Calibration with confidence -
the assurance of temperature accuracy
Taylor Instrument / Consumer-IndustrialProducts / Sybron Corporation
Arden, North Carolina 28704
Subject index: Calibrationmethods, general.
Most technical papers arewritten to advance knowledge in a given field, and are written primarilyto benefit the few working actively in, and are most familiar with, thatfield.
In contrast, this paper iswritten to restore to general understanding a knowledge of long-standingcalibration fundamentals that are familiar to experienced professionalsin the field, but are not generally understood by many who have a "needto know."
The task of assuring accuracyin temperature measurement is critically important. Safety or health wouldbe compromised, equipment damaged or product wasted in many processes ifthe temperature were incorrect. And no matter how precise the measurementor careful the operator, if the device is not calibrated correctly, theresult is wrong.
II. DEFINITION OF TERMS
- "Precision" in temperature measurementhas to do with detecting very small changes, and also with the abilityto repeat measurements again and again with similar results. It may ormay not imply knowledge of a correct temperature.
- "Accuracy," on the other hand,refers to a knowledge of true temperature, and implies confidences in thesimilarity of measurements in one location and another. For example, aheat sterilization temperature may be determined in the laboratory, andthen monitored in the manufacturing area. Instruments used in either orboth locations may indicate temperature to a small fraction of a degree.If either or both, however, are not correctly calibrated or are subjectto drift, there may be failure of the process because of inaccuracy. Precisionoften 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 widelyaccepted primary standards are those used to define the International PracticalTemperature Scale -- IPTS-68. (ref.1)
- "Reference" is a term used intwo ways in temperature measurement. First, it is used to describe theprocess of comparing the reading of one instrument with another -- mostcommonly the indication of an instrument being calibrated with the "known"temperature of a primary standard material or thermometer. Second, it isused 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 calibratetemperature devices under "daily use" conditions in laboratory and industry.Either way, the term "reference" refers to the comparison processby which correct calibration is assured.
III. BASIC APPROACH
- Compare -- under conditions asclose as possible to actual operation, the indication of an operating instrumentwith a "working standard" -- a master reference thermometer whose accuracyis known with very small uncertainty.
- Periodically check the accuracyof the master reference thermometer in accordance with the manufacturer'sinstructions -- either by reference to a primary standard of temperaturesuch as an ice bath, or by having the instrument re-calibrated at NISTor a respected testing laboratory.
IV. CALIBRATION FUNDAMENTALS
- Insure that conditions of installationof the sensing element approximate actual use conditions as closely aspossible. Degree of immersion, ambient temperature, shielding and housing(protective shield or other installation accessory) all may affect theheat flux around the sensing element and thus influence its calibration.Much calibration work is done by using a rapidly-agitated liquid bath asan approximation of actual use conditions. Such baths are the least expensiveway to provide a stable, uniform, easily regulated temperature transfermedium. They may or may not closely simulate actual sensing element heatflux conditions. For instance -- consider a sensing element that is ina metal-shielded housing (with a substantial heat flux through the housingto cooler surroundings and hence a higher-than-normal reading. On the otherhand, oil has much poorer heat transfer capability than water or steamdue to its insulating properties, and hence may supply less heat to maintainthe heat flux, resulting in a lower-than-normal reading.
- Insure that the equipment usedfor calibration, and the surroundings and procedures, contribute the smallesterror that is possible. This usually includes having a relatively largemass of liquid medium, agitated vigorously to insure good heat transferand 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 besophisticated or expensive. For example -- for calibration between ambientand, say, 140 degrees F, a large, insulated food/beverage container providedwith a kitchen food mixer and simple paddle for agitation, and with thetemperature controlled by manually opening and closing a hot water faucet-- can become, in the hands of a skilled operator, a precision calibrationbath useful for calibration at uncertainty levels less than 0.1 degreesC.
