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Calibration
with confidence -
the assurance of temperature accuracy
R.D. Collier
Taylor Instrument
/ Consumer-Industrial Products / Sybron Corporation
Arden, North Carolina
28704
ABSTRACT
Highly sensitive
temperature devices, particularly those with multi-digit electronic
display, give the illusion of accuracy. However, knowledge
of true temperature -- the real concern of measurement accuracy
-- is only indirectly related to sensitivity or precision.
To assure temperature accuracy, it is necessary to maintain
a temperature reference standards capability. this must include
equipment and procedures that permit calibration of operating
devices with temperature standards in a way that insures minimum
uncertainty. For most requirements the creation and maintenance
of such capability is neither expensive or difficult, but
lack of understanding often results in expense and inaccuracy.
Equipment and procedures are discussed that permit calibration
with confidence at three levels of accuracy; an uncertainty
level of +/- 1.0 degrees Celsius, +/- 0.1 degrees Celsius,
and +/- 0.01 degrees Celsius, respectively.
Subject index:
Calibration methods, general.
I. INTRODUCTION
This paper outlines
temperature instrument calibration fundamentals that apply
to "daily use" conditions in laboratory and industry. In style,
language, and content, therefore, it differs from the majority
of papers on temperature measurement.
Most technical
papers are written to advance knowledge in a given field,
and are written primarily to benefit the few working actively
in, and are most familiar with, that field.
In contrast,
this paper is written to restore to general understanding
a knowledge of long-standing calibration fundamentals that
are familiar to experienced professionals in the field,
but are not generally understood by many who have a "need
to know."
The task of assuring
accuracy in temperature measurement is critically important.
Safety or health would be compromised, equipment damaged
or product wasted in many processes if the temperature were
incorrect. And no matter how precise the measurement or
careful the operator, if the device is not calibrated correctly,
the result is wrong.
II. DEFINITION
OF TERMS
The assurance of
temperature accuracy begins with an understanding of four
key concepts: "Accuracy," "Precision," "Reference" and "Standards,"
and the relationship between these terms:
- "Precision"
in temperature measurement has to do with detecting very
small changes, and also with the ability to repeat measurements
again and again with similar results. It may or may not
imply knowledge of a correct temperature.
- "Accuracy,"
on the other hand, refers to a knowledge of true temperature,
and implies confidences in the similarity of measurements
in one location and another. For example, a heat sterilization
temperature may be determined in the laboratory, and then
monitored in the manufacturing area. Instruments used
in either or both locations may indicate temperature to
a small fraction of a degree. If either or both, however,
are not correctly calibrated or are subject to drift,
there may be failure of the process because of inaccuracy.
Precision often brings a false sense of accuracy.
- "Standards"
of temperature are the key to assurance of accuracy. Two
types are important -- primary standards and secondary,
or reference standards. The most widely accepted primary
standards are those used to define the International Practical
Temperature Scale -- IPTS-68. (ref.1)
- "Reference"
is a term used in two ways in temperature measurement.
First, it is used to describe the process of comparing
the reading of one instrument with another -- most commonly
the indication of an instrument being calibrated with
the "known" temperature of a primary standard material
or thermometer. Second, it is used as a term describing
a thermometer itself -- a "master reference thermometer"
or "Secondary reference thermometer." This is a high-accuracy
instrument -- commonly a specially-made mercury-in-glass
thermometer -- used to calibrate temperature devices under
"daily use" conditions in laboratory and industry. Either
way, the term "reference" refers to the comparison
process by which correct calibration is assured.
III. BASIC
APPROACH
Most temperature
measurement involves use of a measuring instrument of some
type, usually a thermometer. Assurance of accuracy of that
instrument involves basically a two step process:
- Compare --
under conditions as close as possible to actual operation,
the indication of an operating instrument with a "working
standard" -- a master reference thermometer whose accuracy
is known with very small uncertainty.
- Periodically
check the accuracy of the master reference thermometer
in accordance with the manufacturer's instructions --
either by reference to a primary standard of temperature
such as an ice bath, or by having the instrument re-calibrated
at NIST or a respected testing laboratory.
