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CONTROL & MEASUREMENT


from a further feature of process transmitters, which can be turned down in order to set the span individually. In the context of liquid level measurements, this means that a tank operator scales the transmitter’s measuring range to the span that is actually relevant for the process. The measurement signal, for instance 4…20 mA, is then utilised optimally within this span.


At the same time, it should be remembered that the measuring accuracy decreases proportionally if the transmitter is turned down past a certain limit. However, provided it does not exceed 5:1, there are no restrictions on the use of the turn-down feature. If the set span covers 200 mbar (= 2 m of water column), therefore, the measuring instrument can be used with 0…1 bar without affecting the accuracy of, say, 0.1%; at 200 mbar, the measurement uncertainty will be no more than 0.2 mbar or 2 mm.


Added versatility is provided by the characteristics of electronic differential pressure measurements. Apart from


actually recording liquid levels, this method is mainly suited for applications with diverse media, for example in the manufacture of fruit juices, where the mixture ratio of concentrate to water can be determined based on the density and hence pressure change. Ships are another good illustration of this, where bilge tanks have to be monitored because seawater is mixed with fluids and lubricants. The differential pressure due to the medium densities which occur in the tank indicates when the water level reaches the defined maximum limit.


Given these measuring characteristics, it seems an obvious idea to also use a master-slave arrangement for flow measurements based on differential pressure, for instance at filters or orifice plates in pipelines. Whereas in tanks static pressure conditions prevail, comparatively high process pressures are the norm in pipes. However, the pressure differences which are decisive for this type of measurement tend to be in the region of a few millibars. To be able to record the static pressure and use it for the calculation, the transmitters would


Master und Slave


Elektronische Differenzdruckmessung mit zwei Prozesstransmittern


therefore have to be designed with far too large a measuring range for the measurements in question. These measurements could then no longer be described as accurate.


Classic differential pressure transmitters are recommended for measuring flow rates for this reason. They determine differential pressure based on the pressure drop in millibars, whereas static pressure can easily be 150 bar or more.


Conclusion: A differential pressure measuring system with two process transmitters in a master-slave relationship expands the options for monitoring liquid levels in closed tanks or in vessels with mixed media with an extremely flexible alternative. It is mainly suited for applications with high accuracy requirements, where temperature effects must be avoided if possible.


andrea.suhrcke@wika.com www.wika.de


Enquiry No. 14


Für die Überwachung von Füllständen hält der Markt eine breite Auswahl an Messmethoden bereit. Bei Applikationen mit geschlossenen Tanks oder vergleichbaren Behältern wird das Niveau des Inhalts vielfach über einen Differenzdrucktransmitter ermittelt. Diese Methode hat sich bewährt. Sie ist häufig einsetzbar, vor allem dort, wo ein Messgerät, zum Beispiel wegen eines Mahlwerks oder eines aggressiven Mediums im Behälterinneren nicht in Frage kommt.


Zur Erfassung des Füllstands wird der Differenzdruck zwischen der flüssigen und der gasförmiger Phase im Behälter ermittelt. Für die genaue Berechnung des Inhaltes müssen neben dem hydrostatischen Druck noch die Tankgeometrie (liegender oder stehender Behälter, verschiedene Deckel und Böden) sowie die spezifische Dichte des Messstoffs in die Kalkulation einbezogen werden.


Bildbeschreibung: ©iStockphoto.com


Die Füllstandskontrolle mittels Differenzdruckmessung hat eine neue Qualität bekommen: ein System mit zwei Prozesstransmittern in einer Master-Slave-Anordnung. Im Vergleich


14


zu einer konventionellen Messlösung wird bei dieser Variante die Installation vereinfacht, der Temperaturfehler minimiert und eine höhere Genauigkeit ermöglicht.


www.reviewonline.uk.com


Differenzdrucktransmitter verfügen über zwei nebeneinander angeordnete 1/4NPT- Prozessanschlüsse gemäß IEC61518. Diese Konstruktion entspricht der genuinen Aufgabe solcher Messgeräte, einen Druckabfall zum Beispiel in Leitungen, an Filtern oder


INDUSTRIAL PROCESS REVIEW


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