Investigation of New Level Technologies in Single Use, Disposable Systems
Different technologies have been utilized to measure level, volume, and/or mass in single use, disposable bags in the biotech industries. The most common include:
1. Load Cells (weight)
2. Floor Scales (weight)
3. Pressure Transmitters (liquid head or weight)
4. Graduated Marks (manual measurement of level)
5. Guided Wave Radar (level)
Each of the above technologies has its benefits as well as its limitations. The purpose of this article is to only discuss experience with guided wave radar transmitters – not to discuss the first four listed technologies.
There are currently several hundred guided wave radar level installations on single use, disposable bags in cGMP facilities.
Guided Wave Radar (GWR) is a well known technology that has been available for process measurement for more than 20 years. Basically, a microwave pulse is sent down a wave guide (also referred to as an antenna or probe). Energy is then reflected back from a surface where there is a change in dielectric. The amount of energy reflected is proportional to the difference in dielectric between air and the process medium. Typically, the wave travels first through air (dielectric of 1) and then is reflected off of a surface of a liquid (WFI has a dielectric of approximately 12).
In the case of biotech-type liquids, which are typically high dielectric, almost all of the energy on the wave guide probe is reflected back up the probe, resulting in a very strong signal and maximum accuracy of measurement.
By knowing the time of flight required for the speed-of-light microwave pulse to travel down the wave guide, reflect off of the surface of the liquid and then travel back, one can determine the distance to the surface of the liquid. Level in the vessel (or bag in this case) can then be mathematically calculated.
Simplified diagrams of guided wave technology are shown in the Figures below.
Potential Single Use Bag GWR Difficulties
In single use, disposable bag level technology, the GWR probe is not in contact with the liquid – but rather, is in direct contact with the outside of the bag. This non-contact application can result in difficulties that are not normally experienced with typical GWR probes that are in direct contact with the process:
1. The microwave pulse is concentrated around the outside of the probe. Since the probe is separated from the process liquid by the thickness of the bag, only the portion of the reflected microwave pulse is actually available for processing as the level signal. This reflected signal strength is typically less than 50% of the amplitude that would be processed if the wave guide was in direct contact with the liquid.
2. The bag may not lay flat against a probe when filling or emptying – causing a deformity in the bag wall. This deformation of the bag can be caused by obstructions within the tote or bin, the GWR probe itself, the shape of the bag as related to the tote/bin shape, and even the seam of the bag. As a result, the signal propagation (speed) of the microwave pulse will vary as it traveling through both the air due to bag deformity and through the plastic of the bag itself. Signal propagation will vary as the inverse of the square of the material dielectric constant. Since the speed of the microwave pulse will be reduced when in contact with the bag as compared to when it is in contact with air, the time that it takes the microwave pulse to travel down the probe, reflect off of the surface of the liquid in the bag, and travel back up the probe will be variable depending on the position of the bag with reference to the probe. The results will not always be the same, system to system or bag to bag.
3. A disposable single use bag tends to “curve away” from the probe near the top of the fill level. Then, as the bag fills or empties, less of the bag sidewall containing liquid becomes in direct contact with the probe. This causes two issues. The first issue is that level is not proportional to volume. The second issue is that since the actual liquid level is not in direct contact with the side wall of the bag and probe, the GWR transmitter measures a different level due to the change in speed of the microwave pulse. This difficulty is avoided if the bag is preformed to the exact shape of the bin/tote/container and the bag is pre-inflated.
4. Measuring low liquid level is also challenging for the GWR since the GWR needs to fit the bottom shape of the tank. The transition between the sidewall and the bottom results in a near 90 degree bend of the GWR antennae. Tighter bends seem to reduce the GWR signal resulting in a poor level signal. A five inch or greater radius for ¼” cable seems to deliver best performance.
5. Bag seams also create a challenge especially at the bottom to sidewall transition. The bag seam is generally more rigid and does not lay completely flat to the GWR cable, which in turn leaves an air gap. This air gap generally occurs in the middle of the GWR bend radius and reduces signal.
To review the rest of the findings click here.
Reference: Ladoski, D., & Klees, D. (2014, September 1). Investigation of New Level Technologies in Single Use, Disposable Systems. Pharmaceutical Engineering Magazine, 28-37.