point level detection Archives - AUTOMATION INSIGHTS

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.

The Evolving Technology of Capacitive Sensors

In my last blog post, Sensing Types of Capacitive Sensors, I discussed the basic types of capacitive sensors; flush versions for object detection and non-flush for level detection of liquids or bulk materials. In this blog post, I would like to discuss how the technology for capacitive sensors has changed over the past few years.

The basic technology of most capacitive sensors on the market was discussed in the blog post “What is a Capacitive Sensor”. The sensors determine the presence of an object based on the dielectric constant of the object being detected. If you are trying to detect a hidden object, then the hidden object must have a higher dielectric constant than what you are trying to “see through”.

Conductive targets present an interesting challenge to capacitive sensors as these targets have a greater capacitance and a targets dielectric constant is immaterial. Conductive targets include metal, water, blood, acids, bases, and salt water. Any capacitive sensor will detect the presence of these targets. However, the challenge is for the sensor to turn off once the conductive material is no longer present. This is especially true when dealing with acids or liquids, such as blood, that adheres to the container wall as the level drops below the sensor face.

Today, enhanced sensing technology helps the sensors effectively distinguish between true liquid levels and possible interference caused by condensation, material build-up, or foaming fluids. While ignoring these interferences, the sensors would still detect the relative change in capacitance caused by the target object, but use additional factors to evaluate the validity of the measurement taken before changing state.

These sensors are fundamentally insensitive to any non-conductive material like plastic or glass, which allows them to be utilized in level applications. The only limitation of enhanced capacitive sensors is they require electrically conductive fluid materials with a dipole characteristic, such as water, to operate properly.

Enhanced or hybrid technology capacitive sensors work with a high-frequency oscillator whose amplitude is directly correlated with the capacitance change between the two independently acting sensing electrodes. Each electrode independently tries to force itself into a balanced state. That is the reason why the sensor independently measures the capacitance of the container wall without ground reference and the capacitance of the conductivity of the liquid with ground reference (contrary to standard capacitive sensors).

Up to this point, capacitive sensors have only been able to provide a discrete output, or if used in level applications for a point level indication. Another innovative change to capacitive sensor technology is the ability to use a remote amplifier. Not only does this configuration allow for capacitive sensors to be smaller, for instance 4mm in diameter, since the electronics are remote, they can provide additional functionality.

The remote sensor heads are available in a number of configurations including versions that can withstand temperature ranges of -180°C up to 250°C. The amplifiers can now provide the ability to not only have discrete outputs but communicate over an IO-Link network or provide an analog output. Now imagine the ability to have an adhesive strip sensor that can provide an analog output based on a non-metallic tanks level.

For additional information on the industry’s leading portfolio of capacitive products 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.