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  2. Methods of measurement at level   (17/06/2010)

This news article was originally written in Spanish. It has been automatically translated for your convenience. Reasonable efforts have been made to provide an accurate translation, however, no automated translation is perfect nor is it intended to replace a human translator. The original article in Spanish can be viewed at Métodos de medida a nivel
The fundamental criterion to choose the most suitable measuring principle is the means to measure

Methods of measurement at level

June 17, 2010

The process of measurement and level control can be based on different technologies: mechanics, capacitive, conductive, pressure, ultrasonic, microwave, optical and radar would be example only some of them. The fundamental criterion to choose the most suitable measuring principle is the means to measure. Other concepts such as measurement with or without contact with the environment as well as measurement of continuously or only by points complete the selection of the method. Ultrasonic sensors and radar technology, as well as some optical and capacitive, sensors can be used for the measurement without contact with the medium, while all other technologies, the sensors will have contact with the medium.
Emilio Martínez, director of Spain and Portugal of Baumer

Level of contact with continuous level

One of the main questions to be answered is whether one wants to measure at a point or continuously. Level switches have a contact of proximity that contains one or two switching points and indicate if it has been reached or vice versa a certain level. While this method only reflects the ability of the tank at some point, a continuous measurement provides more detailed information by measuring the level through an analog output signal. This very precise method is often applied if liquids should be added or mixed, or settings, consumption or losses must be monitored.

When you choose appropriate measurement methods and technologies, several factors should be considered. The most important are the size of the deposit, the type and the material, the moving parts as devices of mixture in the tank, the distance between the minimum and maximum, level the consistency of the medium (conductivity or not, homogeneity or heterogeneity)(, temperature, etc.). It may be to consider the additional factors such as the formation and detection of foam, the risk of explosion due to gas or dust, existence of inert N2, the accuracy of the measurement required and the necessary certificates (especially for applications in the fields of marine industry)(, food or areas classified ATEX).

Level measurement based on conductivity

If the process is a conductive liquid, you can do the switching point or continuous measurement using principle of conductivity sensors. In conductive level switches, an output signal is connected to an isolated electrode. When the environment comes into contact with the electrode, a minimum current flows through the middle to the box. An amplifier for electrical voltage collects this stream and sends it as a sign.

The most convenient device for continuous level measurement is the LSP 05 x from Baumer. Safe and easy-to-assemble sensor leads very exact measurements and is applicable to all media with a minimum conductivity of 1mS/cm. The measuring method is based on a rod of resistive measurement is introduced in a conductive liquid. An oscillator generating an alternating current, which is fed to all the measuring rod. Depending on the respective level, a proportional current flow is captured between the rod and the metal wall of the tank or a reference electrode and then directed to an amplifier. The resistance of the liquid decreases proportionally to the submerged part of the rod. If the fluid is homogeneous, it provides a linear measurement of the full level and, therefore, a linear signal 4 to 20 mA output. This level measurement device is ideal for use with resistant or even grassy substances like honey or toothpaste. If the formation of foam prevents the measurement, a layer of Teflon on the part of the rod measuring does not ensure that wrong signals there is no.

foto
Application of conductivity LSP.
The fact that it is not affected by the pressure, density, temperature or the dielectric constant open a wide variety of applications for this technology. Because of the connection process apt 3A, the device can be used in the food industry and beverages, pharmaceutical and biochemical industry. An example of a special case is the measurement of whipped cream of chocolate, where the horizontal container implied that the measuring rod could only be installed on side. Therefore, the rod had an angle of 45 ° in the media and remained unfilled to ensure a distance of signal which was used to register the level.

Hydrostatic level measurement

The level control with the help of pressure measurement is an established method from for a long time, and the hydrostatic measurement is an easy to use method and economic. During this process, pressure sensors detect the hydrostatic pressure at the bottom of the tank which is directly proportional to the density and height of the liquid, allowing therefore the measurement of the level of liquid. The devices are allowed for hygienic applications and can be built in containers open or closed as a standard feature.

