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Mechanical pressure switches

Definitions

Definitions

Switching differential

The switching differential (hysteresis) is the difference in pressure between the

switching point (SP)

and the

reset point (RSP) of a pressure switch. Switching differential tolerances occur due to

tolerances in the microswitches, springs and pressure bellows. Therefore the data in the product

summaries always refers to average values. In the case of limiter functions the switching differential

has no significance, as one is only interested in the switching point at which cutoff occurs, not the

reset point. For a

controller function, i. e. in the case of pressure switches used to switch a burner,

pump etc.

on and off, a pressure switch with an adjustable switching differential should be

chosen. The switching frequency of the burner or pump can be varied by changing the switching

differential.

Adjustable switching differential/ calibration

In the case of pressure switches with adjustable switching differential, the hysteresis can be set within

the specified limits. The switching point (SP)

and reset point (RSP) are precisely definable. When

setting the pressure switch, the switching differential situation and the type of factory calibration must

be taken into account. Some pressure switches (e.g. minimum pressure monitors of the DCM series)

are calibrated under "falling" pressure, i.e. switching under falling pressure takes place at the scale

value with the switching differential being above it. The device switches back at scale value +

switching differential. If the pressure switch is calibrated under rising pressure, switching takes place

at the scale value and the device switches back at scale value - switching differential (see direction of

action). The calibration method is indicated in the data sheets.

Direction of action

In principle, any pressure switch can be used for both maximum pressure and minimum pressure

monitoring. This excludes pressure limiters, whose direction of action (maximum or minimum) is

predefined. The only thing to remember is that the scale reading may deviate by the amount of the

switching differential. See example at bottom left: The scale value is 2.8 bar.

Maximum pressure monitoring

With rising pressure, switching takes place once the preset switching pressure is reached (SP).

The reset point (RSP) is lower by the amount of the switching differential.

Minimum pressure monitoring

With falling pressure, switching takes place once the preset switching pressure is reached (SP).

The reset point (RSP) is higher by the amount of the switching differential.

Direction of action in vacuum range

It is particularly important to define the direction of action in the vacuum range.

Rising does not mean a rising vacuum, but rising pressure (as viewed from absolute "0"). "Falling"

pressure means a rising vacuum.

For example: Vacuum switch set to -0.6 bar falling means: Switching (SP) takes place under falling

pressure (rising vacuum) at -0.6 bar. The reset point is higher by the amount of the switching

differential (e.g. at -0.55 bar).

Setting a pressure switch

To define the switching point of a pressure switch exactly, it is necessary to determine the direction of

action in addition to the pressure. "Rising" means that switching takes place at the set value when the

pressure rises.

The reset point is then lower by the amount of the switching differential. "Falling" means exactly the

opposite.

Please note when specifying the setting of a pressure switch:

In addition to the switching point it is also necessary to specify the direction of action (falling or rising).

Example for selection of a pressure switch:

A pump is to be turned on at 2.8 bar and off again at 4.2 bar.

Chosen type: DCMV6 according to data sheet DCM. Setting: Scale pointer to 2.8 bar (lower switching

point). Switching differential to 1.4 bar (set according to pressure gauge).

Cutoff point: 2.8 bar +1.4 bar = 4.2 bar.

Maximum pressure monitoring

RSP = SP – xd

Minimum pressure monitoring

RSP = SP + xd