However, present differential pressure transmitters show one disadvantage: The zero point always needs to be reset manually again and again in order to compensate the drift caused by ageing, temperature, or by system-related influences. The new sensors of the 984A and 985A series are now self-zeroing. Thus, the sensor can do without maintenance at regular intervals – an advantage that is not inconsiderable – as the removal of false ceilings or contamination caused by the maintenance technician will be avoided. Particularly for systems that require utmost hygienic or clean-room conditions or even high security in connection with access limitations, this is a decisive advantage in terms of operating costs.
The new differential pressure transmitter is available in two different designs, the 984A with a round (85 x 58 mm in diameter) or the 985A with a rectangular IP65 enclosure (81 x 83 x 60 mm). Both models use a small valve for zeroing or offset compensation done automatically at regular intervals. Their operating voltage is 24 VAC/VDC. Their three-wire output signal can be configured either for 0...10 V or for 4...20 mA (20...500 Ohm burden) with the aid of a jumper. The maximum current consumption of the LED display model is below 150 mA, while the one without display even draws less than 60 mA. These sensors are suitable for use in air or non-aggressive gases. They can be switched over between two different pressure ranges. Depending on the respective model, the measuring range is between 0...25 Pa and 0...250 kPa. By means of jumpers, one can also set the response time on site, for example, to suppress the display of short pressure surges. Also with the aid of a jumper, the user can choose between a linear or a square-rooted signal. The latter is useful for measuring volume flows. The round sensor enclosures meet the requirements of protection category IP 54, while the rectangular ones are even in conformity with IP 65.