While in open containers, a pressure measurement device mounted at the bottom of the tank detects the level of fluid above the point of measurement, the internal pressure in closed containers is measured through a second pressure transmitter mounted in the Tower of the container. The difference between two values is the pressure of the level of the liquid, which is indicated as the level of filling, volume and density through a viewer of downstream or a regulator. Hydrostatic level measurement is independent of viscosity and conductivity of the medium and the formation of foam or other substances in the tank, applies in almost all areas of industrial operation.

Registrar of ultrasonic reads

With ultrasonic level detection, are electrically activated piezoelectric crystals in the sensor to send ultrasonic impulses. These impulses are reflected by the medium that is being measured and then reach again to the sensor after a short period of time, and what this delay option is used to measure the level. Therefore, this method of non-contact measurement can be used in open or closed tanks. The only factor to consider is the sonic velocity which depends on the composition and temperature of the gas. The Ultrasonic sensors are used primarily for the powder, paste or liquid level control. For applications with aggressive gases or liquids, standard sensors should only be used with instruments of measurement with adequate protection. To comply with this requirement, the sensors Unar Baumer are equipped with a layer of chemically resistant parileno with excellent organic acid solutions, solvents, inorganic barrier effects and water vapor.
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Unar range of Ultrasonic sensors resistant to chemical substances for control of level in aggressive environments.

Capacitive sensors and optical sensors

Capacitive sensors detect a means making use of its inductive capacity that is different from the air. If the sensitive ability of a capacitor's plates expands due to external circumstances, an RC oscillator vibrating. Resulting investment flows is analysed by the functional unit downstream and then led - according to the signal - to a switching circuit. This occurs if the liquid or solid to be measured is located at some distance from the active area. The digital switching point can be adjusted through a potentiometer directly on the sensor. Capacitive sensors do not typically require maintenance and are easy to install. They are used to control the level of different means of contact or non contact through a wall of the tank made of plastic or glass and can be used in non conductive substances such as porcelain, wood, clay, the plywoodthe paper, cardboard, dust, grain, tea or chocolate. The main fields of application are chemical, food and plastic industries.
foto
Capacitive sensors can measure the levels of many different substances.
On the other hand, the optical sensors contain a head of quartz crystal that bears a diode IR emitter and a sensitive semiconductor to light as a receiver. With these sensors, the levels can be controlled without contact from fluid through transparent tubes or transparent media or tubes that use version FFDK16 of special design. Given that the light is used for level detection, the optical method can measure conductive and non-conductive liquids. If the sensor head is surrounded by air, the angle of limit for the total reflectance of light changes. On the other hand, if the head made of borosilicate glass comes in contact with the liquid, the beam of light is diverted to the fluid and the output of the sensor changes its status of switching. The FFAR of Baumer is available with a variety of equipment covered in situ for different conditions of application, for example, the stainless steel makes it very resistant to a large number of aggressive liquids. The liquid can be conductive or non conductive, while its index of reflection is not too high.

Leakage sensors

Leakage sensors can detect safely and easily leaking fluids starting with a minimum of 1 ml, using the optoelectronic method. This procedure he is achieved a fast response time to make the least possible loss of fluid and avoiding contamination of the facility. Leakage sensors can be mounted directly at the bottom of the tank or on a support device. A reliable system for leakage control is very important in many areas, such as warehouses, conditioning cabinets, fuel tanks and industrial machines. Only if the leaks are detected quite early, can be taken and avoid damage, for example with disconnections of emergency for pump systems.
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Leak detection sensors are based on optoelectronic method.

More methods for the control of level

In addition to the methods of measurement referred to so far, there are other technologies used for the detection and level control. Another method of contact is not the radar which relies on the operating time from emission to reception of the reflected wave. For radiometric measurements, radioactive compounds are used as source of radiation and vibration sensors are switches of level based on the principle of the vibrating fork. There are also based on magnetic meter level switches and flow sensors, mainly used in the process technology to measure the volume and flow mass necessary to count and measure in the load process.

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