1、TECHNICAL REPORT ISO/TR 18931 First edition 2001-06-01 Imaging materials - Recommendations for humidity measurement and control Matriaux pour limage - Recommandations pour le mesurage er le contrle de lhumidit Reference number ISOTTR 18931 :2001 (E) - Em= o IS0 2001 ISOiTR 18931 :2001 (E) PDF discla
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6、he requester. IS0 copyright office Case postale 56 CH-121 1 Geneva 20 Tel. + 41 22 749 O1 11 Fax +41 227490947 E-mail copyrightiso.ch Web www.iso.ch Printed in Switzerland II 0 IS0 2001 -All rights reserved ISO/TR 18931 2001 (E) Contents Page Foreword . iv Introduction . v 1 Scope 1 Moisture content
7、 of gases 2 2 3 4 Measuring systems . 2 5 Sensor location 5 6 Recommendations . 5 Annex A Importance of relative humidity . 6 Annex B Humidity control in storage areas 7 Bibliography 8 Terms and definitions . 1 O IS0 2001 -All rights reserved iii ISO/TR 18931:2001(E) Fo rew o r ci IS0 (the Internati
8、onal Organization for Standardization) is a worldwide federation of national standards bodies (IS0 member bodies). The work of preparing International Standards is normally carried out thorugh IS0 technical committees. Each member body interested in a subject for which a technical committee has been
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10、rdization. International Standards are drafted in accordance with the rules given in the ISOAEC Directives, Part 3. The main task of technical committees is to prepare International Standards. Draft International Standards adopted by the technical committees are circulated to the member bodies for v
11、oting. Publication as an International Standard requires approval by at least seventy-five percent of the member bodies casting a vote. In exceptional circumstances, when a technical committee has collected data of a different kind from that which is normally published as an International Standard (
12、“state of the art”, for example), it may decide by a simple majority vote of its participating members to publish a Technical Report. A Technical Report is entirely informative in nature and does not have to be reviewed until the data it provides are considered to be no longer valid or useful. Atten
13、tion is drawn to the possibility that some of the elements of this Technical Report may be the subject of patent rights. IS0 shall not be held responsible for identifying any or all such patent rights. ISOTTR 18931 was prepared by Technical Committee ISOTTC 42, Photography. iv O IS0 2001 - All right
14、s resewed ISO/TR 18931 :2001 (E) Introduction Some tests in photographic International Standards are carried out at a specified temperature and relative humidity (RH). A typical test condition is 23 OC k 1 OC and (50 f 2) % RH. Temperature is relatively easy to measure and control to within f 1 OC.
15、Accurate thermometers of several types, which have been calibrated by a national standards laboratory or by the vendor and traceable to a standards laboratory, are readily available. Humidity is much more complex. Calibration by national standards laboratories can be expensive, and the relatively lo
16、ng turn-around time conflicts with the need for frequent recalibration of the most useful humidity sensors. Some instrument vendors are now providing calibration traceable to the National Institute of Standards and Technology (NIST) at moderate cost. In other situations, the standards user may wish
17、to do his own calibration. It should be noted that calibration is complicated by the lack of useful reference points; relative humidities of O % and 100 %, for example, are not readily measurable. The accurate and precise determination of relative humidity is usually done indirectly and the results
18、converted to relative humidity. This Technical Report discussed devices used as hygrometers and humidistats in the measurement and control of relative humidity. The importance of relative humidity as opposed to other moisture parameters is discussed in annex A. O IS0 2001 -All rights reserved V TECH
19、NICAL REPORT ISO/TR 18931:2001(E) Imaging materials - Recommendations for humidity measurement and control 1 Scope This Technical Report discusses devices in photographic standardization that are used as hygrometers and humidistats in the measurement and control of relative humidity (RH) in test cha
20、mbers and storage areas. Special attention is given to situations where a photographic standard specifies controlling relative humidity to f 2 % RH or better. Electric hygrometers are recommended for their precision, low cost, and accuracy when properly calibrated. Calibration can be done either by
