If levels are measured in very small containers or also in tubes, then the usual hydrostatic pressure transmitters fail because of the small signals. Pasty or highly adhesive media equally frequently cause faulty measurements especially when using capacity level sensors. The most suited for problems of this type is the potentiometric measuring, continuous level sensor NSK-… from Negele, that can capture levels up from 50mm.
Note: The NSK sensor series has been replaced by the newly developped and more efficient NSL series.
The so-called potentiometric measuring procedure is used by Negele in the continuous level sensor NSK and is especially suited to small levels. The sensor in its single rod variant can be used from above and if required from below in metallic containers of up to a level of 2 m. The sensor can be supplied variably in any rod length according to the height of the tank. For plastic containers, with slanting installation or irregular tank shapes there is also a two rod sensor with separate earthing rod, which replaces the metal sides of the tank. As a measuring signal there is an adjustable 4…20mA signal directly on the sensor available to the user. Its special feature is the rapidly transmitted update of the measured values. With the NSK an updated level measurement is given 10 times every second.
Because of this feature the sensor is especially well adapted to level control for containers of every type. The process connection of the sensor is carried out with the tried and tested, elastomer-free, 3-A certified hygiene build-in unit from Negele. Additionally there are numerous suitable adapters such as TriClamp, Milchrohr (Milk tube), Varivent, DRD and may others available. Thus it is any easy step to adapt the sensor to existing process connections or to match it with an existing plant standard. The measuring rod consists of stainless steel 1.4404, the insulating element that simultaneously functions as a watertight cone is made of FDA approved PEEK plastic. Because of the high hygiene standard of the modular unit and choice of materials the NSK is therefore best suited for sterile.applications and fully CIP-capable, and in the optional high temperature version, also SIP-capable.
The sensor with its robust stainless steel housing is resistant to even the toughest daily requirements of equipment in the foodstuffs industry.
It is a truism that no measurement procedure works correctly in every situation absolutely and with no hitches. This is of course also valid for conductive potentiometrics.In homogeneous media it works excellently: as soon as the distribution of the conductivity in the medium becomes extremely inhomogeneous, there results a measurement error. Rises in temperature on the measuring rod itself have very little effect, due the an appropriate selection of the material, and are usually negligible. It must be noted that the medium should not fall below a certain, very low conductivity, because otherwise it would result in an empty signal for safety reasons. The system voltage would otherwise be affected by malfunction. Whereby for example distilled water can always still be measured without problem. In some extreme situations there can be especially on the upper rod end conductive bridges from the rod to the wall of the tank or to the device earthing, which then cause a faulty measured value. In this case however an optional insulation of the measuring rod at the upper rod end acts as a remedy in most cases.
The NSK continuous level sensor is usually applied for small pressurised tanks such as are found in breweries. As mentioned earlier, the hydrostatic measurement of pressure difference does not always fulfil the accuracy requirements of the users. A contactless measurement is often too expensive for such applications. A highly accurate and also more economical solution is the NSK continuous level sensor from Negele.The main application of the NSK is in presettling containers of filling equipment and supplies an important parameter for the regulation of the feeding pressure there. Very accurate dosage procedures in filling equipment can be carried out with the NSK on account of its very short response time, since the changing level and its associated feeding pressure can be very rapidly adjusted on account of its short response time.Particularly for filling equipment containing changing and pasty products the NSK is most suitable, remembering the above mentioned reasons, for an accurate level measurement with the highest standards of hygiene. The sensors in the application photo are installed into the presettling tanks of a filling machine in a dairy. Here they are utilised through optional milk tube adapters in the existing process connection. The sensors in this case carry out the level measurement which is the essential criterion for the regulation of feeding pressure. Here, media such as curds and yoghurt, for example, are being filled. It is important for successful operation that adhesions that always occur with such media do not corrupt the measurement. The NSK meets this requirement on account of the measurement principle.When a medium is changed such as it is often the case here, the sensor can also contribute considerably to the flexibility of the user, since, as already described, no re-adjustments are needed when changing the product. This increases speed and thus the efficiency of the unit. The short response time, together with proven filler unit technology is a guarantee that, tub by tub, the same quantity will be delivered.