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Calibration is the validation of specific measurement techniques and equipment. At the simplest level, calibration is a comparison between measurements-one of known magnitude or correctness made or set with one device and another measurement made in as similar a way as possible with a second device.

The device with the known or assigned correctness is called the standard. The second device is the unit under test (UUT), test instrument (TI), or any of several other names for the device being calibrated.

Contents

History

Many of the earliest measuring devices were intuitive and easy to conceptually validate. The term "calibration" probably was first associated with the precise division of linear distance and angles using a dividing engine and the measurement of gravitational mass using a weighing scale. These two forms of measurement alone and their direct derivatives supported nearly all commerce and technology development from the earliest civilizations until about 1800AD.

The Industrial Revolution introduced wide scale use of indirect measurement. The measurement of pressure was an early example of how indirect measurement was added to the existing direct measurement of the same phenomena.

Direct reading design
Indirect reading design from front
Indirect reading design from rear, showing Bourdon tube

Before the Industrial Revolution, the most common pressure measurement device was a hydrostatic manometer, which is not practical for measuring high pressure. Eugene Bourdon filled the need for high pressure measurement with his Bourdon tube pressure gage.

In the direct reading hydrostatic manometer design on the left, unknown pressure pushes the liquid down the left side of the manometer U-tube (or unknown vacuum pulls the liquid up the tube, as shown) where a length scale next to the tube measures the pressure, referenced to the other, open end of the manometer on the right side of the U-tube. The resulting height difference "H" is a direct measurement of the pressure or vacuum with respect to atmospheric pressure. The absence of pressure or vacuum would make H=0. The self-applied calibration would only require the length scale to be set to zero at that same point.

In a Bourdon tube shown in the two views on the right, applied pressure entering from the bottom on the silver barbed pipe tries to straighten a curved tube (or vacuum tries to curl the tube to a greater extent), moving the free end of the tube that is mechanically connected to the pointer. This is indirect measurement that depends on calibration to read pressure or vacuum correctly. No self-calibration is possible, but generally the zero pressure state is correctable by the user.

Even in recent times, direct measurement is used to increase confidence in the validity of the measurements.

In the early days of US automobile use, people wanted to see the gasoline they were about to buy in a big glass pitcher, a direct measure of volume and quality via appearance. By 1930, rotary flowmeters were accepted as indirect substitutes. Visible in the picture above on the right (above the price digit register) is a hemispheric viewing window where consumers could see the blade of the flowmeter turn as the gasoline was pumped. By 1970, the windows were gone and the measurement was totally indirect.

Indirect measurement always involve linkages or conversions of some kind. It is seldom possible to intuitively monitor the measurement. These facts intensify the need for calibration.

Most measurement techniques used today are indirect.

Calibration versus Metrology

There is no consistent demarcation between calibration and metrology. Generally, the basic process below would be metrology-centered if it involved new or unfamiliar equipment and processes. For example, a calibration laboratory owned by a successful maker of microphones would have to be proficient in electronic distortion and sound pressure measurement. For them, the calibration of a new frequency spectrum analyzer is a routine matter with extensive precedent. On the other hand, a similar laboratory supporting a coaxial cable manufacturer may not be as familiar with this specific calibration subject. A transplanted calibration process that worked well to support the microphone application may or may not be the best answer or even adequate for the coaxial cable application. A prior understanding the measurement requirements of coaxial cable manufacturing would make the calibration process below more successful.

Basic calibration process

The calibration process begins with the design of the measuring instrument that needs to be calibrated. The design has to be able to "hold a calibration" through its calibration interval. In other words, the design has to be capable of measurements that are "within engineering tolerance" when used within the stated environmental conditions over some reasonable period of time. Having a design with these characteristics increases the likelihood of the actual measuring instruments performing as expected.

The exact mechanism for assigning tolerance values varies by country and industry type. The measuring equipment manufacturer generally assigns the measurement tolerance, suggests a calibration interval and specifies the environmental range of use and storage. The using organization generally assigns the actual calibration interval, which is dependent on this specific measuring equipment's likely usage level. A very common interval in the United States for 8–12 hours of use 5 days per week is six months. That same instrument in 24/7 usage would generally get a shorter interval. The assignment of calibration intervals can be a formal process based on the results of previous calibrations.

The next step is defining the calibration process. The selection of a standard or standards is the most visible part of the calibration process. Ideally, the standard has less than 1/4 of the measurement uncertainty of the device being calibrated. When this goal is met, the accumulated measurement uncertainty of all of the standards involved is considered to be insignificant when the final measurement is also made with the 4:1 ratio. This ratio was probably first formalized in Handbook 52 that accompanied MIL-STD-45662A, an early US Department of Defense metrology program specification. It was 10:1 from its inception in the 1950s until the 1970s, when advancing technology made 10:1 impossible for most electronic measurements.

Maintaining a 4:1 accuracy ratio with modern equipment is difficult. The test equipment being calibrated can be just as accurate as the working standard. If the accuracy ratio is less than 4:1, then the calibration tolerance can be reduced to compensate. When 1:1 is reached, only an exact match between the standard and the device being calibrated is a completely correct calibration. Another common method for dealing with this capability mismatch is to reduce the accuracy of the device being calibrated.

