A pressure switch does not measure a pressure, it watches for one. It holds a contact in one state until the pressure reaches a set figure, transfers the contact, and holds it there until the pressure falls back past a second figure set by the differential adjustment. The instrument on this page does that mechanically, with a spring and a sensing element rather than electronics, in three range versions covering vacuum to 1.0 MPa.
The three versions exist because the differential available is proportional to the range. PK503 covers -0.05 to 0.3 MPa with a differential settable from 0.035 to 0.15 MPa, PK506 covers -0.05 to 0.6 MPa with 0.06 to 0.4 MPa, and PK510 covers 0.1 to 1.0 MPa with 0.1 to 0.3 MPa. All three switch an SPDT contact set, reset automatically, and are proof tested to 1.65 MPa.
How a mechanical pressure switch works
Pressure acts on a sensing element, and the element pushes against a spring. The spring force is set by the RANGE screw, so the pressure at which the element has moved far enough to transfer the contacts is adjustable at the face of the instrument. A second screw, marked DIFF, sets how far the element has to travel back before the contacts transfer the other way. That second screw is what makes the device useful: without it, a switch would chatter at its set point as the pressure wandered either side of the figure.
The contact set is single pole double throw, brought out to terminals 1, 3 and 5. At low pressure the common terminal is connected to one of the outer terminals; as the pressure rises past the set point the common transfers to the other. That means one switch can both make a circuit and break another, which is why a compressor control can use a single switch to stop the motor and light an indicator without a relay between them.
The dial carries two scales, MPa and kgf/cm², which is not decoration: refrigerant and air system data are published in both units in different places, and a switch that shows only one of them forces a conversion at the point where a mistake is easiest to make.
How to select the right pressure switch
Begin with the direction of the pressure, not the size of it. PK503 and PK506 both reach down to -0.05 MPa and can therefore be set to act on a vacuum; PK510 begins at 0.1 MPa and cannot. A duty that pulls a vacuum at any point in its cycle rules PK510 out regardless of how high the positive pressure goes, and this is the single most common way to specify the wrong version.
Then bracket the working pressure inside the range. The differential is expressed as a fraction of the range, so a version chosen well above the working pressure cannot be set finely enough to hold a tight band, and one chosen too close to the working pressure leaves no headroom for the swing the process actually has. The factory set points - OFF and ON figures for each version - are a starting point for a common duty rather than a recommendation.
Then settle the connection and the electrical side. The standard connection is PT1/4, with M12×1.25, 7/16-20NPT and a capillary available by suffix, and push-in tube fittings for 4 mm to 12 mm tube where the system is assembled from tube. On the electrical side the contact rating has to cover the load actually being switched, including the inrush of a motor or a solenoid, because the inrush figures published for the contacts are transient values and say nothing about how long the contacts will last at a given switching frequency.
Setting and adjusting the switch
The two screws interact, so they are set in an order. The differential is set first, from how much the process is allowed to swing before the switch resets, and the set point is brought to the required figure afterwards, since changing the differential moves the point at which the switch resets and therefore where the next cycle starts. A setting outside the published differential band will not hold, whatever position the screws are left in.
The factory figures are documented for each version, so a switch ordered without a stated setting arrives at those figures. If a setting is required before dispatch, give both the switching point and the differential, since supplying one without the other does not fully define the behaviour. The dial’s DIFF scale and its RANGE scale are separate, and a reading taken from the wrong one is the usual reason a field adjustment does not do what was intended.
Measurement limitations and selection considerations
The sealing is IP40, so the enclosure keeps solid objects out and does not keep water out. An outdoor location, a washdown area or a condensing plant room needs an enclosure rather than an accessory, and it is better raised at the enquiry than discovered at commissioning.
The medium is limited to refrigerant, air and non-corrosive liquids within a fluid temperature range of -10 to 100°C. A corrosive medium is not covered, and the supplied material does not state the housing or diaphragm material, so the enquiry should say what the medium is rather than assume compatibility from the general description.
A mechanical switch is also not the right answer where the pressure has to be known rather than acted on. If the control system needs the value, a transmitter produces a continuous signal and the switching can be done in the controller, where the set point can be changed without a screwdriver. Where a local indication and a switch are both wanted, a gauge and a switch are still usually cheaper and clearer than an instrument doing both.
What to state on the enquiry
The medium, the pressure range in both directions if the cycle crosses zero, the figure at which the switch should act, and the differential that the process tolerates. Then the contact function, the voltage and the load being switched, the process connection or tube size, and whether the set point has to be adjusted before dispatch. With those the first reply can be a configuration rather than a question.
Every value below is taken from source documentation. Where a parameter is not stated in the available material it is marked as information required rather than estimated.
