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#Tech Pressostats

(Pressure switches of the A/C system)

A Pressostat is an electromechanical safety device. They are actually switches operated by pressure which acts on a bellow or on a diaphragm and they are used for the protection of the air conditioning system; sensitive to pressure variations in the refrigerant circuit.  A/C Pressostats and they are usually installed between the condenser and the expansion valve at the high pressure side.

The purpose of the pressure switch in the refrigerant circuit of the A/C system, is to:

  • protect the circuit and the compressor from excessively high or low pressures
  • set the condensing pressure of the refrigerant by controlling the condenser’s fan speed

For these purposes, the pressure switch communicates directly with the central control unit (ECU) or the control unit of an automatic air conditioning system.

They are classified according to the type and the number of adjustments that can be made:

  • maximum pressure switch
  • minimum pressure switch
  • binary pressostat
  • trinary pressostat
  • quadrinary pressostat
  • linear pressostat

Maximum pressure switch

The maximum pressure limit switch is calibrated to the maximum allowable pressure (25 bar or 2.5 MPa), beyond which the system may be damaged. When the pressure exceeds the pressure limit, the switch disengages the compressor, by acting on the compressor’s clutch, causing the pressure to drop and it engages the clutch only when the pressure is dropped at 2 MPa (20 bar). It is installed on the HIGH PRESSURE tubing.

Minimum pressure switch

The minimum pressure switch is sensitive to a specific minimum pressure value that allows the system operation (0.22 Mpa or 2.22 bar). When the pressure is lower than the pressure limit, the switch disengages the compressor, by acting on the compressor’s clutch. It is installed on the HIGH PRESSURE tubing.

Binary pressostat (two functions) combines the functions of the minimum pressure and the maximum pressure switch.

Trinary pressostat (three functions) is equipped with 4 electric pins; two of them act on the compressor coil (binary function), while the two others, using a relay, control the condenser ventilator. Therefore the trinary pressostat performs three functions: minimum and maximum pressure switch, similar to those previously examined, and activating the condenser ventilator when the pressure level is higher than 1,5 MPa (15 bar) and turning it off when it comes down to 1,2 MPa (12 bar).

Quadrinary Pressostat (four functions) has the same functions as the trinary but it contols a condenser ventilator with low and high speed. 

Linear pressostats, replacing the quadrinary pressostats, they are used to provide continuous monitoring and intervention on the a/c circuit. They continuously monitor the pressure of the a/c circuit, informing in real time the ECU about the pressure variations, making the management of the activation thresholds more flexible.To each pressure variation corresponds a voltage signal used by the engine control unit to actuate the electric fan speed and to switch off the compressor if the pressure rises or falls more than the allowed limits (safety function). The range of use of the linear sensor varies from 3,018 bar up to 29.508 bar (intervention values ​​of GRANDE PUNTO 1.2 8v).
The above values will vary according to the type of pressure switch (they are reference values).

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