Bypass Sensor In Conventional Technology (Types 8710 / 8700) - Burkert 8626 Instructions De Service

Bypass sensor in conventional technology (Types 8710 / 8700)

Measurement is also on the bypass principle. A laminar flow element in the main
channel generates a small pressure drop. This drives a small flow proportional to
the main flow through the actual sensor tube.
On this narrow tube are wound two heater resistors which are connected in
measuring bridge. In the zero-flow state, the bridge is balanced, but with finite flow,
heat is transported in the flow direction and the bridge becomes unbalanced.
I
Br
R R
1 2
sensor tube
gas flow p p
1 2
laminar flow element
prefilter
Fig.: Schematic diagram of bypass measurement
The dynamics of the measurement are determined by the tube walls, which act as
a thermal barrier. They are hence significantly poorer, on principle, than with
sensors having resistors placed directly in the medium flow. Through use of
suitable software in the controller, correction times are obtained that are adequate
for a large part of the applications (in the range of a few seconds).
With contaminated media, we recommend installing filter elements upstream. This
avoids changes in the division ratio between main flow and sensor tube, as well as
changes in the head transmission chaused by deposits on the walls.
With these sensors, even aggressive gases can be controlled, since all essential
parts in contact with the medium are fabricated in stainless steel.
MFC/MFM - 9