Details of the methods
and equipment needed to accomplish these two procedures with
confidence, depend on the level of uncertainty required. We
will consider in this paper the equipment and procedures needed
to calibrate to three uncertainty levels; an uncertainty (or
maximum expected error) of +/- 1 degrees Celsius, of +/- 0.1
degrees Celsius, and +/- 0.01 degrees Celsius. However, calibration
at all three levels involves this same basic two-step approach.
IV. CALIBRATION
FUNDAMENTALS
There are four important
fundamental considerations that are most important in assuring
good calibration procedure:
- Insure that
conditions of installation of the sensing element approximate
actual use conditions as closely as possible. Degree of
immersion, ambient temperature, shielding and housing
(protective shield or other installation accessory) all
may affect the heat flux around the sensing element and
thus influence its calibration. Much calibration work
is done by using a rapidly-agitated liquid bath as an
approximation of actual use conditions. Such baths are
the least expensive way to provide a stable, uniform,
easily regulated temperature transfer medium. They may
or may not closely simulate actual sensing element heat
flux conditions. For instance -- consider a sensing element
that is in a metal-shielded housing (with a substantial
heat flux through the housing to cooler surroundings and
hence a higher-than-normal reading. On the other hand,
oil has much poorer heat transfer capability than water
or steam due to its insulating properties, and hence may
supply less heat to maintain the heat flux, resulting
in a lower-than-normal reading.
- Insure that
the equipment used for calibration, and the surroundings
and procedures, contribute the smallest error that is
possible. This usually includes having a relatively large
mass of liquid medium, agitated vigorously to insure good
heat transfer and minimum temperature gradient; insulation
to aid in temperature stability; and a sensitive proportioning
temperature control system to minimize fluctuation. Depending
on temperature range and conditions, the equipment need
not be sophisticated or expensive. For example -- for
calibration between ambient and, say, 140 degrees F, a
large, insulated food/beverage container provided with
a kitchen food mixer and simple paddle for agitation,
and with the temperature controlled by manually opening
and closing a hot water faucet -- can become, in the hands
of a skilled operator, a precision calibration bath useful
for calibration at uncertainty levels less than 0.1 degrees
C.
- The master
reference thermometer must have an accuracy such that
its level of uncertainty is a small fraction of the allowable
calibration error desired; preferably on the order of
one to two tenths. This means that for calibration of
thermometers or temperature control devices to within
a maximum expected error of +/- 1 degree C, the reference
thermometer should have a maximum error of no more than
a few tenths of one degree; for the calibration error
to be less than +/- 0.1 degree C, the reference thermometer
must have a maximum error of no more than a few hundredths
of a degree, and so forth. Equally important is the long-term
stability of the master reference standard. It must be
able to be used with confidence for a practical period
of time between its own calibration checks, and with reasonable
certainty that it is not subject to short-term variations
in calibration.
- The above
three fundamental considerations are all involved with
the process of comparing a temperature sensing instrument
to a master reference standard thermometer, to do with
the second step, that of insuring that the master reference
instrument is itself continuing to be accurate. This assurance
of the accuracy of the standard themometer is again done
by comparison. In this instance, however, the comparison
is usually done by referencing its indications to a primary
standard or near equivalent. The most commonly-used of
these are the triple point of water, or for most laboratory
and industrial use, its near equivalent, the ice point.
It is commonly understood that an ice point may have uncertainties
on the order of 0.01 deg C. However, James L Thomas of
NIST, in 1939 performed an exhaustive test that indicated
that with care, the ice point could be realized with an
uncertainty of little more than the triple point of water.
(ref 2) Moreover, an ice bath is so
much easier to prepare and use than any of the standard
fixed points that it has become the common choice for
reference standard calibration. However, as with any procedure
in high-accuracy work, care must be taken. It is therefore
appropriate to describe procedures that will insure minimum
error.
V. REALIZATION
OF ICE POINT
The basic steps
required to insure ice-point accuracy are:
- Insuring
water purity
For most purposes,
ice made from ordinary culinary water is sufficient. However,
since most dissolved minerals affect the freezing point,
it is common to use only ice and water that has been demineralized.
For an ice point with less than 0.01 deg C uncertainty,
only distilled water, and ice made from distilled water
should be used and the container should be of carefully-cleaned
glass or stainless steel. As little as 12PPM of some salts
can cause a 0.01 deg C reduction in the ice point.