For example, a gage with 3% manufacturer-stated accuracy can be changed to 4% so that a 1% accuracy standard can be used at 4:1. If the gage is used in an application requiring 16% accuracy, having the gage accuracy reduced to 4% will not affect the accuracy of the final measurements. This is called a limited calibration. But if the final measurement requires 10% accuracy, then the 3% gage never can be better than 3.3:1. Then perhaps adjusting the calibration tolerance for the gage would be a better solution. If the calibration is performed at 100 units, the 1% standard would actually be anywhere between 99 and 101 units. The acceptable values of calibrations where the test equipment is at the 4:1 ratio would be 96 to 104 units, inclusive. Changing the acceptable range to 97 to 103 units would remove the potential contribution of all of the standards and preserve a 3.3:1 ratio. Continuing, a further change to the acceptable range to 98 to 102 restores more than a 4:1 final ratio.

This is a simplified example. The mathematics of the example can be challenged. It is important that whatever thinking guided this process in an actual calibration be recorded and accessible. Informality contributes to tolerance stacks and other difficult to diagnose post calibration problems.

Also in the example above, ideally the calibration value of 100 units would be the best point in the gage's range to perform a single-point calibration. It may be the manufacturer's recommendation or it may be the way similar devices are already being calibrated. Multiple point calibrations are also used. Depending on the device, a zero unit state, the absence of the phenomenon being measured, may also be a calibration point. Or zero may be resettable by the user-there are several variations possible. Again, the points to use during calibration should be recorded.

There may be specific connection techniques between the standard and the device being calibrated that may influence the calibration. For example, in electronic calibrations involving analog phenomena, the impedance of the cable connections can directly influence the result.

All of the information above is collected in a calibration procedure, which is a specific test method. These procedures capture all of the steps needed to perform a successful calibration. The manufacturer may provide one or the organization may prepare one that also captures all of the organization's other requirements. There are clearinghouses for calibration procedures such as the Government-Industry Data Exchange Program (GIDEP) in the United States.

This exact process is repeated for each of the standards used until transfer standards, certified reference materials and/or natural physical constants, the measurement standards with the least uncertainty in the laboratory, are reached. This establishes the traceability of the calibration.

See metrology for other factors that are considered during calibration process development.

After all of this, individual instruments of the specific type discussed above can finally be calibrated. The process generally begins with a basic damage check. Some organizations such as nuclear power plants collect "as-found" calibration data before any routine maintenance is performed. After routine maintenance and deficiencies detected during calibration are addressed, an "as-left" calibration is performed.

More commonly, a calibration technician is entrusted with the entire process and signs the calibration certificate, which documents the completion of a successful calibration.

Calibration process success factors

The basic process outlined above is a difficult and expensive challenge. The cost for ordinary equipment support is generally about 10% of the original purchase price on a yearly basis, as a commonly accepted rule-of-thumb. Exotic devices such as scanning electron microscopes, gas chromatograph systems and laser interferometer devices can be even more costly to maintain.

The extent of the calibration program exposes the core beliefs of the organization involved. The integrity of organization-wide calibration is easily compromised. Once this happens, the links between scientific theory, engineering practice and mass production that measurement provides can be missing from the start on new work or eventually lost on old work.

The 'single measurement' device used in the basic calibration process description above does exist. But, depending on the organization, the majority of the devices that need calibration can have several ranges and many functionalities in a single instrument. A good example is a common modern oscilloscope. There easily could be 200,000 combinations of settings to completely calibrate and limitations on how much of an all inclusive calibration can be automated.

Every organization using oscilloscopes has a wide variety of calibration approaches open to them. If a quality assurance program is in force, customers and program compliance efforts can also directly influence the calibration approach. Most oscilloscopes are capital assets that increase the value of the organization, in addition to the value of the measurements they make. The individual oscilloscopes are subject to depreciation for tax purposes over 3, 5, 10 years or some other period in countries with complex tax codes. The tax treatment of maintenance activity on those assets can bias calibration decisions.

New oscilloscopes are supported by their manufacturers for at least five years, in general. The manufacturers can provide calibration services directly or through agents entrusted with the details of the calibration and adjustment processes.

Very few organizations have only one oscilloscope. Generally, they are either absent or present in large groups. Older devices can be reserved for less demanding uses and get a limited calibration or no calibration at all. In production applications, oscilloscopes can be put in racks used only for one specific purpose. The calibration of that specific scope only has to address that purpose.

This whole process in repeated for each of the basic instrument types present in the organization, such as the digital multimeter (DMM) pictured below.

A DMM (top), a rack-mounted oscilloscope (center) and control panel

Also the picture above shows the extent of the integration between Quality Assurance and calibration. The small horizontal unbroken paper seals connecting each instrument to the rack prove that the instrument has not been removed since it was last calibrated. These seals are also used to prevent undetected access to the adjustments of the instrument. There also are labels showing the date of the last calibration and when the calibration interval dictates when the next one is needed. Some organizations also assign unique identification to each instrument to standardize the recordkeeping and keep track of accessories that are integral to a specific calibration condition.

