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What Measuring Methods Come into Consideration?

In single-use applications, it is still common practice to determine the level by weighing. However, the big disadvantage of load cells is that their measuring results are easily corrupted. This can happen, for example, if, due to irregular expansion of the flexible bioprocess bag, the parts of the bag touching the measuring system shift. Load cells used for containers that are constantly in motion also require relatively frequent recalibration.

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The alternative to weighing, capacitive measurement, also has disadvantages. Capacitive sensors are often not sufficiently accurate or responsive for the small production quantities of substances that are increasingly considered “highly active”. They can even damage or destroy the single-use bag when they are removed. And just like with load cells, the typical irregular filling and folding of single-use bags has a negative effect on accuracy. In capacitive contact measurement, the electrode in the sensor tip, together with the GND electrode, functions as a capacitor plate; for this reason it works like an open capacitor. As a probe, it has to be inserted into the measured medium from the outside of the bag.

Special single-use sensor systems are another variant that, although convincing in terms of price, lags behind other methods in terms of reliability and accuracy. Insufficient ruggedness and durability are among the disadvantages of the available models that can adversely affect the ongoing process.

The Advantages of 80-GHz Radar

Continuous level measurement with 80-GHz radar technology, in contrast, reliably helps to achieve the required high process efficiency. Vegapuls 64 radar sensors measure contactlessly from outside the process, right through the plastic container. This works even if the bag surface has wrinkles. By avoiding all contact with the medium, the danger of cross-contamination can be effectively minimized. The work involved in installation and setup is also minimal: with little or no adjustment, the high-frequency radar sensors start measuring immediately and react flexibly to process changes.

Small Containers, High Accuracy

Stirring and mixing in small containers with heights of less than 2 m are typical processes in single-use applications. With an antenna size of DN50, Vegapuls 64 has a beam angle of only three degrees. This creates the strong focusing that allows the sensors to measure right past internals and deliver reliable results in such applications. What is more, Vegapuls 64 can measure the level very close to the container bottom with an accuracy of +/-2 mm. This is particularly advantageous for the sensor in compact applications like the standard culture bags.

The dynamic range of a radar sensor, i.e. the difference between the largest and smallest signal, is a good indication of which applications the sensor can be used in. The greater the dynamics, the wider the application range of the sensor and the higher the measurement certainty. With a dynamic range of 120 dB, the best on the market, Vegapuls 64 can even cope with the condensate that typically forms on the inside of container bags.

What Are the Advantages of Using Multiple Sensors?

Reusable sensors are the higher quality alternative for single-use applications — provided they are as easy to use as their disposable competitors. 80-GHz radar sensors can be placed outside single-use bags, while the bags themselves are coupled via special plug-in connectors. This guarantees the necessary sterile barrier: the bags, however, remain effectively transparent to the strongly focused and highly dynamic sensor signals. The sensor is thus able to query the contents of the bioreactor with high precision — always with the advantage of being reusable and not in contact with the medium.

Useful accessory: External Display and Adjustment Unit

If a measuring point is located at a poorly accessible location, the external, digital indicating and adjustment unit Vegadis 81 can be connected to the electronics of the radar sensor. The instrument is mounted in a distance of up to 50 m away from the sensor in an easily accessible position. Due to the direct connection to the electronics, the measuring cable remains uninterrupted. Optionally there is the possibility of wireless operation via Bluetooth with Smartphone/Tablet/PC as well as magnetic pen operation in case of housing lid with inspection window. Typical applications of the Vegadis 81 are:

• visual inspection of level and pressure measured variables

• display device for diagnostic information and sensor parameters

• adjustment unit for the connected radar sensor

Last not least radar sensors can also be optimally integrated into many other sterile processes besides single-use. Their hygienically flawless antenna connections allow them to be mounted front-flush. Process fittings that use only PTFE as the media-contacting material are also available for use in hygienic applications. The materials thus meet the requirements of both FDA and EC 1935/2004. The design of the sensors complies with the specifications of the 3-A standards.

Milestones in Chemical, Pharma and Process Engineering: Vega has been the number 1 in radar level measurement for years and was awarded the milestone "Instrumentation" by PROCESS.

Milestone Instrumentation

Successfully Focusing on the Core Business

* The author works in the marketing department at Vega Grieshaber, Schiltach/Germany.

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