In the pharmaceutical industry, it's not uncommon for raw materials to have volatile properties not easily managed by standard manufacturing components. Case in point: a glass cylinder containing a liquid pharmaceutical that can be activated by a light source. The liquid must be protected in some way to prevent the potential problems that activation may cause. It is also important to monitor the status of the substance, as product run-dry can lead to equipment damage or failure. The amount of material in the container must therefore be accurately gauged, but the mode of measurement must not disturb the contents.
Of the sensing methods used to detect level in standard applications, the most common are probes, linear displacement transducers (LDTs), and photoelectric sensors. However, they are not acceptable for pharmaceuticals because probes are typically inserted directly into the substance being sensed and photoelectric sensors require a light source to do their work.
One pharmaceutical manufacturer decided to try capacitive level sensors. These devices sense through materials with lower dielectric properties to detect those with higher dielectric properties. They are often used for liquid level detection in manufacturing applications. The only problem is that if the higher dielectric material sticks to the inside of a container, the capacitive sensor still senses the material as present and produces a false readout. And that was precisely what happened.
After the pharmaceutical company ruled out capacitive sensors, it turned to a unique ultrasonic sensing method that mounts to the outside of the container, rather than above it as with conventional ultrasonic sensors. Moreover, this sensor can detect level through the container wall. It is nonintrusive and not misled by residue on the inside of the container wall.
Principle of Operation
The sensor works by generating a high-frequency ultrasonic pulse that is transferred into the container wall and influenced by the container's contents. The information is then analyzed and compared to the sensor's preset conditions.
Turck ultrasonic see-through sensors operate in two modes: reverb and echo.
Reverb Mode. This mode evaluates the ultrasonic pulse as it reverberates within the container wall. The pulse travels through the container wall until it reaches the inner wall. The reverb mode is best suited for liquids with low viscosity, and when stirring devices are used inside the container.
Echo Mode. In this mode, the ultrasonic pulse is evaluated as it travels through the liquid and echoes off the opposite wall of the container. The echo mode is best suited for liquids with high viscosity, as it can see through media that may have coated the container's inner wall.
The sensor provides point level detection, meaning that it detects a single point relative to its placement. Single-point detection does not provide a continuous reading or measurement, but rather indicates whether contents are present in a container at some particular point.
For the pharmaceutical application, it was necessary to determine level at three points within the glass container to distinguish between full and empty conditions. The sensors were mounted near the top, near the bottom, and in between those two positions (Figure 1). Their data prompted the controller to add or stop adding the liquid.
Figure 1. For point level sensing, ultrasonic sensors were mounted in three positions on the outside wall of the pharmaceutical container
For correct operation, the sensor must make solid contact with the container wall. The sensors are spring loaded, and coupling gel is applied to the transducer faces before flush mounting to the container wall. They are held in place by mounting brackets, which in turn are secured by stainless steel straps.
By investigating level sensing options, the pharmaceutical company was able to find the sensor best suited for its needs—one that didn't compromise the application or the product.