speedwave microwave digestion: patented optical pressure control (OPC)

Optical pressure measurement inside the speedwave XPERT

Development of the Berghof microwave pressure sensor system OPC

Berghof Products + Instruments GmbH set new standards, not only with inventing and patenting the contactless in-situ temperature measurement of a sample inside of a pressure/sample vessel, but also at monitoring the internal pressures of vessels of a microwave digestion.

Monitoring the reaction parameters pressure and temperature is crucial to the reproducibility of microwave digestions. Using the contactless in-situ temperature measurement of a sample inside of a pressure/sample vessel, which was patented under the number DE 4412887C2 in 1998 and is still in use today under the name speedwave® DIRC inside of the speedwave XPERT and ENTRY, Berghof ensured the monitoring of the reaction parameter temperature. Now, the challenge lied in integrating a technology for monitoring pressure inside of the existing concept, while ensuring the replaceability of individual components. Since microwave digestion is connected with high chemical and mechanical stress, the user has to be able to replace wear parts of the pressure monitoring system without any significant physical or financial expenses.

Most common sensors were ruled out because they didn’t feature any acid resistance. Furthermore, if force sensors were used, the user would have to additionally wire the sensors before each digestion, which would massively impair handling. For this reason, Berghof used an optical effect, which originated in the investigation of mechanical stresses. Material structures deform under pressure, therefore, increasing pressure in laboratory glass changes its polarization plane of light. If the change in the plane of polarization is monitored, the ambient pressure of the laboratory glass is monitored at the same time.

Using this optical effect to monitor pressure in the digestion vessels proved to be considerably more difficult than implementing the contactless in-situ temperature measurement. The optical precision required for this pressure measurement method was successfully implemented after a series of tests. For this purpose, the vessel cap is fitted with a prism to which a polarization foil and a ring of laboratory glass is attached. The prism transmits a light signal through the polarization foil and the laboratory glass ring towards two light conductors, which are mounted inside the microwave. The pressure will be determined on the basis of the electrical conversion of the signal. If the pressure in the digestion vessel increases, the material structure of the laboratory glass ring changes. This causes the light to be refracted differently, which will be detected by the light conductor system. By comparing the change in light refraction with a reference value, the internal vessel pressure can be precisely determined. No reference vessel is required for the use of this optical pressure monitoring system. In addition, this technology can be upgraded and extended to any desired number of vessels at any time.

The patent DE 19710499A1 for optical pressure monitoring was granted on 21.02.2008. Under the name speedwave OPC (Optical Pressure Control), pressure monitoring is implemented in the speedwave XPERT.