4. AC motors starting and protection
6.10 Pressure switches and vacuum switches
6 - Data acquisition:
detection
6.10 Pressure switches and vacuum switches
b What is pressure?
Pressure is the result of a force applied to a surface area. If P is the pressure, F the force and S the surface area, we obtain the relation P=F/S.
The earth is surrounded by a layer of air which has a certain mass and therefore exerts a certain pressure called “Atmospheric pressure” equal to 1 bar at sea level.
Atmospheric pressure is expressed in hpa (hectopascal) or mbar.
1hP = 1mbar.
The international unit of pressure is the Pascal (Pa): 1 Pa = 1N/1m2 A more practical unit is the bar: 1bar = 105Pa = 105N/m2 = 10N/cm2 Pressure switches, vacuum switches and pressure transmitters are used to monitor, control or measure pressure or a vacuum in hydraulic or pneumatic circuits.
Pressure switches and vacuum switches convert a change in pressure into a discrete electrical signal when the displayed set-points are reached.
Their technology can be electromechanical or electronic (CFig.59).
Pressure transmitters (also called analogue sensors), which use electronic technology, convert pressure into a proportional electrical signal.
b Pressure control detectors v Principle
Electromechanical devices use the movement of a diaphragm, piston or bellows to actuate electrical contacts mechanically (CFig.60).
Telemecanique electronic pressure detectors are equipped with a piezo- resistive ceramic cell (CFig.61). The distortion caused by the pressure is transmitted to the “thick-film” resistors on the Wheatston bridge screen- printed onto the ceramic diaphragm. The variation in resistance is then processed by the built-in electronics to give a discrete signal or a signal proportional to the pressure (e.g. 4-20mA, 0-10V, etc.).
Pressure control or measurement is the result of the difference between the prevailing pressures on both sides of the element under pressure.
Depending on the pressure reference, the following terms are used:
Absolute pressure:measurement relative to a sealed value, usually vacuum.
Relative pressure:measurement in relation to atmospheric pressure.
Differential pressure:measurement of the difference between two pressures.
Note that the electrical output contacts can be:
- power, 2-pole or 3-pole contacts, for direct control of single-phase and 3-phase motors (pumps, compressors, etc.),
- standard, to control contactor coils, relays, electrovalves, PLC inputs, etc.
v Terminology (CFig.62)
• General terminology - Operating range
The interval defined by the minimum low point (LP) adjustment value and the maximum high point (HP) adjustment value for pressure switches and vacuum switches. It corresponds to the measurement range for pressure transmitters (also called analogue sensors). Note that the pressures displayed on the device are based on atmospheric pressure.
AFig. 59 Example of pressure detectors (Telemecanique),
A: XML-B electromechanical pressure switch
B: XML-F electronic pressure switch C: XML-G pressure transmitter
AFig. 60 Principle of an electromechanical detector (Telemecanique)
AFig. 61 Section through an electromechanical pressure detector
AFig. 62 Graphic illustration of commonly-used terms
a b c
6.10 Pressure switches and vacuum switches
6 - Data acquisition:
detection
- Rating
Maximum value of the operating range for pressure switches.
Minimum value of the operating range for vacuum switches.
- High set-point (HP)
The maximum pressure value, selected and set on the pressure switch or vacuum switch, at which the output will change status when the pressure rises.
- Low set-point (LP)
The minimum pressure value, selected and set on the pressure switch or vacuum switch, at which the output will change status when the pressure drops.
- Differential
The difference between the high (HP) and low (LP) set-points.
- Fixed differential devices
The low point (LP) is directly linked to the high point (HP) by the differential.
- Adjustable differential devices
The differential can be adjusted to set the low point (LP).
• Electromechanical terminology (CFig.63) - Accuracy of set-point display (CFig.63a)
The tolerance between the displayed set-point value and the actual value at which the contact is activated. For an accurate set-point (first installation of a product), use the reference of a rating device (manometer, etc.).
- Repeatability (R) (CFig.63b)
The variation in the operating point between two successive operations.
- Drift (F) (CFig.63c)
The variation in the operating point over the entire lifetime of the device.
• Electronic terminology
- Between pressure values measuredby the transmitter. It ranges from 0 bars to the pressure corresponding to the transmitter rating.
- Accuracycomprises linearity, hysteresis, repeatability and adjustment tolerances. It is expressed as a percentage of the measuring range of the pressure transmitter (% MR).
- Linearityis the greatest difference between the actual and rated curves of the transmitter (CFig.64a).
- Hysteresisis the greatest difference between the rising and falling pressure curves (CFig.64b).
- Repeatabilityis the maximum scatter band obtained by varying the pressure in specified conditions (CFig.64c).
- Adjustment tolerancesare the tolerances for zero point and sensitivity adjustment specified by the manufacturer (curve gradient of the transmitter output signal).
6
AFig. 63 Graphic illustration of electromechanical terms
AFig. 64 Graphic illustration:
a/ linearity b/ hysteresis c/ repeatability a
b
c
6.10 Pressure switches and vacuum switches
6 - Data acquisition:
detection
- Permitted maximum pressure in each cycle (Ps)
The pressure a detector can withstand in each cycle with no adverse effect on its lifetime. It is equal to at least 1.25 times the device rating.
- Permitted maximum accidental pressure
The maximum pressure, excluding pressure surges, which the detector can withstand occasionally without causing damage to the device.
- Break pressure
The pressure beyond which the detector risks developing a leak or bursting.
All these definitions of pressure are essential for choosing the right sensors for an application, in particular for ensuring they can be used in hydraulic circuits where severe transitory phenomena can occur, such as water hammer.
v Other features of presence detectors
This document has described the range of detection technologies.
Each has its own advantages and limitations.
There are other criteria to consider when choosing one. These are laid out in selection tables in the manufacturers’ catalogues. Particularly relevant ones, depending on the detectors, include:
- Electrical characteristics, - Environmental conditions, - Options and ease of use.
b Choice criteria
The paragraphs below give some examples of criteria which, though not central to the basic function, are advantageous for implementation and operation.
v Electrical characteristics
• Supply voltage, AC or DC, the range of which varies.
• 2-wire or 3-wire load switching techniques (CFig.66).
2-wire technique: the detector is powered in series with the load, so it is subject to a residual current in the open state and a voltage drop in the closed state. The output can be normally open or normally closed (NO/NC).
The maximum intensity of the switched current at the AC or DC output can be higher or lower, with or without short-circuit protection.
3-wire technique: the detector has two wires for power supply and one for transmitting the output signal (or more for products with more than one output). The output can be transistorised PNP or NPN.
Both techniques are used by many manufacturers, but it is important to pay special attention to residual currents and voltage drops at the detector terminals: low values ensure greater compatibility with all types of load.
v Environmental conditions
• Electrical:
- immunity to line interference, - immunity to radio frequencies, - immunity to electrical shocks, - immunity to electrostatic discharge.
• Thermal
Usually from -25 to +70° but can be as much as -40 to +120°C.
• Moisture/dust
Degree of protection of the enclosure (seal): e.g. IP 68 for cutting oil in machine tooling.
AFig. 66 2-wire and 3-wire connections