Accurate and reliable liquid-level measurement is often critical for life sciences applications such as the manufacturing of pharmaceuticals, vaccines, and other biotherapies. In fact, keeping the liquid level in a container within acceptable parameters is often essential for maintaining the safety and repeatability of a process. However, measuring liquid levels in mobile tanks and single-use vessels can present a unique set of challenges and requirements that could make or break your process or cost you valuable time and money to engineer a solution. In this article, we compare several technologies currently used for measuring liquid levels in mobile tanks and single-use vessels and make a case for why digital-capacitive load cells are often the superior solution.
Unique Challenges for Measuring Liquid Level in Mobile Tanks and Single-Use Vessels
Measuring liquid levels in mobile tanks and single-use vessels presents a unique set of challenges. First, mobility introduces dynamic conditions such as movement and vibration that can interfere with the accuracy of measurements. The constant shifting of liquids due to transportation or handling can create waves, causing fluctuating readings and making it difficult to obtain a stable measurement. Furthermore, an impulse force from rolling over a threshold or bumping into a wall (or other obstruction) for example can lead to inconsistent results, the need to recalibrate, and even damage to equipment or walls.
Additionally, bioprocess bags pose their own hurdles. These vessels are typically made from flexible films which may not offer a rigid and consistent surface for mounting measurement devices. Also, since these vessels are disposed of after one use, the level measurement solution needs to be cost-effective and easy to install without compromising on accuracy and reliability. As such, sensors must be employed in a way they can be re-used which can tend to disqualify some technologies such as invasive sensors, for example, due to burdensome sterilization requirements.
Moreover, both mobile tanks and single-use vessels often contain different types of liquids, varying in properties like viscosity, temperature, and dielectric constant. These variations can necessitate a versatile level measurement solution capable of handling diverse liquid properties under changing conditions. In the next section, we will delve into various technologies available for measuring liquid levels in mobile tanks and single-use vessels and explain why Acuity typically recommends digital-capacitive load-cell systems for these applications.
A Summary of Existing Level-Measurement Technologies Typically Used in Life Sciences
Sight gauges are widely utilized in the life sciences industry due to their straightforward design and reliable performance. They consist of a transparent tube or window that allows direct visual observation of the liquid level. Sight gauges typically require a clear line of sight for accurate readings and may not be appropriate for hazardous environments. They also offer only limited precision and generally require manual readings which can introduce possible errors and limit automation capabilities.
Float gauges are commonly employed in the life sciences and utilize a buoyant float that moves up and down with the liquid level, providing a visual indication or signal to an external controller for calculating the liquid level. Float gauges are known for simplicity and reliability, although they may not be suitable for measuring highly viscous or corrosive liquids and can be impractical for use in single-use vessels due to direct contact with the fluid and a lack of precision.
Like float gauges, displacers also rely on the principle of buoyancy. A typical displacer apparatus might include a tube that is situated inside of a vessel with a spring mechanism for measuring bouyancy forces. As the liquid level rises and falls, the buoyancy force acts on the spring system to enable continuous measurement of the liquid level. Displacer technology is favored for its accuracy and versatility in handling a wide range of liquids including those with varying viscosities and densities. However, their drawbacks can include a limited suitability for smaller tanks, high-maintenance requirements, potential for mechanical wear, installation and set up complexity, high cost and contamination concerns.
Radar level measurement is a non-contact method that utilizes electromagnetic waves to determine the liquid level. This technology is widely used in the life sciences industry due to its high accuracy and suitability for both mobile tanks and single-use vessels. However, it is important to note that radar may have limitations when it comes to measuring liquids with high levels of foam or where obstructions in the vessel are present. It may also have limited success in agitated environments, have a limited measurement range, and can be prone to electrical interference.
