Q: What are the main causes of system failure on site for permanent noise monitoring?
Your noise management plan relies on them, but there are some common threats to your permanent noise monitoring installations that can significantly disrupt your data collection.
Animals, the weather and even people all represent a risk to your monitoring stations, which are usually in remote locations.
Mooing cows are loud and interfere with noise measurements, but they can also cause physical damage to your setup and the surrounding chain link fence.
Cockatoos are not only noisy, but have an impressive habit of chewing on anything exposed; whether that is a cable, a windsock, or even a bird spike.
Ants have also been known to make their way into sound level meter cavities, shorting across circuits and causing major damage to the electronics inside.
In one case NVMS helped a client where a mouse cut measurement short by severing cables in the monitoring set, including the power supply. The enclosure was full of rodent droppings and the electronic equipment was in tatters.
NVMS recommends that you install a fence around your setup, to ensure that there is no unauthorized access whether bovine or human (who often attempt to steal solar panels and batteries). Electronics need to be contained in sealed boxes that keep insects, rodents and water out.
Beyond external threats, there are other common barriers to getting the data you need: inadequate solar installations that don’t deliver enough power to properly recharge batteries, or a communications system that simply isn’t fit for purpose.
NVMS can help you specify the right systems and enclosures for your remote noise measurement to ensure it is a task you only need to do once. For existing set ups, NVMS can perform site surveys to ensure your equipment will provide reliable data.
The 2250 and 2270 sound level meters from Brüel & Kjær are industry leading in quality and usability, but did you know that you can make voice, text and image notes that are appended to measurements?
The function helps keep notes organised with your measurements as they are all saved automatically alongside the data. When downloading your sound measurement they show up as time-stamped items in B&K’s Measurement Partner Suite.
The text notes are made on the screen using the stylus, while the the voice notes are made by pressing and holding the “Voice Annotation” button at the top right of the key pad while speaking into the back of the meter. A tiny voice microphone is mounted in the back of the unit above the battery. This allows you to make voice notes even if your measurement microphone is on an extension lead some distance away.
A useful feature of the 2270 unit is the inbuilt camera that allows you to take photos during a measurement.
Text notes can be applied to a measurement, a folder or a template, allowing useful descriptions of setups as well as the measurements themselves.
Whether you’re sending your sound level meter up to site or to a calibration laboratory, shipping an instrument worth thousands of dollars can be nerve-wracking. Freight companies never guarantee safety and freight insurance covers up to only a few hundred dollars at most, so it’s important to pack instruments carefully. If in doubt, over-pack: it’s better for your instrument to arrive in safely in five layers of packing than damaged in two.
Regulations – and good measurement practice – require that you perform a ‘field calibration’ on your sound level meter or noise dose meter each time it is used.
A field calibration will detect microphone defects or any other issue in the measurement chain before you start your measurements, and will make minor adjustments to the overall sensitivity of the meter if required, to ensure accurate measurements are taken. A post-measurement check is also required, to demonstrate that instrument sensitivity was consistent during the entire period.
Using an acoustic calibrator is easy – you simply couple it to the microphone, switch on both instruments, and the calibrator applies an acoustic signal at a known frequency and known sound pressure level. Most sound level meters and dosimeters will then automatically adjust the displayed level to match the input level to ensure that subsequent measurements are reliable and trustworthy.
Field calibration does have its limitations though: it checks the operation of the meter at just one point amongst a very wide range of measurement capabilities. So, in addition to field calibration, regulations also require a full ‘periodic verification’ in an accredited lab to test your sound level meter, dose meter, calibrator or filter set across all of its settings and functions.
Periodic verification is typically required every two years (depending on applicable state legislation) and must usually be done by a NATA-accredited facility, such as NVMS’s Calibration Laboratory in Perth. Combined with routine field calibrations, this ensures the accuracy and legal validity of your measurements at all times.
Sound level meters are fragile pieces of equipment and a damaged microphone will mean your measurements are defective or worthless. To ensure your measurements are up to standard, keep the microphone in excellent condition:
Always use a windshield
Apart from reducing wind-induced ‘noise’, which is its key purpose, a windshield helps maintain your microphone by providing a degree of physical protection in the case of bumps or knocks and also reducing the likelihood of dust, dirt and moisture from reaching the fragile diaphragm – a one micron thick sheet of stainless steel.
