Endress+Hauser Waterpilot FMX21 - Page 29

Waterpilot FMX21 Electrical connection
Endress+Hauser 29
6.7.4 Connecting for air pressure compensation with external
measured value
FXN520
1 2 3 4
A0018757
1 Fieldgate FXA520
2 Multidrop connector FXN520
3 Cerabar
4 Waterpilot FMX21
For applications in which condensation may occur, the use of an absolute pressure probe is
recommended. For level measurement using an absolute pressure probe, the measured
value is affected by fluctuations in the ambient air pressure. To correct the resulting
measured error, you can connect an external absolute pressure sensor (e.g. Cerabar) to the
HART signal cable, switch the Waterpilot to burst mode and operate the Cerabar in "Electr.
Delta P" mode.
By switching on the "Electr. Delta P" application, the external absolute pressure sensor
calculates the difference between the two pressure signals and can thus determine the
level precisely. Only one level measured value can be corrected in this way.
If using intrinsically safe devices, the regulations which apply to interconnecting
intrinsically safe circuits as outlined in IEC 60079-14 (proof of intrinsic safety) must
be observed.
6.7.5 Connecting an external temperature sensor/temperature head
transmitter for density compensation
The Waterpilot FMX21 can correct measured errors that result from fluctuations in the
density of the water caused by temperature. Users can choose from the following options:
Use the internally measured sensor temperature of the FMX21
The internally measured sensor temperature is calculated in the Waterpilot FMX21 for
density compensation. The level signal is thus corrected according to the density
characteristic line of the water.
Use the optional internal Pt100 temperature sensor for density compensation in a
suitable HART master (e.g. PLC)
The Waterpilot FMX21 is available with an optional Pt100 temperature sensor. Endress
+Hauser additionally offers the TMT182 temperature head transmitter to convert the
Pt100 signal to a 4 to 20 mA HART signal. The temperature and pressure signal is
requested by a HART master (e.g. PLC), where a corrected level value can be generated
using a stored linearization table or density function (of a chosen medium).