Endress+Hauser Prosonic M FMU40 - Measuring Principle

Appendix Prosonic M - FOUNDATION Fieldbus
96 Endress+Hauser
11.9 Measuring principle
L00-FMU4x-15-00-00-xx-001
E: Empty distance; F: Span (full distance); D: Distance from sensor membrane - product surface; L: Level; BD: Blocking
distance
11.9.1 Time-of-flight method
The sensor of the Prosonic M transmits ultrasonic pulses in the direction of the product surface.
There, they are reflected back and received by the sensor. The Prosonic M measures the time t
between pulse transmission and reception. The instrument uses the time t (and the velocity of sound
c) to calculate the distance D between the sensor membrane and the product surface:
D = c  t/2
As the device knows the empty distance E from a user entry, it can calculate the level as follows:
L = E - D
An integrated temperature sensor compensates for changes in the velocity of sound caused by
temperature changes.
11.9.2 Interference echo suppression
The interference echo suppression feature on the Prosonic M ensures that interference echos (e.g.
from edges, welded joints and installations) are not interpreted as a level echo.
11.9.3 Calibration
Enter the empty distance E and the span F to calibrate the device.
20 mA
100%
4mA
0%
D
L
F
E
BD
Sensor BD Max. range fluids Max. range bulk materials
FMU40 0.25 m (0.8 ft) 5 m (16 ft) 2 m (6.6 ft)
FMU41 0.35 m (1.1 ft) 8 m (26 ft) 3.5 m (11 ft)
FMU42 0.4 m (1.3 ft) 10 m (33 ft) 5 m (16 ft)
FMU43 0.6 m (2.0 ft) 15 m (49 ft) 7 m (23 ft)
FMU44 0.5 m (1.6 ft) 20 m (66 ft) 10 m (33 ft)