21、the vendor or in-house by a dew-point measurement. Where the budget permits, dew-point combined with ambient temperature measurements (converted to relative humidity) may be the only sensor system. 2 Terms and definitions For the purposes of this Technical Report, the following terms and definitions
22、 apply. 2.1 absolute humidity mass of water vapour per unit volume of wet gas NOTE availble for chemical activity. It is a measure of the amount of water present as part of the chemical analysis of the space, .e., how much water is 2.2 accuracy degree of conformity of a measurement to an accepted st
23、andard or ideal (true) value 2.3 desiccant drying agent 2.4 dew-point temperature to which moisture-laden air must be cooled to induce condensation 2.5 dry-bulb temperature true temperature of the air at rest, .e., the temperature as measured with ordinary instrumentation 2.6 frost-point temperature
24、 to which moisture-laden air must be cooled for frost or ice formation 2.7 humidistat device that senses the moisture content of the air for the purpose of controlling it O IS0 2001 -All rights reserved 1 ISOTTR 18931 :2001 (E) 2.8 humidity general term for the amount of water vapour in the air 2.9
25、hygrometer instrument that measures the moisture content of an air specimen 2.1 o mixing ratio mass of water vapour per unit mass of dry air 2.1 1 mole ratio number of moles of water vapour per mole of dry gas 2.12 percent saturation mass of water vapour present relative to the mass at saturation NO
26、TE Often confused with relative humidity. 2.13 precision measure of repeatability; the degree of closeness of a series of measurements under the same operating conditions 2.14 relative humidity ratio, expressed as a percentage, of the existing partial vapour pressure of water to the vapour pressure
27、at saturation 2.1 5 wet-bulb temperature temperature indicated by a temperature sensor covered by a wetted wick 3 Moisture content of gases The moisture content of a gas specimen can be expressed in a variety of ways. Details are given in the literature (see l in the bibliography) and are beyond the
28、 scope of this Technical Report. Some methods have been defined above and include absolute humidity, relative humidity, dew-point, etc. They are interrelated, and each has its place with scientists, engineers, meteorologists, etc. Films and papers respond directly to relative humidity, and this just
29、ifies its specification in International Standards concerning photography (see annex A). 4 Measuring systems Humidity devices serve one or more of the following purposes: - measurement; - control; - calibration. 2 O IS0 2001 - All rights resewed ISO/TR 18931 :2001 (E) Of the more than ten fundamenta
30、lly different ways to measure moisture content (see 2, 3, 4 in the bibliography), only the most significant for International Standards concerning photography will be discussed. Applicability to each of the above purposes will be included. Agreement among laboratories to within k 2 % RH is obviously
31、 not possible with instrument accuracies of only k 5 %. This Technical Report will, therefore, stress accuracy. 4.1 Gravimetric train (calibration only) In this method, technicians weigh a small amount of a powerful drying agent. A moist air specimen is then passed through so as to remove all its mo
32、isture. The drying agent is then reweighed. Equilibrium moisture content is achieved when a repeat measurement at a greater time interval shows no change. The difference in weight determines the moisture content. Although simple in principle, the procedure is complex in practice and a single measure
33、ment can take hours, days or weeks to perform. The apparatus fills a room and is used by national standards laboratories to provide the ultimate standard of accuracy. 4.2 Dew-point/Frost-point hygrometers; also called condensation hygrometers (secondary calibration, measurement and control) This is
34、the most accurate off-the-shelf method to calibrate working humidity sensors. Dew-point is the temperature at which moisture from the carrier gas condenses on a chilled surface. When the ambient temperature is also measured, relative humidity can be calculated either off-line or by an internal micro
35、processor. Commercial instruments use a mirror chilled by a thermoelectric cooler together with a light-emitting diode and a photocell which receives the reflected image. Moisture condensation causes the light to scatter, at which point the feedback circuit from the photocell controls the cooling so
