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7828 Density Transmitter Technical Manual
Maintenance
78285000_AA
6-5
Great care must be taken not to reduce the sensitivity of the transmitter 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 7828’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:
(a) the latest measured value falls back to the level of the original value, indicating that the
transient has passed; or
(b) 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 (see section 4).
Modbus Register 138:
contains the maximum allowable change in the time period
between readings, specified in
µ
s. The preset value is 10.
Modbus Register 137:
contains the Time period count, which is the maximum number of
measurements to be rejected before resuming normal operation; the preset value is 2. If
the value is set to 0, TPT is disabled, and the output will always follow the time period
measurement. If you want to program another value, it should be determined
experimentally, and be equal to the length of the longest undesirable transients which are
likely to arise. If the value is set too high, the 7828 will be slow to respond to genuine
changes in the fluid properties.
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