Differential pressure measurement is widely used as a method 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 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 next principle: For the level of the contents in the vessel, the differential pressure between your liquid and the gaseous phase is determined. 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 level measurement, 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 info on the mixing ratio of liquids.
Connection via signal cable
When monitoring the level using electronic differential pressure measurement, alternatively, both measuring points have one process transmitter each. For example, 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 interior bus.
Level monitoring in a closed tank
Application example
The figure on the left shows an average application example for level monitoring with electronic differential pressure measurement on a liquid tank. The Secondary instrument 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. Cheap from P1 and P2. Utilizing the measured value and taking into account the other parameters mentioned, it determines the volume in the tank as an indication of the particular level. This value could be output either as an analogue or digital signal to the control room and/or to the display of the principal instrument.
Benefits 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. Regarding the WIKA models, they are up to 0.05 % of the set span. In addition, there is the possibility of ?turndown?, the individual setting of a specific measuring span (that 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. In contrast, the capillaries found in the conventional measuring method are a lot 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 does not require any test run.
Reduced maintenance effort
The maintenance effort is lower: In case of a fault, only the process transmitter concerned must be replaced. In contrast, with measuring differential pressure with capillaries, the entire measuring arrangement should be replaced.
Note
More info on the model CPT-2x and IPT-2x process transmitters, which are ideal for electronic differential pressure measurement for level monitoring, can be acquired on the WIKA website.g