DJB Instruments

Instrumentation - VT2

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Calculator Size Vibration Meter Reads • Wideband/Bandwidth Limited • Acceleration, Velocity, Displacement Autoranging Digital Readout • Rechargeable Battery

Industrial plant such as rotating machinery will generate a vibration signature comprising spectral components associated with specific primary vibration sources-rotors, bearings, moving parts. These will further excite passive, static elements such that the overall spectrum, although containing information relevant to say, balance, bearing wear, electrical faults, may be difficult to interpret. Frequency selective measurement provides the means for recovering meaningful source dependant data buried in wideband signals, and to therefore diagnose the state of specific machine components.

The VT2 adds frequency selective displacement, velocity and acceleration (DVA) measurement capability to the broadband VT1 DVA level meter. Each of D, V, A may be displayed as a wideband or bandwidth limited level, the latter allowing targeting of discrete vibration sources.
A prime application of the VT2 is in the periodic monitoring of rotating/reciprocating plant. The standard VT2 provides a bandwidth limited velocity measurement characteristed to the requirements of ISO 2954-1975. This stipulates an RMS velocity measurement within the restricted bandwidth 10Hz/1kHz.

In addition the standard VT2 provides bandwidth limited displacement 10Hz/100Hz, isolating balance type (fundamental shaft frequency) problems, and acceleration 2kHz/20kHz, the latter aimed at detection of roller bearing faults of a pulse burst nature.

ISO 10816-1 provides general guidelines for instrumentation used for machine vibration measurement, specifically where the vibration spectrum includes low and high frequency components best evaluated as displacement and acceleration. A strategy applicable to the VT2 would configure bandpass D, V, and A as continuous sectors of the overall bandwidth, each parameter thus containing data relevant to specific machine components.

The VT2 is therefore offered as an application specific vibration monitor with diagnostic capability, subject of course to engineering constraints.

Format

D, V, A select is identical to the VT1, with additional keypad selectable bandpass filters for each of D, V, A, plus reading hold. Wideband/ bandpass mode is identified on the display.

Custom Filters

D, V and A filters each comprise separate 4 pole high/lowpass sections and are totally decoupled. Filter positioning within the overall VT2 bandwidth is constrained solely by physical size of filter components, noise, and signal limiting. The latter two are specific to integration (velocity, displacement conversion) and place an upper frequency limit on the lowpass section of each filter.

Application Notes

fixed transducer scaling (25pC/g nom.) plus autoranging reduces operational procedure to D, V, A and wideband/filter selection minimising error. Hold retains a snapshot of the display reading and is useful if levels are to be logged.

Spatial consistency is important in the gathering of vibration data.
Orthogonal readings allow computation √(v12+v22+v32) of absolute value.

Use of transducer probes and magnetic mountings degrade data above 1/5kHz (see P.V.A hand held meter accessory data sheet for typical A/20 + probe, magnet frequency responses). For collation of meaningful high frequency data transducers should be rigidly attached to the structure.

Instrumentation-VT2