Differential pressure measurement is trusted as a way for monitoring the level of closed vessels. Oftentimes, operators use individual instruments with an output signal or on-site display. In contrast, a more precise and fault-immune solution may be 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 next principle: For the amount of the contents in the vessel, the differential pressure between the liquid and the gaseous phase is set. Furthermore, the hydrostatic pressure, the precise 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 Courageous , 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 utilized flexibly, e.g. for info on the mixing ratio of liquids.
Connection via signal cable
When monitoring the particular 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. Excruciating are arranged as Primary instrument and Secondary instrument. They’re connected, purely electrically, with a signal cable and are therefore not susceptible to disturbances. THE PRINCIPAL instrument supplies the Secondary instrument with power. The Secondary instrument is parameterised with a serial interface or through the display on the principal instrument. Finally, communication takes place via an interior bus.
Level monitoring in a closed tank
Application example
The figure on the left shows a typical application example for level monitoring with electronic differential pressure measurement on a liquid tank. Atrocious measures the pressure P1, in cases like this the gas pressure, and transmits it to the Primary instrument. This detects the pressure P2, here the pressure in the liquid column in the bottom of the tank. THE PRINCIPAL 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 being an indication of the particular level. This value could be output either being an analogue or digital signal to the control room and/or to the display of the Primary instrument.
Advantages of the measuring arrangement with two process transmitters
The technique for level monitoring described here has several advantages when compared to a conventional method, for example, with a differential pressure transmitter:
Higher accuracy and ?turndown?
Process transmitters deliver higher degrees of accuracy. In the case of the WIKA models, these are around 0.05 % of the set span. In addition, there is the chance of ?turndown?, the average person setting of a specific 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 much more susceptible here. This, in turn, can ultimately affect the measuring result.
Fast commissioning
The electronic measuring arrangement could be commissioned faster. Contrary to a solution with capillaries, it generally 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 must be replaced.
Note
More info on the model CPT-2x and IPT-2x process transmitters, which are suitable for electronic differential pressure measurement for level monitoring, can be acquired on the WIKA website.