Pressure sensors rely on a pressure reading, typically at the bottom of the vessel, to measure the liquid level. Benefits include continuous measurement; the ability to measure level and pressure; and the ability to be used with various tank shapes. Although pressure sensors are invasive, contamination can be minimized by installing a “single-use” pressure sensor on to the bag assembly and then sterilizing the assembly as a complete unit. Some drawbacks include the invasiveness of the sensor; susceptibility to issues such as clogging or leaking; sensitivity to pressure fluctuations; and the need to model the system’s internal geometry which can sometimes be complex.
Capacitance transmitters rely on the principle of capacitance to measure the liquid level. A probe inserted into the tank or vessel measures the changes in capacitance caused by the liquid level fluctuation. This technology offers high accuracy, can handle many corrosive or viscous liquids, and is suitable for both mobile tanks and single-use vessels. Some drawbacks include sensitivity to changes in temperature, pressure, and dielectric constant of the medium, costly calibration requirements, invasive probes, and limited range.
Magnetostrictive Level Transmitter
Magnetostrictive level transmitters utilize the magnetostrictive effect to determine the liquid level. A float with a magnetostrictive wire is immersed in the liquid, generating a pulse that can be detected and measured as the float moves. This technology provides precise and reliable level measurement, making it suitable for various life sciences applications. Some benefits include the ability to provide high-accuracy and reliable measurements, long lifespan, and the ability to handle a wide range of media such as corrosive liquids. Some drawbacks include a relatively higher cost than other technologies, high complexity, sensitivity to magnetic interference, and limited suitability for some types of liquids including highly conductive, high viscosity, and high temperature and pressure liquids. Its invasive nature may also not be ideal in many bioprocess applications.
Ultrasonic Level Transmitters
Ultrasonic level transmitters use ultrasonic waves to measure the liquid level. By emitting waves that bounce off the liquid surface and measuring the time it takes for the waves to return, the liquid level can be determined. This non-contact technology offers high accuracy and can be employed in various tank configurations. Some benefits of ultrasonic level measurement include non-contact measurement, continuous measurement, and relatively easy installation in large and small tanks, while some drawbacks include sensitivity to interference from obstructions, foam, surface turbulence, or changes in atmospheric conditions such as humidity and temperature of the air between the media and the sensor.
Laser-level transmitters employ laser beams to measure the liquid level. The transmitter emits a laser beam that reflects off the liquid surface and is detected by a receiver. By calculating the time it takes for the laser light to travel, the liquid level can be accurately determined. Laser technology benefits include high accuracy, non-contact measurement and the ability to measure levels over long distances, while some drawbacks include a relatively higher cost than most solutions, sensitivity to inference from elements such as dust, fog, or other particles in the air, high-calibration requirements, and relatively high-power consumption.
Digital-Capacitive Load Cells – A Superior Solution
Digital-capacitive load cells have become a preferred choice for measuring liquid levels in mobile tanks and single-use vessels. In the past, less capable strain-gauge load cells have been employed with varying levels of success due to their tendency to be affected by environmental factors. However, digital-capacitive load cells have redefined what is possible and are not subject to the same drawbacks as strain-gauge load cells.
One of the main advantages of digital-capacitive load cells over other technologies like laser, ultrasonic, or radar is their direct measurement of weight. They measure force directly, translating it into a weight reading without relying on quantities like distance, capacitance, or buoyancy forces for example. Weighing by difference, as is done with digital-capacitive load cells, is a highly reliable method because it directly measures the change in mass of a system. This eliminates many of the variables that can affect other measurement methods. For example, high viscosity of the fluid, foam, or internal tank dimensions do not affect the results when using load cells.
Furthermore, digital-capacitive load cells are robust, durable, and resistant to factors such as shock and overload. They also have excellent long-term stability, maintaining their accuracy over time without the need for frequent recalibration. This makes them a cost-effective choice for many applications, as they require less maintenance and replacement than other types of sensors. Acuity’s AccuCart weighing carts are just one example of how Acuity utilizes digital-capacitive load cells for mobile weighing. Contact us today to learn how we can help to simplify your process.
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