Siemens SITRANS LG270 Operating Instructions Manual page 17

Pressure
Bypass tubes
PBD-51041064
Keep in mind that a min. distance must be maintained below the refer-
ence plane and possibly also at the end of the probe - measurement
in these areas is not possible (dead band). The length of the cable
can be used all the way to the end only when measuring conductive
products. These blocking distances for different mediums are listed
in chapter "Technical data". Keep in mind for the adjustment that the
default setting for the measuring range refers to water.
The process fitting must be sealed if there is gauge or low pressure in
the vessel. Before use, check if the seal material is resistant against
the measured product and the process temperature.
The max. permissible pressure is specified in chapter "Technical
data" or on the type label of the sensor.
Standpipes or bypass tubes 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 tube corresponds to a coax measuring probe. Lateral inlets
in bypass tubes do not influence the measurement.
Measuring probes can be mounted in bypass tubes up to DN 200.
For bypass tubes, select the probe length such that the blocking
distance (dead band) of the probe is above and below the lower
lateral filling openings of the bypass tube. You can thus measure the
complete range of the medium in the bypass tube (h). When design-
ing the bypass tube, keep the blocking distance of the probe in mind
and select the length of the bypass tube above the upper lateral filling
opening accordingly.
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 bypass tubes, 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 buildup can form on the spacers. Strong buildup
can influence the measurement.
SITRANS LG270 - Operating Instructions
17