Level sensing in hygienic applications

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New technologies make providing accurate and reliable level sensing in hygienic applications easier than ever. There are, however, still important decisions to be made, as Andy Walker of ifm electronic explains.

Accurately and reliably monitoring and controlling levels in tanks and other vessels is a common requirement in the food, beverage, and pharmaceutical industries. There are many ways this can be achieved, but in these industries where absolute freedom from the risk of product contamination is a prime requirement, two technologies are now dominant: hydrostatic measurement and non-contact radar sensing. These technologies operate in very different ways, and each has its own set of benefits, which is important to consider when deciding on the best option for a particular application.

Hydrostatic level measurement is the more traditional option and uses a pressure sensor to determine the level. The sensor is located at the bottom of the vessel, and as the level of liquid in the vessel increases, so does the pressure the liquid exerts on the sensor. Therefore, provided that the density of the liquid is fairly constant and the shape of the vessel is properly considered, the liquid level can be calculated from the pressure measured by the sensor.

This approach can work very well and has the benefits of being relatively inexpensive and producing results that are largely unaffected by the presence of foam on the surface of the liquid or from mixing agitation. Nevertheless, it also has limitations, the biggest being changing media density as the sensor is generally calibrated to a single media/density and vessels with either dished or coned ends.

For example, the calculation relating pressure to level can be slightly more complicated and may require linearisation to provide the best accuracy for a continuous level measurement. In all applications, the sensor is dynamically calibrated in situ to allow for the characteristics of the vessel and the density of the media. Hydrostatic level measurement can also be used if the vessel is pressurised or subjected to vacuum. But requires a second sensor to measure the top pressure, either positive or vacuum.

A final consideration, which is particularly significant in hygienic applications, is that the sensor comes into direct contact with the media, which shouldn’t present too much of a problem, as completely flush mounting pressure/level sensors, specifically designed for hygienic applications, are now readily available, and allow easy and complete cleaning via CIP/SIP. Despite this, some users have a strong preference for non-contact measurement as this minimises the risk of contamination and allows for quick and easy removal should it be required.

Non-contact options

Fortunately, non-contact sensing is one of the key benefits of radar level measurement, which uses FMCW (frequency-modulated continuous wave) technology.

With this, the sensor, which is mounted above the surface of the liquid, continuously sends out a microwave signal that, in the latest versions, rapidly varies in frequency from 77 to 81GHz. This signal is reflected from the surface of the liquid and returns to the sensor, but because the frequency of the transmitted signal is changing rapidly, there is a frequency difference between the transmitted and received signals. The sensor uses this difference, which is directly related to the time it takes the signal to travel to the surface of the liquid and back, to calculate the distance between the sensor and the liquid.

In addition to offering totally non-contact operation, radar sensors have many other benefits. They can work over long ranges and deliver very high accuracy – millimetre accuracy at a range of 10m is easily achieved. They are unaffected by changing temperature or density of the media and are also unaffected in applications where the vessel is under positive or vacuum pressure. They can also be installed or removed without emptying the vessel as they are fitted above the surface of the liquid, unlike hydrostatic sensors, which need to be at the bottom of the liquid. In fact, radar sensors can often be installed above the vessel, rather than inside, in applications where vessels are constructed of plastic, such as IBC containers or chemical storage tanks.

Radar sensors clearly offer an extensive range of benefits, many of which are particularly attractive in hygienic applications, but are they always the best option? Sometimes they are not because, like every other type of sensor, they have limitations.

Although they are unaffected by the presence of steam, vapours or condensation, radar sensors work less well with foaming media. Also, the microwave signal sent out by the sensor will only be reflected if the liquid has a dielectric constant ≥ 2. In practice, radar sensors will operate reliably with any liquid having a dielectric constant greater than two and, in many cases, liquids with a slightly lower dielectric constant, but this does depend on the sensing range and other details of the application.

Cost considerations

A final consideration relating to radar sensors is cost as, although prices have fallen significantly in recent times, they are still more expensive than the single sensors used for hydrostatic level measurement in vented vessels. In some applications, the extra cost is well worth paying for the benefits radar level sensors offer, and in others, the cost difference may be less than it may first appear. Radar sensors, for example, can provide continuous level information, where a single radar sensor can often provide a level measurement that would need two or more hydrostatic sensors in pressurised applications. Radar sensors are also now very easy to configure, and the reduction in commissioning time translates into cost savings.

The information provided in this article will, it is hoped, make it easier for those involved with the specification of equipment for hygienic process plant to choose the best type of level sensor for their applications. In many cases, the latest radar sensors will represent the ‘gold standard’, but there are still instances where hydrostatic level measurement provides an economical and effective solution.

www.ifm.com

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