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36 Micro Motion 3098 Gas Specific Gravity Meter
Maintenance and Fault Finding
5.3.2 Instrument under-reads
This is most probably due to a gas leak from the reference chamber. Before dismantling the
instrument it is desirable to locate the leak, the cause of which may be one of the following:
(i) Reference chamber to sample gas path
Parts affected are:
•Diaphragm
• Reference chamber valve
• Reference chamber metalwork.
This type of leak can be identified by using the following test.
Charge the reference chamber to a high pressure (up to 7 Bar A maximum) and then isolate by closing
the reference chamber valve. Vent the sample gas path at the instrument’s inlet and outlet to
atmosphere, then seal by closing the inlet and outlet line valves.
If gas is leaking into the sample gas flow path, this will be indicated by the change in output signal
from the density meter.
Alternatively, if the leak rate is influenced by whether the sample gas path is at atmospheric pressure
or at the line operating pressure, then this is indicative of a leak into the sample gas flow path.
(ii) Reference chamber to atmosphere
Parts affected are:
•Diaphragm
• Sealing gasket (meter)
• Reference chamber valve pipework
• Reference c+hamber metalwork
This type of leak can often be traced by the application of a soap solution, or 'Snoop', and bubble
observation. Unlike the previous type of leak this will not be influenced by sample gas path pressure.
If the leak is due to a faulty gasket seal, diaphragm, or reference chamber valve then a serviceable
replacement should be fitted.
If in doubt, advice should be sought from the factory – contact details are on the back page of this
manual.
(iii) Erratic instrument readings
These can be caused by:
• Electronic fault
This can exist in either the meter or its associated electronics.
If an independent frequency generator is available, this can be used to check the performance
of the flow computer/signal converter.
If the fault is in the meter amplifier, this can be changed with no degradation in performance.
• Vibrating cyclinder
If the sample gas flow is stopped by closing the inlet valve, the time period signal should drop
slightly to a steady value or, if there is a small leak, continue to drop slowly. Should the
reading remain erratic, it is likely that there is deposition on the vibrating cylinder which needs
to be stripped, cleaned and re-assembled.
• Pressure control valve