When the instruments being calibrated are integrated with computers, the integrated computer programs and any calibration corrections are also under control.

In the United States, there is no universally accepted nomenclature to identify individual instruments. Besides having multiple names for the same device type there also are multiple, different devices with the same name. This is before slang and shorthand further confuse the situation, which reflects the ongoing open and intense competition that has prevailed since the Industrial Revolution.

The calibration paradox

Successful calibration has to be consistent and systematic. At the same time, the complexity of some instruments require that only key functions be identified and calibrated. Under those conditions, a degree of randomness is needed to find unexpected deficiencies. Even the most routine calibration requires a willingness to investigate any unexpected observation.

Theoretically, anyone who can read and follow the directions of a calibration procedure can perform the work. It is recognizing and dealing with the exceptions that is the most challenging aspect of the work. This is where experience and judgement are called for and where most of the resources are consumed.

Quality

To improve the quality of the calibration and have the results accepted by outside organizations it is desirable for the calibration and subsequent measurements to be "traceable" to the internationally defined measurement units. Establishing traceability is accomplished by a formal comparison to a standard which is directly or indirectly related to national standards (NIST in the USA), international standards, or certified reference materials.

Quality management systems call for an effective metrology system which includes formal, periodic, and documented calibration of all measuring instruments. ISO 9000 and ISO 17025 sets of standards require that these traceable actions are to a high level and set out how they can be quantified.

Instrument calibration

Calibration can be called for:

  • with a new instrument
  • when a specified time period is elapsed
  • when a specified usage (operating hours) has elapsed
  • when an instrument has had a shock or vibration which potentially may have put it out of calibration
  • whenever observations appear questionable

In non-specialized use, calibration is often regarded as including the process of adjusting the output or indication on a measurement instrument to agree with value of the applied standard, within a specified accuracy. For example, a thermometer could be calibrated so the error of indication or the correction is determined, and adjusted (e.g. via calibration constants) so that it shows the true temperature in Celsius at specific points on the scale.

International

In many countries a National Metrology Institute (NMI) will exist which will maintain primary standards of measurement (the main SI units plus a number of derived units) which will be used to provide traceability to customer's instruments by calibration. The NMI supports the metrological infrastructure in that country (and often others) by establishing an unbroken chain, from the top level of standards to an instrument used for measurement. Examples of National Metrology Institutes are NPL in the UK, NIST in the United States, PTB in Germany and many others. Since the Mutual Recognition Agreement was signed it is now straightforward to take traceability from any participating NMI and it is no longer necessary for a company to obtain traceability for measurements from the NMI of the country in which it is situated.

To communicate the quality of a calibration the calibration value is often accompanied by a traceable uncertainty statement to a stated confidence level. This is evaluated through careful uncertainty analysis.

See also

References


1911 encyclopedia

Up to date as of January 14, 2010

From LoveToKnow 1911

CALIBRATION, a term primarily signifying the determination of the "calibre" or bore of a gun. The word calibre was introduced through the French from the Italian calibro, together with other terms of gunnery and warfare, about the 16th century. The origin of the Italian equivalent appears to be uncertain. It will readily be understood that the calibre of a gun requires accurate adjustment to the standard size, and further, that the bore must be straight and of uniform diameter throughout. The term was subsequently applied to the accurate measurement and testing of the bore of any kind of tube, especially those of thermometers.

In modern scientific language, by a natural process of transition, the term "calibration" has come to denote the accurate comparison of any measuring instrument with a standard, and more particularly the determination of the errors of its scale. It is seldom possible in the process of manufacture to make an instrument so perfect that no error can be discovered by the most delicate tests, and it would rarely be worth while to attempt to do so even if it were possible. The cost of manufacture would in many cases be greatly increased without adding materially to the utility of the apparatus. The scientific method, in all cases which admit of the subsequent determination and correction of errors, is to economize time and labour in production by taking pains in the subsequent verification or calibration. This process of calibration is particularly important in laboratory research, where the observer has frequently to make his own apparatus, and cannot afford the time or outlay required to make special tools for fine work, but is already provided with apparatus and methods of accurate testing. For non-scientific purposes it is generally possible to construct instruments to measure with sufficient precision without further correction. The present article will therefore be restricted to the scientific use and application of methods of accurate testing.

Table of contents

General Methods and Principles

The process of calibration of any measuring instrument is frequently divisible into two parts, which differ greatly in importance in different cases, and of which one or the other may often be omitted. (1) The determination of the value of the unit to which the measurements are referred by comparison with a standard unit of the same kind. This is often described as the Standardization of the instrument, or the determination of the Reduction factor. (2) The verification of the accuracy of the subdivision of the scale of the instrument. This may be termed calibration of the scale, and does not necessarily involve the comparison of the instrument with any independent standard, but merely the verification of the accuracy of the relative values of its indications. In many cases the process of calibration adopted consists in the comparison of the instrument to be tested with a standard over the whole range of its indications, the relative values of the subdivisions of the standard itself having been previously tested. In this case the distinction of two parts in the process is unnecessary, and the term calibration is for this reason frequently employed to include both. In some cases it is employed to denote the first part only, but for greater clearness and convenience of description we shall restrict the term as far as possible to the second meaning.