- Insuring
minimum heat flux
To insure that
the sensor being tested is unaffected by ambient conditions,
it should be placed in the center of a relatively large
mass of ice and water (normally two liters or more), well
away from the walls of the container, and the container
should be insulated to minimize melting of ice. The sensing
element being calibrated should be immersed adequately to
minimize heat transfer through its housing (remembering
the rule that calibration conditions should approximate
use conditions).
- Insuring
equilibirium
To guard against
temperature rise due to insufficient ice, and to insure
against poor heat transfer due to air in the bath, the following
procedure is recommended:
- Fill the
container with crushed or chipped ice.
- Fill the
container with water to an overflow condition.
- Add more
ice until ice is tightly paced to bottom of container,
allowing water to overflow.
- Insert
sensor to be calibrated and allow temperature to reach
equilibrium (normally 5 minutes or more).
- If test
continues more than a few minutes, add more ice periodically,
as before, insuring that ice is packed tightly to bottom
of container each time. The goal is to insure that at
all times the sensor is in contact with an ice/water
mixture over its entire surface.
VI. CALIBRATION
PROCEDURES
Application of fundamentals
discussed above to the calibration of specific temperature
sending elements will vary somewhat, depending on the level
of accuracy required. It is uneconomical and unnecessary to
take the time and care needed for extremely precise calibration,
when not required by the needs of the process being monitored,
or when the sensor has substantial built-in inaccuracy. The
important consideration is the amount of inaccuracy (or, more
properly, the level of uncertainty) that is permissible. For
convenience, we will discuss procedures for three levels of
uncertainty: +/- 1.0 deg C, +/- 0.1 deg C, and +/- 0.01 deg
C.
- Calibration
within +/- 1.0 deg C uncertainty:
For many uses
where an uncertainty of the order of +/- 1 deg C is acceptable,
thermometers and controllers are purchased having specifications
that claim inaccuracies no greater than that amount. The
instruments are then used for extended periods of time
without calibration -- often, in fact, until breakage
or major malfunction occurs. If in fact, and accuracy
of +/- 1 deg C is important, this is a dangerous practice,
since few instruments will remain in calibration for extended
periods unless specifically made for long-term stability.
Even many glass thermometers, generally accepted as "correct
unless broken," are no longer regularly made with the
expensive glass annealing and aging steps that insure
the necessary stability.
The simplest
calibration procedure for such instruments is to make
a periodic ice point check, if 0 deg C is included in
the instrument range, and/or to compare desired readings
with that of a high-quality mercury-in-glass thermometer
such as the ASTM precision series, ASTM 62C through 70C
(or F). These reference thermometers have scale graduations,
in the moderate ranges, of +/- 0.1 deg C or +/- 0.2 deg
F and hence are within the accuracy range (an order of
magnitude more accureat than the instrument to be calibrated)
needed for such service.
- Calibration
within +/- 0. 1 deg C uncertainty:
In order to
insure that routine temperature measurements with operating
instruments are accurate to within +/-0. 5 deg C to +/-1.
0 deg C, it is necessary for the instrument itself to
be calibrated to an uncertainty of no more than +/-0.
1 deg C. Since this is the accuracy range most commonly
needed in industrial use, the calibration procedures will
be described in more detail than those above.
- Equipment:
Care must be taken in selecting and using equipment
for calibration at this level of uncertainty, since
the reference thermometer, temperature controller and
other items must introduce errors of no more than a
few hundredths of a degree.
The following
items are recommended:
- Reference
Standard Thermometer: One of two types of instrument
is commonly used; a high-accuracy mercury/glass thermometer
accompanied by a signed certificate of calibration
with corrections to the nearest 1/5 of a graduation
division, or a precision platinum resistance probe
with high-accuracy indication system, also accompanied
by a NIST-traceable calibration record. Since there
is a cost difference of between 10: 1 and 50: 1 between
the two instruments, the mercury/glass thermometer
is most commonly used.
- Ice
bath: The same ice bath can be used as described
above, as long as care is taken to avoid contamination
of the water or ice. One additional piece of equipment
is needed, a 10X microscope and stand, to allow reading
of the mercury/glass thermometer without parallax
and to permit careful interpolation to at least the
nearest 1/5 of a graduation division.
