4) »
Dry calibration«
A dry calibration for hydrostatic sensors requires:
• the »empty« level at which the measurement should start
• the maximum height of the liquid column at »full« level and
• the density of the liquid
• the calculated offset and sensitivity of the display.
Caution!
• The sensor constants must have been entered as per Section 4.1.
• Check the calibration during the first filling of the tank! If your calculations are
incorrect the levels measured will be incorrect also!
Example: display in %
display for p
zero
= 0
Calculate the pressure in mbar acting at
the sensor for the desired »empty« level
and the span (»full« - »empty«):
p
mbar
= 10 x ρ (kg/dm
3
) x g (m/s
2
) x ∆h (m)
For 0.45 m water, display = 0%,
For 10.45 m water, display = 100%
Span (0%
…
100%) = 10 m
• p
zero
= 10 x 1.0 x 9.807 x 0.45
= 44.13 mbar
p
span =
10 x 1.0 x 9.807 x 10.00
= 980.7 mbar
• Offset = p
zero =
44.13 mbar
Sensitivity = p
span
/span = 980.7/100
9.807 mbar/%
Sensor calibration
# Matrix Entry Remarks
1 V0H2 e.g. 100 Full level (100%)
2 - »E« Register entry
3 V3H1 e.g. 44.13 Offset in mbar
4 - »E« Register entry
5 V3H2 e.g. 9.807 Sensitivity mbar/%
6 - »E« Register entry
7 V0H0 **.** Measured value %
The Prolevel now measures 0 at 44.13 mbar
Next step… Set analogue outputs and relays in %, see
p. 30…33.
Caution!
sensor zero, f
0
frequency (V3H5)
f (Hz)
P mbar
level
BA142_33
offset
V3H1
display sensitivity
mbar/digit, V3H2
sensor sensitivity ∆f, S
(V3H2)
Fig. 4.7:
Offset and sensitivities for dry calibration
VH
00
VH
00
100
0
»Empty«, e.g. 0.45 m
»Full« e.g. 10.45 m
p
empty
= ρ x g x 0.45
e.g. = 44.13 mbar
span = 100%
= e.g. 10 m
BA142_34
sensor zero
point
display
sensor
offset
Fig. 4.8:
Parameters for dry calibration, display in %
ρgh
Chapter 4: Level Measurement Prolevel FMC 661
28 Endress+Hauser
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