The methods of standardization or calibration employed have much in common even in the cases that appear most diverse. They are all founded on the axiom that "things which are equal to the same thing are equal to one another." Whether it is a question of comparing a scale with a standard, or of testing the equality of two parts of the same scale, the process is essentially one of interchanging or substituting one for the other, the two things to be compared. In addition to the things to be tested there is usually required some form of balance, or comparator, or gauge, by which the equality may be tested. The simplest of such comparators is the instrument known as the callipers, from the same root as calibre, which is in constant use in the workshop for testing equality of linear dimensions, or uniformity of diameter of tubes or rods. The more complicated forms of optical comparators or measuring machines with scales and screw adjustments are essentially similar in principle, being finely adjustable gauges to which the things to be compared can be successively fitted. A still simpler and more accurate comparison is that of volume or capacity, using a given mass of liquid as the gauge or test of equality, which is the basis of many of the most accurate and most important methods of calibration. The common balance for testing equality of mass or weight is so delicate and so easily tested that the process of calibration may frequently with advantage be reduced to a series of weighings, as for instance in the calibration of a burette or measure-glass by weighing the quantities of mercury required to fill it to different marks. The balance may, however, be regarded more broadly as the type of a general method capable of the widest application in accurate testing. It is possible, for instance, to balance two electromotive forces or two electrical resistances against each other, or to measure the refractivity of a gas by balancing it against a column of air adjusted to produce the same retardation in a beam of light. These "equilibrium," or "null," or "balance" methods of comparison afford the most accurate measurements, and are generally selected if possible as the basis of any process of calibration. In spite of the great diversity in the nature of things to be compared, the fundamental principles of the methods employed are so essentially similar that it is possible, for instance, to describe the testing of a set of weights, or the calibration of an electrical resistance-box, in almost the same terms, and to represent the calibration correction of a mercury thermometer or of an ammeter by precisely similar curves.

Method of Substitution

In comparing two units of the same kind and of nearly equal magnitude, some variety of the general method of substitution is invariably adopted. The same method in a more elaborate form is employed in the calibration of a series of multiples or submultiples of any unit. The details of the method depend on the system of subdivision adopted, which is to some extent a matter of taste. The simplest method of subdivision is that on the binary scale, proceeding by multiples of 2. With a pair of submultiples of the smallest denomination and one of each of the rest, thus I, I, 2, 4, 8, 16, &c., each weight or multiple is equal to the sum of all the smaller weights, which may be substituted for it, and the small difference, if any, observed. If we call the weights A, B, C, &c., where each is approximately double the following weight, and if we write a for observed excess of A over the rest of the weights, b for that of B over C+D +&c., and so on, the observations by the method of substitution give the series of equations, A - rest =a, B - rest = b, C - rest =c, &c... (1) Subtracting the second from the first, the third from the second, and so on, we obtain at once the value of each weight in terms of the preceding, so that all may be expressed in terms of the largest, which is most conveniently taken as the standard B =A/2 +(b - a)/2, C=B/2+(c - b)2, &c... (2) The advantages of this method of subdivision and comparison, in addition to its extreme simplicity, are (1) that there is only one possible combination to represent any given weight within the range of the series; (2) that the least possible number of weights is required to cover any given range; (3) that the smallest number of substitutions is required for the complete calibration. These advantages are important in cases where the accuracy of calibration is limited by the constancy of the conditions of observation, as in the case of an electrical resistance-box, but the reverse may be the case when it is a question of accuracy of estimation by an observer.

In the majority of cases the ease of numeration afforded by familiarity with the decimal system is the most important consideration. The most convenient arrangement on the decimal system for purposes of calibration is to have the units, tens, hundreds, &c., arranged in groups of four adjusted in the proportion of the numbers I, 2, 3, 4. The relative values of the weights in each group of four can then be determined by substitution independently of the others, and the total of each group of four, making ten times the unit of the group, can be compared with the smallest weight in the group above. This gives a sufficient number of equations to determine the errors of all the weights by the method of substitution in a very simple manner. A number of other equations can be obtained by combining the different groups in other ways, and the whole system of equations may then be solved by the method of least squares; but the equations so obtained are not all of equal value, and it may be doubted whether any real advantage is gained in many cases by the multiplication of comparisons, since it is not possible in this manner to eliminate constant errors or personal equation, which are generally aggravated by prolonging the observations. A common arrangement of the weights in each group on the decimal system is 5, 2, I, I, or 5, 2, 2, I. These do not admit of the independent calibration of each group by substitution. The arrangement 5, 2, I, I, I, or 5, 2, 2, I, I, permits independent calibration, but involves a r larger number of weights and observations than the I, 2, 3, 4, grouping. The arrangement of ten equal weights in each group, which is adopted in "dial" resistance-boxes, and in some forms of chemical balances where the weights are mechanically applied by turning a handle, presents great advantages in point of quickness of manipulation and ease of numeration, but the complete calibration of such an arrangement is tedious, and in the case of a resistance-box it is difficult to make the necessary connexions. In all cases where the same total can be made up in a variety of ways, it is necessary in accurate work to make sure that the same weights are always used for a given combination, or else to record the actual weights used on each occasion. In many investigations where time enters as one of the factors, this is a serious drawback, and it is better to avoid the more complicated arrangements. The accurate adjustment of a set of weights is so simple a matter that it is often possible to neglect the errors of a well-made set, and no calibration is of any value without the most scrupulous attention to details of manipulation, and particularly to the correction for the air displaced in comparing weights of different materials. Electrical resistances are much more difficult to adjust owing to the change of resistance with temperature, and the calibration of a resistance-box can seldom be neglected on account of the changes of resistance which are liable to occur after adjustment from imperfect annealing. It is also necessary to remember that the order of accuracy required, and the actual values of the smaller resistances, depend to some extent on the method of connexion, and that the box must be calibrated with due regard to the conditions under which it is to be used. Otherwise the method of procedure is much the same as in the case of a box of weights, but it is necessary to pay more attention to the constancy and uniformity of the temperature conditions of the observing-room.