- Temperature
bath: There are three important criteria in good
bath construction: First, that the heating/cooling
elements be isolated from the test area; second, that
the bath be well insulated to minimize heat transfer
load and controller stabilization needs; and third,
adequate agitation. As a rule of thumb, on all baths
except those at low temperatures where the medium
is highly viscous, adequate agitation is insured when
the liquid surface has the appearance of water at
a "rolling boil" condition. Also, in order to insure
stability, most well-designed baths have a minimum
exposed surface area. If this is not possible, a well-insulated
cover should be made to cover all but the minimum
exposed surface area.
- Temperature
Controller: Since the advent of solid-state electronics,
vast improvement in proportioning controllers has
come about. The best for calibration bath purposes
have a visual indicator--a flickering lamp that indicates
control status (off when temperature is above control
point, on when below, and flickering intermittently
when at control point) . For control temperature below
ambient, it is common to install a throttle-able refrigeration
system for gross control (continuous operation) and
an electric heater with sensitive controller to override
for fine control. As noted under "A" ." above, manual
control can also be used if calibration is infrequently
done and the cost of an adequate proportioning action
must be simulated by a variable resistance unit that
allows a varying heat input rates rather than "on-off"
control.
Procedures:
Actual calibration procedure for achievement of less than
+/-0. 1 deg C uncertainty is quite simple--still following
the "BASIC APPROACH" described at the beginning of this
paper. The major effort centers around extra precautions
taken to insure that each error and uncertainty is less
than a few hundredths of a degree, so that the sum of
all uncertainties is less than one tenth. The degree of
difficulty in achieving this result depends on the temperature.
It is not difficult--with proper equipment and training--in
the range from 0 deg C through 90 deg C, more difficult
in the range 0 deg C to -40 deg C and 90 deg C to 200
deg C, and extremely difficult outside those ranges due
to equipment limitations.
Greatest
attention will be given to procedures using the most
dependable and economical components; (a) a rapidly-agitated
liquid bath or baths for temperature comparison, and
(b) a master reference standard thermometer or set of
thermometers that are mercury-in-glass units built to
ASTM Precision-series standards but calibrated and certified
accurate to the nearest 1/5 of the smallest scale division--with
certification directly traceable to the NIST. Comments
are in two groups corresponding to the two steps of
the Basic Approach; comparison of thermometer to be
tested with the reading of the master reference thermometer,
and when calibration check of the master reference thermometer:
(a) Comparison
of thermometer to be calibrated with master reference
thermometer in agitated liquid bath. This involves primarily
attention to details that could influence the accuracy
of results, including:
- Periodic
check of temperature bath to insure negligible temperature
gradients.
- Understanding
bath temperature control system and adjusting to
insure negligible short-term fluctuation.
- Learning
technique of taking readings on slowly rising temperature
to minimize effects of mechanical hysteresis in
the mercury/glass thermometer.
- Understanding
time response and thermal lag of instruments to
be sure that enough stabilization time is allowed.
- Learning
the technique of interpolation of mercury/glass
thermometer scales so that readings of both instruments
can be made consistently to the nearest 1/5 (and
eventually 1/10) of the smallest graduation division.
- Checking
skill of technician and dependability of equipment
by making multiple tests and by comparing one person's
results with another with the same equipment.
- Assuring
consistent immersion of sensor, and consistent ambient
conditions that both simulate actual operating conditions
as exactly as possible (or if not possible, determining
a reliable correction factor to apply to calibration
results)
- Understanding
the relative stability of each sensor to be calibrated,
so that recalibration cycle is timed properly; and
keeping calibration records to support timing decisions
- Taking
care to properly apply calibration corrections from
the calibration certification of the master reference
thermometer to the test readings.
- Insuring
adequate lighting for maximum visibility.
- Taking
adequate precautions to insure against parallax
errors in reading both reference and test thermometers.
(b) Calibration
check of master reference thermometer: If the master
thermometer is a high-accuracy mercury-in-glass unit
that has been properly made and certified, this calibration
check is primarily a matter of making a periodic ice
point check under carefully- controlled conditions (described
below); and recalculating calibration corrections if
necessary. Normally, such a thermometer can be used
for decades without needing to be returned to the factory
or laboratory for recalibration. If a platinum resistance
thermometer is used as a master reference, it should
be completely recalibrated (at least at all temperatures
needed for use) once per year or oftener.