- The master reference thermometermust have an accuracy such that its level of uncertainty is a small fractionof the allowable calibration error desired; preferably on the order ofone to two tenths. This means that for calibration of thermometers or temperaturecontrol devices to within a maximum expected error of +/- 1 degree C, thereference thermometer should have a maximum error of no more than a fewtenths of one degree; for the calibration error to be less than +/- 0.1degree C, the reference thermometer must have a maximum error of no morethan a few hundredths of a degree, and so forth. Equally important is thelong-term stability of the master reference standard. It must be able tobe used with confidence for a practical period of time between its owncalibration checks, and with reasonable certainty that it is not subjectto short-term variations in calibration.
- The above three fundamental considerationsare all involved with the process of comparing a temperature sensing instrumentto a master reference standard thermometer, to do with the second step,that of insuring that the master reference instrument is itself continuingto be accurate. This assurance of the accuracy of the standard themometeris again done by comparison. In this instance, however, the comparisonis usually done by referencing its indications to a primary standard ornear equivalent. The most commonly-used of these are the triple point ofwater, or for most laboratory and industrial use, its near equivalent,the ice point. It is commonly understood that an ice point may have uncertaintieson the order of 0.01 deg C. However, James L Thomas of NIST, in 1939 performedan exhaustive test that indicated that with care, the ice point could berealized with an uncertainty of little more than the triple point ofwater.(ref 2) Moreover, an ice bath is so much easier toprepare and use than any of the standard fixed points that it has becomethe common choice for reference standard calibration. However, as withany procedure in high-accuracy work, care must be taken. It is thereforeappropriate to describe procedures that will insure minimum error.
V. REALIZATION OF ICE POINT
The basic steps required toinsure ice-point accuracy are:
- Insuring water purity
- Insuring minimum heat flux
- Insuring equilibirium
For most purposes, ice madefrom ordinary culinary water is sufficient. However, since most dissolvedminerals affect the freezing point, it is common to use only ice and waterthat has been demineralized. For an ice point with less than 0.01 deg Cuncertainty, only distilled water, and ice made from distilled water shouldbe used and the container should be of carefully-cleaned glass or stainlesssteel. As little as 12PPM of some salts can cause a 0.01 deg C reductionin the ice point.
To insure that the sensorbeing tested is unaffected by ambient conditions, it should be placed inthe center of a relatively large mass of ice and water (normally two litersor more), well away from the walls of the container, and the containershould be insulated to minimize melting of ice. The sensing element beingcalibrated should be immersed adequately to minimize heat transfer throughits housing (remembering the rule that calibration conditions should approximateuse conditions).
To guard against temperaturerise due to insufficient ice, and to insure against poor heat transferdue to air in the bath, the following procedure is recommended:
- Fill the container with crushedor chipped ice.
- Fill the container with waterto an overflow condition.
- Add more ice until ice is tightlypaced to bottom of container, allowing water to overflow.
- Insert sensor to be calibratedand allow temperature to reach equilibrium (normally 5 minutes or more).
- If test continues more than afew minutes, add more ice periodically, as before, insuring that ice ispacked tightly to bottom of container each time. The goal is to insurethat at all times the sensor is in contact with an ice/water mixture overits entire surface.
VI. CALIBRATION PROCEDURES
- Calibration within +/- 1.0 degC uncertainty:
- Calibration within +/- 0. 1 degC uncertainty:
- Equipment: Care must betaken in selecting and using equipment for calibration at this level ofuncertainty, since the reference thermometer, temperature controller andother items must introduce errors of no more than a few hundredths of adegree.
- Reference Standard Thermometer:One of two types of instrument is commonly used; a high-accuracy mercury/glassthermometer accompanied by a signed certificate of calibration with correctionsto the nearest 1/5 of a graduation division, or a precision platinum resistanceprobe with high-accuracy indication system, also accompanied by a NIST-traceablecalibration record. Since there is a cost difference of between 10: 1 and50: 1 between the two instruments, the mercury/glass thermometer is mostcommonly used.