Avoid knocks and bumps
Avoiding knocks and bumps to a sound level meter will help ensure its effectiveness. The microphone is especially fragile and can be destroyed by hitting it against the wall or dropping it on the floor. Once damaged, the diaphragm will lose frequency response or will return no data at all. For valid results, a new microphone is needed, which necessitates a new NATA-accredited calibration for the sound level meter, so a bump incident will interrupt measurements.
Keep contaminants away from the diaphragm
Dust and debris that settles on the diaphragm of the microphone can seriously affect the quality and accuracy of the measurements. You may notice unusual readings or you may observe that the protection grid is damaged or dirty. This is a good indicator that you should have the meter cleaned, checked, and if necessary, recalibrated. Note: it is strongly recommended that you do not unscrew the protection grid as it can lead to more damage to the unprotected diaphragm.
Typical microphone damage
From left to right:
1: A few specks of dust but otherwise clean and undamaged – works fine.
2: Significant dirt build-up and small crease in right hand edge of diaphragm– mic may still appear to work but high frequency response out of spec, so needs replacing.
3: Creases and a tear in the diaphgram – mic is dead. May register a level on the SLM but will be wrong as both overall sensitivity and low frequency response will be severely affected. Needs replacing.
4: Major tear in diaphragm – mic is dead. Unlikely to register anything on the SLM. Needs replacing.
Machines show symptoms when their health is compromised. With rotating machinery such as pumps, motors, turbines, compressors, fans and gearboxes these may present as changes in noise, smell, vibration, oil contamination, heat or variations in current. Of these, vibration and temperature are particularly useful for many reasons, two of which are:
- Vibration is seen in a wide number of possible faults, or failure modes – more so than any other symptom (see table below).
- Vibration and temperature are very easily converted into an analogue electrical signal which accurately describes the physical condition being monitored, whereas other symptoms are subjective, impractical to measure continuously or simply cannot be quantified objectively.
Which symptoms allow for accurate fault detection?:
A conformance check on a vibration analyser is performed to verify the accuracy of the unit under test as per the performance check from the manufacturer. Results should be +/- 1% of nominal expected value. Condition monitoring professionals test their units to give them confidence in their equipment, and to demonstrate to others that their instruments have been checked for accuracy. Instruments used for verification are calibrated and traceable to National (and international) Standards.
NVMS perform conformance tests on the GE Commtest VB range and GE Bently Nevada SCOUT units. We can also verify the associated accelerometers. Note that these are not NATA certified calibrations.
It’s a question we hear often:
Can we use a standard proximity probe on our machine? The shaft material used is [insert material here]. It’s close to AISI 4140. Can we just adjust the scale factor?
The calibration target for all standard proximity probe systems is AISI / SAE 4140.
The fact is, although the scale factor may be close, the small differences in material composition can dramatically affect linearity across the measurement range. This non-linearity could affect the diagnosis of faults, cause incorrect baseline measurements, cause false trips or (more worryingly) missed trips.
A material calibrated system has the scale factor adjusted and the new material’s linearity response is compensated for – all from the factory. By purchasing a proximity system that is calibrated for your machine’s shaft material, you can have confidence in the measurements collected.
For more information about probes calibrated for non-standard shaft material, please contact us.
Structural Testing & Data Acquisition
Reduce your installation costs
A single fibre can handle multiple measurements. With optical sensors you can reduce wiring requirements. Several optical sensors can be used on a single fibre. The optical measurement chain is individually adapted to suit your requirements.
Get high-precision test results even in difficult environments
Optical FBG sensors enable you to test the fatigue strength of your components. Stress testing can be conducted even in materials with high levels of strain, and with high numbers of load cycles. You get consistent and accurate test results even in adverse ambient conditions, such as in high-voltage systems.
Acquire reliable data whatever the distance between your optical strain gauges and the measurement system
The effects of distance and cable length on the test result are not an issue with optical strain gauges. Even if your data acquisition system is located hundreds of metres from the measuring points, the use of optical sensor technology will not negatively affect the quality of your test results.