Why Calibration Checks Are Essential for Reliable Seismic Measurements

Your preventative or predictive maintenance program depends on sensors providing you accurate and timely data and information. Ensuring your seismic sensors are calibrated properly is key to an effective maintenance program. Checking the calibration of vibration measurement sensors is essential to obtain accurate readings. If you want to trust your seismic sensors, it helps to schedule regular calibration checks. Below, we’ll examine why it matters and how to plan your schedule.

Why Calibration Checks Matter
The rotating and reciprocating equipment in industrial settings wear with age, this means the stiffness of the machine changes with time. This change in stiffness will result in a change in vibration amplitude, so changes in vibration level are expected. However, one does not want an erroneous reading due to the sensor’s own imperfections. Fortunately, manufacturers have done an excellent job of providing robust vibration measurement sensors. Even though the sensors are of high quality, trust in their readings comes through verification. Metrix recommends verifying the calibration of your seismic sensors annually. This includes the sensors on your critical machines, as well as your portable data collector.

Understanding Seismic Sensor Calibration Checks
Seismic sensors, namely accelerometer and velocity transducers, should be checked against a known standard. The standard is usually created by a governmental entity that sets the accuracy, to which measurement devices must meet. When Metrix calibrates a Hardy Shaker, it calibrates the Shaker to the NIST (National Institute of Standards and Technology) standard. The Hardy Shaker is calibrated to the NIST standard so the device that it validates can have a calibration check sticker placed on it. The Metrix Hardy Shaker brand has been a standard in the industry for over 30 years. The Hardy Shaker can provide a known input in both amplitude and frequency to help determine if the seismic sensor is reading accurately. Usually this is done by selecting a frequency range of interest, as well as an amplitude to measure against. For example, a velocity sensor (100 mV/ips, 3.93 mV/mm/s) may have a measurement range of 0 to 1 ips (0 to 25 mm/s), with a frequency range of 5 to 1,000 Hz. The velocity sensor would be mounted onto the Hardy Shaker, then the person conducting the test would adjust the amplitude to a fixed value, usually 0.5 ips (12.5 mm/s), and then record output values for each frequency checked. Typically, 5 frequencies are checked within the frequencies of interest. In this case, 30 Hz, 60 Hz, 100 Hz, 500 Hz and 750 Hz. We usually don’t choose low and high frequency limits because of the - 3db downturn in amplitude due to sensor characteristics. At each point, with the amplitude set at 0.5 ips (12.5 mm/s), one should measure 50 mV +/- 5% for this sensor’s scale factor and generated output. If the sensor accuracy is outside this range for the selected frequencies, one must use judgement to determine if the sensor should be placed back in service. Most technicians and engineers will allow a 5% deviation and note the deviation in a report. More than a 10% deviation from what is expected, usually means the sensor should be replaced.

Regardless of plant or site size, whether you work in food and beverage or oil and gas, technicians and engineers can't afford to skip calibration checks as part of their standard maintenance schedule. In addition to consistent vibration and resonance, sensors are impacted by their environment, including temperature, humidity, dust, and electromagnetic interference.

This partly explains why the average lifespan of an accelerometer can vary so significantly. If you have sensors operating under harsh circumstances, for which they were not designed, they don’t have to be in operation for long before they start to show signs of distress. Metrix produces a full line of reliable and durable seismic vibration products that are meant to survive in harsh environments.

Impact of Calibration Checks on Sensor Performance
Sensor calibration checks help ensure a seismic vibration sensor’s measurements are accurate:
● Linear measurements: Calibration verification ensures that the sensor’s input and the output match. If the accelerometer or velocity sensor is misinterpreting the energy surrounding it, it can lead to misunderstanding about the reality of the machine’s performance.
● Sensor drift correction: Depending upon the sensor, sensor drift can occur without warning, and the initial discrepancies may not even be noticeable for quite some time. With regular calibration checks, at least annually, you can catch even the most minor errors in readings, catching sensor drift before it becomes a significant problem.
● Reliable data: Accelerometers and velocity sensors don’t just report on general vibration levels, they’re used for a much larger analysis of the machine’s performance. Being able to obtain accurate waveforms from your vibration sensors allows an analyst to interrogate the frequency content of the vibration signal. Accurate seismic sensor readings are necessary for proper waveform and spectrum interrogation. With regular calibration, you can ensure that your reports are as rich and detailed as they are reliable.

