± 1000g depending on sensitivityrequirements (10pC/g is typical).Standard accelerometers are also available with velocityoutputs in either metric or imperial format,although for these configurations the required measurement range must be understood if the correct sensitivity is to be selected (0-20mm/s,0-50mm/s etc.).Since the integration function to convert the acceleration to velocity is carried out within the accelerometer,the low frequency performance is typically limited to around a few hertz. The limited complexity of the conditioning circuitry that can be included within the accelerometer combined with the integrated noise tend to be the limiting factors.For direct integration with SCADA and PLC basedsystems,most manufacturers offer direct 4-20mA outputs covering a factory set range of either acceleration or velocity vibration.This can be an extremely cost effective solution (provided the required measurement range is again well understood) since no signal conditioning unit is required to drive the transducer or process the resultingmeasurement.The resulting current loop output istypically either a peak or an RMS representation of the vibration signal and therefore signal frequency analysis isnot possible. ± 70g,which is suitable for most applications.Larger dynamic ranges can be achieved through lowering the sensitivity at the expense of signal to noise ratio, 10mV/g as an example,or utilising current output devices, which can provide Accelerometers are available in various outputconfigurations.The industry standard drive utilises a 2- wire current source interface operating at a nominal bias voltage of +12Vdc typ.,with an output sensitivity of 100mV/g.This configuration limits the dynamic range to around

The ATEX Directive

The ATEX directive 94/9/EC defines the specification requirements of equipment intended for use in potentially explosive atmospheres.Equipments supplied to meet the directive are approved by an authorized external body, the manufacturer must also maintain a quality system tomeet with the standard.Sensonics produce a wide range of ATEX approved accelerometers and eddy currentproximity probes for various intrinsically safe applications.The accelerometer is of particular concern whenoperating within a potentially explosive atmosphere because of the self-generating nature of piezoelectric devices and the high potential voltages that can be generated under shock conditions.For this reason the ATEX directive specifies complete encapsulation of the inner transducer body and limited capacitive capability within the electrical interface to minimize this effect.The inner industrial crystal construction is voltage limited through the addition of diodes,as is the electrical interface.The construction of the device and the internal features will be specific to the approved temperature range and zone of operation.

Velocity Transducer

The velocity transducer is inherently different to theaccelerometer with a conditioned velocity output.This device operates on the spring-mass-damper principle,is usually of low natural frequency and actually operates above its natural frequency.The transducing element is either a moving coil with a stationary magnet,or a stationary coil with a moving magnet.A voltage is produced in a conductor when the conductor cuts a magnetic field and the voltage is proportional to the rate at which the magnetic lines are cut.Thus,a voltage is developed across the coil,which is proportional to velocity.This type of transducer can provide sensitivities of up to20mV/mm/s and is convenient because it generates a signal without an external power supply and the signal usually does not require further amplification.The sensitivity vs.frequency response curve of a velocitypickup is limited at low frequencies by the optimum damping of the first natural frequency;at high frequencies its response is limited by the amount of motion necessary to overcome the inertia of the system,as well as by the presence of higher order natural frequencies. For practical purposes,a typical velocity pick-up is limitedto frequencies between approximately 10 and 1500 Hz. This has an advantage in certain applications where high frequency vibration (generated from steam noise for example) can saturate standard accelerometers with abuilt-in integration function.It is possible to obtain moving coil type transducerswhich operate in any axis.The high specification units tend to be only available for operation in either the vertical or horizontal plane due to the arrangement of the sprung mechanism and orientation during factory calibration.
The ATEX approved PZS4

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