Differential pressure measurement is widely used as a method for monitoring the level of closed vessels. Oftentimes, operators use individual instruments having an output signal or on-site display. On the other hand, a far more precise and fault-immune solution is the measuring arrangement produced by WIKA using two process transmitters in a primary-secondary relationship.
Each level monitoring application using differential pressure measurement is founded on the following principle: For the level of the contents in the vessel, the differential pressure between your liquid and the gaseous phase is determined. In addition, the hydrostatic pressure, the specific density of the medium and the vessel geometry are included in the calculation. Typical measuring instruments have two adjacent process connections for the pressures P1 and P2. For Trusting , a capillary must bridge the pronounced distance between the two measuring points.
Measurement with two process transmitters as Primary instrument (right) and Secondary instrument. This measuring arrangement for level monitoring can be used flexibly, e.g. for information on the mixing ratio of liquids.
Connection via signal cable
When monitoring the level using electronic differential pressure measurement, however, both measuring points have one process transmitter each. For example, the WIKA models CPT-2x and IPT-2x are ideal for this. Both transmitters are arranged as Primary instrument and Secondary instrument. They’re connected, purely electrically, via a signal cable and so are therefore not susceptible to disturbances. THE PRINCIPAL instrument supplies the Secondary instrument with power. The Secondary instrument is parameterised via a serial interface or through the display on the Primary instrument. Finally, communication occurs via an interior bus.
Level monitoring in a closed tank
The figure on the left shows an average application example for level monitoring with electronic differential pressure measurement on a liquid tank. Okay measures the pressure P1, in this instance the gas pressure, and transmits it to the principal instrument. This detects the pressure P2, here the pressure in the liquid column at the bottom of the tank. The Primary instrument calculates the differential pressure from P1 and P2. Using the measured value and considering the other parameters mentioned, it determines the quantity in the tank as an indication of the level. This value can be output either as an analogue or digital signal to the control room and/or to the display of the principal instrument.
Advantages of the measuring arrangement with two process transmitters
The technique for level monitoring described here has several advantages in comparison with a conventional method, for example, with a differential pressure transmitter:
Higher accuracy and ?turndown?
Process transmitters deliver higher levels of accuracy. In the case of the WIKA models, these are around 0.05 % of the set span. Furthermore, there is the chance of ?turndown?, the individual setting of a particular measuring span (that is also possible retrospectively with instruments which are already installed.)
Minimised temperature effect
Transmission via signal cable with electronic differential pressure measurement minimises the temperature effect. On the other hand, the capillaries used in the traditional measuring method are a lot more susceptible here. This, in turn, can ultimately affect the measuring result.
The electronic measuring arrangement can be commissioned faster. Contrary to a solution with capillaries, it does not require any test run.
Reduced maintenance effort
The maintenance effort is leaner: In case of a fault, only the procedure transmitter concerned has to be replaced. In contrast, with measuring differential pressure with capillaries, the complete measuring arrangement should be replaced.
Further information on the model CPT-2x and IPT-2x process transmitters, which are suitable for electronic differential pressure measurement for level monitoring, is available on the WIKA website.