Pressure measurment
Pressure transducers were used for the first time in 50 years. They were covered by patent protection until recently, which affected the availability and prices in the market. Nowadays, they are available and produced by different manufacturers.
Principles of transducer operation are based on tensometric technique. Pressure transmission from built-in membrane flow canal takes place through closed capillary system filled with mercury, NaK or oil. The capillary is finished with a second membrane with a built-in tensometer which changes linearly electrical resistance accompanied by the pressure change. The proportional resistance change is used to make measurements. For transducers with built-in amplifiers we obtain normalised final signals (0-10 VDC, 0-20 mA or 4-20 mA).
Pressure sensors function as security devices for extruders in case of dangerous pressure excess which may lead to the extruder damage. Through border signals the extruder’s driving system may be switched off once the pre-defined pressure is exceeded. For this reason, every line, for its own security, should be equipped with at least one pressure sensor.
Installation area:
- In the extruder’s barell – for further development and modernization of screws, installation of sensors along the barrel is indispensable. The obtained data may be used to define material features in specific areas of barrel’s heating. A sensor installed at the end of the extruder indicates the level of material tension.
- Screen changer – through the measurement of pressure differences before and after screen changer, we are sure when filter screen should be changed. A transducer installed at the end of the extruder allows the to plan moment of screen change (e.g. after extrusion of the necessary section length).
- Melt pump – when using pumps, pressure differences have to be measured at the inlet and outlet from the pump. The correct pressure indication prevents damaging the pump, and it helps to increase the extruder’s performance.
- Die – an optimum place to install the transducer. In the die, the transducer indicates pressure necessary to obtain a correct section. Pressure and temperature are the main parameters influencing the material flow and quality. We use both parameters to control material changes, quality, screw and barrel consumption and thermal technology changes. Pressure change has a predominant impact on final product dimensions.