- Ice bath: The same icebath can be used as described above, as long as care is taken to avoidcontamination of the water or ice. One additional piece of equipment isneeded, a 10X microscope and stand, to allow reading of the mercury/glassthermometer without parallax and to permit careful interpolation to atleast the nearest 1/5 of a graduation division.
- Temperature bath: Thereare three important criteria in good bath construction: First, that theheating/cooling elements be isolated from the test area; second, that thebath be well insulated to minimize heat transfer load and controller stabilizationneeds; and third, adequate agitation. As a rule of thumb, on all bathsexcept those at low temperatures where the medium is highly viscous, adequateagitation is insured when the liquid surface has the appearance of waterat a "rolling boil" condition. Also, in order to insure stability, mostwell-designed baths have a minimum exposed surface area. If this is notpossible, a well-insulated cover should be made to cover all but the minimumexposed surface area.
- Temperature Controller:Since the advent of solid-state electronics, vast improvement in proportioningcontrollers has come about. The best for calibration bath purposes havea visual indicator--a flickering lamp that indicates control status (offwhen temperature is above control point, on when below, and flickeringintermittently when at control point) . For control temperature below ambient,it is common to install a throttle-able refrigeration system for grosscontrol (continuous operation) and an electric heater with sensitive controllerto override for fine control. As noted under "A" ." above, manual controlcan also be used if calibration is infrequently done and the cost of anadequate proportioning action must be simulated by a variable resistanceunit that allows a varying heat input rates rather than "on-off" control.
For many uses where anuncertainty of the order of +/- 1 deg C is acceptable, thermometers andcontrollers are purchased having specifications that claim inaccuraciesno greater than that amount. The instruments are then used for extendedperiods of time without calibration -- often, in fact, until breakage ormajor malfunction occurs. If in fact, and accuracy of +/- 1 deg C is important,this is a dangerous practice, since few instruments will remain in calibrationfor 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 agingsteps that insure the necessary stability.
The simplest calibration procedurefor such instruments is to make a periodic ice point check, if 0 deg Cis included in the instrument range, and/or to compare desired readingswith that of a high-quality mercury-in-glass thermometer such as the ASTMprecision series, ASTM 62C through 70C (or F). These reference thermometershave 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 magnitudemore accureat than the instrument to be calibrated) needed for such service.
In order to insure thatroutine temperature measurements with operating instruments are accurateto within +/-0. 5 deg C to +/-1. 0 deg C, it is necessary for the instrumentitself 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 thoseabove.
The following items arerecommended:
Greatest attention will begiven 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 thermometersthat are mercury-in-glass units built to ASTM Precision-series standardsbut calibrated and certified accurate to the nearest 1/5 of the smallestscale division--with certification directly traceable to the NIST. Commentsare in two groups corresponding to the two steps of the Basic Approach;comparison of thermometer to be tested with the reading of the master referencethermometer, and when calibration check of the master reference thermometer:
(a) Comparison of thermometerto be calibrated with master reference thermometer in agitated liquid bath.This involves primarily attention to details that could influence the accuracyof results, including:
- Periodic check of temperaturebath to insure negligible temperature gradients.
- Understanding bath temperaturecontrol system and adjusting to insure negligible short-term fluctuation.
- Learning technique of takingreadings on slowly rising temperature to minimize effects of mechanicalhysteresis in the mercury/glass thermometer.
- Understanding time response andthermal lag of instruments to be sure that enough stabilization time isallowed.
- Learning the technique of interpolationof mercury/glass thermometer scales so that readings of both instrumentscan be made consistently to the nearest 1/5 (and eventually 1/10) of thesmallest graduation division.
- Checking skill of technicianand dependability of equipment by making multiple tests and by comparingone person's results with another with the same equipment.
- Assuring consistent immersionof sensor, and consistent ambient conditions that both simulate actualoperating conditions as exactly as possible (or if not possible, determininga reliable correction factor to apply to calibration results)
- Understanding the relative stabilityof each sensor to be calibrated, so that recalibration cycle is timed properly;and keeping calibration records to support timing decisions
- Taking care to properly applycalibration corrections from the calibration certification of the masterreference thermometer to the test readings.
