Which is the best level sensing technology to measure my medium?

Optical reflection measurement exploits the refraction of light on the medium to detect the level of liquids. Refraction is the phenomenon where light rays change direction as they pass from one material to another. This occurs because the speed of light is different in different materials. Optical reflectance sensors consist of a light emitter and a photodetector that measures the reflected light. When a liquid is present, the light is refracted and thus only a portion of the light is reflected, resulting in a corresponding change in the electrical signal.

This is an invasive method that requires contact with the liquid, but is an inexpensive and practical solution for level measurement and leak detection at high temperatures and pressures. Such sensors can only detect a certain level and are susceptible to contamination.

These level sensors emit ultrasound so that the distance between the sensor and the liquid can be determined based on the travel time of the reflected sound wave. This is an accurate, non-contact method that is not affected by the type of material being measured. However, level measurement by ultrasonics is complex and expensive. In addition, problems occur with foaming liquids and complex container shapes.

Moreover, the method does not work with vacuumed containers, since sound cannot propagate in the airless space. As with all measurement methods based on the ToF method, there is a dead zone in the upper area of the container, so that the container cannot be filled completely.

This method is particularly suitable for liquids as well as for powdery and fine-grained solids in the mining, chemical and food industries. Vibration sensors have a fork-shaped probe that vibrates at its natural frequency. A change in frequency indicates a change in level. This method is inexpensive and compact, but requires contact with the material.

Since you can only measure a certain level per sensor, measurement by vibration usually only measures discrete levels, such as a certain limit level or 40%, 60%, and 80%. Vibration sensors are also not very repeatable.

Another sensor for easy detection of a high or low level is the float switch. This rises or falls as the level in the vessel or tank increases or decreases. This mechanical sensing principle requires no power supply and is inexpensive.

However, compared to other sensors, float switches are relatively large and less reliable because they can become dirty and stuck in the tank.

As a specialist in level measurement, CAPTRON uses capacitive as well as ToF laser technology for level measurement.

As the figure on the right shows, the operating principle of capacitive level measurement is based on the change in capacitance of a capacitor. In this example, the probe rod and the vessel wall form the two electrodes, while the medium acts as the dielectric. The change in level causes a change in capacitance. An empty vessel has a low capacitance, while a filled vessel has a high capacitance.

Selection Criteria for Capacitive Probes

The selection criteria depend on numerous factors – 1-rod probes are used for metal vessels, while double-rod probes or a 1-rod probe with a cladding tube are used for plastic vessels. The environmental conditions, such as temperatures above 100 °C, and the required protection class, e.g. IP67, determine whether an external amplifier is required or what type of instrument connection is needed.

Special designs according to customer specifications

In every industry, very different requirements are placed on products and materials. That is why CAPTRON offers a range of probe systems that can be combined and customized. Depending on the application and type of vessel, the company's systems can be used to configure probes that meet very specific customer requirements - right down to probe lengths that are accurate to the millimeter. Custom designs are an important consideration, especially for different vessel sizes. CAPTRON meets these requirements and offers probes or even dual probes in the desired length and with sensors that measure many different liquids and powders. Due to the material options, corrosion does not occur. Special designs are developed according to customer specifications, especially the rod length can be determined to the centimeter and delivered within two weeks. CAPTRON's level sensors are extremely reliable and very accurate. There are systems that have been in operation for over 20 years. CAPTRON also takes care of the precise calibration of probes with regard to the vessel geometry, the measured material (DK value) and other influencing factors, such as temperature or the movement of an agitator, among others. Compact probes with integrated electronics in the housing or probes with external amplifiers are available. The external amplifier is connected via a shielded, temperature-resistant Teflon cable to the rod probe. Due to the spatial separation of the rod probe and the amplifier, these probes are suitable for temperatures of up to 230 °C.