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

Bypass sensor in conventional technology "capillary"

(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.
gas flow
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 resistor 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. With this sensor
prinziple it is also possible to convert between different gases. A choice of some
gases are listed in the table below, others on request. Q(gas) = f x Q (N
Gas Factor f
N 1.00
2
Air 1.00
O 0.99
2
H 1.01
2
Ar 1.4
He 1.41
CO 0.76
2
prefilter
By using the gas factors it is possible
that the accuracy is not within the
datasheet specification. For applications
which need high accuracy it is
recommended to calibrate under
application conditions.
sensor tube
).
2
MFC/MFM - 11