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Vibration monitors provide much-valued data, but calibration is key

 
By Jim Timlick

 

It’s often said the biggest things come in the smallest packages.

While it’s unlikely that whoever came up the phrase had vibration monitors in mind when they first coined it, it’s an apt description of a device (some as small as a cellphone or computer tablet) that has become a ubiquitous piece of equipment on construction sites where deep foundation work is conducted.

Vibration monitors can help companies prevent serious property damage, avert project delays and avoid costly litigation. While most such monitors can cost hundreds or even thousands of dollars, one expert suggests it’s a small price to pay for the peace of mind they provide to the companies that use them.

“Vibration monitoring allows the contractor, the engineer and the construction team to really reduce their risk by understanding the effect of what they’re doing on adjacent structures,” said Kenny Campbell, director of business development for Geo-Instruments, a user and distributor of Ottawa-based Instantel’s line of vibration monitors.

“All too often we get calls from homeowners or building owners next door to a construction site, and they think their building is going to fall down. The reality is when you go out with a vibration monitor that’s just not the case and people are hypersensitive when there’s construction next door. By having that ability to have a metric or a measurement and share that with all relevant parties … you can say this is what we’re measuring and it’s not in fact as bad as you might perceive it to be.”

Most vibration monitors consist of two pieces: a base unit and a geophone. Inside the geophone are three individual transducers. Each transducer contains a coiled spring and a gravity-weighted pendulum. Once the unit starts to pick up vibrations, the pendulum begins to shake. The coiled spring then applies a magnetic current to stabilize the pendulum. The amount of current required to stabilize the pendulum is then transposed into a vibration measurement that tells the user how much vibration is occurring at a site.

Those traditional analog monitors are starting to get some competition in the form of new accelerometer-based devices that provide digital vibration measurements. While this emerging technology is still in its infancy, Campbell says it’s starting to gain traction in North America and is already fairly dominant in Europe.

Although the premise behind vibration monitors is relatively simple, Campbell says it is extremely important that individuals responsible for monitor installation know how to do so properly and in the correct location.

“It’s really important that the person installing the geophone has suitable training as to how to do it. If it’s not installed correctly it’s going to measure things inaccurately,” he said. “For example, if it’s just sitting on a concrete slab and it’s loose, it’s going to bounce up and down and completely distort the measurement. We see a lot of installations, especially in big cities, where there’s poor installation standards and poor workmanship, and it really impacts the validity of the data.”

Campbell says Geo-Instruments and a few other companies offer one-day vibration monitoring training courses that are geared to field technicians. Although Geo-Instruments recently paused the courses due to the pandemic and other factors, it hopes to begin offering them again later this year or in early 2023.

“It really educates the field engineer or installer on why they’re doing what they’re doing, how the technology works and how to confidently operate the equipment, install the equipment and what the data means at the end of the day,” he said.

Like any kind of technology, vibration monitors require regular maintenance. Perhaps the most important requirement is that many units must be regularly recalibrated. How often recalibration should occur depends largely on the manufacturer. In most cases recalibration is recommended every one or two years, although some manufacturers stipulate it should be done as often as every six months, and a few claim their devices don’t ever require recalibration.

Regardless of the type of unit being used, Campbell says it’s important that the monitor is regularly recalibrated to ensure the accuracy of the data it provides.

“A good, competent company that does this kind of work is going to want to look after its equipment because ultimately it’s out there being exposed to all kinds of weather and environments, and the last thing you need is for it to not be functioning correctly or letting in water or whatever,” he said, adding that units should be inspected if they become water-damaged or appear to be providing any kind of questionable data.

Recalibration is a relatively simple process for most vibration monitors. The process begins by placing the unit on a calibrated shake table. The table creates a known level of vibration which is then compared to the vibration level measured by the geophone. If the geophone’s reading is off, the device is then recalibrated so that its reading matches that of the calibrated table.

Campbell says it’s crucial that any inspection or recalibration be performed by the original equipment manufacturer rather than a third party to ensure it’s being done correctly.

“It’s really important when it does come time to calibrate the unit that it goes to the manufacturer and not a third-party calibration house. Not only do they calibrate it, but they test the entire system and service it and do any updates to its firmware that may be required,” he said, adding there have been several instances in which U.S. companies have been successfully sued after using third parties to recalibrate their monitors.

With construction booming in many parts of the country, it’s important that no piece of equipment a company requires is out of service for an extended period and that includes vibration monitors. The good news is that most vibration monitors manufactured in North America can be serviced and recalibrated in a week or less, says Campbell. It can take longer in the case of European-manufactured monitors because of the additional shipping time required.

Like most things, vibration monitor servicing has been impacted by the Covid-19 pandemic. Early on, most manufacturers instituted some form of sanitary isolation requirements for incoming devices which meant customers had to wait longer – in some cases weeks longer – to get their devices back. Thankfully, things have since returned to relative normal and wait times are mostly back to what they were pre-pandemic, Campbell says.

One final bit of advice from Campbell on how to ensure you get the most out of your investment in a vibration monitor: you get out of it what you put into it.

“If you look after them properly, they’ll give you the data that you need. If they’re not functioning properly or regularly calibrated, they’re pretty much useless.” 

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Piling Canada is the premier national voice for the Canadian deep foundation construction industry. Each issue is dedicated to providing readers with current and informative editorial, including project updates, company profiles, technological advancements, safety news, environmental information, HR advice, pertinent legal issues and more.