Structural health monitoring sampling frequency is only one of three clocks: sensor sampling, data aggregation and network reporting. Choose each from the physical behavior and the decision it supports. A dashboard updated every minute can hide faster raw measurements—or conceal a measurement process that is itself too slow.
Table of Contents
Begin with the physical quantity
A slow displacement trend and a vibration waveform are different measurement tasks. The first may emphasize stable references and long-duration change; the second needs adequate frequency response and temporal resolution.
For a band-limited signal, the ideal sampling requirement is greater than twice the highest frequency being represented. Practical acquisition also needs suitable filtering and margin; a bare numerical rate is not a complete measurement design. National Instruments’ explanation of aliasing describes why frequencies outside the supported sampling range can appear as misleading lower-frequency content.
The purchasing question for deformation monitoring equipment is therefore not “what is the fastest rate?” It is “which behavior must this installed sensor and recorder preserve?”
Distinguish sampling, aggregation and reporting
A system can sample rapidly, calculate a one-minute statistic and upload every five minutes. All three intervals are valid descriptions, but they refer to different stages.
| Clock or setting | What it controls | Example consequence |
|---|---|---|
| Sensor bandwidth | Physical variation the measurement can follow | Fast changes may be attenuated before recording |
| Sampling interval | Discrete observations | A short event can be missed between samples |
| Aggregation interval | Statistic retained | An average can suppress a short peak |
| Reporting interval | Network delivery schedule | A valid result may reach the dashboard late |
| Alarm persistence | Decision logic over time | Confirmation adds delay while reducing transient triggers |
The CR6 data logger manual is an example of documentation that treats measurement, processing, storage and communication as configurable functions. The chosen instrument’s implementation determines which combinations are actually supported.
How does the interval affect detection delay?
Suppose a threshold event can begin at any point between instantaneous samples spaced 60 seconds apart. Ignoring all other delays, the next observation can occur almost immediately or almost 60 seconds later. Under a uniform arrival assumption, the average sampling wait is 30 seconds.
If the logic requires three consecutive qualifying samples, confirmation adds two further sample intervals after the first qualifying observation. Network delivery and operator response add their own delays. This example explains the timing structure; it is not a recommended alarm setting.
Aliasing has a separate signature. If a periodic movement occurs faster than the sampling rate can represent, it can appear as a slower oscillation in the record. Increasing the reporting rate after acquisition does not remove that false pattern. Check the sensor bandwidth and the acquisition filter together, then verify with an independent high-rate measurement during commissioning.
The alarm-design guide addresses thresholds and actions. Sampling design should feed that workflow with a stated observation and decision delay.
Preserve statistics that match the question
A mean value can describe a sustained state while losing the amplitude of a brief excursion. A maximum preserves an extreme but loses its duration and shape. A count of threshold exceedances differs from either statistic.
For a mining monitoring program, define which variables need raw retention, which can be summarized, and how triggered records are preserved. A high-frequency buffer can be useful when an event requires a closer look, provided its pre-event and post-event coverage is defined.
Storage and power budgets then follow from the data requirement. Reducing uploads may save communications energy without reducing sensor sampling, but the exact saving depends on the hardware and duty cycle.
An exported number should carry enough meaning that a later analyst can tell whether it is an instantaneous reading, a mean, a maximum or a delayed report. Record the rate, averaging window, units, timestamp convention and missing-data behavior in the technical resource package before the first baseline is accepted.
FAQs
Can faster uploads recover an event that was never sampled?
No. Network frequency cannot reconstruct missing physical observations. Sampling must first capture the behavior of interest, with appropriate sensor bandwidth and filtering.
Does a one-minute average preserve a one-second peak?
Usually not as a peak value. An average changes the statistic. If short peaks matter, retain suitable raw data or additional statistics such as maximum values, with their time basis.
Should every monitoring instrument use the same interval?
No. Slow deformation trends, vibration and environmental variables can require different sampling and reporting. Their timestamps and aggregation definitions still need to be compatible for interpretation.
How should triggered records be stored?
Define the pre-event and post-event buffer length, the trigger condition and the retention period. A triggered record should preserve the raw samples around the event, not only the aggregated value that crossed the threshold.
