bürkert 8626 Bedienungsanleitung Seite 10

Inline sensor (Types 8626 / 8006 / 8716 / 8706)
This sensor works as a hot-film anemometer in the so-called CTA (Constant
Temperature Anemometer) mode. Two resistors with exactly defined temperature
coefficients in the medium flow form a resistance bridge with three resistors
situated outside the flow.
The first resistor in the medium flow (R
second, lower resistance resistor (R
temperature a certain amount above that of the medium. The heating current
required is a measure of the heat dissipation by the flowing gas and represents
the primary measurement value.
Suitable flow conditioning inside the MFC or MFM and calibration with high-quality
flow standards guarantee that the quantity of gas flowing per unit time can be
derived with high precision from the primary signal.
gas flow
prefilter
Fig.: Functional diagramm of the Inline sensor
Direct medium contact by the resistors R
excellent dynamics for the devices with response times of a few hundred
miliseconds on sudden changes in the setpoint or process value. Owing to the
arrangement of the resistors on a glass support lying tangential to the flow, the
sensor is only slightly prone to contamination. The measurement range of the
Inline sensor is limited at the bottom end by instrinsic convection in the flow
channel, which also occurs when the control valve is closed. It is hece unsuitable
for devices whose working range must extend to flow rates below ca. 1 l
The signal from intrinsic convection in the flow channel depends on the installation
position of the device. In order that high precision can be obtained at low flow
rates, the actual installation position should be identical to that specified on
ordering 1) . For the same reason, the operating pressure should not differ too much
from the calibration pressure.
1 )
The device is calibrated in the installation position stated in the questionnaire to be found in the Annex to the Data
Sheet.
10 - MFC/MFM
All manuals and user guides at all-guides.com
) measures the fluid temperature; the
T
) is always heated to maintain its
S
sensor with
electronics
flow conditioning
T
R R
2 1
PID
R
K
R R
T s
and R in the main flow assure
S
I
s
/min.
N