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How do capacitive sensors work and how can we get the best from this technology for level measurement? In this article, we’ll aim to explain the technology and the benefits of using capacitive technology in your tanks.

 

As we explained in a previous article, capacitive sensors can be very accurate, robust and reliable for level measurement and can be used for both liquids and bulk materials in many applications. These sensors depend on the principle of capacitors, which consist of two parallel conductive plates, or electrodes, separated by an insulating material, known as the dielectric. When a voltage is applied to the plates, the charges on the plates result in the capacitor storing a quantity of electrical charge corresponding to the applied voltage and the surface area of the plates. This charge would vary according to the dielectric, with what’s known as a DK value (dielectric constant) indicating the material’s ability to store a charge.

In level measurement, this characteristic is ideal for determining the level in a tank where the probe and tank wall form the electrodes, and the liquid or material in the tank forms the dielectric. A change in level causes a change in capacitance, with an empty tank having low capacitance, while a filled tank would have high capacity. As the level of the material rises and falls, the capacitance changes accordingly, providing an accurate measure of the level of the material in the tank.

 

Different materials will have different DK values – for example, hydrogen cyanide has a DK value of 158, while printing ink has a value of just 4.6. Or, fiber glass powder has a DK value of 1.1, while ceramic compound has a value of 17. For foods, ketchup has a value of 24 while cacao beans have a value of 1.8.

This understanding is important as it will guide us in selecting the right types of probes, depending on the application, tank type, and material being measured. Capacitive level sensors can be used in almost any environment and measure any kind of medium – including very dry materials, extreme or harsh environments. Hence CAPTRON provides sensors in rods based on both PTFE (Teflon) as well PEEK (a thermoplastic material). Unlike steel, PTFE or PEEK rods are not subject to corrosion. This is especially important for the chemical industry and the food and pharmaceutical industry, which require both resistance to corrosion plus adherence to safety standards and regulation.

 

The benefits of PTFE probes include high chemical stability against alkalis, alcohols, ketone and petroleum, and the low surface tension means very few substances will stick to the probes. The PTFE material is also physiologically harmless and can resist frost down to -270°C and can operate up to a maximum temperature of 260°C. Meanwhile, PEEK probes offer high chemical stability against almost all organic and inorganic chemicals and are not flammable. They can operate from as low as -65°C and up to 240°C; in medical applications, a particularly useful feature is they are biocompatible.

Probes from CAPTRON, whether PTFE or PEEK, are provided either in a compact system which integrates all the electronics; or where they need to operate in harsher conditions, such as in high temperatures, they are available as probes with external amplifiers which contain the electronics.

So, you have different rod types and they can come as compact or with external amplifier. The next consideration to determine is the tank material. For metal tanks, since the tank wall acts as an electrode, single rod probes can be used. In plastic tanks, a two-rod probe or jacket tube would be needed.

Next is the rod length. A unique feature of CAPTRON capacitive level sensors is the ability to customize the probes to the exact length required (up to 3000mm) to the nearest millimetre, which means they can be used for any shape and size of tank. This is enabled through a web site configurator. This is unlike other probes that come in standard sizes so may not be useable for all shapes of tank, or exactly meet a specific custom need or requirement.In summary, capacitive sensing provides reliable and accurate readings of fill level for almost any materials. The CAPTRON systems, which integrate sensors and electronics in a compact probe or with external electronics, are housed in rods that are uniquely customizable in length, meaning they can be used for any shape or size of tank. Added benefits include the use of non-corrosive materials for the rods. This means they are able to provide level measurement for almost any material, both liquids and bulk materials.

 

To learn more about our Capacitive Level Measurement Sensors please contact us via email: sales@captron.us or give us a call at (914)-619-5422. We're excited to help you pick a level sensing solution tailored to your material's needs!

 

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