Reliable level measurement with
electromechanical plumb bob system
It is not quite easy to measure the level in a closed, non-transparent container, such as a silo. Only those who really know their stock are able to order supplies in time and to avoid bottlenecks in supply or production. However, such measurements are usually time-consuming and, moreover, often not very exact. Now, a new plumb bob is a good choice as a cost-effective, automated solution with a price and performance particularly adjusted to this range. It provides very reliable results and, at the same time, helps to improve logistic processes.
Depending on how often the level has to be measured, such time-consuming methods quickly become no longer viable. Apart from this, regular bustling around in such a height – usually these silos have a height of up to 30 m – is rather risky. For this reason, the UWT GmbH have developed the plumb bob system Nivobob 4000 (Fig. 1) in particular for the building materials as well as the feed and grain industries. The development work was focussed on the production of a measuring instrument with all the necessary and practical functionalities for these applications at a favourable price.
The devices of the plumb bob technology basically apply a similar measuring method, i.e. they use a lowered weight fixed on a rope. Essentially the measuring instrument consists of an electromechanically driven reel, a rope or a tape and a sensing weight at the end. The instruments are mounted on the roof of the silo. When lowering the sensing weight into the container, the tape or the rope are reeled off until the sensing weight hits on the material to be monitored. The rope tension indicates if the sensing weight is still freely moving in the air or if it already contacts the material filled in. Based on the reeled-off rope or tape length, the level of the container can be calculated. When reeling up again, the revolutions are also counted. Thus, the measured value is verified and, at the same time, the sensing weight is returned to its starting position.
With this technology the measurement can be started comfortably from the bottom or from the office and exact values will be obtained. Alternatively the measured values can also be transmitted to a PC equipped with a corresponding interface card. The plumb bob technique is particularly suitable for these applications because dust inside the silo neither influences the results of measurement nor the conductivity of bulk solids. Changing moistures of the bulk solids as well as adherent material are not a problem for the measurement method either. Furthermore, the plumb bob equipment is very rugged and is distinguished by its high operational reliability and exactness.
The plumb bob is accommodated in an aluminium casing of the protective system IP66 to cope with the harsh conditions on the silo roof. Temperatures between – 40 and + 80 °C are no problem. An additional weather protection cover is a protection against climatic influences, such as rainwater, formation of condensate, excessive heating due to insolation and extreme cold in winter. So as to avoid a contamination of the electronic equipment, the technical and mechanical ranges are strictly separated from each other in the apparatus (Fig. 2). They offer an alternative with rope, which is particularly suitable for dry products, and an alternative with measuring tape for sticky products. Automatic tape cleaning is included for the latter preventing foreign bodies from penetrating into the instrument. The instruments are available for supply voltages of 230 or 115 V AC as well as 24 V DC. Measuring ranges of up to 30 m are possible. All materials from a density of 300 g/l can be measured. An optional pin on the sensing weight prevents the weight from slipping down or tilting at the steep angles of repose, e.g. with powders (Fig. 3). Due to its ATEX and FM certification the instrument may also be used in dust explosion areas. As regards the menu prompt, in addition to German, they also offer English, French, Italian and Russian.
It was also important for the experts of level measurement technology that such an instrument could be installed without any major expenditure and that it can be retrofitted even with silo roofs without sockets. A master pattern supplied facilitates the mechanical mounting. Only a few holes have to be drilled, a pre-defined range must be sawn out, the instrument has to be inserted and the screws must be tightened. Since it is not easy to align the plumb bob system exactly horizontally due to the frequently conical silo roofs, a special aiming flange (Fig. 4) has been developed and is offered for such cases. It can be adjusted between 0 ° and 50 ° and also facilitates the installation. Then only the cables have to be laid and connected for the power supply and data transmission. Finally, the individual properties of the silos can be adjusted in the device menu. The typical parameters in this case are the silo height, the cone height and the maximum reel-off length. The latter will ensure that the sensing weight will not pass into the discharge opening. A timer is also adjusted in the menu starting the measurement in certain intervals. Thanks to the easy installation no additional costs for service technicians will accrue.
Without maintenance for years
Not only the installation of the Nivobob is easy, it is also extremely easy to maintain during daily use. For reasons of safety the tape has to be replaced after 250 000 measuring cycles. With this tape version and a measuring range of 10 m it means that the instrument will operate without maintenance for years with one measurement per hour. If for certain applications the measurements are carried out in clearly shorter intervals, this is no problem either. The operator himself can replace the motor, board as well as the tape and rope reel without any difficulty.
In the meantime the Nivobob 4000 has proved a success in various pilot projects. It has been tested by operators of the agricultural, feed and cement sector and during the storage of chips in the wood industry. Now the instrument will be presented to the public at the Powtech and it will be available worldwide from the beginning of May. In the future the levels in the buildings materials, feed and grain industries can easily and precisely be determined. Since the measured data are available on the PC, the subsequent order logistics can also be simplified. It will be possible in the future to order automatically if the level has reached a certain value.
Halle 5 Stand 5-326