capacitive sensors Archives - AUTOMATION INSIGHTS

Protecting photoelectric and capacitive sensors

Supply chain and labor shortages are putting extra pressure on automation solutions to keep manufacturing lines running. Even though sensors are designed to work in harsh environments, one good knock can put a sensor out of alignment or even out of condition. Keep reading for tips on ways to protect photoelectric and capacitive sensors.

Mounting solutions for photoelectric sensors

Photoelectric sensors are sensitive to environmental factors that can cloud their view, like dust, debris, and splashing liquids, or damage them with physical impact. One of the best things to do from the beginning is to protect them by mounting them in locations that keep them out of harm’s way. Adjustable mounting solutions make it easier to set up sensors a little further away from the action. Mounts that can be adjusted on three axes like ball joints or rod-and-mount combinations should lock firmly into position so that vibration or weight will not cause sensors to move out of alignment. And mounting materials like stainless steel or plastic can be chosen to meet factors like temperature, accessibility, susceptibility to impact, and contact with other materials.

When using retroreflective sensors, reflectors and reflective foils need similar attention. Consider whether the application involves heat or chemicals that might contact reflectors. Reflectors come in versions, especially for use with red, white, infrared, and laser lights, or especially for polarized or non-polarized light. And there are mounting solutions for reflectors as well.

Considering the material and design of capacitive sensors

Capacitive sensors must also be protected based on their working environment, the material they detect, and where they are installed. Particularly, is the sensor in contact with the material it is sensing or not?

If there is contact, pay special attention to the sensor’s material and design. Foods, beverages, chemicals, viscous substances, powders, or bulk materials can degrade a sensor constructed of the wrong material. And to switch perspectives, a sensor can affect the quality of the material it contacts, like changing the taste of a food product. If resistance to chemicals is needed, housings made of stainless steel, PTFE, and PEEK are available.

While the sensor’s material is important to its functionality, the physical design of the sensor is also important. A working environment can involve washdown processes or hygienic requirements. If that is the case, the sensor’s design should allow water and cleaning agents to easily run off, while hygienic requirements demand that the sensor not have gaps or crevices where material may accumulate and harbor bacteria. Consider capacitive sensors that hold FDA, Ecolab, and CIP certifications to work safely in these conditions.

Non-contact capacitive sensors can have their own special set of requirements. They can detect material through the walls of a tank, depending on the tank wall’s material type and thickness. Plastic walls and non-metallic packaging present a smaller challenge. Different housing styles – flat cylindrical, discs, and block styles – have different sensing capabilities.

Newer capacitive technology is designed as an adhesive tape to measure the material inside a tank or vessel continuously. Available with stainless steel, plastic, or PTFF housing, it works particularly well when there is little space available to detect through a plastic or glass wall of 8mm or less. When installing the tape, the user can cut it with scissors to adjust the length.

Whatever the setting, environmental factors and installation factors can affect the functionality of photoelectric and capacitive sensors, sometimes bringing them to an untimely end. Details like mounting systems and sensor materials may not be the first requirements you look for, but they are important features that can extend the life of your sensors.

Capacitive Sensors: Versatile enough for most (but not all) detection applications

capacitive 1

Capacitive sensors are versatile for use in numerous applications. They can be used to detect objects such as glass, wood, paper, plastic, ceramic, and more. Capacitive sensors used to detect objects are easily identified by the flush mounting or shielded face of the sensor. This shielding causes the electrostatic field to be short and conical shaped, much like the shielded version of an inductive proximity sensor.

capacitive 2 Just as there are non-flush or unshielded inductive sensors, there are non-flush capacitive sensors, and the mounting and housing look the same. The non-flush capacitive sensors have a large spherical field which allows them to be used in level detection, including detection of liquids and granular solids. Levels can be detected either directly with the sensor making contact with the medium, or indirectly with the sensor sensing the medium through a non-metallic container wall.

Capacitive sensors are discrete devices so once you adjust the sensitivity to detect the target while ignoring the container, the sensor is either on or off. Also remember that the sensor is looking for the dielectric constant in the case of a standard capacitive sensor or the conductivity of a water based liquid in the case of the hybrid technology.