Method of Equal Steps

In calibrating a continuous scale divided into a number of divisions of equal length, such as a metre scale divided in millimetres, or a thermometer tube divided in degrees of temperature, or an electrical slide-wire, it is usual to proceed by a method of equal steps. The simplest method is that known as the method of Gay Lussac in the calibration of mercurial thermometers or tubes of small bore. It is essentially a method of substitution employing a column of mercury of constant volume as the gauge for comparing the capacities of different parts of the tube. A precisely similar method, employing a pair of microscopes at a fixed distance apart as a standard of length, is applicable to the calibration of a divided scale. The interval to be calibrated is divided into a: whole number of equal steps or sections, the points of division at which the corrections are to be determined are called points of calibration. Calibration of a Mercury Thermometer. - To facilitate description, we will take the case of a fine-bore tube, such as that of a thermometer, to be calibrated with a thread of mercury. The bore of such a tube will generally vary considerably even in the best standard instruments, the tubes of which have been specially drawn and selected. The correction for inequality of bore may amount to a quarter or half a degree, and is seldom less than a tenth. In ordinary chemical thermometers it is usual to make allowance for variations of bore in graduating the scale, but such instruments present discontinuities of division, and cannot be used for accurate work, in which a finely-divided scale of equal parts is essential. The calibration of a mercury thermometer intended for work of precision is best effected after it has been sealed. A thread of mercury of the desired length is separated from the column. The exact adjustment of the length of the thread requires a little manipulation.

The thermometer is inverted and tapped to make the mercury run down to the top of the tube, thus collecting a trace of residual gas at the end of the bulb. By quickly reversing the thermometer the bubble passes to the neck of the bulb. If the instrument is again inverted and tapped, the thread will probably break off at the neck of the bulb, which should be previously cooled or warmed so as to obtain in this manner, if possible, a thread of the desired length. If the thread so obtained is too long or not accurate enough, it is removed to the other end of the tube, and the bulb further warmed till the mercury reaches some easily recognized division. At this point the broken thread is rejoined to the mercury column from the bulb, and a microscopic bubble of gas is condensed which generally suffices to determine the subsequent breaking of the mercury column at the same point of the tube. The bulb is then allowed to cool till the length of the thread above the point of separation is equal to the desired length, when a slight tap suffices to separate the thread. This method is difficult to work with short threads owing to deficient inertia, especially if the tube is very perfectly evacuated. A thread can always be separated by local heating with a small flame, but this is dangerous to the thermometer, it is difficult to adjust the thread exactly to the required length, and the mercury does not run easily past a point of the tube which has been locally heated in this manner.

Having separated a thread of the required length, the thermometer is mounted in a horizontal position on a suitable support, preferably with a screw adjustment in the direction of its length. By tilting or tapping the instrument the thread is brought into position corresponding to the steps of the calibration successively, and its length in each position is carefully observed with a pair of reading microscopes fixed at a suitable distance apart. Assuming that the temperature remains constant, the variations of length of the thread are inversely as the variations of cross-section of the tube. If the length of the thread is very nearly equal to one step, and if the tube is nearly uniform, the average of the observed lengths of the thread, taking all the steps throughout the interval, is equal to the length which the thread should have occupied in each position had the bore been uniform throughout and all the divisions equal.

I

Calibration by Method of Gay Lussac. The error of each step is therefore found by subtracting the average length from the observed length in each position. Assuming that the ends of the interval itself are correct, the correction to be applied at any point of calibration to reduce the readings to a uniform tube and scale, is found by taking the sum of the errors of the steps up to the point considered with the sign reversed.

In the preceding example of the method an interval of ten degrees is taken, divided into ten steps of I° each. The distances of the ends of the thread from the nearest degree divisions are estimated by the aid of micrometers to the thousandth of a degree. The error of any one of these readings probably does not exceed half a thousandth, but they are given to the nearest thousandth only. The excess length of the thread in each position over the corresponding degree is obtained by subtracting the second reading from the first. Taking the average of the numbers in this line, the mean excess-length is - 10.4 thousandths. The error of each step is found by subtracting this mean from each of the numbers in the previous line. Finally, the corrections at each degree are obtained by adding up the errors of the steps and changing the sign. The errors and corrections are given in thousandths of I °.

No. of

Step.