- Insuring adequate lighting formaximum visibility.
- Taking adequate precautions toinsure against parallax errors in reading both reference and test thermometers.
(b) Calibration checkof master reference thermometer: If the master thermometer is a high-accuracymercury-in-glass unit that has been properly made and certified, this calibrationcheck is primarily a matter of making a periodic ice point check undercarefully- controlled conditions (described below); and recalculating calibrationcorrections if necessary. Normally, such a thermometer can be used fordecades without needing to be returned to the factory or laboratory forrecalibration. If a platinum resistance thermometer is used as a masterreference, it should be completely recalibrated (at least at all temperaturesneeded for use) once per year or oftener.
The continued use of mercury-in-glassthermometers for the majority of applications as master reference standardsis due to this unique feature--the face that if proper records are maintainedand procedures followed, the accuracy of the thermometer can be known withconfidence for several decades without the need for a full recalibration.This is true of few other temperature devices. The following explanationmight help understand this unique feature:
a. All measurementdevices are subject to change with time and usage. This includes the resistanceelements of platinum resistance thermometers and bridges as well as theglass of mercury-in-glass thermometers. The important criterion is to beable to measure and know the magnitude of these changes.d. To permit this simple calibrationcheck, mercury-in-glass thermometers made for use as master reference standardsinclude the following features:
b. The ideal way to know howmuch change has occurred in a device is to compare it periodically withsomething that does not change--a "primary standard."
c. This brings us to a pairof interesting phenomena that combine to provide the unique capabilityof the high-accuracy mercury-in-glass thermometer as a master referencestandard:(1) High-accuracyglass thermometers have been made for over a hundred years, and duringthat time manufacturing techniques have been developed and tested thathave been time-proved to assure a remarkable capability: That is, thatessentially all measureable change that will affect the temperature indicationwill occur in the bulb of the thermometer. It is possible, then, if thetemperature representing the freezing point of water (the ice point), isincluded within the thermometer scale, that a careful calibration checkat that one temperature will, in effect, provide a calibration check ofthe entire scale, since there will be no relative change of indicationof one part of the scale over another.
(2) It is relatively easyand inexpensive to realize the temperature of freezing water to a levelof 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 assureits accuracy over its entire temperature scale can be recalibrated indefinitelyby simply making ice point checks and applying any correction needed toall other temperatures indicated by the instrument.
(1) An auxiliary"ice point" scale if 0 deg is not included in the range.For a greaterunderstanding of thermometry practice using mercury-in-glass thermometers,refer to NBS Monograph 150 (ref 3) and for greaterunderstanding of high-accuracy thermometry using platinum resistance elements,see NBS Monograph 126. (ref 4)
(2) Unusual care in manufacturing--upto 75 or more manufacturing steps including aging and annealing operationscompared with 20 or less steps in making "laboratory" glass thermometers.
(3) An individually-graduatedscale etched into the glass surface. Each individual graduation may bespaced slightly differently than the adjacent graduation to exactly matchvariations in the glass bore diameter.
(4) A signed certificate ofcalibration resulting from a retest of the thermometer at a number of pointsthroughout the scale range, under extremely carefully controlled conditions,using a reference thermometer kept in calibration through a high-levelrecalibration and Measurement Assurance Program as described under "Calibrationwithin O .010 uncertainty" in the main body of this paper. Any correctionsnoted on the certificate should then be applied to appropriate readingsof the thermometer, with interpolation between certification points.
For a high accuracy ice pointcheck of a mercury/glass master reference thermometer, the following shouldbe observed:
-- Insuring thatonly demineralized water and ice are used, that the bath is kept full ofice and water, that precautions are taken to insure minimum heat flux andcomplete stability, as described above under "Realization of Ice Point."
