On the Level: Selecting the Right Sensor for Level Detection

We’ve probably all experienced having the “pot boil over” or “run dry” at one time or another. The same is frequently true on a much larger scale with many industrial processes. These large events can prove costly whether running dry or overflowing, resulting in lost product, lost production time, damage to the tank, or even operator injury. And then there is the cleanup!

The fact is, many procedures require the operator to monitor the bin or tank level – especially on older equipment. This human factor is prone to fail due to inattention, distractions, and lack of proper training. With today’s employee turnover and the brain drain of retirements, we need to help the operators out.

Multiple solutions exist that can provide operators with sufficient warning of the tank and bin levels being either too low or too high. This article provides a framework and checklist to guide the selection of the best technology for a specific application.

What type of monitoring is necessary?

First, consider whether the application requires or would benefit from continuous monitoring, or is point-level monitoring all that is needed?

    • Point-level monitoring is the simplest. It is essentially sensing whether the product is present at specific detection point(s) in the tank or bin. If the goal is to avoid running dry or overflowing, monitoring the bin or tank point level may be all that is required. Point-level sensors typically are best if the product levels can be detected through the wall or inside the tank or bin itself. A number of sensors can prevent false readings with products that are viscous, leaving residue on the sensor, and even ignore foam.
    • Continuous-level monitoring detects levels along a range – from full to empty. This is required when the exact level of the product must be known, such as for batch mixing.

Checklist for sensor selection

The checklist below can help guide you to what should be the appropriate technologies to consider for your particular application. Frequently, more than one type of technology may work, given the media (or product) you’re detecting, so it may make sense to test more than one.

Checklist for sensor selection

Ultimately, the sensor(s) you select must reliably sense/detect the presence of the subject product (or media). Which solution is least costly is frequently a big consideration, but remember there can be a hefty cost associated with a sensor that gives a false reading to the operator or control system.

Choosing sensors for washdown or clean-in-place environments

For products that will be consumed or entered into the human body, further selection considerations may include sensors that must survive in washdown or clean-in-place environments without contaminating the product.

The encouraging news is that sensors exist for most applications to detect product levels reliably. The finesse is in selecting the best for a given application when multiple technologies can do the job.

Again, there may be some trial and error at play but this checklist should at least narrow the field and pointed you to the better solution/technology.

Requirements for Sanitary Fill Level Sensors

In a previous entry here on the SensorTech blog, we discussed the concept of liquid level sensing, and the difference between discrete liquid level detection and continuous liquid level monitoring.  In this entry, we are going to talk about the requirements for liquid level sensors that are used to measure or monitor liquid products that will ultimately be consumed by humans.

In these applications, it is necessary and critical that sanitary standards be met and maintained.  Sensor designed for sanitary applications are usually designed from the ground up to meet these requirements.

Basically, there are two key criteria that come into play when considering the suitability of a sensor to be used in a sanitary environment:

  • Cleanability – Sanitary filling systems typically need to be regularly cleaned and/or sterilized to prevent the growth of potentially harmful bacteria. It is desirable in most cases that the cleaning/sterilization process be done as quickly and as easily as possible, without having to remove components (including sensors) from the system.  For this reason, many sanitary fill sensors are designed to withstand “cleaning-in-place” (CIP).  Factors such as water-tightness, and ability to withstand elevated cleaning solution temperatures come into play for CIP suitability.
  • Mechanical Sensor Design – Sensors for sanitary fill applications are usually designed such that there are no mechanical features that would allow liquid or debris to collect. Crevices, grooves, seams, etc. can all act as collection points for liquid, and can ultimately lead to contamination.  For this reason, sanitary sensors are designed without such features.  The physical make-up of the sensor surface is also important.  Exterior surfaces need to be very smooth and non-reactive (e.g. high-grade stainless steel).  Such materials also contribute to cleanability.

Consistent standards for sanitary equipment, products, and processes are defined and maintained by 3-A SSI, a not-for-profit entity that provides consistent, controlled, and documented standards and certifications for manufacturers and users of sanitary equipment, particularly in the food, beverage, and pharmaceutical industries.  Equipment that meets these sanitary standards will usually display the 3-A symbol. For more information on this solution visit the Balluff website.