I

2

3

4

5

6

7

8

9

10

Ends of 1

+ oio

-. o16

- 020

-. 031

+016

+ 008

+ 013

+ 017

+ 004

- .088

thread. S

1 + 038

+ 017

- 003

- 022

+ 010

+ 005

+ 033

*018

+ 013

- 003

Excess-

length.

- 028

-. 033

-. 017

- 009

+006

- 003

- 020

- 001

- 004

+ 005

Error of

step.

- 17.6

- 22.6

- 6.6

1.4

+16'4

7'4

- 9' 6

9'4

6 '4

+15'4

Correc-

tion.

+17'6

+4 0 ' 2

+4 6.8

+45'4

+29.0

+21 6

+31.2

+21.8

+15.4

o

Complete Calibration

The simple method of Gay Lussac does very well for short intervals when the number of steps is not excessive, but it would not be satisfactory for a large range owing to the accumulation of small errors of estimation, and the variation of the personal equation. The observer might, for instance, consistently over-estimate the length of the thread in one half of the tube, and under-estimate it in the other. The errors near the middle of the range would probably be large. It is evident that the correction at the middle point of the interval could be much more accurately determined by using a thread equal to half the length of the interval. To minimize the effect of these errors of estimation, it is usual to employ threads of different lengths in calibrating the same interval, and to divide up the fundamental interval of the thermometer into a number of subsidiary sections for the purpose of calibration, each of these sections being treated as a step in the calibration of the fundamental interval. The most symmetrical method of calibrating a section, called by C. E. Guillaume a "Complete Calibration," is to use threads of all possible lengths which are Table integral multiples of the calibration step. In the example already ampoules, were calibrated by Chappuis in five sections of 20° each, given nine different thread s Com were IL - Complete used, and Calibration the of Interval nteYVa o l f of Io each r y as to determine the corrections at the points 20°, 40°, 60°, 80°, which observed in as many positions as possible. Proceeding in this may be called the "principal points" of the calibration, in terms of manner the following numbers were obtained for the excess length the fundamental interval. Each section of 20° was subsequently of each thread in thousandths of a degree in different positions, calibrated in steps of 2°, the corrections being at first referred, as in starting in each case with the beginning of the thread at o°, and the example already given, to the mean degree of the section itself, moving it on by steps of °. The observations in the first column and being afterwards expressed, by a simple transformation, in terms are the excess lengths of the thread of 1 ° already given in of the fundamental interval, by means of the corrections already illustration of the method of Gay Lussac. The other columns found for the ends of the section. Supposing, for instance, that the give the corresponding observations with the longer threads. corrections at the points o° and 10° of Table III. are not zero, but The simplest and most symmetrical method of solving these C° and C' respectively, the correction C, at any intermediate point observations, so as to find the errors of each step in n will evidently be given by the formula, terms of the whole interval, is to obtain the differences of Cn= C°+cn+ (C' - C°)n/Io. (3) the steps in pairs by subtracting each observation from the one Table C, I Is The Correction Already Given In The Table.

Le Ii. ° In To Steps. If The Corrections Are Required To The Thou Sandth Of A Degree, It Is Necessary To Tabulate The Results Of The Calibration At Much More Frequent Intervals Than 2°, Since The Correction, Even Of A Good Thermometer, May Change By As Much As 20 Or 30 Thousandths In 2°. To Save The Labour And Difficulty Of Calibrating With Shorter Threads, The Corrections At Intermediate Points Are Usually Calculated By A Formula Of Interpolation. This Leaves Much To Be Desired, As The Section Of A Tube Often Changes Very Suddenly And Capriciously. It Is Probable That The Graphic Method Gives Equally Good Results With Less Labour.

Slide Wire

The Calibration Of An Electrical Lengths Of Threads.

I° 2° 3° 4° 5° 6° 7° 8° 9° Above It. This Method Eliminates The Unknown Lengths Of The Threads, And Gives Each Observation Approximately Its Due Weight. Subtracting The Observations In The Second Line From Those In The First, We Obtain A Series Of Numbers, Entered In Column I Of The Next Table, Representing The Excess Of Step (I) Over Each Of The Other Steps. The Sum Of These Differences Is Ten Times The Error Of The First Step, Since By Hypothesis The Sum Of The Errors Of All The Steps Is Zero In Terms Of The Whole Interval. The Numbers In The Second Column Of Table Iii. Are Similarly Obtained By Subtracting The Third Line From The Second In Table Ii., Each Difference Being Inserted In Its Appropriate Place In The Table. Proceeding In This Way We Find The Excess Of Each Interval Over Those Which Follow It. The Table Is Completed By A Diagonal Row Of Zeros Representing The Difference Of Each Step From Itself, And By Repeating The Numbers Already Found In Symmetrical Positions With Their Signs Changed, Since The Excess Of Any Step, Say 6 Over 3, Is Evidently Equal To That Of 3 Over 6 With The Sign Changed. The Errors Of Each Step Having Been Found By Adding The Columns, And Dividing By 10, The Corrections At Each Point Of The Calibration Are Deduced As Before.

