ASTM D-5032-11 Standard Practice for Maintaining Constant Relative Humidity by Means of Aqueous Glycerin Solutions

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1.   Scope

  1. This practice describes a method for obtaining constant relative humidity ranging from 30 to 98 % at temperatures ranging from O to 70°C in relatively small containers by means of an aqueous glycerin solution.

l.2 This practice is applicable for closed systems such as environmental conditioning containers.

1.3 This practice is not recommended for the generation of continuous (flowing) streams of constant humidity unless precautionary criteria are followed to ensure source stability.

1.4 This standard does not pwport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appro­ priate safety and health practices and determine the applica­ bility of regulatory limitations prior to use.

2.   Referenced Documents

  • ASTM Standards:2

D618 Practice for Conditioning Plastics for Testing D4023 Terminology Relating to Humidity Measuremcnts3

D6054 Practice for Conditioning Electrical Insulating Ma- terials for Testing

EI04 Practice for Maintaining Constant Relative Humidity

hy Means of Aqueous Solutions

2.2        Other Documents:

DIN50008 Constant Climates over Aqueous Solutions4 Part I: Saturated Salt and Glycerol Solutions

Pait 2: Sulfuric Acid Solutions (1981)

‘ This practice is under the jurisdiction of ASTM Committee 009 on Electrical and Electronic Insulating Materials and is the direct responsibility of Subcommittee

1)09.12 on Electrical Tests.

Current edition approved Aug. I, 2011. Published Seplember 2011. Originally approved in 1990. Last previous edition approved in 2003 as D5032 -97(2003). DOI: IO.l520/D5032-II.

2 For referenced ASTM standards. visit the ASTM website, For A111111al Book of ASTM S1a11dards volume information. refer lo the standard Document Summary page on the ASTM website.

3 Withdrawn. The last approved version of this historical standard is referenced

on W\

4 Available f m De111sc/1es h1s1i1111 f11r Norm1111g, 4-10 811rggre11ze11strasse Postfac/1 1107, D-1000 Berlin, Germany. Also available from American National Standards Institute, 25 W. 43rd St., 4th Floor, New York, NY I0036..

3.   Summary of Practice

  • J Controlled relative humidity environment<, are gener­ ated using mixtures of glycerin and water.

3.2 Practice El04 contains methods for maintaining con­ stant relative humidity environments using aqueous saturated salt solutions or various strength sulfuric acid-water systems.

4.    Significance and Use

  • Controlled relative humidity environments are impor­ tant for conditioning materials for shelf-life studies or for investigating the change in physical or dielect1ic properties after exposure.
    • The use of aqueous-glycerin solutions reduces the pos­ sibility of contamination of the materials or corrosion of electrode systems which would be more likely to result from saturated salt or acid water solutions.
    • Applicable material specifications should state the ex­ posure conditions, including time, temperature and relative humidity that a mate1ial should be subjected to before subse­ quent testing. Typical conditions are given in Practice D6l8 or D6054.

5.    Apparatus

  • Container, airtight, of a material not acted upon by copper sulfate (or with the glyce1in solution contained in a tray made of a material not acted upon by copper sulfate).
    • Refract meter, covering the range of 1.33 to 1.47 (sodium) with an accuracy of 0.0003.

6.     Glycerin Solution

  • Use a good industrial grade of glycerin (“high gravity” and “dynamite” grades have been found to be satisfactory) in distilled water. Calculate tlie concentration in terms of the refractive index, (R), at 25°C for the desired relative humidity at any temperature between 0 and 70°C as follows:


 =(Y(l00+   A)2+A2-(H+A)2-A)715.3+  1.3333



T         temperature of the solution, °C,

= 25.60-0.1950T+0.0008T2, and

H = relative humidity, percent.

  • This will give the desired relative humidity with an accuracy of ±0.2 % ata constant temperature of 25°C. At other constant temperatures, the error, if any, may increase with the deviation of the temperature from 25°C. The relative humidity values at 0, 25, 50 and 70°C for a number of refractive index values are given in Table I. Obtain the refractive index for intermediate values of relative humidity and temperature by plotting curves from the values in the table or by calculating from the above formula.
    • To prevent fungus growth in the solution, add about

0.1 % by weight of copper sulfate to the glycerin solution. The most convenient way of measuring the copper sulfate is to prepare a saturated solution in water and add four drops of the saturated solution per 100 mL of the glycerin solution. Use a container. or tray holding the glycerin solution, made of a material that will not react with the copper in the copper sulfate. If the copper is removed, fungus growth can occur, which will cause lowering of the humidity value of the glycerin solution.

  • Loss of water through evaporation when the container is opened can reduce the humidity value of the solution. The rate of loss with the container open is quite low and is negligible for the normal time the container would be opened for loading and unloading (Note 1).

NOTE 1-A solution adjusted to produce a 96 % relative humidity atmosphere at 25°C in an open container. in a still atmosphere of 50 % relative humidity at 25°C, will lose water at the rate of approximately 0.01 mL/h/in.2 of solution surface area. This rate will reduce the relative humidity valueof a 96 % solution having a depth of I in. by 0.5 % relative humidity in 26 h.

