VEGA VEGAPULS 45 4 … 20 mA; HART compact sensor - Manual (Page 17)

VEGAPULS 42, 44 and 45 – 4 … 20 mA 17
Welding beads too large
Tube connection protrudes
Additional connection in the bypass tube in one plane
Use of guide tubes
(VEGAPULS 44)
In case of very rough inner surfaces in exist-
ing bypass tubes (e.g. due to corrosion),
large connection openings as well as bypass
tubes with more than 100 mm inner diameter,
the use of a guide tube inside the existing
bypass tube is recommended. This reduces
the noise level and increases reliability con-
siderably. The flange of the guide tube can
be easily mounted as a sandwich flange
between vessel and sensor flange.
If enough liquid (300 … 800 mm) remains in
the blind lower end of the tube, the portion of
the signal that penetrates the liquid and re-
flects from the tube bottom is sufficiently
damped - the sensor can then easily distin-
guish it from the echo of the liquid surface. In
cases where there is not enough liquid at the
bottom of the tube, a deflector situated there
will carry out the same function. It deflects
signals that reach the tube bottom into the
standard connection opening.
Connections to the bypass tube
The connections to the bypass tubes must
be fashioned in such a way that only minimal
reflections are caused by the walls of the
connecting tubes. This is especially important
for the breather connection in the upper part
of the tube. Observe the following points:
• Use small openings for the connection.
• The diameter of the connecting tubes
should not exceed 1/3 of the bypass tube
diameter.
• The tube connections must not protrude
into the bypass tube.
• Large welding beads in the tubes should
be avoided.
• Additional connections to the bypass tube
must lie in the same plane as the upper
and lower vessel connection (above each
other or displaced by 180°).
Mounting and installation
Optimum connection to the bypass tube