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Choosing the right level sensor for every job

Choosing the right level sensor for every job
From hydrostatic to opto-electronic to modern capacitive sensors, level sensor technology covers a wide range of options. But not all sensor systems are suitable for every job or application. And as the experts at Sensortechnics explain here, it's not just a question of technology.

Determining which method of level measurement is best suited to a given application depends on the medium being measured, the type of measurements required (continuous or limit value), needs for data transfer and interfaces and finally also on the budget. The characteristics of the individual applications can vary widely too: aggressive, powdery or sterile media, foam or differing densities of fluids, giving rise to additional requirements, for example. Let's look at the key level measurement technologies in detail.

Essentially, a hydrostatic sensor is installed as a dip probe directly above the base of the container or body of water and measures the pressure of the liquid column load, thereby determining the fluid level. Depending on the product, the measurement data is transmitted as analogue or digital signals in a continuous measurement. 
If the container base is not suitable for drilling, dip probes can be lowered directly into the media from the top, using the cable. Dip probes used for hydrostatic measurement are highly technically developed and are largely resistant to aggressive substances, as - depending on the environmental medium - the sensor casings may be made from stainless steel or plastic. Good manufacturers offer the right casing for all media, pressures and temperature conditions, together with an extensive range of cables. 

Hydrostatic sensors can be fitted in relatively large casings and can therefore easily be connected up with other sensor systems. For instance, roof tanks on the skyscrapers of New York have dip probes to measure rainwater levels and at the same time take temperature measurements. If the water in the cisterns falls below a minimum temperature, heating is switched on to prevent the cisterns freezing. Hydrostatic sensors are also found in applications such as the balancing of ballast tanks in ships or as warning alarms for dykes in the Netherlands, where coastal defence has a long history.

However there are also limits on the use of hydrostatic dip probes. For instance, dip probes are not suitable if an agitator is operating. In this case, a hydrostatic pressure transmitter can be fastened onto the base of the container through the wall - although only where the media and applications allow the container wall to be drilled. 

Opto-electronic sensors are a simpler and lower cost variant for level measurement. The principle is that a light diode sends light to a prism, where it is split and reflected back at a phototransistor. Where the container is not full and the prism is exposed to the air, the phototransistor receives the entire light. On the other hand, if the prism is covered by a liquid, the angle of refraction of the LED light increases. The phototransistor then receives less light and reports this to the evaluation system. Opto-electronic level sensors are small, easy to install and can be used for many purposes. Manufacturers also offer many kinds of casing for optimal media compatibility and ease of installation.

Optical sensors are however only limit switches with 'on/off' or 'full/not full' statuses. They cannot measure continuously. Also, because they have to be fitted on the tank or container wall, this usually has to be drilled through and equipped with a screw thread. This may be unsuitable where there are high hydrostatic pressures or other structural restrictions. For sterile or powdery media too, as well as for liquids with moving surfaces, optical switching sensors are often no use, or at best only of limited use. Also, when in operation, dirt can give rise to failures.

A more recent development is capacitive sensors and switches, such as the CLC and CLW series from Sensortechnics: these are capacitive sensors which can measure levels without contact and at the same time continuously using capacitance differences. The measurement signal depends on the permittivity of the medium. The effects of temperature and humidity are compensated by means of a reference electrode. Capacitive sensors are small, affordable and measure without contact, as they can be attached to the container wall from the outside. Therefore they are ideal for the continuous measurement of sterile media in small containers, as happens in medical technology or the pharmaceutical industry. Products which have ISO 13485:2003 certification meet the high standards and quality requirements set for medical products.

Where continuous measurement of only a few millilitres is required, capacitive sensors can be calibrated to monitor the levels of the smallest quantities to the nearest millimetre. They can also be adjusted either to measure or to ignore foam on the surface of the liquid, as required. In industrial applications, the potential of contact-free capacitive sensor systems has barely been touched on, even though there are many potential uses thanks to their precision, flexibility and ease of calibration. One well-known use is the measurement of fluid levels in industrial printing machinery. Another example is the control of pumps, which prevents damage arising from water entering the system instead of oil. The limits of such two-phase mixtures can be exactly recorded by capacitive CLC sensors. Furthermore, capacitive sensors are enormously flexible and robust in use. For instance, they can be stuck to the outer walls of non-metallic containers and calibrated very easily. 

On the other hand, capacitive sensors are less suited to large fluid levels, as the rod electrodes (pads) become prone to failure if they are more than 10cm long, unless suitably shielded. By 'daisy-chaining' however, the measurement range of the CLC sensor can be expanded. Standard solutions based on hydrostatic or opto-electronic sensors can be carried today by integrators as catalogue products. The same is not true, however, where the application demands special adjustments for the client or the system, or where ultra-precise, capacitive measurement is concerned. 

Adjustments like these start with simple cable superstructures, cable lengths and windings - but even here customers can reach the limits of service of catalogue producers. Another important aspect is the interface to the evaluation system. If the sensor system manufacturer provides the measurement data via the appropriate interface and in the required format, this can save on development costs.

Premium sensor technology manufacturers also offer skilled developer support. Here, a number of engineers contribute their expertise in sensor systems and many years of experience with applications to the developer teams - in shared co-developments or as sub-projects up to the award of CE on the basis of specifications. In this way customers profit with design-in, for example from skills in the areas of self-test functions, remote measurement, combination sensor systems and in the integration of sensor systems/actuators in the total solution. Also valuable to the customer's company is the experience of the precision and lifespan of the sensor systems - because the plant and equipment developer does not have to tediously build up this specialist knowledge in their own enterprise.
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