1、AMERICAN NATIONAL STANDARD Temperature and Humidity Environment for Dimensional Measurement ANSI 889.6.2 -1973 REAFFIRMED 1995 FOR CURRENT COMMITIEE PERSONNEL PLEASE SEE ASME MANUAL AS-11 SECRETARIAT THE AMERICAN SOCIETY OF MECHANICAL ENGINEERS PUBLISHED BY THE AMERICAN SOCIETY OF MECHANICAL ENGINEE
2、RS United Engineering Center 345 East 47th Street New York, N.Y. 10017 No part of this document may be reproduced in any form, in an electronic retrieval system or otherwise, without the prior written permission of the publisher. Copyright 1974 by TilE AMERICAN SOCIETY OF MECHANICAL ENGINEERS Printe
3、d in U.S.A. FOREWORD American National Standards Committee 889 on Dimensional Metrology, organized under the procedures of the American National Standards Institute, was formed to develop certain minimum stand ards for the various parameters in metrology and represents the consensus of United States
4、 industry. The various subcommittees of Committee 889 deal with the different parameters, i.e., environment, angle, length, geometry, etc. Subcommittee 889.6 is assigned the task of developing standards in physical environ ment and the effects of this environment and other extraneous influences on a
5、ccuracy and precision of dimensional measurements. This standard for temperature and humidity is the work of the ANSI 889.6.2 Working Group. The results of its cooperative efforts are expressed in this document. The effect of heat flow and resulting temperature gradients, differences and variation f
6、rom measure ment to measurement can result in errors of dimensional measurement because of the thermal expansion properties of materials. By international agreement the true size and shape of an object is that which exists at a uniform temperature of 68 F (20 C). The purpose of this standard is to p
7、rovide American industry with practical requirements procedures, and methods by which the intent of the international agreement can be satisfied without compromise to economical operation. In discharging its responsibilities, the Working Group has recognized two basic needs of industry. First, it re
8、cognizes the need for standard approaches to the buying and selling of artificially controlled en vironments. Second, it recognizes the need for the qualification of individual measurements regarding errors induced by non-ideal temperature conditions. Standard specifications for artificially control
9、led environments, in terms of the quality of temperature control, are especially necessary as a means of communicating metrological requirements to construction agencies such as heating and air-conditioning contractors. In specific instances, sufficient experience has been obtained such that require
10、d dimensional accuracies can be translated directly into temperature control specifications. However, the Working Group has concluded that no general set of temperature control specifications can be stated that will simultaneously assure levels of measurement accuracy and avoid the risk of overdesig
11、n or underdesign. Indeed, no recommendation can be made on which type of artificial en vironment, or even whether one is necessary or not, that would represent the most satisfactory engineering for every application. Consequently, the Working Group has chosen to list those properties of an artificia
12、lly controlled environment that must be specified for an adequate description, to specify standard procedures for the administration of the required specifications, and to provide advisory information in the form of guidelines that the users of this standard may find helpful in the development of sp
13、ecifications adapted to individual needs. The metrologist, his management, or a potential customer of a metrological service has, each for his own purpose, a need and a right to know the magnitude of measurement errors induced by the thermal en vironment. Therefore, this standard includes a descript
14、ion of procedures for the estimation of the error con tributions caused by various defects of the thermal environment. Further, there is a need for a convenient means of communication between these parties. For this purpose, the Working Group has provided a stand ard figure of merit, the Thermal Err
15、or Index. Because this document, for the first time, presents the Thermal Error Index for use by industry at large, the methods for its determination and use are carefully developed in an appendix. Recommendations for the control of humidity in metrological environments are included in this document
16、, because it is often directly affected by and related to the control of temperature, especially in the design of room enclosures. After approval by the 889 National Standards Committee and submittal to public review the Stand ard was approved by ANSI as a National Standard on October 30, 1973. iii
17、AMERICAN NATIONAL STANDARDS COMMITTEE 889 DIMENSIONAL METROLOGY (The following ia the roster of the Committee at the time of approval of this Standard) OFFICERS E. G. Loewen, Chairman J. K. Emery, J st Vice-Choirmon J. C. Moody, 2nd ViceChoirmon Mory Hoskins,_ /!fxec:ufive Secretory AEROSPACE INDUST
