Make Clear Water Visible to Your Sensors

In some industries such as life sciences it is necessary to detect clear water or clear liquids in a container or tube. This is even more challenging when the diameters of the tube are small, and the tube thickness is nearly as large as the stream of liquid.

The attenuation or gradual reduction of the intensity of the light beam in water and air can be directly compared. The attenuation of light in water can be attributed to light entering water at any angle other than at a right angle and can be refracted. The measurement of light through a tube is different because not only is the light attenuated by the liquid, but depending on where the light passes through the tube it can be refracted, diverted and or focused. As a result, the signal differences can be low.

Attenuation is typically the first choice if the liquids are opaque or colored. The requirements of the shape of the light beam and the alignment of the sensor add more complication to the application. The attenuation effect appears weaker in clear liquids. The principle does not work with reflective sensors since reflection is a surface effect and the light must pass through the liquid.OPTO_appl_08_sw-water

From spectral analysis it’s has been determined that the attenuation characteristics of water are heavily dependent on the wavelength of the light that is conducted through it. Sensors were developed for such applications. Typically, these sensors utilize LED’s in the upper infrared range of 1,450 nm. At this wavelength water literally absorbs the light and becomes opaque making detection more simplified and reliable.

This principle even works for fine capillaries and microchannels. Liquid detection can be very precise depending on the sensor size and the effective light beam. Light beams as small as 0.4mm can provide high resolution for small thin tubes typically found in microfluidics applications.

Versions of these sensors exist for applications that involve less transparent or semi-transparent vessels. Light at the 1450nm wavelength can pass through these containers or tubes and can be attenuated by the water. The main factor is that enough light makes its way through the walls of the container.

Through-beam sensors were developed for applications such as detecting clear liquids. These sensors are also available in extremely small dimensions and usually require an amplifier, or they can be supplied in a rugged fork sensor housing. The required sensor dimensions conform to the geometry of the vessel or container.

For more information on sensors for these types of applications contact your local Balluff representative or contact us at www.balluff.com.

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.

Image 1
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.

Figure 2
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.

Figure 3
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.

Figure 4
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.

Figure 5
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.

Figure 6
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.

Figure 7
Level detection via ultrasonic sensor

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.

Figure 8
Level monitoring in storage tanks

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

Level Detection Basics – Part 2

In the first blog on level detection we discussed containers and single point and continuous level sensing.  In this edition we will discuss invasive and non-invasive sensing methods and which sensing technologies apply to each version.  Keep in mind that when we are talking about level detection the media can be a liquid, semi-solid or solid with each presenting their own challenges.

tankInvasive or direct level sensing involves the sensing device being in direct contact to the media being sensed.  This means that the container walls or any piping must be violated leading to issue number one – leakage.  In some industries such as semiconductor and medical the sensing device cannot contact the media due to the possibility of contamination.

level_btl-sf-wThe direct mounting method could simplify sensor selection and setup since the sensor only has to sense the medium or target material properties.  Nonetheless, this approach imposes certain drawbacks, such as costs for mounting and sealing the sensor as well as the need to consider the material compatibility between the sensor and the medium.  Corrosive acids, for example, might require a more expensive exotic housing material.level_bsp_w

Invasive sensing technologies that would solve level sensing applications include capacitive, linear transducers, hydrostatic with pressure sensors.

In many cases the preferred approach is indirectly or non-invasively mounting the sensor on the outside of the container.  This sensing method requires the sensor to “see” through the container walls or by looking down at the media from above the container through an opening in the top of the container.  The advantages for this approach are easier mounting, lower cost and easier to field retro-fit.  The container wall does not have to be penetrated, which leaves the level sensor flexible and interchangeable in the application.  Avoiding direct contact with the target material also reduces the chances of product contamination, leaks, and other sources of risk to personnel and the environment.

level_bglIn some cases a sight glass is used which is mounted in the wall of the tank and as the liquid media rises it flows into the sight glass.  When using a sight glass a fork style photoelectric sensor can be used or a capacitive sensor can be strapped to the sight glass.

The media also has relevance in the sensor selection process.  Medical and semiconductor applications involve mostly water-based reagents, process fluids, acids, as well as different bodily fluids.  Fortunately, high conductivity levels and therefore high relative dielectric constants are common characteristics among all these liquids.  This is why the primary advantages of capacitive sensors lie in non-invasive liquid level detection, namely by creating a large measurement delta between the low dielectric container and the target material with high dielectric properties.  At the same time, highly conductivity liquids could impose a threat to the application.  This is because smaller physical amounts of material have a larger impact on the capacitive sensor with increasing conductivity values, increasing the risk of false triggering on foam or adherence to the inside or outside wall.

Non-invasive or indirect level sensing technologies include photoelectrics, capacitive, linear transducers with a sight glass and ultrasonics.

For more information visit www.balluff.com.

Level Detection Basics – Where to begin?

Initially I started to write this blog to compare photoelectric sensors to ultrasonic sensors for level detection. This came to mind after traveling around and visiting customers that had some very interesting applications. However, as I started to shed some light on this with photoelectrics, sorry for the pun but it was intended, I thought it might be better to begin with some application questions and considerations so that we have a better understanding of the advantages and disadvantages of solutions that are available. That being said I guess we will have to wait to hear about ultrasonic sensors until later…get it, another pun. Sorry.

Level detection can present a wide variety of challenges some easier to overcome than others. Some of the questions to consider include the following with some explanation for each:

  • What is the material of the container or vessel?
    • Metallic containers will typically require the sensor to look down to see the media. This application may be able to be solved with photoelectrics, ultrasonics, and linear transducers or capacitive (mounted in a tube and lowered into the media.
    • Smart LevelNon-metallic containers may provide the ability for the sensors look down to see the media with the same technologies mentioned above or by sensing through the walls of the container. Capacitive sensors can sense through the walls of a container up to 4mm thick with standard technology or up to 10mm thick using a hybrid capacitive technology offered by Balluff when detecting water based conductive materials. If the container is clear or translucent we have photoelectric sensors that can look through the side walls to detect the media.
  • What type of sensing is required? The short answer to this is level right? However, there are basically two different types of level detection. For more information on this refer to the Balluff Basics on Level Sensing – Discrete vs. Continuous.
    • Single point level or point level sensing. This is typically accomplished with a single sensor that allows for a discrete or an on-off signal when the level actuates the sensor. The sensor is mounted at the specific level to be monitored, for instance low-low, low, half full (the optimistic view), high, or high-high. These sensors are typically lower cost and easier to implement or integrate into the level controls.
    • Example of in-tank continuous level sensor
      Example of in-tank continuous level sensor

      Continuous or dynamic level detection. These sensors provide an analog or continuous output based on the level of the media. This level detection is used primarily in applications that require precise level or precision dispensing. The output signals are usually a voltage 0-10V or current output 4-20mA.  These sensors are typically higher cost and require more work in integrating them into system controls.  That being said, they also offer several advantages such as the ability to program in unlimited point levels and in the case of the current output the ability to determine if the sensor is malfunctioning or the wire is broken.

Because of the amount of information on level detection this will be the first in a series on this topic. In my next blog I will discuss invasive vs non-invasive mounting and some other topics. For more information 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
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.