Pressure switch is a component, which compares current pressure or pressure difference on input with preset level and output returns information if input pressure is below or above this level. In comparison with mechanical switches this electronic one has higher accuracy and reliability and better long term stability. With power actuator proper connection may this switch operate as a boolean feedback regulator.
This instrument is designed for a wide application in a branch of measurement and regulation in a range of very small pressures, especially in air-conditioning, ventilation, combustion process control, appliance design, etc. Many applications are offered in a medical technology and in the laboratory measurements. This transducer is designed for measurement of the differential pressure and this fact also includes a requirement for measurement of the relative pressures. However, it is not determined for a high common pressure measurement, such as measurement on the orifice gauges in the pressure distribution systems. It is possible to manufacture switches with pressure range higher then 100kPa, but only as a gage. The medium being measured may be a non-aggressive gas. Use of the non-aggressive liquid, which is permitted in the pressures higher than 2.5 kPa, should be consulted with manufacturer.
The whole transducer, i.e. a pressure sensor, supply, compensation and amplifying circuits as well as a filter for increase of the noise immunity are placed in a small plastic box from polycarbonate, which may be installed on a panel or DIN strip. The pressure supplies are realized by the nickel-plated brass inlets with a diameter 5 mm, which are suitable for use of a small hose, possibly a quick coupling for higher pressures. In addition, the medium comes into contact with silicon, silicone rubber and a plastic substance polyetherimide.
Electric connection into the measuring circuit is realized by a sealing arrested miniature
connector type DIN 43650 - C with a cable outlet PG7, which enables to use a cable with a diameter max. 6.5 mm.
This type of instrument measures the pressure by means of a silicon diaphragm on a piezoelectric principle. Therefore this transducer attains good overload endurance, is resistant against vibrations and may work in an arbitrary position. It is an advantage for some applications to measure both the positive and negative pressure with one transducer. Electronic circuitry is realized by a surface mount technology.
Output is realized as NPN transistor with open collector. This type of output allows simply realize logic levels for direct input to regulator or connect relay for power actuator driving. On the right side of the case is LED, which indicates transistor switch-ON state. Implicitly the switch is set to transistor switch-ON with increasing pressure over preset level. Output signal can be negated by shunting jumper. Fixed hysteresis cca 1,5% from nominal range is built-in to prevent oscillations during pass over switching level. Switching level adjustment is realized by multi-turn trim-pot, which is accessible through the hole in right side, beside LED. To keep high IP protection this hole is covered by M3 screw, which must be during adjustment dismount. Switching level adjustment can be ordered directly at the manufacturer, can be adjusted experimentally or with external pressure meter, respectively by external DC voltmeter with measuring internal voltage and recalculating according formula, which is placed on the rear side of this document. Pressure switch fitted with LCD allow displaying adjusted level by press the internal button.
The switches may be supplied by a DC voltage in a range 12–36 V. Change of the supply voltage within this range has not any practical impact on the measurement accuracy. Under the cover, there are placed short-circuiting jumpers for selection of the damping time constant.
