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56 Micro Motion 7829 Viscomaster
®
and Viscomaster Dynamic
™
Viscosity Meters
General Maintenance
6.3.2 Mechanical servicing
This mainly comprises the cleaning of any deposition or corrosion from the tines. Deposition is
removed by the use of a suitable solvent. For corrosion, solvent and the careful use of a fine abrasive
will usually be sufficient. Take care not to damage the PFA lamination if installed. However where
extensive corrosion has been treated, it is highly recommended that a full calibration is carried out to
check the meter characteristics.
6.3.3 Time period trap
Disturbances in the fluid caused by bubbles, cavitations or contaminants can cause sudden changes in
the measured output, which may, under some circumstances, give rise to instability (i.e. hunting) in a
control system relying on the measurement. The meter can maintain the analog output during such
perturbations by ignoring the aberrant measurement, and maintaining the output at the last good
measured value. This facility is known as the Time Period Trap (TPT).
Under all normal circumstances, the factory settings for the TPT should be used. However, in extreme
cases it may be necessary to alter the settings to meet the demands of a particular system. This should
only be done after monitoring the behavior of the system for some time, to establish the normal
running conditions.
Great care must be taken not to reduce the sensitivity of the meter so that normal response to
fluctuations in the fluid is impaired.
The time period trap facility works as follows:
After each measurement of the time period (of the meter’s vibrating tines) the new value is
compared with the previous value. If the difference between them is smaller than the allowable
tolerance, the output is updated to correspond to the new measured value, and the TPT remains
inoperative; i.e., operation is normal. If the difference exceeds the allowable tolerance, the output
remains at the its previous level, and does not follow the apparent sudden change in value.
This process is repeated until either of the following:
• The latest measured value falls back to the level of the original value, indicating that the
transient has passed; or
• The TPT count is reached. At this point it is assumed that the change in value is not due to a
random disturbance, and the output adopts the value of the latest reading.
Two Modbus Registers control the operation of the Time Period Trap facility. These can be changed,
if necessary, using ADView’s Register Read/Write facility.
• Modbus Register 138: contains the maximum allowable change in the time period between
readings, specified in μs. The preset value is 10.
Care is essential in handling the meter during transit, installation, and removal from the pipeline/tank.