Recent technology advances with remote amplifiers have allowed capacitive sensors to provide an analog output or a digital value over IO-Link. As previously mentioned, these sensors are based off of a dielectric constant so the analog value being created is dependent on the media being sensed.

While capacitive sensors are versatile to work in many applications, they are not the right choice for all applications.

Recently a customer inquired if a capacitive sensor could detect the density of an substance and unfortunately the short answer is no, though in some applications the analog sensors can detect different levels of media if it can be separated in a centrifuge. Also, capacitive sensors may not detect small amounts of media as the dielectric constant of the media must be higher than the container that holds the media.

There are three important steps in applying a capacitive sensor — test it, test it and test it one more time. During your testing procedures be sure to test it under the best and worse conditions. Also like any other electronic device temperature can have an affect although it may be negligible there will be some affect.

For more information on capacitive sensors visit 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.

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

Sensing Types of Capacitive Sensors

Similar to inductive sensors, capacitive sensors are available in two basic versions. The first type is the flush or shielded or embeddable version however with capacitive sensors they are sometimes referred to as object detection sensors. The second type is the non-flush or non-shielded or non-embeddable version however again with capacitive sensors they are sometimes referred to as level detection sensors.

CapacitiveTypes1

The flush or object detection capacitive sensors are shielded and employ a straight line electrostatic field. This focused field is emitted only from the front face of the sensor allowing the sensor to be mounted in material so that only the face of the sensor is visible.

The highly focused electrostatic field is perfect for detecting small amounts of material or material with low dielectric constant. The typical range of a flush 18mm capacitive sensor is approximately 2 to 8mm depending on the objects dielectric constant. As with any capacitive sensor the sensor should be adjusted after installation.

CapacitiveTypes2

If the sensors are mounted adjacent to each other the minimum gap should be equal to the diameter or the adjusted sensing distance whichever is less. These sensors can also be mounted opposing each other however the distance should four times the diameter of the adjusted sensing distance whichever is less.

CapacitiveTypes3 Shielded or flush capacitive sensors are perfect for detecting solids or liquids through non-metallic container walls up to 4mm thick. If you are detecting liquid levels through a sight glass with the sight glass mounting bracket then the flush mounted sensor is the preferred choice.

CapacitiveTypes4 The non-flush or level detection capacitive sensors are not shielded and employ a spherical electrostatic field. This field is emitted from the front face of the sensor and wraps around to the sides of the sensor head. Unlike the flush sensor this version cannot be mounted in material where only the face of the sensor is visible. Non-flush sensors have better characteristics and better performance in applications with adhering media.

CapacitiveTypes5 The spherical electrostatic field provides a larger active surface and is perfect for detecting bulk material and liquid either directly or indirectly. The typical range of a flush 18mm capacitive sensor is approximately 2 to 15mm depending on the objects dielectric constant. As with any capacitive sensor the sensor should be adjusted after installation.

CapacitiveTypes6 If the sensors are mounted adjacent to each other the minimum gap should be equal to three times the diameter or the adjusted sensing distance whichever is less. These sensors can also be mounted opposing each other however the distance should four times the diameter of the adjusted sensing distance whichever is less.

Shielded or flush capacitive sensors are perfect for detecting solids or liquids through non-metallic container walls up to 4mm thick. If you are detecting liquid levels through a sight glass with the sight glass mounting bracket then the flush mounted sensor is the preferred choice.

Capacitive sensors are perfect for short range detection of virtually any object regardless of color, texture, and material.

To learn more about capacitive sensors visit www.balluff.com.

What is a Capacitive Sensor?

Capacitive proximity sensors are non-contact devices that can detect the presence or absence of virtually any object regardless of material. They use the electrical property of capacitance and the change of capacitance based on a change in the electrical field around the active face of the sensor.

Capacitive sensing technology is often used in various detection tasks:

Flow
Pressure
Liquid level
Spacing
Thickness
Ice detection
Shaft angle or linear position
Dimmer switches
Key switches
X-y tablet
Accelerometers

Principle of operation

A capacitive sensor acts like a simple capacitor. A metal plate in the sensing face of the sensor is electrically connected to an internal oscillator circuit and the target to be sensed acts as the second plate of the capacitor. Unlike an inductive sensor that produces an electromagnetic field a capacitive sensor produces an electrostatic field.

