Subject index: Calibration
methods, general.
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.
The basic steps required to insure ice-point accuracy are:
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.
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.
The following items are
recommended:
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:
(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.d. To permit this simple calibration check, mercury-in-glass thermometers made for use as master reference standards include the following features: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.
(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 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.
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.
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.