NV100 Acoustic Mill Monitor

NVMS Mill Monitors have found their place not only listening to ball and rod mills but also to crushers and any piece of plant machinery which exhibits certain characteristic noise during operation. Examples of this include the condition of the mill's contents and the characteristic tones produced as the steel balls hit the mill lining, or the noise produced by an ore crusher when unwanted objects are introduced.

The NVMS industrial noise monitors are essentially electronic versions of the human ear, but without the inconsistency between operators. The electronics are housed in a sealed IP66 rated stainless steel enclosure and the microphone is protected by an outdoor weather proof assembly. Power is drawn from a standard 110/240V supply and the measured sound level in decibels is converted to a 4-20mA signal which can be readily accessed by DCS systems such as Scan 3000 and LCN.

At the heart of the system is a Bruel & Kjaer Sound Level Meter. This instrument is available in several versions and these complement perfectly the typical applications plant operators have in mind. A basic version of the sound level meter will provide a simple overall noise level whereas enhanced versions offer such features as octave and 1/3rd octave band filtering as well as data logging and frequency analysis. In many instances octave band or 1/3rd octave band filters are indispensable for plant monitoring, since being able to narrow down the instrument's 'hearing' sensitivity to a particular range of frequencies can allow characteristic tones to be monitored which are otherwise obscured by the general back-ground noise surrounding the equipment.

The entire solution is developed and manufactured by NVMS which means that customised solutions based upon noise monitoring can be readily available. This may include such options as a miniature computer to monitor, log, analyse and act upon results, alarm triggering, activating tape recorders... the flexibility is limited only by imagination

Just Listen...

Operators may wish to place microphones at strategic positions around the mill or specified equipment. In this case the cost of the installation can be reduced by including more than one Sound Level Meter (SLM) in the enclosure. Three versions of the NV100 are available; Models A, B and C. These correspond to enclosures containing 1,2 or 3 SLMs. This way the NV100 can be strategically located and the microphones positioned as required. Up to 50 meters of microphone extension cable can be used to position the microphones at nominated locations.

Hostile Environments...

As the microphone is typically located in a hostile environments, with mud, spray and other potential contaminants a constant hazard; NVMS have developed the NV100MT Hostile Microphone Housing. This device protects the delicate microphone and pre-amplifier from damage and corrosion whilst providing an acoustic window (Upilex) which can be readily replaced.

NV100MT Housing

In order to determine the acoustic effect of the NV100MT a series of tests were conducted. From these results a set of correction curves were established which can be viewed here:

The protective housing affects absolute levels to varying degrees across the audible spectrum.

If absolute levels are required they can be calculated from the correction values available. In most instances however this is not required as most applications require a subjective comparison between changing levels, not the actual true sound pressure level.

Specialist Installers...

NVMS can provide these solutions either directly or through 3rd parties who specialise in instrumentation installation and commissioning. Please contact us for further information.

NV100F Industrial Noise Ear - Full Noise Spectrum

Octave and One-third Octave Analysis...

The system above monitors a single octave or 1/3rd octave band of frequencies (band pass filtering) which is ideal in any situation where a price of equipment produces a defined tone under optimised operating conditions. A system we proposed recently provides a spectral output instead of a single 4-20mA signal. This is detailed as follows:

This design is ideal if you wish to monitor many frequencies quasi-simultaneously, using only ONE sound level meter. The principal difference in the system is that the output would no longer be a single 4-20mA constant current signal, it would be an ASCII STRING sent via RS-232. This 'text string' would be a comma-separated list of numbers which would be collected, trended, charted and interpreted by your plants computing infrastructure. The list represented the current SPL (Sound Pressure Level) in each frequency band. An example of a transmitted string would be:

70.2,72.4,77.0,79.9,89.3,90.3,92.9,97.7,99.2,101.0,99.4,99.1,99.7,100.3,98.6,96.6,94.8,92.8,89.9,90.3

Although your practical requirements may be for less frequencies the system can be readily configured to produce a string containing any number of frequencies. It is possible that having such a detailed signature of the noise characteristics of the equipment may provide operators with a powerful tool to optimise the running conditions.

The communication protocol provided (RS-232) is fairly arbitrary because at minimal cost you can convert this to virtually any telemetry method possible; TCP/IP for example. You would need to work with the plant DCS or Scan personnel in order to present this data in a meaningful manner (such as via Citect or as a Scan 3000 graph)