Methods of Level Measurement
14 July 2009
The process of level measuring and monitoring can be based on various technologies – capacity, conductivity, pressure, ultrasonic, light or radar being a few of them. A basic criterion for the choice of the most suitable measuring principle is the medium to be measured. Depending on that, further factors such as measuring or switching and contact or non-contact sensors play an important role.
Ultrasonic sensors, radar technology as well as some optical and capacitive sensors can be used for non-contact measurement, whereas with all other technologies, the sensors will have contact with the medium.
Point Level vs. Continuous Level
One of the main questions to be answered is whether one wants to measure at a point, or continuous level. Level switches have a proximity switch containing either one or two switch-points and indicate whether a certain level has been reached. Whereas this method only reflects the tank capacity at a certain point, a continuous measurement provides more detailed information by measuring the level via an analogue output signal. This very accurate method is often applied if liquids are to be dosed or mixed, or if adjustments, consumption or leakage are to be monitored.
When choosing appropriate technologies and measuring methods, various factors have to be considered. The most important ones being container size, type and material, moving parts such as stirring devices in the container, the distance between minimum and maximum level, the medium’s consistence (conductive or non-conductive, homogenous or heterogeneous, temperature etc.). Additional factors like foam formation and foam detection, explosion hazard due to gas or dust, the accuracy of measurement required and the certificates needed (especially for applications in the fields of marine and food industry) may also need to be considered.
Conductivity Based Level Measurement
If the process medium is a conductive liquid, continuous or switch point measurement can be made using conductivity principle sensors. In conductive level switches, an output signal is connected to an insulated electrode. When the medium contacts the electrode, a minimal current flows through the medium to the housing. An electric booster collects this current and outputs it as a signal.
A most suitable device for continuous level measurement is Baumer’s LSP 05x. The reliable, easy to mount sensor conducts highly accurate measurements and is applicable with all mediums with a minimum conductivity of 1μS/cm. The measuring method is based on a low resistive measuring rod which dips into a conductive liquid. An oscillator generates an alternating current, which is fed to the whole measuring rod. Depending on the respective level, a proportional current flow is captured between the rod and the metallic tank wall or a reference electrode and then routed to an amplifier. The liquid’s resistance decreases proportionally to the immersed part of the rod. If the liquid is homogeneous, an absolute linear measurement of the fill level and thus a linear signal output of 4 to 20 mA is provided. This level measurement device is ideal for use with tough or even pasty substances such as honey or tooth-paste. If foam formation impairs the measurement, a Teflon coating of the non-measuring part of the rod ensures there are no false signals.
The fact that it is not affected by pressure, density, temperature or DK value opens up a wide application range for this technology. Due to the 3A-suitable process connection, the device can be used in the food and beverage industry as well as in pharmaceutical and biochemistry. One example for a special case is a company’s measurement of chocolate mousse, where a horizontal vessel meant the measuring rod could only be installed sideways. Therefore, the rod was angled 45° into the media and remained uncoated to ensure a signal path which served to record the level.
Hydrostatic Level Measurement
Level monitoring with the help of pressure measurement is a long-established method, with hydrostatic measurement being an easy-to-use and cost-efficient method. During this process, pressure sensors detect the hydrostatic pressure at the tank bottom which is directly proportional to density and height of the liquid, therefore enabling the measurement of the liquid level. The devices have approval for hygienic applications and can be built into open or closed vessels as a standard feature.
Whereas in open containers, a pressure measuring device mounted at the bottom of the tank detects the liquid level above the metering point, the internal pressure in closed containers is being measured via a second pressure transmitter mounted in the container tower. The difference between both values is the liquid level pressure, which is being indicated as fill level, volume or specific weight via either a downstream display or a controller. Since hydrostatical level measurement is independent of the medium’s viscosity and conductivity, foam forming or other substances in the tank, it is being applied in almost all industrial operation areas.