  • Loss of water by absorption by the material being conditioned, can reduce the humidity value of the solution. Proper precaution must be taken to prevent the reduction of humidity by a material being conditioned that will absorb a large amount of water. If it is estimated that the reduction in humidity will be greater than desired, one or both of the following options must be done: Reduce the loading below that suggested in 7.5 or increase the depth of the solution.

NOTE 2-For example, a loss of 0.26 mL water/in.3 of a glyceria-water solution adjusted to produce a 96 % relative humidity at 25°C will reduce the humidity by 0.5 % relative humidity.

TABLE 1 Relative Humidity Over Glycerin Solutions

Refractive                                               Relative Humidity, %

Index at 25•co·c25•c5o•c10°c
1.4387 53.3 55.0 56.5 57.6 O) 48.3 <…SOJiJ 51.5 52.6 1.4486 43.3 45.0 46.6 47.7    

– 9-)                        58.4               CoP-01          61.4                  62.5

                                      38.3                 <.@           41.6                    42.7  

7.     Precautions

7.1   Container:

7.I. 1 Make the container small so that the temperature throughout the container will be the same as that of the solution. Keep the volume of the air space per unit area of surface of solution low. Ten cubic inches or less per in.2 of solution surface is advisable unless a larger volume is neces­ sary because of the device to be conditioned.

7. l.2 Although an airtight container is recommended, it is desirable to have a vent under certain conditions of test or with some kinds of containers. (Changes in pressure may produce undesirable cracks in some types of containers.) Make the vent as small as practical as there will be a continual loss of vapor through the vent. Check the concentration of the solution periodically and adjust if necessary in this case.

7.1.3 Make the swface creepage distance between the solution and the material being conditioned long enough to prevent the solution from creeping on to the material being conditioned.

7.2   Temperature Fluctuations:

  • Avoid temperature fluctuations. Best results are ob­ tained in a controlled temperature room where the average temperature is constant and the fluctuations are of relatively short duration. Cover the container to shield from drafts. Drafts may cause temperature differences inside the container. Chang­ ing ambient temperature causes a temperature difference be­ tween that of the solution and the air above it. As a rule. changes in the solution temperature lag behind that of the air in the container. This results in a low humidity with rising temperature and a high humidity with falling temperature.
    • If a controlled temperature room is not available. place the container in a location having the mi:1imum change in temperature and thermally insulating the container with a minimum of I in. of glass wool, or the equivalent. Reducing the volume of air space in the container per unit area of solution surface will also reduce the effect of changing tem­ perature.
    • A glass desiccator covered with a corrugated paper box will stand shor1 time (30 min or less) fluctuations of temperature of ± I°C without changing the relative humidity over :!::0.1 %. Where larger fluctuations or long time fluctua­ tions are encountered, thermally insulate the container. It is estimated that a thermally insulated container will withstand fluctuations of temperature of ±3°C without changing the relative humidity over ±0.1 %.
    • A thick aluminum cover or base plate, or both, on the container will also effectively dampen temperature fluctua­ tions.
    • Temperature Above Room Temperature-Operating at temperatures above room temperatw·e is not as satisfactory as operating at room temperature, because of the greater possi­ bility of the air in the container not being equivalent to the solution temperature and not being the equivalent throughout the container. However, with proper care, humidities at tem­ peratures above room temperature are attainable by heating the container in an oven. Thermally insulate the container as described in 7.2 and adjust the oven air circulation so as to have as nearly uniform temperature throughout the container as possible. Load the container while at room temperature.

Norr 3-For example, with a solution for a relative humidity of 96 %, a spot having a temperature 0.3°C higher than that of the solution would have a relative humidity or 94 %. while that having a temperature 0.3°C lower would have a relative humidity of 98 %.

  • Temperatures Below Room TemperatureOperating at temperatures below room temperature is not as satisfactory as operating at room temperature, because of the greater possi­ bility of the air in the container not being equivalent to the solution temperature and not being the equivalent throughout the container. However, with proper care, humidities at tem­ peratures below room temperature are attainable by cooling the container in a chamber. Thermally insulate the container as described in 7.2 and adjust the chamber air circulation so as to have as nearly uniform temperature throughout the container as possible. Load the container by reducing the temperature of the container below the conditioning temperature before loading.
  • Loading-Do not overload the container as this will decrease the rate of rise of the humidity in the container to such an extent that an unreasonably long time is required for the humidity to reach a steady state. The limit of loading cannot very well be specified as this depends upon the amount of moisture the material will absorb and this will differ by material. As a general rule, make the overall area of the material less than the surface area of the solution.
    • Opening of the Chamber During Test-Avoid opening the chamber dming a test since the rate of establishing equilibrium after reclosing the chamber is not known. Equilib­ rium in the chamber depends on the ratio of chamber volume to solution surface area, type of material in the chamber, amount of matetial in the chamber and temperature difference between the solution and the chamber atmosphere.

8.    Keywords

  • aqueous glycerin solutions; conditioning; constant rela­tive humidity; glycerin; relative humidity