Use of Keller Sensors in the Design of a Liquid Flow Meter

Overpressure-Protected Flow Controller – Flexibility as a Result of Digitization

The highest level of solution optimization is achieved when close cooperation is established between the manufacturer of individual components and the manufacturer of finished devices at the product development stage. A confirming example is the differential pressure measurement module, which was developed in response to a customer request and later used in the production of a new flow controller. The flexibility of both parties and, most importantly, digital signal processing brought remarkable results in this case.

Initially, for sheet metal processing, there was a need to create a spraying system that would allow precise dosing of lubricants. This task became a pilot project for Bürkert and served as the starting point for the development of a series of liquid flow controllers. In one compact device, designed for continuous uninterrupted operation (which is a standard requirement for measurement technologies), it was necessary to combine a sensitive element, control technology, actuator, and standard electronic process interfaces.

Flow Measurement Using Differential Pressure

Since the characteristics of the working environment required a fairly robust design and high operational reliability, it was decided to calculate the flow by measuring the pressure drop of the medium as it passes through a constricted orifice of a specified diameter. For this purpose, two independent pressure sensors were installed. By that time, Bürkert already had very good relations with the German branch of Keller AG, and this circumstance played an extremely positive role in this situation. "At that time, we already presented our differential pressure transducer (Series PD-39X), which had all the qualities necessary to solve this task – the main one being resistance to pressure overloads," recalls Executive Director Wolfgang Braun.

Pressure Difference or Differential Pressure Transducer

In standard differential pressure transducers, both sides of the measuring membrane come into contact with the measuring medium, representing one inlet for "high" and one inlet for "low" pressure. At standard differential pressure measurement ranges of 500 mbar and a standard reference pressure of 10 bar, a brief disappearance of pressure at the high or low inlet can lead to a twentyfold overload of the membrane. To withstand such a load, the sensor must have a special complex design (and, consequently, be expensive). The Bürkert specialists developing the liquid control system wanted to eliminate such risks, so they became seriously interested in the KELLER differential pressure measurement module.

KELLER differential pressure transducers consist of two sealed silicon pressure sensors installed 20 mm apart. They separately send output signals to the microprocessor inputs: after direct 16-bit analog-to-digital conversion, the microprocessor's computing power allows eliminating virtually any reproducible deviations from linearity and temperature dependencies. Thanks to this method, Keller differential pressure transducers have a total error of less than ±0.1% FSO over a wide temperature range. The module's analog output signal is updated 200 times per second, and a good dynamic reserve becomes available for subsequent processing. It can be said that the differential pressure measurement range in this case should be approximately 20% of the reference pressure.

In addition to standard analog signals 4…20 mA and 0…10 V, the processor outputs a digital signal via a half-duplex RS485 interface. This interface can be used, for example, to output temperature and pressure values from individual sensors, so the output values will not be limited only to differential pressure values. Digitization provides flexible adaptation of the analog output signal range to the desired input signal range (differential pressure).

The mechanical connection between the pressure sensors and the main channel of the flow controller is made via a capillary tube (blown through during a special cleaning process), which also acts as a low-pass filter for pressure spikes. All parts in contact with the medium (except sealing rings) are made of high-quality stainless steel.

Close cooperation between Keller and Bürkert engineers ensured a result that fully meets the jointly developed specification. In addition, Keller engineers made some additional changes according to the client's wishes. Thus, the variability of designs for flexible printed circuit boards with electrically connectable parts and mechanical integration was agreed upon jointly, as was the output signal at the minimum flow value, which varies significantly among different Keller products at a voltage of 2.5 V.

Integration

Bürkert produces liquid flow controllers on request, as specialized measuring devices for specific applications. Using only three differential pressure transducers with different equipment, final flow values from 0.9 l/h to 36 l/h can be achieved, depending on the magnitude of the reference pressure. Measurement ranges are finely tuned using special holes in the flow channel; the required difference between inlet and outlet pressures is typically about 500 mbar.

As the project progressed, features of digital signal processing, as well as the availability of separate signals for inlet and outlet pressures, led to unexpected advantages. It became possible to use these features to set ranges, detect overloads, or implement other diagnostic tools. In addition, during flow measurement calibration (usually performed using water or another liquid with the same velocity as the working medium), it became possible to override calibration data in the pressure transducer processor, thus performing direct calibration of the flow controller, not just the pressure sensor.

Operating principle of measurement: the measurement is based on the differential pressure principle. A constriction in the main channel causes a pressure drop in the flow, which is detected by differential pressure sensors. The sensor outputs an accurate, temperature-compensated signal with measurement data, allowing subsequent flow calculation.

Summary/Conclusions

Two specialists – one in flow control and one in pressure measurement – worked closely and productively to find a joint solution to a highly specific task set by the client. Signal processing performed by the microprocessor of the Keller differential pressure transducer, which has two sensors, simplified the integration of the device into the flow controller designed for continuous and long-term operation in technological processes; moreover, this allowed the implementation of a whole range of new functions in the controller. To date, the device has been used to solve a large number of diverse tasks and has demonstrated significant advantages over standard differential pressure transducers with only one membrane – especially regarding overload resistance. In particular, for non-standard tasks, digital signal processing offers many advantages, the implementation of which will affect the final cost of the device. 

KELLER dual sensor model, series PD-9 FLX, with electronic components