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3098 Technical Manual Maintenance & Fault Finding
5-1
Chapter 5
Maintenance & Fault Finding
5.1 Introduction
This chapter deals with the recommended servicing and maintenance that can be carried out under field conditions,
including calibration checks, faultfinding procedures and simple maintenance. If a fault is traced to a reference chamber
malfunction, it is strongly recommended that the repair of the faulty unit be restricted to a qualified engineer or that the
faulty unit be returned to the factory (see Appendix C for Returns Forms).
If a calibration check reveals a significant error, the cause of this error (e.g. reference chamber leak, deposition on the
vibrating cylinder) should be thoroughly investigated before any re-calibration attempt is made.
5.1.1 Calibration Check
It is normally good practice to carry out periodic checks on the system accuracy. This is simply achieved by passing a gas
of known specific gravity through the instrument as previously detailed under the Calibration section of the Installation
Procedure (Chapter 2). It is preferable that the specific gravity of this calibration gas lies within the specific gravity range
of the system under test since this will simplify the system check procedure. However, a gas whose specific gravity is
outside this range can be used if its characteristics are similar to those of the system line gas.
5.1.2 Fault Finding
If any adverse or suspect readings occur upon checking the calibration, the possible causes for this can be summarised
into 4 groups:
• Instrument Over-reads;
• Instrument Under-reads;
• Erratic Instrument readings;
• Meter Faults.
a) Instrument Over-reads
This is generally due to deposition, condensation or corrosion on the vibrating cylinder walls.
The effects of deposition and condensation can be removed from the cylinder by carefully cleaning the cylinder walls
(once the density meter has been removed from the 3098) although corrosion cannot be dealt with this way.
If the cylinder is corroded or damaged in any way (i.e. dents, scratches etc.) then it must be replaced with a new unit.
b) 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: