If environmental monitoring equipment is inaccurate, measurements taken with it are as good as
worthless. Equipment’s accuracy may not be given too much consideration when making a purchase.
The specifications may be a driving force and precision assumed, with buyers tending to rely on
manufacturer’s claims regarding accuracy.
Sound level meters that are mass produced in The Far East may be bought on the Internet for as little as
£10 but recognised makes of sound level meters are designed and built to meet exacting international
standards, for example IEC61672. The price of these latter instruments is likely to be many hundreds if
not thousands of pounds. Precision and stability are key to cost and a manufacturer’s calibration
certificate may be supplied with the sound level meter to assure the purchaser of their unit’s accuracy.
Many less costly sound level meters are sold with claims that they meet a build standard and a class of
accuracy that they patently do not, nor ever could do. For example, microphone capsules meeting Type
1 accuracy of IEC61672 will normally be priced at £500 or more, just for the capsule. The pre-amplifier
may well be an additional £200 and the meter itself £2000 to £3000.
Seismographs currently have no national or international build standards, so manufacturers build or try
to build their instruments so that they will accurately monitor in compliance with the relevant
methodology standards such as BS:7385 Part 2, 1993 and BS:5228 Part2 2009. A manufacturer’s
certificate of calibration will almost always accompany a newly purchased blast monitor.
It is however, worth remembering that all that is needed to provide a calibration certificate is a piece of
paper and a pen. An accredited calibration by an independent laboratory is a sensible choice if accuracy
is paramount. UKAS is the UK’s National Accreditation Body. It is responsible for: “determining, in the
public interest, the technical competence and integrity of organisations such as those offering testing,
calibration and certification services”. Traceability is the main criterion for ensuring accuracy of
calibration. A UKAS accredited calibration laboratory will provide a trustworthy service and provide a
calibration certificate that is traceable and a true reflection of the instrument’s accuracy. There are
many UKAS accredited laboratories offering calibration of sound level meters.
For seismographs there is no UK accredited body able to offer calibration of the velocity sensors
(geophones) that seismographs employ. There is however a laboratory in Europe that does hold
accreditation. The laboratory was in 2014 awarded ILAC accreditation to the ISO/IEC 17025 Norm. It is
the first company in the world to earn the highest level of accreditation for seismographs’ velocity
transducer calibration. It took the laboratory almost 5 years to achieve accreditation and, needless to
say, hundreds of thousands of Euros in test equipment and set up costs.
A seismograph for use in the UK must be able to measure Peak Particle Velocity (PPV) in mm/s from 4Hz
up to 100Hz. The frequency response should be as linear as possible in each of the three axes.
The picture below is from a calibration certificate showing the Vertical channel’s response of a
seismograph, calibrated in October 2107. The Vertical channel of this seismograph has an almost
perfectly linear response between 4Hz and 100Hz. To be acceptable, any deviation must be within the
upper and lower blue lines. In this example, the other two axes, Transverse and Radial, also have a very
similar linear response.
Since its accreditation in 2014 the laboratory mentioned above has calibrated many makes and models
of seismographs currently available. The seismographs from the world’s two largest manufacturers are
very accurate and meet the required specifications well. However the lab has seen a few astonishingly
inaccurate instruments from manufacturers whose specifications claim that they meet in full the
parameters necessary for accurate monitoring in compliance with methodology standards.
The results below show the Vertical response of a widely available seismograph. The blue line indicates
where the red line should be in order to achieve ideal linearity. The other two axes each showed a
Accuracy, and crucially between 4Hz and 50Hz, is essential in order to avoid damage to property or
structures. The non-linear readings made by the above seismograph will mean in practice that any
measurements made will be far lower than the true levels of vibration actually were.
In the case above, the laboratory stated that in their opinion the electronics used in the design of the
seismograph, could never enable it to achieve a linear response. Instead of using digital signal processing
it appeared to use the direct output from the 4½Hz geophones through an a to d converter, using
resistors as a means of adjustment but this design could never achieve a linear response.
In summary, if there is a suspicion that monitoring equipment is inaccurate or there is doubt about
manufacturer’s calibration techniques, an independent accredited laboratory should be used.