Differential pressure measurement is widely used as a way for monitoring the amount of closed vessels. In many cases, operators use individual instruments with an output signal or on-site display. On the other hand, a far 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 based on the following principle: For the amount of the contents in the vessel, the differential pressure between the liquid and the gaseous phase is set. In addition, the hydrostatic pressure, the specific density of the medium and the vessel geometry are contained in the calculation. Typical measuring instruments have two adjacent process connections for the pressures P1 and P2. For level measurement, a capillary has to 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 home elevators the mixing ratio of liquids.
Connection via signal cable
When monitoring the level using electronic differential pressure measurement, on the other hand, both measuring points have one process transmitter each. For instance, the WIKA models CPT-2x and IPT-2x are suitable for this. The two transmitters are arranged as Primary instrument and Secondary instrument. They’re connected, purely electrically, with 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 with a serial interface or through the display on the principal instrument. Finally, communication takes place via an internal 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. The Secondary instrument measures the pressure P1, in cases like this the gas pressure, and transmits it to the principal 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 Intense measured value and taking into account the other parameters mentioned, it determines the volume in the tank being an indication of the level. This value can be output either being 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 method for level monitoring described here has several advantages when compared to a conventional method, for instance, with a differential pressure transmitter:
Higher accuracy and ?turndown?
Process transmitters deliver higher degrees of accuracy. Regarding the WIKA models, they are around 0.05 % of the set span. In addition, there is the chance of ?turndown?, the individual setting of a particular measuring span (this is also possible retrospectively with instruments that 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 found in the conventional measuring method are much more susceptible here. This, in turn, can ultimately affect the measuring result.
Fast commissioning
The electronic measuring arrangement can be commissioned faster. Unlike a solution with capillaries, it generally does not require any test run.
Reduced maintenance effort
The maintenance effort is lower: In the event of a fault, only the procedure transmitter concerned must be replaced. In contrast, with measuring differential pressure with capillaries, the complete measuring arrangement should be replaced.
Note
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.