VEGA VEGAFLEX 81 Profibus PA User Manual

Page 21

4 Mounting

VEGAFLEX 81 • Profibus PA

44217-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