Consequences of Using Uncalibrated Sensors
Accelerometers and velocity sensors that have not had their calibration verified run a huge risk for providing inaccurate vibration measurements. The longer verification checks are not done, the more likely that the plant’s equipment will sustain significant damage. In addition to increased operating and maintenance costs, incorrect readings can impact the safety of the staff. Hazardous sites are also more likely to be fined and, depending on which organization steps in, can be financially detrimental — especially for smaller operations.

Various Methods for Checking the Calibration of Seismic Sensors
There are several calibration methods for seismic sensors:
● Factory: Factory calibration checks take place in a controlled environment, usually at the manufacturer's facility. While they'll test it to a high degree of accuracy, the logistics of shipping back the sensors don’t make it a popular option. Plus, factories can’t account for the array of mounting and environmental conditions.
● On-site calibration: On-site calibration checks use portable equipment to ensure the sensor’s accuracy. It’s important to choose the right person for this, as the job requires a high degree of skill to use the equipment and verify the sensor’s functionality.
● Comparison: The comparison method uses a calibrated reference sensor to ensure that the accelerometer or velocity sensor readings match that of a known quantity. This is typically a reliable way to check calibration, though you need to be sure that the reference sensor’s readings are accurate before testing.
● Shaker table: The shaker table will create vibrations so that you can verify the sensor’s readings. Once you know exactly how much output the shaker is generating, you can use that data to cross-check the sensor accuracy.
● Back-to-back: Back-to-back calibration links two accelerometers, subjects them to the same signals, and then calculates any difference in output. This calibration method has a specified degree of tolerance, meaning the outputs do not have to exactly match to declare the sensor as accurate.

Frequency of Calibration Checks
Calibration verification is a specific exercise that depends on many factors. Instrument type, environmental conditions, usage frequency, and industry standards can all influence your verification schedule.

If you’re checking the calibration of aerospace, automotive, pharmaceutical, oil/gas, semiconductor, or food/beverage vibration sensors, you’ll need at least annual calibration verifications to ensure that your critical machinery is operating within range. Ideally, most vibration sensors will not show major deviations from normal for at least 10 years of operation. More frequent verifications may be needed if the annual survey shows a degrading trend over time. Your verification schedule should reflect not just which instruments to test but the most critical findings from past calibrations (e.g., significant sensor drift, etc.).

As you perform calibration checks, you should document every finding for clear traceability in the event of an issue.

Accelerometer and Velocity Sensor Linearity Checks
Each accelerometer or velocity sensor has its own scale factor. Metrix has found that the best way to find out if its sensor is working properly is to break up the measurement range into increments and determine if the scale factor is within specification. For example, using an accelerometer with a 100 mV/g scale factor, and the practical measurement range of 0 to 10 g, then you’d test the output at 0.0, 2.5, 5.0, 7.5 and 10.0 g’s at a frequency of 200 Hz. The voltage obtained at each g value is recorded, and the voltage difference between points is divided by the change in g’s, in this case, 2.5 g’s. The calculated incremental scale factor should equal the scale factor of the sensor as determined by the manufacturer’s datasheet. If the scale factor measured is outside the limits of the datasheet, judgement needs to be used to determine if the vibration sensor should be replaced. Caution should be employed in using large increments to determine scale factor. For example, in the above example, we picked 4 increments, we could have just picked 0 to 10 g’s and looked at the overall scale factor. The problem with this approach is that most vibration modes are usually manifested within smaller increments. Using a large increment to determine scale factor can result in errors in the measurement, so smaller increments are suggested.

Technological Advancements for Seismic Sensor Calibration Verifications
If you’re looking for an easy and accurate way to check the calibration of your sensors, use the Metrix Hardy Shakers. One can easily verify the calibration of an accelerometer or velocity sensor using the incremental scale factor methodology described above. Verifying the calibration of your seismic sensors will give you the confidence necessary for an effective predictive or proactive maintenance program. With the right equipment, you can cut back on manual labor and analysis; while improving the uptime and output of the plant due to the confidence you have in your vibration monitoring equipment. At Metrix Vibration, we can help you find the equipment that you need, regardless of your industry, production schedule, site conditions, equipment configuration, or size.