The continued
use of mercury-in-glass thermometers for the majority
of applications as master reference standards is due
to this unique feature--the face that if proper records
are maintained and procedures followed, the accuracy
of the thermometer can be known with confidence for
several decades without the need for a full recalibration.
This is true of few other temperature devices. The following
explanation might help understand this unique feature:
a.
All measurement devices are subject to change with time
and usage. This includes the resistance elements of
platinum resistance thermometers and bridges as well
as the glass of mercury-in-glass thermometers. The important
criterion is to be able to measure and know the magnitude
of these changes.
b. The
ideal way to know how much change has occurred in
a device is to compare it periodically with something
that does not change--a "primary standard."
c. This
brings us to a pair of interesting phenomena that
combine to provide the unique capability of the high-accuracy
mercury-in-glass thermometer as a master reference
standard:
(1)
High-accuracy glass thermometers have been made for
over a hundred years, and during that time manufacturing
techniques have been developed and tested that have
been time-proved to assure a remarkable capability:
That is, that essentially all measureable change that
will affect the temperature indication will occur
in the bulb of the thermometer. It is possible, then,
if the temperature representing the freezing point
of water (the ice point), is included within the thermometer
scale, that a careful calibration check at that one
temperature will, in effect, provide a calibration
check of the entire scale, since there will be no
relative change of indication of one part of the scale
over another.
(2) It
is relatively easy and inexpensive to realize the
temperature of freezing water to a level of uncertainty
of a few thousandths of a degree in any laboratory
or office. Thus, a temperature instrument that needs
only an ice point check to assure its accuracy over
its entire temperature scale can be recalibrated
indefinitely by simply making ice point checks and
applying any correction needed to all other temperatures
indicated by the instrument.
d. To permit
this simple calibration check, mercury-in-glass thermometers
made for use as master reference standards include the
following features:
(1)
An auxiliary "ice point" scale if 0 deg is not included
in the range.
(2) Unusual
care in manufacturing--up to 75 or more manufacturing
steps including aging and annealing operations compared
with 20 or less steps in making "laboratory" glass
thermometers.
(3) An
individually-graduated scale etched into the glass
surface. Each individual graduation may be spaced
slightly differently than the adjacent graduation
to exactly match variations in the glass bore diameter.
(4) A signed
certificate of calibration resulting from a retest
of the thermometer at a number of points throughout
the scale range, under extremely carefully controlled
conditions, using a reference thermometer kept in
calibration through a high-level recalibration and
Measurement Assurance Program as described under "Calibration
within O .010 uncertainty" in the main body of this
paper. Any corrections noted on the certificate should
then be applied to appropriate readings of the thermometer,
with interpolation between certification points.
For
a greater understanding of thermometry practice using mercury-in-glass
thermometers, refer to NBS Monograph 150 (ref
3) and for greater understanding of high-accuracy thermometry
using platinum resistance elements, see NBS Monograph 126.
(ref 4)
For a high
accuracy ice point check of a mercury/glass master reference
thermometer, the following should be observed:
--
Insuring that only demineralized water and ice are used,
that the bath is kept full of ice and water, that precautions
are taken to insure minimum heat flux and complete stability,
as described above under "Realization of Ice Point."
-- Use a
10X microscope, carefully aligned to insure that the
microscope axis is perpendicular to the axis of the
thermometer. This insures against parallax error, and
allows accurate interpolation of mercury column height
to 1/10 of the smallest graduation division.
-- Also using
a 10S or 20X microscope, examine the bulb and bore of
the thermometer to insure that there is no evidence
of "air" in the bulb or mercury column, and no droplets
of mercury separated from the column.
-- Keep adequate
records to gradually gain confidence in the stability
of the master reference thermometer. A good plan is
to check the ice point at least every four months until
a shift of less than 0.2 of the smallest division occurs
between checks, then extend to an annual check. However,
if annual checks show a change of 0.2 division or more,
return to four-month checks until again stabilized.
