Instrumentation
4–20mA and 0–10V, the processor offers a digital RS485 half-duplex interface. This interface can be used, for example, to output the measured pressure and temperature values from the individual sensors, ie output is not limited to the differential pressure values. Digitisation enables flexible adaptation of the interval for the analogue output signal to the desired interval for the input signal (differential pressure).
SCHEMATIC CROSS-SECTION OF A LIQUID FLOW CONTROLLER
Bürkert manufactures the Liquid Flow Controllers to order, as customised process measurement devices for each specific application. Based on only three differential pressure transmitters with variant equipment, final flow rate values of between 0.9l/h and 36l/h can be achieved, depending on the common- mode pressure. The measuring ranges are fine-tuned with the help of special orifices integrated into the flow channel; the targeted difference between input pressure and output pressure is typically about 500mbar. As the project progressed, the inherent benefits of
Connection plugs for setpoint, actual value, operating voltage, etc.
Control electronics Proportional valve
Fieldbus plug Flow sensor
High-grade stainless steel base body
Fluid input with integrated filter
Fig. 2. Schematic cross-section of a liquid flow controller.
The mechanical connection between the pressure sensors and the main channel of the flow controller is implemented in each case with a capillary tube (vented by means of a defined purging process), which is also de-signed as a low-pass filter for pressure peaks. All the parts that come into contact with the measuring medium (except for the sealing rings) are made of high- grade stainless steel. Keller implemented additional details in line with
the customer’s wishes, eg, the delivery format for the flexible PCB with electrical connection details and the mechanical integration were specified by joint agreement, as was the output signal at nominal flow rate, which at its current value of 2.5V varies s from the Keller catalogue product.
digital signal processing and individual sensor signals for input pressure and output pressure produced unexpected benefits. For example, these can be utilised internally within the flow controller to set limits, detect overloads or to implement other diagnostic functions. Further, during calibration of the flow rate measurement (usually carried out with water or a fluid with similar viscosity to that of the process liquid), the calibration data can be entirely re-characterised in the processor of the differential pressure transmitter, thereby resulting in an ‘end-to-end’ calibration of the flow controller, not just the pressure sensor
Fig. 4. KELLER dual sensor module, Series PD-9 FLX, with electronics.
Outlook Signal processing based on a microprocessor as deployed in Keller’s differential pressure transmitter, which operates with two pressure sensors, made it far easier to integrate the solution into a flow controller intended for continuous operation in the process technology segment, and this approach also made it possible to implement a whole series of functionalities. The module has now been deployed in numerous
applications, where it has demonstrated its benefits over classical differential pressure transmitters with only one diaphragm – especially as regards overload capability. For customised applications in particular, digitised sensor signal processing offers a host of benefits. n
For more information ✔ at
www.engineerlive.com/epe
Fig. 3. Operating principle for capture of the measured values: measurements are based on the differential pressure method. An orifice in the main channel responds to the flow by triggering a decrease in pressure, which is captured by the differential pressure sensor that is in place. The differential pressure sensor supplies a precise, temperature-compensated measurement signal that provides the basis for calculating the flow rate.
Bernhard Vetterli is with KELLER AG für Druckmesstechnik, Winterthur, Switzerland.
www.keller-druck.com
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