Blocking Distance High And Blocking Distance Low Guidance; Blocking Distance High (Bdh) Guidance; Blocking Distance Low (Bdl) Guidance; Blocking Distance, Full Tank Detection And Latching Modes - Honeywell SmartLine SLG 700 User Manual

2.8 Blocking distance high and blocking distance low guidance

The minimum blocking distances and accuracy in the transition zones may be compromised in
certain applications. In these cases and in order to meet the required specifications, it is
necessary to follow the guidance provided below. If the sensor performances in the transition
and blocking zones are compromised due to the presence of an obstruction and if the
performances cannot be improved, it is advisable to increase the configured blocking distances
accordingly. The latter will prevent the sensor from finding erroneous reflections in these
regions.

Blocking distance high (BDH) guidance

Meeting the minimum blocking distance high specification may require recording a field
background. A field background is captured when the tank is empty or when the tank
level is below the minimum required distance (See Section 2.4.2). The field background
records the reflected signal near the process connector. The background data is
subsequently removed from the echo curve that is used for analysis.
The length of the field background depends on the type of sensor, the length of the probe
and the length of the nozzle (if used). A field background may be automatically updated if
the Dynamic Background option is turned on.

Blocking distance low (BDL) guidance

To meet the blocking distance low specifications, the following three conditions apply:

The probe type must be configured properly. For example, it is necessary to
indicate if a coax, a rod or a wire is used and what is the termination (end weight,
centering disk, etc.).

The probe length must be configured accurately. The probe length is set at factory
time however if the probe has been cut or a centering disk was added it may be
necessary to reconfigure the probe length. See section
disks and configured probe length.

In some cases, it might be necessary to adjust the end of probe model. For
example, if the end of probe is close to the bottom of the tank or to an obstacle, the
default end of probe model parameters might not be suitable. Refer to the SLG 700
HART Option manual, #34-SL-25-06 or the SLG 700 FOUNDATION Fieldbus
manual, #34-SL-25-07, for details on adjusting model parameters.

Blocking Distance, Full Tank Detection and Latching modes

When levels are lost into blocking zones the gauge display will report the level status as
uncertain, the level is reported as the blocking distance (rather than the actual level). A
digital indication will also be produced to indicate that the transmitter is detecting that the
level is in the blocking zone. Refer to the appropriate user's manual for details of interface
specific behavior.
Echos are generally not processed when the level is inside these zones except when Full
Tank Detection is turned on. When the level lowers outside of the upper blocking zone or
raises above the lower blocking zone, a small hysteresis distance is applied (2 cm and 3
cm for the upper and lower zones respectively) so that the sensor output remains
consistent as the level hovers near the blocking distance and does not toggle back and
forth between output states.
Page 34
SLG 700 SmartLine Level Transmitter User's Manual
0 for details on centering
Revision 8