Influence Of Gas Layer; Gas Phase Compensation With External Pressure Sensor - Endress+Hauser Levelflex FMP51 Technical Information

Influence of gas layer

Gas layer Temperature
°C °F 1 bar (14.5 psi) 10 bar (145 psi) 50 bar (725 psi) 100 bar (1450 psi) 200 bar (2900 psi) 400 bar (5800 psi)
Air 20 68
200 392
400 752
Hydrogen 20 68
200 392
400 752
Gas layer Temperature
°C °F
Water 100 212
(saturated
120 248
steam)
152 306
180 356
212 414
264 507
311 592
366 691
Gas phase compensation
with external pressure
sensor (PROFIBUS PA /
FOUNDATION Fieldbus)
40
High pressures reduce the propagation velocity of the measuring signals in the gas/vapor above the
fluid. This effect depends on the kind of gas/vapor and of its temperature. This results in a
systematic measuring error that gets bigger as the distance increases between the reference point of
the measurement (flange) and the product surface. The following table illustrates this measured
error for a few typical gases/vapors (with regard to distance; a positive value means that too large a
distance is being measured):
0.00 % 0.22 %
-0.01 % 0.13 %
-0.02 % 0.08 %
-0.01 % 0.10 %
-0.02 % 0.05 %
-0.02 % 0.03 %
1 bar 2 bar
(14.5 psi) (29 psi) (72.5 psi)
0.26 % -
0.23 % 0.50 %
0.20 % 0.42 %
0.17 % 0.37 %
0.15 % 0.32 %
0.12 % 0.26 %
0.09 % 0.22 %
0.07 % 0.18 %
PROFIBUS and FOUNDATION Fieldbus devices can receive the signal of an external pressure sensor
through the bus and use it to perform a pressure dependent time-of-flight correction. In the case of
saturated steam in the temperature range from 100 to 350 °C (212 to 662 °f), for example, the
measuring error of the distance measurement can be reduced by this method from up to 29 %
(without compensation) to less than 3 % (with compensation).
Pressure
1.2 % 2.4 %
0.74 % 1.5 %
0.52 % 1.1 %
0.61 % 1.2 %
0.37 % 0.76 %
0.25 % 0.53 %
Pressure
5 bar 10 bar 20 bar
(145 psi) (290 psi)
- -
- -
1.14 % -
0.99 % 2.10 %
0.86 % 1.79 % 3.9 %
0.69 % 1.44 % 3.0 %
0.58 % 1.21 % 2.5 %
0.49 % 1.01 % 2.1 %
Levelflex FMP51, FMP52, FMP54
4.9 %
3.0 %
2.1 %
2.5 %
1.6 %
1.1 %
50 bar 100 bar
(725 psi) (1450 psi)
- - -
- - -
- - -
- - -
- -
9.2 % -
7.1 % 19.3 %
5.7 % 13.2 %
9.5 %
6.0 %
4.2 %
4.9 %
3.1 %
2.2 %
200 bar
(2900 psi)
-
-
-
-
-
-
-
76 %
Endress+Hauser