Keller developed a differential pressure measuring module for integration into a production flow controller in response to a customer’s request.
The client, Bürkert needed to develop a series of liquid flow controllers for a spraying system that allowed precise metering of lubricants and was required for sheet metal working. The sensor and control technology as well as the actuator and the custo¬mary electrical process interfaces had to be accommodated in one compact device that was to be designed for trouble-¬free continuous operation.
Flow rate measurement using differential pressure
Since the process environment called for a fairly robust design and high operating reliability, it was decided to measure the flow volume on the basis of the pressure drop in the measuring medium as it passed through a metering orifice with a defined diameter. Two individual pressure sensors were to be deployed for this purpose.
Pressure difference or differential pressure transmitters
In conventional differential pressure transmitters, both sides of a measuring diaphragm are exposed to the measuring medium; i.e., one high and one low pressure input. With typical differential measuring ranges of 500 mbar and common mode pressures of up to 10 bar, a momentary interruption of either high or low pressure could result in a 20-fold over¬load on the diaphragm. Such an overload can only be absorbed with complex (and therefore expensive) adaptations to the structural design, without which the transmitter would inevitably be destroyed. Bürkert’s fluid control systems specialists wanted to exclude risks of this sort, so they were very interested in Keller’s differential pressure measurement module.
Differential pressure transmitters operate with two selec¬ted encapsulated silicon pressure sensors, installed at a distance of about 20mm from each other. They deliver their respective output signals to the inputs of a micro-processor: after a straightforward 16 bit A/D conversion, the microprocessor’s computing power is sufficient to eliminate virtually all reproducible nonlinearities and temperature dependencies, as determined during calibration by mathematical means. Thanks to this method, Keller’s differential pressure transmitters attain a total error band of better than ±0.1%FS across broad temperature ranges. The analogue output signal from the module is up¬dated as many as 200 times per second, and a good dynamic reserve is available for subsequent processing. As a rule of thumb, the measuring range for differential pressure measurements of this sort should be approximately 20% of the common-mode pressure.
In addition to standard analogue signals of 4…20 mA and 0…10 V, 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, i.e. output is not limited to the differential pressure values. Digitisation enables flexible adaptation of the interval for the analogue output signal to the desired inter¬val for the input signal (differential pressure).
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 designed 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.
A close working relationship between engineers at Keller and Bürkert ensured the end result would be compliant with the jointly-developed specification. Keller’s engineers also implemented additional details in line with the customer’s wishes; e.g., 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 2.5 V varies significantly from the Keller catalogue product.
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 0.9 l/h and 36 l/h can be achieved, depen¬ding 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 500 mbar.
As the project progressed, the inherent benefits of digital signal processing and individual sensor signals for input pressure and output pressure produced unexpected bene¬fits; e.g., these can be utilised internally within the flow con¬troller to set limits, detect overloads or 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.
Two specialists – one in flow control and the other in pressure measurement – collaborated on a joint solution for a highly specific customer requirement. 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 pro¬cess technology segment; 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 considerable superiority over classical differential pressure transmitters with only one diaphragm. For customised applications in particular, digitised sensor signal processing offers a host of benefits that are also reflected in the overall cost calculation.
Written by Bernhard Vetterli, Dipl. El.-Ing. HTL.