Siemens SITRANS LG250 Operating Instructions Manual page 16

Measuring range
Pressure
Bypass tubes
16
Fig. 8: Mounting of the sensor with inflowing medium
The reference plane for the measuring range of the sensors is the
sealing surface of the thread or flange.
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.
SITRANS LG250 - Operating Instructions PBD-51041388