Endress+Hauser Levelflex FMP54 - Influence of Gas Layer; Gas Phase Compensation with External Pressure Sensor

Levelflex FMP51, FMP52, FMP54
40 Endress+Hauser
Influence of gas layer
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):
Gas layer Temperature Pressure
°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 0.00 % 0.22 % 1.2 % 2.4 % 4.9 % 9.5 %
200 392 -0.01 % 0.13 % 0.74 % 1.5 % 3.0 % 6.0 %
400 752 -0.02 % 0.08 % 0.52 % 1.1 % 2.1 % 4.2 %
Hydrogen 20 68 -0.01 % 0.10 % 0.61 % 1.2 % 2.5 % 4.9 %
200 392 -0.02 % 0.05 % 0.37 % 0.76 % 1.6 % 3.1 %
400 752 -0.02 % 0.03 % 0.25 % 0.53 % 1.1 % 2.2 %
Gas layer Temperature Pressure
°C °F 1 bar
(14.5 psi)
2 bar
(29 psi)
5 bar
(72.5 psi)
10 bar
(145 psi)
20 bar
(290 psi)
50 bar
(725 psi)
100 bar
(1450 psi)
200 bar
(2900 psi)
Water
(saturated
steam)
100 212 0.26 % - - - - - - -
120 248 0.23 % 0.50 % - - - - - -
152 306 0.20 % 0.42 % 1.14 % - - - - -
180 356 0.17 % 0.37 % 0.99 % 2.10 % - - - -
212 414 0.15 % 0.32 % 0.86 % 1.79 % 3.9 % - - -
264 507 0.12 % 0.26 % 0.69 % 1.44 % 3.0 % 9.2 % - -
311 592 0.09 % 0.22 % 0.58 % 1.21 % 2.5 % 7.1 % 19.3 % -
366 691 0.07 % 0.18 % 0.49 % 1.01 % 2.1 % 5.7 % 13.2 % 76 %
Gas phase compensation
with external pressure
sensor (PROFIBUS PA /
FOUNDATION Fieldbus)
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).