Table Iii. Solution Of Complete Calibration Slide Wire Into Parts Of Equal Resistance Is Precisely Analogous To That Of A Capillary Tube Into Parts Of Equal Volume. The Carey Foster Method, Employing Short Steps Of Equal Resistance, Effected By Transferring A Suitable Small Resistance From One Side Of The Slide Wire To The Other, Is Exactly Analogous To The Gay Lussac Method, And Suffers From The Same Defect Of The Accumulation Of Small Errors Unless Steps Of Several Different Lengths Are Used. The Calibration Of A Slide Wire, However, Is Much Less Troublesome Than That Of A Thermometer Tube For Several Reasons. It Is Easy To Obtain A Wire Uniform To One Part In 500 Or Even Less, And The Section Is Not Liable To Capricious Variations. In All Work Of Precision The Slide Wire Is Supplemented By Auxiliary Resistances By Which The Scale May Be Indefinitely Extended. In Accurate Electrical Thermometry, For Example, The Slide Wire Itself Would Correspond To Only T °, Or Less, Of The Whole Scale, Which Is Less Than A Single Step In The Calibration Of A Mercury Thermometer, So That An Accuracy Of A Thousandth Of A Degree Can Generally Be Obtained Without Any Calibration Of The Slide Wire. In The Rare Cases In Which It Is Necessary To Employ A Long Slide Wire, Such As The Cylinder Potentiometer Of Latimer Clark, The Calibration Is Best Effected By Comparison With A Standard, Such As A Thomson Varley Slide Box.

Step

No.

I

2

3

4

5

6

7

8

9

Io

I

O

5

Ii

20

34

2 5

7

26

23

32

2

5

0

16

23

39

29

12

31

28

37

3

Ii

16

0

8

24

1 3

4

15

13

22

4

20

23

8

0

1 5

5

12

1 7

4

13

5

34

39

24

15

0

9

26

8

To

2

6

25

29

13

5

9

0

17

2

1

8

7

7

12

4

12

26

17

O

{ 19

16

26

8

26

31

15

7

8

2

19

0

3

6

9

23

28

13

4

10

I

16

3

0

9

Io

32

37

22

13

2

8

26

6

9

0

E Rror Of

Step.

17.3

22.0

6.4

1 9

16 7

7.1

8.9

6 I

15.1

Correc

Tions.

1 1 7'3

39 3

45 7

43' 8

27.1

20.0

30.1

21.2

15.1

0

Graphic Representation Of Results

The Results Of A Calibration Are Often Best Represented By Means Of A Correction Curve, Such As That Illustrated In The Diagram, Which Is Plotted To Represent The Corrections Found In Table Iii. The Abscissa Of Such A Curve Is The Reading Of The Instrument To Be Corrected. The Ordinate Is The Correction To Be Added To The Observed Reading To Reduce To A Uniform Scale. The Corrections Are Plotted In The Figure In Terms Of The Whole Section, Taking The Correction To Be Zero At The Beginning And End. As A Matter Of Fact The Corrections At These Points In Terms Of The Fundamental In The Advantages Of This Method Are The Simplicity And Symmetry Of The Work Of Reduction, And The Accuracy Of The Result, Which Exceeds That Of The Gay Lussac Method In Consequence Of The Much Larger Number Of Independent Observations. It May Be Noticed, For Instance, That The Correction At Point 5 Is 27.1 Thousandths By The Complete Calibration, Which Is 2 Thousandths Less Than The Value 29 Obtained By The Gay Lussac Method, But Agrees Well With The Value 27 Thousandths Obtained By Taking Only The First And Last Observations With The Thread Of 5°. The Disadvantage Of The Method Lies In The Great Number Of Observations Required, And In The Labour Of Adjusting So Many Different Threads To Suitable Lengths. It Is Probable That Sufficiently Good Results May Be Obtained With Much Less Trouble By Using Fewer Threads, Especially If More Care Is Taken In The Micrometric Determination Of Their Errors.

The Method Adopted For Dividing Up The Fundamental Interval Of Any Thermometer Into Sections And Steps For Calibration May Be Widely Varied, And Is Necessarily Modified In Cases Where Auxiliary Bulbs Or "Ampoules" Are Employed. The Paris Mercury Standards, Which Read Continuously From O° To 1 00° C., Without Intermediate Terval Were Found To Be 29 And 9 Thousandths Respectively. The Correction Curve Is Transformed To Give Corrections In Terms Of The Fundamental Interval By Ruling A Straight Line Joining The Points 29 And 9 Respectively, And Reckoning The Ordinates From This Line Instead Of From The Base Line. Or The Curve May Be Replotted With The New Ordinates Thus Obtained. In Drawing The Curve From The Corrections Obtained At The Points Of Calibration, The Exact Form Of The Curve Is To Some Extent A Matter Of Taste, But The Curve Should Generally Be Drawn As Smoothly As Possible On The Assumption That The Changes Are Gradual And Continuous.

