We get these (and many other) questions frequently, but its fairly straightforward – once you break it
down. Both terms relate to maintaining Accuracy (link to Precision vs. Accuracy) of a device.
You may recall one of the major errors (link to Part 1) is Linearity – the extent a measurement device
deviates from the ideal measurement curve. Ideally, you receive a sensor calibrated to deliver
minimal linearity and other errors – or at least minimized enough to meet the specifications.
Most consider a unit “calibrated” when it meets this standard; however, a lot can happen over
time.
Many sensors (though certainly not all) may drift out of spec over time, depending on design,
component selection and site conditions. Some designs include compensation for temperature
changes, instrument voltage shifts, surges, etc. – but many do not. Receiving a unit in spec is a
good start, but what about performance over time? Should you re-calibrate the device?
Well, as always, it depends.
If you just need to confirm the process is running, then perhaps not.
If you control the process based on the sensor output, then accuracy and stability typically matter
significantly.
If you have to prove the accuracy of your measurement for example, to comply with Energy Star, or
where money changes hands, then you need sensor performance Certification, which should have
accompanied the original order.
When should you confirm a sensor’s calibration? We recommend re-calibration:
Whenever an agency or agreement requires it. EnergyStar and other Federal, as well as State
and Local programs, require recalibration and re-certification at specified intervals. The same is
true where the application has a financial transaction associated with it. Energy billing is a
common example. These are “must do’s”.
Whenever your facility requires it. Many operations define a period in which sensor
performance must be confirmed and re-calibrated, if necessary. Always follow that as a
minimum.
Whenever you conclude the measurement device is an issue. Talk to the manufacturer’s tech
support but, unless you’ve changed the set up prior to the identifying the issue, odds favor
sensor drift.
Whenever you’ve identified a surge in either the measurement current or voltage, or a
sustained temperature outside the device specified operating temperature range.
Whenever the manufacturer recommends it. The market is moving more toward “disposable”
devices so fewer manufacturers will tell you a recalibration interval in hopes you might not
notice measurement changes. If if it’s not a critical application, you may not recognize the
change – for a while. Others will recommend an interval and a few even offer recalibration
services.
Certification is a formal documentation of the sensor’s expected and frequently actual performance.
There are numerous levels (outlined below), but the key is the ability to trace the measurement,
through the calibration, back to a known standard. This makes your measurement believable and
defensible and requires the calibration facility provide documentation of its traceable path back to the
standard. National Institute of Standards and Technology (NIST) is the most common agency for US
entities, while PTB seems to be common for European entities.
Certification has multiple levels and we’ve tried to provide generic examples (below), but each
manufacturer or other entity has their own matrix. Just remember – the work required increases as you
move down the list, so the costs increase:
Certificate of Compliance (C of C) is the first level, and states the unit met its specs when it
shipped from the manufacturer. Some charge for this and some include it in their unit cost.
C of C with Documented Traceability adds traceability to NIST by listing identified calibration
equipment and report numbers on the Certificate.
C of C with Traceability, as above, including multiple “As Left” data points
C of C with Traceability, as above, but including multiple “As Found” and “As Left” data points
C of C with ANSI/NCSL Z540-1-1994 compliance certification, including multiple “As Found” &
“As Left” data points
C of C with ISO/IEC 17025:2005 compliance certification, including multiple “As Found” & “As
Left” data points
The last two are clearly defined standards, which you should see called out in the project specifications.
Your industry and location usually dictate which level you need to operate at, but drop us a line if you’re
not sure and we’ll try to help you sort it.
