Standard Practice for Maintaining Constant Relative Humidity by Means of Aqueous Glycerin Solutions
This standard is issued under the fixed designation D5032; the number immediately following the designation indicates the year of original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A superscript epsilon (Ɛ) indicates an editorial change since the last revision or reapproval.
- Scope
1.1 This practice describes a method for obtaining constant relative humidity ranging from 30 to 98 % at temperatures ranging from 0 to 70⁰C in relatively small containers by means of an aqueous glycerin solution.
1.2 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 purport 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 appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.
- Referenced Documents
2.1 ASTM Standards: ²
D618 Practice for Conditioning Plastics for Testing
D4023 Terminology Relating to Humidity Measurements³
D6054 Practice for Conditioning Electrical Insulating Materials for Testing
E104 Practice for Maintaining Constant Relative Humidity by Means of Aqueous Solutions
2.2 Other Documents:
DIN50008 Constant Climates over Aqueous Solutions⁴
Part 1: Saturated Salt and Glycerol Solutions
Part 2: Sulfuric Acid Solutions (1981)
- Summary of Practice
3.1 Controlled relative humidity environments are generated using mixtures of glycerin and water.
3.2 Practice E104 contains methods for maintaining constant relative humidity environments using aqueous saturated salt solutions or various strength sulfuric acid-water systems.
- Significance and Use
4.1 Controlled relative humidity environments are important for conditioning materials for shelf-life studies or for investigating the change in physical or dielectric properties after exposure.
4.2 The use of aqueous-glycerin solutions reduces the possibility of contamination of the materials or corrosion of electrode systems which would be more likely to result from saturated salt or acid water solutions.
4.3 Applicable material specifications should state the exposure conditions, including time, temperature and relative humidity that a material should be subjected to before subsequent testing. Typical conditions are given in Practice D1618 or D6054.
5.Apparatus
5.1 Container, airtight, of a material not acted upon by copper sulfate (or with the glycerin solution contained in a tray made of a material not acted upon by copper sulfate).
5.2 Refract meter, covering the range of 1.33 to 1.47 (sodium) with an accuracy of 0.0003.
- Glycerin Solution
6.1 Use a good industrial grade of glycerin (“high gravity” and “dynamite” grades have been found to be satisfactory) in distilled water. Calculate the concentration in terms of the refractive index, (R), at 25⁰C for the desired relative humidity at any temperature between 0 and 70⁰C as follows:
R = (√ (100 + A) ² + A² - (H + A) ² - A) 1/715.3 + 1.3333
Where: T = temperature of the solution, ⁰C
A = 25.60 – 0.1950T + 0.0008T², and H = relative humidity percent.
6.1.1 This will give the desired relative humidity with an accuracy of ±0.2% at a constant temperature of 25⁰C. At other constant temperatures, the error, if any, may increase with the deviation of the temperature from 25⁰C. The relative humidity values at 0, 25, 50, and 70⁰C for a number of refractive index values are given in Table 1. 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.
6.2 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 100ml 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.
6.3 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 25⁰C in an open container, in a still atmosphere of 50% relative humidity at 25⁰C, 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 value of a 96% solution having a depth of 1 in. by 0.5% relative humidity in 26 h.
6.4 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 glycerin-water solution adjusted to produce a 96% relative humidity at 25⁰C will reduce the humidity by 0.5% relative humidity.
- Precautions
7.1 Container:
7.1.1 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 necessary because of the device to be conditioned.
7.1.2 Although an airtight container is recommended, it is desirable to have a vent under certain conditions of the 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 adject if necessary, in this case.
7.1.3 Make the surface creepage distance between the solution and the material being conditioned long enough to prevent the solution from creeping on the material being conditioned.
7.2 Temperature Fluctuations:
7.2.1 Avoid temperature fluctuations. Best results are obtained 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. Changing ambient temperature causes a temperature difference between 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.
7.2.2 If a controlled temperature room is not available, place the container in a location having the minimum change in temperature and thermally insulating the container with a minimum of 1 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 temperature.
7.2.3 A glass desiccator covered with a corrugated paper box will stand short time (30 min or less) fluctuations of temperature of ± 1⁰C without changing the relative humidity over ±0.1%.
7.2.4 A thick aluminum cover or base plate, or both, on the container will also effectively dampen temperature fluctuations.
7.3 Temperature Above Room Temperature—Operating at temperatures above room temperature is not as satisfactory as operating at room temperature, because of the greater possibility 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 temperatures 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.
Note 3—For example, with a solution for a relative humidity of 96% a spot having a temperature 0.3⁰C higher than that of the solution would have a relative humidity of 94%, while that having a temperature 0.3⁰C lower would have a relative humidity of 98%.
7.4 Temperatures Below Room Temperature--- Operating at temperatures below room temperature is not as satisfactory as operating at room temperature, because of the greater possibility 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 temperatures 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.
7.5 Loading—Do not overload the container as this will decrease the rate of the 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.
7.6Opening of the Chamber During Test – Avoid opening the chamber during a test since the rate of establishing equilibrium after reclosing the chamber is not known. Equilibrium in the chamber depends on the ratio of chamber volume to solution surface area, type of material in the chamber, amount of material in the chamber and temperature difference between the solution and the chamber atmosphere.
- Keywords
8.1 aqueous glycerin solutions; conditioning; constant relative humidity; glycerin; relative humidity humidity
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