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.

Liquid Level Sensing: Detect or Monitor?

Pages upon pages of information could be devoted to exploring the various products and technologies used for liquid level sensing and monitoring.  But we’re not going to do that in this article.  Instead, as a starting point, we’re going to provide a brief overview of the concepts of discrete (or point) level detection and continuous position sensing.

 Discrete (or Point) Level Detection

Example of discrete sensors used to detect tank level
Example of discrete sensors used to detect tank level

In many applications, the level in a tank or vessel doesn’t need to be absolutely known.  Instead, we just need to be able to determine if the level inside the tank is here or there.  Is it nearly full, or is it nearly empty?  When it’s nearly full, STOP the pump that pumps more liquid into the tank.  When it’s nearly empty, START the pump that pumps liquid into the tank.

This is discrete, or point, level detection.  Products and technologies used for point level detection are varied and diverse, but typical technologies include, capacitive, optical, and magnetic sensors.  These sensors could live inside the tank outside the tank.  Each of these technologies has its own strengths and weaknesses, depending on the specific application requirements.  Again, that’s a topic for another day.

In practice, there may be more than just two (empty and full) detection points.  Additional point detection sensors could be used, for example, to detect ¼ full, ½ full, ¾ full, etc.  But at some point, adding more detection points stops making sense.  This is where continuous level sensing comes into play.

Continuous Level Sensing

Example of in-tank continuous level sensor
Example of in-tank continuous level sensor

If more precise information about level in the tank is needed, sensors that provide precise, continuous feedback – from empty to full, and everywhere in between – can be used.  This is continuous level sensing.

In some cases, not only does the level need to be known continuously, but it needs to be known with extremely high precision, as is the case with many dispensing applications.  In these applications, the changing level in the tank corresponds to the amount of liquid pumped out of the tank, which needs to be precisely measured.

Again, various technologies and form factors are employed for continuous level sensing applications.  Commonly-used continuous position sensing technologies include ultrasonic, sonic, and magnetostrictive.  The correct technology is the one that satisfies the application requirements, including form factor, whether it can be inside the tank, and what level of precision is needed.

At the end of the day, every application is different, but there is most likely a sensor that’s up for the task.

Level Detection with Ultrasonic Sensors

Liquid measurement can be a demanding application therefore selecting the most effective sensor technology is imperative. Ultrasonic sensors are the ideal solution for distance measurement or position detection of granules, fluids and powders. They measure fill levels, heights and sag without making contact as well as count and monitor the presence of objects. They are extremely versatile, operate independently of color and surface finish, and are not affected by transparent objects that generate strong reflections. And because they are not affected by dust, dirt and steam, they are the ideal choice for critical applications.

Ultrasonic Sensors operate with propagation of sound waves providing a reliable detection source for level detection applications where liquid measurement monitoring is necessary. They will enhance the flexibility of the application with additional advantages of being a non-contact sensor so there are no mechanical floats or arms that can retain residual liquid build up. This residual material can cause machine application downtime as interruption of production is needed to maintain and clean the mechanical monitoring system. Because the distance to the object is determined via a sound transit time, ultrasonic sensors have excellent background suppression. With their transit time measurement, ultrasonic sensors can record the measured value with highly precise resolution (some sensors to even 0.025 mm). Ultrasonic Sensors can monitor
and detect nearly any liquid.

Level Detection with Ultrasonic Sensors2Level Detection with Ultrasonic SensorsAn ultrasonic sensor is not affected by the color, transparency or glossiness of a surface. They can reliably detect bulk materials, liquids and material surfaces. Good reflective materials include:

  • „„ Water
  • „„ Paint/varnish
  • Wood
  • Metal
  • Plastic
  • Stone/concrete
  • Glass
  • Hard foam rubber

Additional advantages ultrasonic sensors provide when monitoring liquid is their ability to have two independent outputs programmed into one sensor. This is a great way to monitor upper and lower liquid levels with one sensor. This sensor feature allows one independent output setting to indicate a minimum level and the second output to indicate a maximum level concurrently providing constant feedback along with a voltage or current output.