VEGA VEGAFLEX 81 Foundation Fieldbus - Manual (Page 21)

21
4 Mounting
VEGAFLEX 81 • Foundation Fieldbus
44218-EN-130910
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). In measurement technology, such
a tube corresponds to a coax probe. It does not matter if the stand-
pipe is perforated or slotted for better mixing. Lateral inlets in bypass
tubes also do not influence the measurement.
For bypass tubes, select the probe length such that the blocking
distance (dead band) of the probe is above or below the lateral filling
openings. You can thus measure the complete range of the medium in
the bypass tube. When designing the bypass tube, keep the blocking
distance of the probe in mind and select the length above the upper
lateral filling opening accordingly.
Microwaves can penetrate many plastics. For process technical rea-
sons, plastic standpipes are problematic. If durability is no problem,
then we recommend the use of metal standpipes.
When the VEGAFLEX 81 is used in standpipes or bypass tubes,
contact with the tube wall must be avoided. We recommend for this
purpose a cable probe with centering weight.
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.
Pressure
Standpipes or bypass
tubes