Siemens SITRANS LG250 Operating Instructions Manual page 17

Pressure
Bypass tubes
PBD-51041053
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 dead zone
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 designing the bypass tube,
keep the dead zone 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 LG250 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 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 LG250 - Operating Instructions
17