Observed Excess 0°

Lengths Of Threads,I°

In Various Posi 2°

Tions, The Begin 3°

Ning Of The Thread 4°

Being Set Near The 5°

Points. 6°

28

33

17

9

6

3

20

I

4

5

32

21

2

26

3 1

5

7

23

29

67

47

8

5

7

1 5

16

Io

62

28

I

3

4

6

2

Ii

14

26

4 1

45

43

15

8

23

36

49

48

22

6

28

2

21

58

8

24

The Ruling Of The Straight Line Across The Curve To Express The Corrections In Terms Of The Fundamental Interval, Corresponds To The First Part Of The Process Of Calibration Mentioned Above Under The Term "Standardization." It Effects The Reduction Of The Readings To A Common Standard, And May Be Neglected If Relative Values Only Are Required. A Precisely Analogous Correction Occurs In The Case Of Electrical Instruments. A Potentiometer, For Instance, If Correctly Graduated Or Calibrated In Parts Of Equal Resistance, Will Give Correct Relative Values Of Any Differences Of Potential Within Its Range If Connected To A Constant Cell To Supply The Steady Current Through The Slide Wire. But To Determine At Any Time The Actual Value Of Its Readings In Volts, It Is Necessary To Standardize It, Or Determine Its Scale Value Or Reduction Factor, By Comparison With A Standard Cell.

A Very Neat Use Of The Calibration Curve Has Been Made By Professor W. A. Rogers In The Automatic Correction Of Screws Of Dividing Machines Or Lathes. It Is Possible By The Process Of Grinding, As Applied By Rowland, To Make A Screw Which Is Practically Perfect In Point Of Uniformity, But Even In This Case Errors May Be Introduced By The Method Of Mounting. In The Production Of Divided Scales, And More Particularly In The Case Of Optical Gratings, It Is Most Important That The Errors Should Be As Small As Possible, And Should Be Automatically Corrected During The Process Of Ruling. With This Object A Scale Is Ruled On The Machine, And The Errors Of The Uncorrected Screw Are Determined By Calibrating The Scale. A Metal Template May Then Be Cut Out In The Form Of The Calibration Correction Curve On A Suitable Scale. A Lever Projecting From The Nut Which Feeds The Carriage Or The Slide Rest Is Made To Follow The Contour Of The Template, And To Apply The Appropriate Correction At Each Point Of The Travel, By Turning The Nut Through A Small Angle On The Screw. A Small Periodic Error Of The Screw, Recurring Regularly At Each Revolution, May Be Similarly Corrected By Means Of A Suitable Cam Or Eccentric Revolving With The Screw And Actuating The Template. This Kind Of Error Is Important In Optical Gratings, But Is Difficult To Determine And Correct.

Calibration By Comparison With A Standard

The Commonest And Most Generally Useful Process Of Calibration Is The Direct Comparison Of The Instrument With A Standard Over The Whole Range Of Its Scale. It Is Necessary That The Standard Itself Should Have Been Already Calibrated, Or Else That The Law Of Its Indications Should Be Known. A Continuous Current Ammeter, For Instance, Can Be Calibrated, So Far As The Relative Values Of Its Readings Are Concerned, By Comparison With A Tangent Galvanometer, Since It Is Known That The Current In This Instrument Is Proportional To The Tangent Of The Angle Of Deflection. Similarly An Alternating Current Ammeter Can Be Calibrated By Comparison With An Electrodynamometer, The Reading Of Which Varies As The Square Of The Current. But In Either Case It Is Neccessary, In Order To Obtain The Readings In Amperes, To Standardize The Instrument For Some Particular Value Of The Current By Comparison With A Voltameter, Or In Some Equivalent Manner. Whenever Possible, Ammeters And Voltmeters Are Calibrated By Comparison Of Their Readings With Those Of A Potentiometer, The Calibration Of Which Can Be Reduced To The Comparison And Adjustment Of Resistances, Which Is The Most Accurate Of Electrical Measurements. The Commoner Kinds Of Mercury Thermometers Are Generally Calibrated And Graduated By Comparison With A Standard. In Many Cases This Is The Most Convenient Or Even The Only Possible Method. A Mercury Thermometer Of Limited Scale Reading Between 250° And 400 ° C., With Gas Under High Pressure To Prevent The Separation Of The Mercury Column, Cannot Be Calibrated On Itself, Or By Comparison With A Mercury Standard Possessing A Fundamental Interval, On Account Of Difficulties Of Stem Exposure And Scale. The Only Practical Method Is To Compare Its Readings Every Few Degrees With Those Of A Platinum Thermometer Under The Condi Tions For Which It Is To Be Used. This Method Has The Advantage Of Combining All The Corrections For Fundamental Interval, &C., With The Calibration Correction In A Single Curve, Except The Correction For Variation Of Zero Which Must Be Tested Occasionally At Some Point Of The Scale.

Autnorities

Mercurial Thermometers: Guillaume, Thermometrie De Precision (Paris, 1889), Gives Several Examples And References To Original Memoirs. The Best Examples Of Comparison And Testing Of Standards Are Generally To Be Found In Publications Of Standards Offices, Such As Those Of The Bureau International Des Poids Et Mesures At Paris. Dial Resistance Box: Griffiths, Phil. Trans. A, 1893; Platinum Thermometry Box: J. A. Harker And P. Chappuis, Phil. Trans. A, 1900; Thomson Varley Potentiometer And Binary Scale Box: Callendar And Barnes, Phil. Trans. A, 1901. (H. L. C.)


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