Michael R. Sharlon |
Simplified
Calibration
of Radiometric Equipment
Michael
R. Sharlon, President
ACS Calibrations
1215
Sturgis Road
Conway, AR 72034
Tel:
501-513-9901
|
To ensure measurement accuracy, infrared radiometers must
be periodically calibrated. Simply stated, the practice
of comparing one article to another, better-defined article
is calibration. Although thermographers frequently cite
and discuss calibration, there is confusion as to its application
and meaning with respect to radiometric instruments. This
paper discusses the art and science of calibrating infrared
instruments, calibration reference sources and maintaining
records that are traceable to known standards.
Introduction: History of Calibration
A few thousand years ago, Egyptians
developed a system of traceable measurement, now known
as Metrology. In the
days of the Pharaohs and Kings, a carved granite block
measured from a Pharaoh’s forearm to the tip his
index finger plus the width of his palm was made and became
the reference for length (the Royal Cubit Master). Architects
and craftsmen in the building of the Pyramids, tombs, and
temples then compared to this reference. The result of
this measure and the traceability to the Royal Cubit Master
mandated is still one of today’s wonders. The Pyramids
were built to a total tolerance (loosely translated, error)
of +/- 0.05%. That means that for every 125 feet, the Egyptian
builders were off by less than an inch! One might note
that if the holders of the Royal Cubit Master copies did
not bring in their copy weekly for comparison to the master,
the penalty was death! (If interested in the rest of this
bite from history and to give credit where it is due, please
visit: http://www.ncsli.org/misc/cubit.cfm).
What Is Calibration Of An Instrument And What Does It Accomplish?
Calibration of an instrument is
the act of comparing that instrument’s fundamental
unit(s) of measure (that it derives) with another instrument.
This comparison instrument
is capable of an even more accurate reading of the same
measured stimulus and that has itself been compared to
an even more accurate instrument. This chain of ever tightening
comparisons is tied to a national or international source.
In the U.S., this source would be the National Institute
of Standards and Technology (NIST).
NIST uses natural phenomena in physics to establish the
most finite or repeatable unit(s) of measure. In the measure
of volt, amperage, and resistance, the reference cell (providing
an exact voltage) is one workhorse. In temperature, more
finite measurement of temperature incorporates the use
of Standard 100 ohm .0385 Platinum Resistance Detectors
(SPRTD or SRTD) in conjunction with the freezing point
of certain metals or the triple point of water (a phenomena
in nature that few elements or compounds share, the temperature/pressure
point at which distilled water is solid/liquid/gas). For
radiant energy measurements, an environmental enclosure
will be added to a contact temperature detector embedded
in a nearly perfect radiator material. These sources are
referred to as primary standards. A primary standard is
considered as accurate a measure as you are going to obtain.
When accuracies are combined to form a
total uncertainty for your instrument, the result is used
to measure a stimulus with a degree of certainty in your
measurement. This provides you with an opportunity to obtain
a measurement point capable of being repeated (when taken
under similar conditions) at another time and anywhere
on the planet. This further establishes a basis for an
expected occurrence, i.e., ice cubes will begin to thaw
at or around 32 degrees F and higher at sea level.
When discussing calibration of any instrument, the terms
accuracy, tolerance and uncertainty are often interchanged
and assumed similar. They are not. These terms can become
very detailed in explanation and simply said, are subjects
within themselves. For the purpose of this presentation,
accuracy is a statement of possible limits of error for
a given parameter of an instrument under specific conditions.
The total errors for just that instrument would be the
tolerance of the instrument. Most measuring instruments
will have several errors capable of affecting the displayed
measurement. When these errors are combined, first with
each other (those associated directly with one instrument,
tolerance) and then with other total errors of additional
instrument(s) used in conjunction with providing a displayed
value (i.e., an instrument loop), the result is a total
uncertainty of the viewed reading.
Why Calibrate?
Perhaps an example might be in order.
Let’s say
the recommended air pressure in a name brand tire is found
to cause rollovers in a model of vehicle. A subsequent
bulletin informs users of this pressure and the new recommended
pressure. At this point, what provides the user with the
assurance that the tire pressure read by a device is in
fact that pressure or that the tested pressure was in fact
the reported pressure? Taken one step further, what assures
the same user when the pressure is read on another continent
with a different instrument? The answer is a science known
as Metrology; the calibration and records system required
to provide assurance of a traceably accurate (to NIST or
similar) instrument.
Another example might be the chair you are sitting in.
Odds are that the parts used to build, cover and package
the chair were manufactured at more than one site. How
then did the back fit so well into the seat, the paint
or chrome adhere to the metal in an even process, or the
print on the corrugated packaging become so evenly applied
on what must have been quite a few containers? The answer
lies in repeatably accurate measurements from instruments
sharing a similar chain of measured accuracy.
Now then, why calibrate your radiometry? The answer to
this question lies in your use of the equipment and being
right about your viewed possible exceptions.
If you:
-
a. Provide
specific IR related temperatures
-
b. Provide Delta IR
related temperatures
-
c. Provide Images with corresponding
shades implying temperatures of the image displayed
-
d. Provide comparative IR related
data to other instrumentation
You might be a thermographer.
If you are a thermographer, you will provide the most
correct thermally related information. That can ONLY be
accomplished with calibrated instruments. Uncalibrated
instruments are sometimes good enough to provide an indication
that an object might be hot or cold. The fact that it is
could not be proven.
Proving your findings is a fundamental
reason for calibration. If your instrument is calibrated
and your customer (or
their attorney) asks: “Was the object in question
as hot or cold as you reported?” The simple answer
is yes. The more complex answer would include a degree
of tolerance.
