C7.2 - Novel Multi Parameter Sensor Approach for Interface Measurement

SENSOR+TEST Conferences 2011
2011-06-07 - 2011-06-09
Proceedings SENSOR 2011
C7 - Sensor Electronic II
H. Schroth, S. Gorenflo - Endress+Hauser GmbH+Co. KG, Maulburg (Germany)
484 - 489


The position of liquid media interfaces is an important process control parameter in chemical and petrochemical industry, e.g. in oil/water separators. Traditionally, only one physical value is measured with a single sensor as mean value along the height of the vessel, typically capacitance (measured with an isolated probe) or density (measured with a float, differential pressure, or gamma ray absorption). However, since up to four parameters may vary in practice (for each of the two liquids the measured material property and the quantity), assumptions have to be made for three of these parameters o determine the fourth, the interface position. Time domain reflectometry (TDR), which measures the time of travel of high frequency pulses guided along a probe is an alternative method, with which two values can be measured at once, namely the position of the reflection at the upper liquid surface as well as at the interface between both liquids. But also there, the dielectric constant of the upper liquid must be known to correct the speed of light, and in the case of heavy emulsion, the interface signal is lost and only one measurement value remains. A third known method to overcome such problems is to use a large number of similar sensors at different heights in the vessel. However, since this is extremely expensive usability is limited.

We solve the problem described above by using a novel multi parameter sensor approach, the combination of TDR and capacitance, to measure in a comparatively easy way with a single probe up to three physical values, so that only one constraint must be known for the system. Since both physical principles are of electrical nature, but react in different ways on changes of one of the four parameters they ideally complement each other. The multi parameter sensor is based on a standard TDR and capacitance measurement circuitry. The signals are coupled via high and low pass filters, respectively, on a single coaxial probe with isolated inner conductor. Evaluation is done with microcontrollers and different evaluation modes can be chosen depending on the application. For example, for a conductive lower medium and a nonconductive upper medium even in case of emulsion,where TDR does not deliver sufficient information alone, the overall level can be measured with TDR, and the interface with capacitance. In case of clearely separated media, the information can be used for self calibration, plausibility checks, and self diagnostics.

Numerous tests were done with different media in the laboratory as well as in real industrial environments. Especially with emulsion layers and with media with varying dielectric constants the system proved to be clearly advantageous as regards reliability and accuracy.