-- Use a 10X microscope, carefullyaligned to insure that the microscope axis is perpendicular to the axisof the thermometer. This insures against parallax error, and allows accurateinterpolation of mercury column height to 1/10 of the smallest graduationdivision.
-- Also using a 10S or 20Xmicroscope, examine the bulb and bore of the thermometer to insure thatthere is no evidence of "air" in the bulb or mercury column, and no dropletsof mercury separated from the column.
-- Keep adequate records togradually gain confidence in the stability of the master reference thermometer.A good plan is to check the ice point at least every four months untila 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 changeof 0.2 division or more, return to four-month checks until again stabilized.
-- Whenever a careful icepoint check shows shift in calibration of more than 0.2 of a division,the calibration certificate should be amended to add the correction toall calibration values. (For mercury-in-glass thermometers only.) Thiscan be done as a result of over 100 years of experience that verifies thatessentially all change occurs in the glass bulb of the thermometer, andits magnitude is determined by the ice point check. Readings at all otherscale points will have, therefore, shifted the same amount as the ice point.
This is the level of accuracyrequired to perform initial calibration and recalibration of the masterreference thermometers described under "B ." immediately above. Since thislevel of accuracy requires a calibration uncertainty of no more than afew thousandths of one degree, unusual care must be taken. Mercury/glassthermometers cannot be used, due to their lack of resolution as well asmechanical variations. The thermometric standard commonly used is a precisionplatinum-resistance element used with a precision potentiometric bridge.Newer systems such as quartz thermometers and electronic digital indicatingdevices are available, but do not have the long-term performance recordof the platinum resistance element and bridge combination.
Actual calibration proceduresare similar to those described under "B." above except that greater careis taken at each step, and long-term experience in calibration techniquesis required to minimize errors. However, to insure the continued accuracyof the master reference thermometer used for such calibrations requiressophisticated equipment and procedures. Basically, the resistance elementas well as the precision bridge are trouble-free, extremely stable instruments.They are both, however, subject to small changes with time, and these changescan affect the output value at one portion of the range while not affectingit in other areas. This requires a regular recalibration schedule for boththe bridge and resistance elements. At this accuracy level, interlaboratorycorrelation becomes important as part of a Measurement AssuranceProgram.(ref 5)
Such a program is plannedto assure confidence that uncertainty levels of no more than a few thousandthsof a degree are maintained. Beyond that basic element, however, the programincludes a system of checks and double checks to virtually eliminate thepossibility of error due to equipment failure or operator mistake. Thisprogram includes most, or all, of the following steps:
-- Periodic (oftenerthan once per year) checks of working bridges and resistance elements againsta master bridge and element.
-- Calibration check of masterbridge by use of a standard resistor on an annual or more frequent basis.
-- Calibration check of standardresistor by independent testing agency--annually until fully stabilized,then every three to five years.
-- Round-robin interlaboratorycomparison tests of resistance elements.
-- Periodic check of bothworking systems and the master calibration standard system against primarystandards--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 offeringassurance of accuracy to a level of uncertainty of less than 0. 1 deg Ccan be developed at low cost, based on the use of carefully-made and calibratedmercury-in-glass thermometers as master reference standards. This accuracyand economy is possible because of the simplicity of high-accuracy calibrationcheck at the temperature of freezing water (the "ice point") , and theproperty of a mercury-in-glass thermometer that an ice point check insuresthat the magnitude of calibration change is known throughout the entiretemperature range of the thermometer.
- For a completediscussion of IPTS-68, see the authorized text in Metrologica 5,35 (1969). Return to text
- Thomas, JamesL., "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, Monograph150, U.S. Department of Commerce, National Bureau of Standards, January1976. 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 descriptionof considerations in an effective Measurement Assurance Program, see Furukawa,G.T., "A Measurement Assurance Program - Thermometer Calibration," unpublishedASTM Technical Talk, June 25, 1980. Available from Dr. Furukawa, U.S. Departmentof Commerce, National Bureau of Standards. Returnto text