-- Whenever
a careful ice point check shows shift in calibration
of more than 0.2 of a division, the calibration certificate
should be amended to add the correction to all calibration
values. (For mercury-in-glass thermometers only.) This
can be done as a result of over 100 years of experience
that verifies that essentially all change occurs in
the glass bulb of the thermometer, and its magnitude
is determined by the ice point check. Readings at all
other scale points will have, therefore, shifted the
same amount as the ice point.
- Calibration
within +/-0.01 deg C uncertainty:
This is the
level of accuracy required to perform initial calibration
and recalibration of the master reference thermometers
described under "B ." immediately above. Since this level
of accuracy requires a calibration uncertainty of no more
than a few thousandths of one degree, unusual care must
be taken. Mercury/glass thermometers cannot be used, due
to their lack of resolution as well as mechanical variations.
The thermometric standard commonly used is a precision
platinum-resistance element used with a precision potentiometric
bridge. Newer systems such as quartz thermometers and
electronic digital indicating devices are available, but
do not have the long-term performance record of the platinum
resistance element and bridge combination.
Actual calibration
procedures are similar to those described under "B." above
except that greater care is taken at each step, and long-term
experience in calibration techniques is required to minimize
errors. However, to insure the continued accuracy of the
master reference thermometer used for such calibrations
requires sophisticated equipment and procedures. Basically,
the resistance element as well as the precision bridge
are trouble-free, extremely stable instruments. They are
both, however, subject to small changes with time, and
these changes can affect the output value at one portion
of the range while not affecting it in other areas. This
requires a regular recalibration schedule for both the
bridge and resistance elements. At this accuracy level,
interlaboratory correlation becomes important as part
of a Measurement Assurance Program.
(ref 5)
Such a program
is planned to assure confidence that uncertainty levels
of no more than a few thousandths of a degree are maintained.
Beyond that basic element, however, the program includes
a system of checks and double checks to virtually eliminate
the possibility of error due to equipment failure or operator
mistake. This program includes most, or all, of the following
steps:
--
Periodic (oftener than once per year) checks of working
bridges and resistance elements against a master bridge
and element.
-- Calibration
check of master bridge by use of a standard resistor
on an annual or more frequent basis.
-- Calibration
check of standard resistor by independent testing agency--annually
until fully stabilized, then every three to five years.
-- Round-robin
interlaboratory comparison tests of resistance elements.
-- Periodic
check of both working systems and the master calibration
standard system against primary standards--not only
the triple point of water, but other according to need,
such as:
- freezing
point of zinc,
- freezing
point of tin,
- boiling
point of oxygen.
VII. CONCLUSION
In summary, it is
possible to have confidence that temperature-measuring instruments
are accurate by following a simple two-step process: First,
comparison under controlled conditions of an operating temperature
device with a master reference standard thermometer; and second,
periodically checking the accuracy of the master thermometer
by appropriate means.
A calibration
program offering assurance of accuracy to a level of uncertainty
of less than 0. 1 deg C can be developed at low cost, based
on the use of carefully-made and calibrated mercury-in-glass
thermometers as master reference standards. This accuracy
and economy is possible because of the simplicity of high-accuracy
calibration check at the temperature of freezing water (the
"ice point") , and the property of a mercury-in-glass thermometer
that an ice point check insures that the magnitude of calibration
change is known throughout the entire temperature range
of the thermometer.
REFERENCES
- For
a complete discussion of IPTS-68, see the authorized text
in Metrologica 5, 35 (1969). Return
to text
- Thomas,
James L., "Reproducibility of the ice point," in 1941
edition of Temperature, Its Measurement and Industry,
New York, Reinhold Publishing Co., 1941. Return
to text
- J.
Wise, Monograph 150, U.S. Department of Commerce,
National Bureau of Standards, January 1976. Return
to text
- J.
Riddle, G. Furukawa and H. Plumb, Monodgraph 126,
U.S. Department of Commerce, National Bureau of Standards,
April 1973. Return to text
- For
a description of considerations in an effective Measurement
Assurance Program, see Furukawa, G.T., "A Measurement
Assurance Program - Thermometer Calibration," unpublished
ASTM Technical Talk, June 25, 1980. Available from Dr.
Furukawa, U.S. Department of Commerce, National Bureau
of Standards. Return to text
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