18、RIES ASSOCIATION OF AMERICA, INC. M. J, Leight, Metrology Section, Primary Standards Laboratories, Hughes Aircraft Company, Culver City, California AMERICAN ORDNANCE ASSOCIATION J-, C. Moody, Sandia Corporation, Albuquerque, New Mexico AMEICAN SOCIETY FOR QUALITY CONTROL John Novotny, Sperry Gyrosco
19、pe, Great Neck, New York AMERICAN SOCIETY FOR TESTING AND MATERIALS H. J, Stremlxl, Associate Director, Technical Operations, ASTct!re oscillation as a function of the frequc:1cy o; temperature oscillation. 3.8 Drift Test An experiment conducted to determine the actual drift inherent in a measuremen
20、t system under normal operating conditions is called a drift test. Since the usual method of monitoring the environment (see Defmition 3.13) involves the correlation of one or more temperature recordings with drift, the test will usually consist of simultaneous recordings of drift and environmental
21、temperatures. The recommended procedure for the conduct of a drift test is given in 20.3.1. 3.9 Master The standard against which the desired dimension of the part is compared is called the master. The standard may be in the form of the wavelength of light, the length of a gage block, line standard,
22、 lead screw, etc. 3.10 Mastering The action of nulling or setting a comparator with a master is called mastering. 3.11 Mastering Cycle Time The time .between successive masterings of the process is called the mastering cycle time of the process. 3.12 Measurement Cycle Time The time between measuring
23、 and the previous mastering is called measurement cycle time. 3.13 Monitoring To ensure the constancy of the Thermal Error Index (see 3.22), it will be necessary to monitor the process in such a way that significant changes in operating conditions are recognizable. The recommended procedure is to es
24、tablish a particular temperature recording station or stations which have a demonstrable correlation with the mag nitude of the drift. The temperature of the selected station should be recorded continuously during any measurement 2 ANSI 889.6.2-1973 process to which the index is to be applied. If th
25、e recording shows a significant change of conditions, the index is null and void for that process, and a re evaluation of the index should be conducted, or the conditions corrected to those for which the index applies. In addition to continuous monitoring of environ mental conditions, it is recommen
26、ded that efforts be made to establish that the process is properly soaked out. This may be done by checking the temperature of all elements before and after the execution of the measurements. 3.14 Nominal Coefficient of Expansion The estimate of the coefficient of expansion of a body shall be called
27、 the nominal coefficient of expan sion. To distinguish this value from the average co efficient of expansion a (68, t) it shall be denoted by the symbolK. 3.15 Nominal Differential Expansion* The difference between the Nominal Expansion of the part and of the master is called the Nominal Dif ferenti
28、al Expansion: NDE = (NE)part (NE)ma “( Om:!: :D:!:m o-o:D _m-:s:;:DCl mll z-lZ (/)Cz o:D zm-l o ,-Zz :s:o m:X:r )CC/1 Cll!:-l c- :Ds:!Z m-iO :!:- mm:D ZzO -l z (/) CD I! en -10 . w AMERICAN NATIONAL STANDARD TEMPERATURE AND HUMIDITY ENVIRONMENT FOR DIMENSIONAL MEASUREMENT .,_so w J: z w a: I.L. Vl w
29、 w c.: 0 w c z w a: 70 : 60 1- a: w 0 :E w 1-CQ I 50 1-w 40 50 . DRY BULB TEMPERATURE IN DEGREES FAHRENHEIT ANSI 889.6.2-1973 . 100 FIG. 3 Still-air comfort chart of the American Society of Heating, Refrigerating and Air-conditioning Engineers 13 AMERICAN NATIONAL STANDARD TEMPERATURE AND HUMIDITY E
30、NVIRONMENT FOR DIMENSIONAL MEASUREMENT microinches. But it takes 0.53 hour (32 minutes) to change 38 microinches. This slowness of response, or thermal inertia, is important to the specification of environments, because it means that high frequency temperature variation is tolerable. The higher the
31、frequency, the more tolerable it is. Experience shows that most machinery, instrument stands, etc., have a limited range of volume to surface area ratio. Quite good results are often achieved with a frequency of temperature variation (in air) of I 5 to 60 cycles per hour and an amplitude of I degree
32、 Fahrenheit. For the gage block in the above example, this temperature variation would cause a length variation of less than I microinch. Like those of high frequency temperature varia tions, the effects of very low frequency temperature variations are not significant when the part and mastef have c
33、losely similar dimensional response. The most fortunate case possible is that in which the part, master, and comparator (see Section 20) all have the same dimensional response characteristics. Then no temperature variation effect is significant. In general, however, there exists an upper and lower l
