Siemens SITRANS LG270 Operating Instructions Manual page 19

Standpipes
PBD-51041506
Instructions for the measurement:
•
The 100 % point in bypass tubes should be below the upper tube
connection to the vessel.
•
The 0 % point in bypass tubes should be above the lower tube
connection to the vessel.
•
A false signal suppression with installed sensor is generally rec-
ommended to achieve the best possible accuracy.
Standpipes or surge pipes are normally metal tubes with a diameter
of 30 ... 200 mm (1.18 ... 7.87 in). Up to a diameter of 80 mm (3.15
in), such a pipe corresponds to a coax measuring probe. It does not
matter if the standpipe is perforated or slotted for better mixing.
Measuring probes can be mounted in standpipes up to DN 200.
For standpipes, select the probe length such that the upper dead
zone of the probe is above the upper ventilation hole. This allows you
to measure the total level range of the medium in the standpipe. When
designing the standpipe, keep the upper dead zone of the probe in
mind and plan the length above the upper lateral filling opening ac-
cordingly.
Microwaves can penetrate many plastics. This is why plastic tubes are
problematic for measurement applications. If durability is no problem,
we recommend the use of uncoated metal standpipes.
When the SITRANS LG270 is used in standpipes, contact with the
tube wall must be avoided. We recommend for this purpose a cable
probe with centering weight.
Caution:
When mounting, make sure that the cable is straight over its entire
length. A kink in the cable can cause measurement errors and contact
with the tube.
With rod probes, a spacer is generally not required. However, if there
is a risk of the rod probe being pressed against the tube wall by in-
flowing medium, you should mount a spacer at the probe end to avoid
contact with the tube wall. In the case of cable probes, the cable can
be strained.
Keep in mind that the lower dead zone underneath the spacer in-
creases if spacers are used.
Buildup can form on the spacers. Strong buildup can influence the
measurement.
SITRANS LG270 - Operating Instructions
19