The external capacitance between the target and the internal sensor plate forms a part of the feedback capacitance in the oscillator circuit. As the target approaches the sensors face the oscillations increase until they reach a threshold level and activate the output.

Capacitive sensors have the ability to adjust the sensitivity or the threshold level of the oscillator. The sensitivity adjustment can be made by adjusting a potentiometer, using an integral teach pushbutton or remotely by using a teach wire. If the sensor does not have an adjustment method then the sensor must physically be moved for sensing the target correctly. Increasing the sensitivity causes a greater operating distance to the target. Large increases in sensitivity can cause the sensor to be influenced by temperature, humidity, and dirt.

There are two categories of targets that capacitive sensors can detect the first being conductive and the second is non-conductive. Conductive targets include metal, water, blood, acids, bases, and salt water. These targets have a greater capacitance and a targets dielectric strength is immaterial. Unlike an inductive proximity sensor, reduction factors for various metals are not a factor in the sensors sensing distance.

The non-conductive target category acts like an insulator to the sensors electrode. A targets dielectric constant also sometimes referred to as dielectric constant is the measure of the insulation properties used to determine the reduction factor of the sensing distance. Solids and liquids have a dielectric constant that is greater than vacuum (1.00000) or air (1.00059). Materials with a high dielectric constant will have a longer sensing distance. Therefore materials with high water content, for example wood, grain, dirt and paper will affect the sensing distance.

When dealing with non-conductive targets there are three factors that determine the sensing distance.

The size of the active surface of the sensor – the larger the sensing face the longer the sensing distance
The capacitive material properties of the target object, also referred to as the dielectric constant – the higher the constant the longer the sensing distance
The surface area of the target object to be sensed – the larger the surface area the longer the sensing distance

Other factors that have minimal effect on the sensing distance

Temperature
Speed of the target object

Sensing range

A capacitive sensor’s maximum published sensing distance is based on a standard target that is a grounded square metal plate (Fe 360) that is 1mm thick. The standard target must have a side length that is the diameter of the registered circle of the sensing surface or three times the rated sensing distance if the sensing distance is greater than the diameter. Objects being detected that are not metal will have a reduction factor based on the dielectric constant of that object material. This reduction factor must be measured to determine the actual sensing distance however there are some tables that will provide an approximation of the reduction factor.

Rated or nominal sensing distance S_{n is} a theoretical value that does not take into account manufacturing tolerances, operating temperatures and supply voltages. This is typically the sensing distance listed in various manufactures catalogs and marketing material.

Effective sensing distance S_ris the switching distance of the sensor measured under specified conditions such as flush mounting, rated operating voltage U_e, temperature T_a = 23°C +/- 5°C. The effective sensing range of capacitive sensors can be adjusted by the potentiometer, teach pushbutton or remote teach wire.

Hysteresis

Hysteresis is the difference in distance between the switch-on as the target approaches the sensing face and switch-off point as the target moves away from the sensing face. Hysteresis is designed into sensors to prevent chatter of the output if the target was positioned at the switching point.

Hysteresis stated in % of rated sensing distance. For example a sensor with 20mm of rated sensing distance may have a maximum hysteresis of 15% or 3mm. Hysteresis is an independent parameter that is not a constant and will vary sensor to sensor. There are several factors that can influence hysteresis including:

Sensor temperature both ambient and heat generated by the sensor being powered
Atmospheric pressure
Relative humidity
Mechanical stresses to the sensor housing
Electronic components utilized on the printed circuit board within the sensor
Correlated to sensitivity – higher sensitivity relates to higher rated sensing distance and a larger hysteresis

How to determine a capacitive sensor’s sensitivity

Capacitive sensors have a potentiometer or some method to set the sensor sensitivity for the particular application. In the case of a potentiometer, the number of turns does not provide an accurate indicator of the sensors setting for a couple of important reasons. First, most potentiometers do not have hard stops instead they have clutches so that the pot is not damaged when adjusted to the full minimum or maximum setting. Secondly, pots do not have consistent linearity.