36、 as to maintain the mirror temperature at the dew-point. The main disadvantages of this method are cost and a slow response at low frost-points, where the sublimation rate can be slow. Advantages include accuracy and freedom from drift as long as the mirror is kept clean. This is easily achieved by
37、use of a maintenance kit that consists of a small bottle of alcohol and some cotton swabs. Some instruments even have a simple calibration control to cancel out day-to-day accumulations of dust. It is important that the illumination system be shielded from room light. The design should, therefore, i
38、nclude provision to maintain good airflow over the mirror, so that the dew-point at the mirror surface matches that in the room. The ambient temperature measurement is as critical as the dew-point measurement. Therefore, the temperature probe should be recalibrated every few months. Dew-point device
39、s are often used for measurement and occassionally as humidistats for controlling relative humidity. Their accuracy can be in the range of k 1 %O RH. Users find them to be free of drift if proper precautions are taken with the mirror. A major failure of the circuitry is virtually unknown, but could
40、conceivably occur. An inexpensive hygrometer would then be useful to establish whether a sudden change in reading was due to the instrument or to the air-handling system. in the USA, the National Institute of Standards and Technology (NET) provides dew-point certification of instruments used to cali
41、brate commercial devices. Here a “two-pressure generator” is used for generating air at a controlled dew-point which is then sent to the device to be calibrated. 4.3 Wet-bulb/dry-bu1 b thermometers and aspirated psychrometers This inexpensive and widely used method employs two thermometers. One is d
42、ry, while the other is wrapped in a wet-cotton wick. The cooling effect of evaporation of water from the wick causes a temperature depression. Psychrometric charts give relative humidity when the two temperatures are known. Sling psychrometers are a common example. Aspirated psychrometers mount the
43、thermometers in a case with a battery-operated fan to draw air over them. The literature is extensive (see 5, 6 in the bibliography) and in summary states that at dew-points above O OC and under ideal conditions, accuracy is seldom better than 5 % RH. This meets the requirements for storage areas, b
44、ut seldom for measurements where k 2 o/o RH is specified. 0 IS0 2001 - All rights reserved 3 ISO/TR 18931 :2001(E) The cooling effect of evaporation is often not complete, and this minimizes the wet-bulb depression. The calculated relative humidity, therefore, tends to be too high. The effect is mos
45、t pronounced at low humidities. A major problem is swelling of the wick, leading to poor contact with the bulb. The wick should, therefore, be replaced frequently. Other problems include wick contamination by salt deposits and oil from the operators fingers. Web-bulb thermometers obviously cannot be
46、 used in freezer vaults. 4.4 Electric hygrometers (measurement and control) The sensing element changes either its resistance or capacitance with changes in relative humidity. The instrument circuitry makes the conversion to relative humidity. These measuring devices are widely used. They respond qu
47、ickly to small changes in humidity and are sensitive enough to measure changes as small as 0,3 Yo RH. They can be multiplexed; data from multiple locations are often sent to a data logger. The sensors themselves are inexpensive and, depending on instrument design, can be replaced if damaged by moist
48、ure condensation resulting from, for example, failure of the air-conditioning system. Many are portable. Others, including some portables, provide analog or digital data recording. Electric sensors have largely replaced hair hygrometers in humidistats. When properly installed and maintained, they ex
49、ceed the precision requirements for conformity to International Standards concerning photography and do so at a modest cost. These devices may drift with time so accuracy should be checked by periodic calibration, either by the vendor or with a dew-point measurement. 4.5 Hair hygrometers/Mechanical expansion This type of equipment is included for the sake of completeness, but cannot be recommended for any purpose where humidity tolerances are k 2 Yo RH. It is a modification of the oldest form of humidity measurement and is based on the principle that a human hair or
