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Maintenance & Fault Finding 3098 Technical Manual
5-2
(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 BarA 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 Chamber 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.
(iii) Erratic Instrument Readings
These can be caused by:
a) 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.
b) Vibrating Cylinder
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.
c) Pressure Control Valve
If the erratic signal is only present while there is a flow of sample gas through the unit, then the fault is likely to be due
to a malfunction of the pressure control valve, brought about by the presence of dirt. In this case the diaphragm (and
hence valve mechanism) should be stripped down, cleaned and re-assembled. Any poor seals or damaged parts
should be replaced. Alternatively, the gas pressure may be falling below that of the designed input condition.