sensitive
ranges.
tolerates
reasonable
levels
11.1.7 Bridge Current
The working current of the katharometer bridge is 350 mA
supplied from the PSU. This value must remain stable during
normal operation as the katharometer output signal is
approximately proportional to the cube of the bridge current.
11.2 Diagnostic Tests
Warning.
•
These units are part of the certified intrinsically safe
system. Appropriate safety precautions must be taken
to prevent any incendive electrical discharges in the
hazardous area when carrying out this task.
•
Ensure that the proper electrical safety precautions are
taken at all times when undertaking this procedure.
11.2.1 Checking Output of the PSU
Carry out the test procedure given in Section 6.3.1.
11.2.2 Checking Integrity of Zener Diode Safety
Barrier Devices
Carry out the test procedure given in Section 6.3.2.
11.2.3 Checking the Katharometer Output
a) Electrically isolate the monitor unit.
b) Remove the outer cover from the katharometer unit.
c) With the katharometer operating, check if the voltage across
terminals TB1 – 1 and TB1 – 4 is not above 4 V with 350 mA
passing. If the voltage is above this value it is likely that one
or more filaments of the bridge is broken.
d) With the katharometer operating, check that the voltage
across terminals TB1 – 1 and TB1 – 4 is below 2.8 V with 350
mA passing. If the voltage is below this value and there is no
zero adjustment available, it is likely that there is an
accumulation of liquid within the katharometer block.
e) If the reading from the test made at step c is unstable when
the katharometer block is tapped gently, this could indicate
that a filament is damaged but not open circuit.
If any of these tests indicate that the katharometer is faulty the
complete katharometer unit must be returned for repair or
of
replacement.
The span adjustment of katharometer units are sealed and must
not be adjusted unless necessary – see Section 8.4.2.
11.3 Routine Maintenance
11.3.1 Hydrogen Katharometer Calibration
Carry out a calibration check in accordance with Section 8.
This task should be carried out at intervals of 3 months of on-line
use.
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