Ultrasonic Readings Recorder
With ultrasonic level detection, piezoelectric crystals in the sensor are being electrically activated to send out ultrasonic pulses. Reflected by the medium being measured, these impulses reach the sensor again after a short period of time and it is this varying delay that is used to measure the level. Therefore, this non-contact measuring method can be used in open or closed containers. The only factor to be considered is the sonic speed which depends on composition and temperature of the gas. Ultrasonic sensors are mainly used for the level monitoring of granulate, paste or liquid. For applications with aggressive liquids or gases, the standard sensors should only be used in measuring instruments with appropriate protection. To meet this requirement, Baumer’s UNAR sensors are equipped with a chemically resistant parylene coating with excellent barrier effects towards inorganic acid solutions, organic solvents and water vapour.
Capacitive sensors detect a medium by making use of its inductive capacity which is different to that of air. If a plate capacitor’s sensitive capacity is magnified due to external circumstances, a RC oscillator vibrates. The resulting reverse in current is analysed by the downstream functional unit and then leads – depending on the signal – to a circuit switch. This happens if the liquid or solid medium to be measured is located at a certain distance to the active area. The digital switch-point can be adjusted via potentiometer directly on the sensor. Capacitive sensors are usually maintenance-free and easy to install. They are used to monitor the level of diverse contact or non-contact media through a tank wall made of plastics or glass and can be used on non-conductive substances such as porcelain, wood, clay, pellets, paper, carton, powder, grain, tea or chocolate. The main fields of application are the chemical, food and plastics industry.
Optical sensors contain a glass quartz head carrying an infrared diode as a sender and a light-sensitive semiconductor as receiver. With these sensors, levels can be monitored without liquid contact via transparent or half-transparent conduits or tubes using a specially designed FFDK16 version. Since light is being used for the level detection, the optical method can measure conductive as well as non-conductive liquids. If the sensor head is surrounded by air, the limit angle for the total reflectance of the light changes. On the other hand, if the head made of borosilicate glass contacts liquid, the light beam is diverted into the fluid and the sensor output changes its switching status. Baumer’s FFAR is available with a variety of field mountable casings for different application conditions, for example stainless steel makes it extremely resistant to a great number of aggressive liquids. The liquid medium can either be conductive or non-conductive, as long as their reflection rate is not too high.
Leakage sensors can safely and easily detect leaking liquids starting at a minimum of 1 ml, by using the optoelectronic method. With this procedure, a fast response time is achieved to keep the loss of liquid as small as possible and to avoid a contamination of the facility. Leakage sensors can be mounted directly at the tank bottom or on a supporting device. A reliable system for leakage control is very important in many areas, such as warehouses, conditioning cabinets, fuel deposits or with industrial machines. Only if leakages are detected early enough can according measures be taken and damages be prevented, for example by emergency cut-offs for pump systems.
More methods for level monitoring
In addition to the measuring methods mentioned so far, there are further technologies used for level detection and monitoring. Another non-contact method is radar which is based on the running time, from sending to receiving the reflected wave. For radiometric measuring, radioactive compounds are used as radiation source, and vibration sensors are level switches based on the vibrating-fork principle. There are also level switches based on magnetic gauges and flow rate sensors, mainly used in process technology to measure volume and mass flow necessary for accounting and dosing in charge processes.
When deciding for or against a measuring method, several process-related factors have to be considered – one of them being costs. Continuous level measuring is generally more complex and therefore more cost-intensive than proximity switches. However, acquisition costs should not be the decisive criteria, since in the long term, a more expensive measuring system can pay off thanks to low maintenance, high accuracy and integration possibilities in the control loop.
Baumer offers a wide range of efficient measuring methods based on various technologies. Its devices for hydrostatic pressure and pressure differential measurement are based on piezo, ceramics, capacity or thin-film. Level monitoring is also possible with ultrasonic, optical or conductivity based sensors. Whether the production facilities require liquid-contact sensors or non-contact devices – Baumer offers custom-tailored applications and excellent technologies for all demands. The various level sensors are most suitable for measurements in industrial areas, chemical process engineering and food and beverage industry.
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