To determine the sensitivity of a capacitive sensor the sensing distance is measured from a grounded metal plate with a micrometer. The plate is grounded to the negative of the power supply and the target is moved axially to the sensors face. Move the target out of the sensing range and then move it towards the sensor face. Stop advancing the target as soon as the output is activated. This distance is the sensing distance of the sensor. Moving the target away and noting when the output turns off will provide the hysteresis of the sensor.

To learn more about capacitive sensor technology visit www.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

Multiple Sensing Modes for Miniature Capacitive Sensors

In a previous blog post we discussed miniature capacitive sensors and their use for precision and small-part sensing. Here we will discuss the different sensing modes available with separately amplified miniature capacitive sensors.

Standard Switching Mode

Std_Switch_Mode

This is the most commonly used teach method for most sensing applications. As an object is placed statically in front of the sensor at its desired detection point, the amplifier is triggered to teach-in this value as its switch point (SP1). Once the value is taught, the output will then switch when the switch point is reached.

Two-Point Switching Mode

TwoPoint_Switch_Mode

As the name sug
gests this teach method has two separate teach-in points, a switch-on point (SP1) and a switch-off point (SP2). These points can be taught wide apart or close together, depending on the application need. One application example is for fill-level control by teaching in min. and max. fill-level points.

Window Function Mode

Window_Mode

This teach method creates a window between two separate switch points (SP1 and SP2). If the sensor value falls inside this window, the output will switch on. If the sensor value is outside of this window, the output remains off. An application example is material thickness (or multiple layer) detection. If the material is too thin or too thick (i.e., sensor value is outside the window) the output remains off; however, if the material is at the correct thickness (i.e., sensor value falls inside the window) the output switches on.

Dynamic Operation Mode

This mode only responds to moving objects and ignores static conditions. This mode is commonly used to ignore a close background, and only detect objects moving in front of the sensor.

Analog Output Mode
Analog_Mode

Additionally, an analog output (either voltage or current) is available. To utilize the whole analog range, two separate teach points are needed. SAHi, analog signal high, and SALo, analog signal low, are taught accordingly to obtain the full range. An application example would be continuous fill-level detection across the sensing area.

For more information on capacitive sensors and their remote amplifiers, click h
ere.

Miniature Capacitive Sensors for Small Part Detection

As discussed in a previous blog post, miniature sensors are an ongoing trend in the market as manufacturing and equipment requirements continue to demand smaller sensor size due to either space limitations and/or weight considerations. However, size and weight aren’t the only factors. The need for more precise sensing — higher accuracy, repeatability, and smaller part detection — is another demanding requirement and, often times, the actual main focus point.

This post will look specifically at capacitive sensors and how smaller capacitive sensors can lead to better detection of smaller parts.

cap4 — Parallel-plate capacitor equation

Capacitive sensors provide non-contact detection of all types of objects, ranging from insulators to conductors and even liquids. A capacitive sensor uses the principle of capacitance to detect objects. The equation for capacitance takes into account the surface area (A) of either electrode, the distance (d) between the electrodes, and the dielectric constant (ε_r) of the material between the electrodes. In simple terms: a capacitive sensor detects the change in capacitance when an object enters its electrical field. Internal circuitry determines if the gain in capacitance is above the set threshold. Once the threshold is met the sensor’s output is switched.

When looking at small part detection, the size of the capacitive sensor’s active sensing surface plays a significant part. Now there isn’t a defined formula for calculating smallest detectable object for a capacitive sensor because of the numerous variables that need to be considered (as seen in the equation above). However, the general rule for optimal sensing is that the target size should be at least equal to the size of the sensor’s active surface. The reason behind this is if the target size is smaller than the sensor’s active surface, the electric field would travel around the target and cause unreliable readings.

Taking the general rule into consideration and comparing a miniature 4mm diameter capacitive sensor to a standard 18mm diameter capacitive sensor, it’s simple to determine that the 4mm diameter capacitive sensor can reliably detect a much smaller target (4mm) than the 18mm diameter capacitive sensor (18mm).

So when looking at small part detection, the smaller the sensor’s active sensing surface is, the better its ability for small part detection. Therefore, if an application requires detection of a small part, it’s best to start with miniature capacitive sensor.

For more information on miniature capacitive sensors click here.

