Performance Specifications of Vibration Monitors

This article is part 3 of a series dealing with data integrity. The first part discussed the importance of calibration in insuring data integrity, while the second part dealt with the importance of mounting sensors properly. This article will discuss the performance specifications of vibration monitors – after all, if the monitor you have is incapable of properly recording what you are trying to measure, there is no point in utilising it. There are lots of vibration monitors on the market these days.

Which one should be used for your project?

As with most products these days, there is a range of capabilities and a range of prices for ground vibration monitors. While all these monitor types have their place in the monitoring world, not all are suited to your project. So, how do you make this choice? To answer this question, you need to understand what you are trying to achieve with the monitor. There are 2 basic reasons for monitoring:

1. Compliance monitoring, and

2. Diagnostic monitoring.

Compliance monitoring:

Unfortunately, we must accept that almost anything we do in the mining, quarrying, construction, and infrastructure industries will cause disruption to people and their assets. When this happens, as we know, people can react in all kinds of expected and unexpected ways. Our job is to manage these impacts in consistent and responsible ways. While we have a job to do in all cases, we must remember that we are working in a complex environment. The most common impacts we must manage are:

• Impact on humans

• Impact on structures

• Impact on the environment

Impact on humans:

Human comfort levels are usually measured as VDV (vibration dose value) levels. VDV is a measure of exposure to vibration transmitted through floors (and other structures) during a working day – it is a cumulative, weighted (by frequency) acceleration measurement over an 8 hr period. The standard generally utilised in Australia is the NSW EPA “Assessing Vibration: a technical guideline of February 2006, but there is also BS 6472.

Impact on structures:

Structural damage from vibration is not well understood, but there are standards that attempt to manage structural impacts. Probably the most common one is DIN 4150 and this is widely used throughout Australia and New Zealand. However, because it is such a contested concept, if you are working in close proximity to major assets, you should consider contracting a competent structural engineer to provide advice. When monitoring is required, it is most likely that acceleration measurements will be called for.

Impact on the environment:

Perhaps because the requirement for this type of monitoring is almost universal, its importance and complexity are often overlooked. It is, of course, an area covered by many standards and perhaps even more expectations. Throughout Australia and New Zealand the most common standards to be imposed are AS 2187.2 – 2006 and ISEE Performance Specifications For Blasting Seismographs 2017, but DIN and BS standards are sometimes imposed. It is within this area of environmental monitoring that a new monitor type is having an impact. We call the use of these monitor types indicative vibration monitoring. These monitors are usually IoT type devices, or similar. Used in the right context, these monitors can be very effective. They tend to be less expensive and that means that more of them can be deployed around a site to provide a wider coverage.

Legal constraints:

However, the most important thing to remember about compliance monitoring is that these measurements may be required to withstand careful analysis in a court environment. Hence the specifications for these types of monitors are strictly defined by standards such as AS 2187.2 – 2006, ISEE Performance Specifications For Blasting Seismographs 2017, etc. A feature of this type of monitor is that it should have the capability of measuring air overpressure in addition to ground vibration. This permits its use for blasting events. Some of the more important specifications are:

• Sampling rate of monitor 1,000 samples per second, or greater
• Frequency range of vibration sensor 2 to 250 Hz
• Frequency range of microphone (if required) 2 to 250 Hz
• The monitor must be capable of long sample periods to ensure the capture of the full waveform from an event (e.g. in a blast there can be several seconds between the arrival of the ground vibration signal and the arrival of the air overpressure signal).
• The mounting of the vibration sensor is defined.
• The frequency and traceability of calibration results for the monitor and sensors are defined.
• If structural damage and human comfort are being monitored, the monitor must be able to use appropriate accelerometers and VDV sensors.

Diagnostic monitoring:

The critical specification for this type of monitor is that it sample much faster than one used for compliance or indicative monitoring. For example, a typical compliance monitor will take about 1,000 samples per second, but a minimum requirement for a diagnostic monitor would be at least 10,000 samples per second. This is because diagnostic monitoring must be done close to the source (e.g. the blast) where vibration frequencies are much higher. The higher the vibration frequency, the faster the monitor must sample to capture a meaningful signal. Most monitor types used for environmental purposes cannot sample quickly enough for this application. This topic is covered in more detail in our blog “Near-Field Blast Monitoring”. As you would expect, there is overlap across these broad categories. For example, in some circumstances, the diagnostic monitor can function well as a compliance monitor and some compliance monitors can function across all the impact types outlined above (with a change in sensor type). But, as a general rule, choose the monitor type best suited to your objective, remembering that how you think about this selection will determine the utility of the data you collect.

A word of caution on the indicative monitor type:

These monitors are less expensive because they sample quite slowly, but also because they utilise new style accelerometers rather than geophones. Geophones remain quite expensive, but these days accelerometers are being produced extremely cheaply (e.g. the type used in your mobile phone), and while they are useful, they do have limitations. Fairly obviously, accelerometers measure acceleration (i.e. mm/s²), but vibration standards use velocity, as measured by geophones (i.e. mm/s). To demonstrate compliance to a standard, results must be reported as velocity. To change acceleration into velocity, the monitor software must integrate the acceleration results. However, at low frequencies (those commonly experienced in an environmental context), by definition, the change in velocity over time (i.e. acceleration) is very small, so integrated results will often be so low that they blend into background noise levels. To make matters worse, powered sensors (e.g. accelerometers) have higher background noise levels than un-powered sensors (e.g. geophones) – i.e. it is even easier to “lose” signals in the background noise. Consequentially, while these sensors may “technically” meet the requirements set by the various standards, practically, they do not. This means that reported levels will not be accurate at the lower frequencies. In a dispute, these results would not be acceptable. Of course, there is a way to avoid the decision of which monitor type to use – you could utilise a managed service package for your project (such as Guardian or 360 by Texcel) – this means that your service provider makes these decisions. But for those who prefer to own this equipment or those who want to understand what the service provider is doing, it’s worth considering some of the details that we have discussed above. If you have concerns or just want an opportunity to discuss your options, please give us a call on +61 7 3237 8111.