34、imit of frequency between which is a frequency of maximum differential response. Unfortunately, it is not uncommon that gaging systems have their maxi mum differential response at a frequency close to the natural 24-hour day/night cycle period. Consequently, it is usually advisable to specify the to
35、lerance-to-temperature variation in terms of allow able deviations from mean temperature which vary ac cording to the frequency. Closer limits should be applied to low frequency components, and wider limits may be permitted for high frequency com ponents. 10.1.1.4 Mean Temperature. Selection of a me
36、an environmental temperature affects the cost of refriger ation and heating equipment, insulation, and flow dis tribution. Operation at a temperature other than 68 F (20 C) entails consequences in the form of potential errors of measurement that must be carefully evaluated. Evaluation procedures are
37、 described in Section 20. The most common objection to operating a room enclosure at 68 F, other than the cost of the air con ditioning system, is a possible discomfort to person nel. As discussed in Section 10.1.1 .2, a high air velocity can cause a sensation of much lower tem peratures and result
38、in complaints. In order to main tain human comfort without a requirement for special clothing, the velocities to which personnel are subjected should be less than 20 fpm to avoid the sen-14 ANSI 889.6.2-1973 sation of drafts. Conventional registers, at which the velocities may be 8QO fpm or more are
39、 not satisfac tory. Large inlet and outflow areas are recommended. Full-flow ceilings are used successfully to simul taneously provide high flow rate and low velocity. In cases where the needs of measuring equipment (68 F) and human personnel (low velocity) cannot be satisfied simultaneously, it is
40、recommended that the equipment environment and human environment be separated. Use of special air-flow boxes, liquid baths, or localized high-velocity air showers have been used successfully for this purpose. 10.1.1.5 Gradients. Gradients are the most difficult of all non-ideal temperature condition
41、s to assess for possible error effects. The existence of gradients, of course, implies that portions of the environment will not be at the same mean temperature so that the con sequences of mean temperatures other than 68 F (20 C) will be different in different locations in a room. Movement of equip
42、ment or workpieces from one area to another will result in a change in the error pattern. Machinery is affected by gradients in a variety of ways. For example, a machine with a high vertical column (z-motion) where the z-motion is controlled by a lead screw will have a progressive error if there is
43、a high vertical temperature gradient. In addition, if the vertical slide carries a long cantilever arm, the arm will undergo a transient change of length when raised or lowered. Surface plates are affected by vertical gradients in that a temperature difference between the top and bottom of the plate
44、 will cause the plate to bend. For solid surface plates, the amount of bending or out-of flatness, li, is calculated by using the following formula: where ( AT) H I -K-2 L = length of the surface plate H = height or thickness of the surface plate (16) Tu =upper surface temperature AT= Tu _ T1 11 =lo
45、wer surface temperature “ = coefficient of linear thermal expansion R =radius of curvature of the plate. Machine bedways are similarly affected by both vertical and horizontal gradients which cause angular motions (pitch, roil and yaw). AMERICAN NATIONAL STANDARD TEMPERATURE AND HUMIDITY ENVIRONMENT
46、 FOR DIMENSIONAL MEASUREMENT Gradients occur because of heat sources that exist within the boundaries of the environment. For this reason, it is difficult to administer meaningful re quirements by testing in the absence of equipment and personnel that shall exist under normal working conditions. The
47、 main sources of heat are the electrical lighting fixtures, electrical and electronic equipment, motors, and people. Room enclosures with only the electrical lighting fixtures present and operating have been tested with an observed gradient of less than 0.1 F per foot in any direction. However, the
48、same room with equipment installed normally has gradients of over 0.2 F per foot and high gradients of several degrees per foot near the surfaces of surface plates, electronic cabinets, etc. As mentioned in Section 10.1.1.2, increasing the flow rate will decrease the gradients. For example, sur face
49、 plates are observed to have temperatures on the upper surfaces of 2 F or more above the local mean in a flow rate of 1 0 to 15 changes per hour and a 0.5 F or less in 1 00 changes per hour. 1 0.1.2 Humidity. In certain measurement systems, a significant error can occur if an incorrect value for humidity is used in computing a dimension. For example, in the measurement of the length of gage blocks by interferometry, a 10 percent relative humidity uncertainty will introduce an error of 0.1 microinch per inch of length. Therefore, in labora to