The Often Overlooked Proximity Sensor

If someone says proximity sensor, what is the first thing that comes to mind? My guess is inductive and justly so because they are the most used sensor in automation today. There are other sensing technologies that use the term proximity in describing the sensing mode. These include diffuse or proximity photo electric sensors that use the reflectivity of the object to change states and proximity mode of ultrasonic sensors that use high frequency sound waves to detect objects. All of these sensors detect objects that are in close proximity of the sensor without making physical contact.

One of the most overlooked or forgotten proximity sensors on the market today is the capacitive sensor. Why? Perhaps it is because they have a bad reputation from when they were released years ago as they were more susceptible to noise than most sensors. I recently heard someone say that they don’t discuss capacitive sensors with their customers because they had this bad experience almost 10 years ago, however, with the advancements of technology this is no longer the case.

Capacitive sensors are versatile in solving numerous applications. These sensors can be used to detect objects such as glass, wood, and paper, plastic, ceramic, the list goes on and on. The capacitive sensors used to detect objects are easily identified by the flush mounting or shielded face of the sensor. Shielding causes the electrostatic field to be short conical shaped much like the shielded version of the inductive proximity sensor.

Just as there are non-flush or unshielded inductive sensors there are non-flush capacitive sensors, and the mounting and housing looks the same. The non-flush capacitive sensors have a large spherical field which allows them to be used in level detection. Since capacitive sensors can detect virtually anything, they can detect levels of liquids including water, oil, glue and so forth and the can detect levels of solids like plastic granules, soap powder, sand and just about anything else. Levels can be detected either directly, the sensor touches the medium or indirectly where the sensor senses the medium through a non-metallic container wall.

With improvements in capacitive technology sensors have been designed that can compensate for foaming, material build-up and filming of water based highly conductive liquids. Since these capacitive sensors are based on the conductivity of liquids they can reliably actuate when sensing aggressive acids such as hydrochloric, sulfuric and hydrofluoric acids. In addition, these sensors can detect liquids through glass or plastic walls up to 10mm thick are not affected by moisture and require little or no cleaning these applications.

The sensing distance of a capacitive sensor is determined by several factors including the sensing face area, the larger the better. The next factor is the material property of the object or dielectric strength, the higher the dielectric constant the greater the sensing distance. Lastly the size of the target affects the sensing range. Just like an inductive sensor you want the target to be equal to or larger than the sensor.

Most capacitive sensors have a potentiometer to allow adjustment of the sensitivity of the sensor to reliably detect the target. The maximum sensing distance of a capacitive sensor is based on a metal target thus there is a reduction factor for non-metal targets.

Although capacitive sensors can detect metal inductive sensors should be used for these applications. Capacitive sensors are ideal for detecting non-metallic objects at close ranges, usually less than 30mm and for detecting hidden or inaccessible materials or features.

Just remember, there is one more proximity sensor…the capacitive one!

To learn more about Balluff capacitive sensors visit www.balluff.us.

Direct vs Indirect Mounting of Capacitive Sensors

In liquid level sensing applications, capacitive sensors can be mounted directly in contact with the medium or indirectly with no contact to the medium.

Containers made of metal or very thick non-metallic tank walls (more than 1″) typically require mounting the sensor in direct contact with the medium (Fig. 1). In some instances, a by-pass tube or a sights glass is used, and the senor detects the level through the wall of the non-metallic tube (Fig. 2).

The 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.

The preferred approach is indirectly mounting the capacitive sensor flush against the non-metallic wall to detect the target material non-invasively through the container wall. The advantages for this approach are obvious and represent a major influence to specify capacitive sensors. 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.

The target material 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 lies in non-invasive liquid level detection, namely by creating a large measurement delta between the low dielectric container walls 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. SMARTLEVEL sensors offered by Balluff will ignore foaming, filming and material build-up in these applications.

Learn more about Balluff’s capacitive solutions on our website at www.balluff.us.

Tag: capacitive sensors

Protecting photoelectric and capacitive sensors

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Capacitive Sensors: Versatile enough for most (but not all) detection applications

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

Precisely detecting point levels

Continuous level sensing

Product bundle for level monitoring in storage tanks

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Sensing Types of Capacitive Sensors

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