Panametrics DigitalFlow GM868, Service Manual

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Chapter 3. Diagnostics
26 DigitalFlow™ GM868 Service Manual
3.4 Flowcell Problems
If preliminary troubleshooting with the Error Code Messages and/or the Diagnostic Menu indicates a possible flowcell
problem, proceed with this section. Flowcell problems fall into two categories:
•
gas problems
• pipe problems.
Read the following sections carefully to determine if the problem is indeed related to the flowcell. If the instructions in
this section fail to resolve the problem, contact the factory for assistance.
3.4.1 Gas Problems
Most gas-related problems result from a failure to observe the flowmeter system installation instructions, as
described in the
Startup Guide. Refer to Chapter 1, Installation, of the Startup Guide to correct any installation
problems.
If the physical installation of the system meets the recommended specifications, it is possible that the gas itself may
be preventing accurate flow rate measurements. The gas being measured must meet the following requirements:
1.
The gas must be homogeneous, single-phase and relatively clean. Although a low level of entrained particles
may have little effect on the operation of the Model GM868, excessive amounts of solid or liquid particles will
absorb or disperse the ultrasound signals. This interference with the ultrasound transmissions through the gas
will cause inaccurate flow rate measurements. In addition, temperature gradients in the gas flow may result in
erratic or inaccurate flow rate readings.
2. The gas must not excessively attenuate ultrasound signals.
Some gases (i.e., high-purity carbon dioxide, hydrogen, etc.) readily attenuate ultrasound energy. In such a case,
an E1 error code message will appear on the display screen to indicate that the ultrasonic signal strength is
insufficient for reliable measurements.
3.
The gas sound speed must not vary excessively.
The Model GM868 will tolerate relatively large changes in the gas sound speed, as may be caused by variations
in gas composition and/or temperature. However, such changes must occur slowly. Rapid fluctuations in the gas
sound speed, to a value that is considerably different from that programmed into the Model GM868, will result in
erratic or inaccurate flow rate readings. Refer to Chapter 2,
Initial Setup, of the Startup Guide and make sure that
the appropriate sound speed is programmed into the meter.