continuous level sensing Archives

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.

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.

Reviewing options for optimized level detection in the food & beverage industry

Level detection plays an important role in the food and beverage industry, both in production and filling. Depending on the application, there are completely different requirements for level detection and, therefore, different requirements for the technologies and sensors to solve each task.

In general, we can differentiate between two requirements — Do I want to continuously monitor my filling level so that I can make a statement about the current level at any time? Or do I want to know if my filling level has reached the minimum or maximum?

Let’s look at both requirements and the appropriate level sensors and technologies in detail.

Precisely detecting point levels

For point level detection we have three different options.

A through-beam fork sensor on the outside of the tank is well suited for transparent container walls and very special requirements. Very accurate and easy to install, it is a good choice for critical filling processes while also being suitable for foaming materials.

One point level detection through transparent container walls

For standard applications and non-metallic tank walls, capacitive sensors, which can be mounted outside the tank, are often the best choice. These sensors work by detecting the change of the relative electric permittivity. The measurement does not take place in direct contact with the medium.

Minimum/maximum level detection with capacitive sensors

For applications with metal tanks, there are capacitive sensors, which can be mounted inside the tank. Sensors, which meet the special requirements for cleanability (EHEDG, IP69K) and food contact material (FCM) required in the food industry, are mounted via a thread and a sealing element inside the tank. For conductive media such as ketchup, specially developed level sensors can be used which ignore the adhesion to the active sensor surface.

Capacitive sensors mounted inside the tank

Continuous level sensing

Multiple technologies can be used for continuous level sensing as well. Choosing the best one depends on the application and the task.

Continuous level detection can also be solved with the capacitive principle. With the aid of a capacitive adhesive sensor, the level can be measured from the outside of the tank without any contact with the sensor. The sensor can be easily attached to the tank without the use of additional accessories. This works best for tanks up to 850 mm.

Continuous level detection with a capacitive sensor head

If you have fast and precise filling processes, the magnetostrictive sensing principle is the right choice. It offers very high measuring rate and accuracy. It can be used for tank heights from 200 mm up to several meters. Made especially for the food and beverage industry, the sensor has the Ecolab, 3A and FDA certifications. Thanks to corrosion-free stainless steel, the sensor is safe for sterilization (SIP) and cleaning (CIP) in place.

Level detection via magnetostrictive sensing principle

If the level must be continuously monitored from outside the tank, hydrostatic pressure sensors are suitable. Available with a triclamp flange for hygienic demands, the sensor is mounted at the bottom of the tank and the level is indirectly measured through the pressure of the liquid column above the sensor.

Level detection via hydrostatic pressure sensor

Level detection through ultrasonic sensors is also perfect for the hygienic demands in the food industry. Ultrasonic sensors do not need a float, are non-contact and wear-free, and installation at the top of the tank is easy. Additionally, they are insensitive to dust and chemicals. There are even sensors available which can be used in pressurized tanks up to 6 bar.

Product bundle for level monitoring in storage tanks

On occasion, both types of level monitoring are required. Take this example.

The tanks in which a liquid is stored at a food manufacturer are made of stainless steel. This means the workers are not able to recognize whether the tanks are full or empty, meaning they can’t tell when the tanks need to be refilled to avoid production downtime.

The solution is an IO-Link system which consists of different filling sensors and a light to visualize the filling level. With the help of a pressure sensor attached to the bottom of the tanks, the level is continuously monitored. This is visualized by a machine light so that the employee can see how full the tank is when passing by. The lights indicate when the tank needs to be refilled, while a capacitive sensor indicates when the tank is full eliminating overfilling and material waste.

To learn more about solutions for level detection visit balluff.com

Level Sensing in Machine Tools

Certainly the main focus in machine tools is on metal cutting or metal forming processes.

To achieve optimum results in cutting processes coolants and lubricants are applied. In both metal cutting and metal forming processes hydraulic equipment is used (as hydraulics create high forces in compact designs). For coolant, lubricant and hydraulic tanks the usage of level sensors to monitor the tank level of these liquids is required.

Point Level Sensing

For point level sensing (switching output) in many cases capacitive sensors are used. These sensors detect the change of the relative electric permittivity (typically a change of factor 10 from gas to liquid). The capacitive sensors may be mounted at the outside of the tank wall if the tank material is non metallic like e.g. plastic or glass. The installation may even be in retrofit applications yet limited to non metallic tanks up to a certain wall thickness.

When using metal tanks the capacitive sensors enter the inner area of the tank via a thread and a sealing component. Common thread sizes are: M12x1, M18x1, M30x1,5, G 1/4″, NPT 1/4″ etc. For conductive liquids specially designed capacitive level sensors may be used which ignore build up at the sensing surface.

Continuous Level Sensing

Advanced process control uses continuous level sensing principles. The continuous sensor signals e.g. 0..10V, 4…20mA or increasingly IO-Link deliver more information to better control the liquid level, especially relevant in dynamic or precise applications.

When using floats the magnetostrictive sensing principle offers very high resolution of the level value. Tank heights vary from typically 200 mm up to several meters. Another advantage of this sensor principle is the high update rate (supporting fast closed loop systems for level sensing)

In many applications the requirements for the level control solutions are not too demanding. In these cases the ultrasonic principle has gained significant market share within the last years. Ultrasonic sensors do not need a float, installation on the top of the tank is pretty easy, there are even sensor types available which may be used in pressurized tanks (typically up to 6 bar). As ultrasonic sensors quite often are used in special applications, field tests during the design in process are recommended.

Finally hydrostatic pressure transducers are an option for level sensing when using non pressurized tanks (typically connected to ambient pressure through a bore in the upper area of the tank). With the sensor mounted at the bottom of the tank the level is indirectly measured through the pressure of the liquid column above the sensor (e.g. 10m of water level resembles 1 bar).

Summary

Concerning level sensing in metalworking applications in the first step it should be decided whether point level sensing is sufficient or continuous level sensing is required. Having chosen continuous level sensing there are several sensor principles available (selection depending on the application needs and features of the liquids and tank properties). It is always a good engineering practice to prove the preselected sensing concept with field tests.

To learn more visit www.balluff.com

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

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

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.