Instrumentation
Customised differential pressure module
Bernhard Vetterli reports on how a differential pressure measuring module was developed in response to a customer’s request and which has now been integrated into a production flow controller.
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ignificantly optimised solutions are obtained if close co-ordination between a component producer and a device manufacturer (to cite one such example) is initiated in a product’s development
phase. Flexibility on both sides and – most important of all – digital signal processing yield benefits. A spraying system that allows precise metering of
lubricants was required for sheet metal working. This application was the starting-point for Bürkert to develop a series of Liquid Flow Controllers, and it served as the pilot project. The sensor and control technology, the actuator and the customary electrical process interfaces had to be accommodated in one compact device designed for trouble-free continuous operation, as is usually the case in process measurement technology.
Flow rate measurement Because 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 passes through a metering orifice with a defined diameter. Moreover, two individual pressure sensors were to be deployed for this purpose. A good relationship with the German branch of
Keller AG für Druckmesstechnik of Switzerland was already in place, and this factor now came fully into play.
In conventional differential pressure transmitters,
both sides of a measuring diaphragm are exposed to the measuring medium; ie, one ‘high’ and the other ‘low’ pressure input. With typical differential measuring ranges of 500mbar and common mode pressures of up to 10bar, a momentary interruption of either the high or low pressure could result in a 20-fold overload 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 the differential pressure measurement module. Differential pressure transmitters operate with two selected, encapsulated silicon pressure sensors, installed
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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 per cent FS across broad temperature ranges. The analogue output signal from the module is updated as many as 200 times per second, and a good dynamic reserve is available for subsequent processing. As a rule of thumb, it can be said that the measuring range for differential pressure measurements of this sort should be abut 20 per cent of the common-mode pressure. In addition to standard analogue signals of
Fig. 1. Bürkert’s Liquid Flow Controllers in operation.
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