Still not convinced? Try telling your customer that they
have a 45 C degree delta in a bolted part of a bus carrying
a few thousand amps. Then find out that they scheduled
a tear down of your reported system with contracted electricians
who report no problem found. You will hear about it. Sometimes
to the tune of a bill!
Accuracy Of Your Imagery And Calibration Standard
The basic statement of accuracy
for your imager will not (typically) include the compiled
inaccuracies of
your equipment. It will typically state the viewed temperature
will be +/- some value (often 2% of full scale reading)
up to a set temperature and some greater value above
this temperature. The calibrating instrument’s
accuracy should be at least 1/3 to 1/4 of your instrument’s
accuracy (i.e., for a 2% instrument, a standard must
be at 0.67 to 0.5% of full-scale reading). As an example,
consider:
An imager’s accuracy is
+/- 2 degrees C to 100 degrees C or +/- 2% of the reading,
whichever is greatest.
With this stated value, at say 250 degrees C, your instrument
may actually be reading 245 to 255 degrees C (+/- 5 deg
C).
When dealing with radiant temperatures, this kind of
accuracy (i.e., 0.5%) is difficult to obtain. The reason
is that Emissivity/Absorbtance (E/A), Reflectivity (R)
and Transmittance (T) tend to induce errors at or greater
than 2% of the displayed value. For this reason, primary
radiant energy standards are usually laboratory blackbodies.
These standards are designed to greatly reduce ERT errors.
R and T in these standards will be negligible and E will
be 0.995 and greater in most. These standards are themselves
set at (typically) +/- 1 to 2% or less of reading or
full scale. It is important to note that these standards
often derive their temperature value and oven/refrigeration
control from imbedded thermometry and because of this
can be calibrated to an even tighter tolerance, if required.
Calibration Certification
Certification of calibration is intended to provide
a quality and legal tool for a manufacturer, user, or
customer’s assurance of a physical values actuality.
In its simplest format, the certification is a document
intended to prove traceable and repeatable accuracy and
identity statements for instrumentation, instrumentation
holder(s) and the performers of the calibration.
Calibration Standard for Infrared Imagery
If you send your equipment in for calibration, odds
are that a blackbody will be used. These primary standards
range in cost from used at approximately $500.00 to new
at $1,500.00 and up, plus the cost of traceable certificates.
So how then can you calibrate an instrument at home or
office, say, less expensively?
One of the more common techniques is to bring distilled
water to a freezing slush or just to a boil and observe
the temperature of the vessel with your radiometry. To
do so requires:
-
1. Assurance
that your vessel is large enough to ensure that your
radiometry’s
spot size is less than the observed surface.
-
2. That the vessel surface observed by your radiometry
is flat and perpendicular
to the observed plane.
-
3. That you can reasonably determine the vessel surface
E (i.e., painting it with a high temperature paint) and
exercising your Level II skills.
-
4. That lighting and other heat sources are minimally
(RT) impacting.
-
5. That you use contact traceable thermometry to ensure
the expected reading
is at or close to approx. 0 or 100 degrees C. Calibrated (and traceable)
thermometry is not that expensive. New and used equipment may be
obtained for under a hundred dollars and calibrated for $50.00 and up
(typically and annually).
1. When heating your
vessel, apply heat such that there is a minimal
chance of convected heat rising along the vessel surface,
between the
vessel and the imager.
2. Reference 5 provides
a simple table depicting temperature correction
for a change in altitude/barometer (pressure). Freezing and boiling
water correlating with 0 and 100 degrees C occurs at sea level (29.92
inches of mercury).
Maintain your records much as you do when maintaining
your reports to your clients. You will need to exercise
repeatability between obtaining your annual readings
so consider the where, when and how when performing your
calibration.
How Often Should The Instrument Be Calibrated Or Validated
Typically IR instruments are calibrated or verified
as, “in cal” annually. Simple single point
validation of your radiometry is often performed prior
to performing a scan. Validation is a self-assurance
concept and typically does not require the time consumed
in calibration or multipoint cal verification. One of
the simplest “slap on the door” validators
can be calibrated, and used daily. It costs about $25.00
(uncalibrated). It is Omni Controls model WD-35625-55
(see enclosure 2).
Cost to Calibrate
The cost to calibrate radiometry
by using a third party, like a calibrations lab or
manufacturer’s calibration
lab is broad at best. Manufacturers will typically be
the only source for correction of radiometry found out
of calibration. This is due to the use of proprietary
software in the radiometry. Calibrations labs outside
of the manufacturer of your radiometry’s loop will
only be able to provide traceable proof that your radiometry
is within manufacturer’s or your specifications.
If you intend to spend the extra dollars to obtain your
own blackbody (highly recommended if you practice Level
II thermography), some sources have been listed in Enclosure
2.
The most comprehensive description of traceability and
an excellent source for key terms in calibration are
provided by NIST on the internet at http://ts.nist.gov/traceability/ and http://ts.nist.gov/traceability/suppl_matls_for_nist_policy_rev.htm
References
1. ANSI/NCSL Z540-2-1997 American National Standard
for Expressing Uncertainty, US Guide to the Expression
of Uncertainty in Measurement
2. NCSL RP-1 January 1996 National Conference of Standards
Laboratories Recommended Practices for Establishment
and Adjustment of Calibration Intervals
3. NCSL RP-3 January
11, 1990 National Conference of Standards Laboratories
Recommended
Practices for Calibration
Procedures for Measuring & Test Equipment, Measurement
Standards, and Measurement/Test Systems
4. Website to the National Institute of Standards and
Technology: http://www.nist.gov 5. Infraspection Institute’s
Level II Certified Infrared Thermographer Reference
Manual
|
