Relative humidity method, water

Relative humidity method. Water absorption is defined as the water absorbed by a dried protein powder with equilibration against water vapor at a known relative humidity. This method, also known as the equilibrium moisture content (EMC) method, was described first by Mellon et al. (10). Huffman et al. (11) used... 

A simple and very useful method of studying humid aging consists of exposing an initially dry sample in a medium at constant temperature and relative humidity (or water activity in immersion) and recording weight changes. Various types of gravimetric curves can be obtained the most frequent ones are shown in Fig. 14.1. 

Ethanol removed by the vapor stream can be recovered by condensation, vapor recompression, or scmbbiag. Ia the first two methods, the coaceatratioa of the recovered ethanol depends on the relative humidity of the sweep stream and the ratio of sweep and permeation rates. In scmbbiag, the rate of water deflvery to the Hquid-gas coatactor affects the ethanol concentration ia the recovered stream. 

The fact that the equilibrium moisture content may be considerable at low humidities is of especial importance in the oven methods. Under ideal conditions no water vapor should be present in the oven, but this is impossible to attain in practice. It is difficult to maintain a dry atmosphere in an air oven, and most commercial vacuum ovens are not air-tight. Thus, the discrepancies in results of different investigators have frequently been traced to different humidities in their ovens. Any attempt to reduce the relative humidity by increasing the oven temperature introduces the danger of error from thermal decomposition. 

In many cases, even the method of conditioning prior to test will influence the ratings. For example, flexural tests run on standard conditioned specimens (50% relative humidity and 73.5°F) may rank materials differently from tests conducted on specimens which have been immersed in water or which have been heated to some elevated temperature after outdoor exposure. 

Generation of water sorption/desorption isotherms in a controlled relative humidity environment can be carried out either gravimetrically or volumetrically. Gravimetric methods require... 

Methods for water absorption are given in ISO 62 [17]. The process is similar in principle to that of ISO 175 [16], but there are three procedures covering exposure to water at 23 °C, boiling water and 50% relative humidity. 

The use of a water-soaking, oven-drying series of cycles for the determination of dimensional stability of wood is a severe test (although it may produce useful data) and it does not necessarily reflect the conditions that wood will encounter in service conditions. As a result, some workers determine dimensional stability by subjecting samples to different relative humidities. It can be readily understood that dimensional stability values determined using different methods will not be comparable and it therefore needs to be explicitly stated how these values were obtained. 

As briefly mentioned in Section 4.3.S.2, Atiyeh et al. [152] performed water balance measurements and calculations to determine the effect of using DLs with MPLs (on either or both cathode and anode sides). In their fuel cell test station, water collection systems were added in order to be able to collect and measure accurately the water leaving both anode and cathode sides of the fuel cell. Based on the operating conditions (e.g., pressures, temperatures, relative humidities, etc.) and the total amount of water accumulated at the outlets of the test station, water balance calculations were performed fo defermine the net water drag coefficient. Janssen and Overvelde [171] used this method to observe how different operating conditions and fuel cell maferials affected... 

The standard states that the measured equilibrium water vapour absorption is substantially the same as the equilibrium absorption which would be obtained by immersion in an aqueous solution and which would be in equilibrium with the vapour (apart from effects due to extraction of water soluble constituents), i.e. in a solution which would maintain the test humidity. As this implies, the equilibrium water absorption of rubber is reduced if the water is not pure and this test method, because 100% R.H. is not readily maintained, does not measure the absorption of pure water. When approaching 100% R.H. the effect is rapid and even very small amounts of a salt in solution will significantly lower the equilibrium absorption. Hence, tests intended to simulate the use of rubber in contact with an aqueous solution, rather than pure water, should be made with that solution or with one having the same equivalent relative humidity. 

Percent relative humidity is the best known ami perhaps the most widely used method for expressing the water vapor content of air. Percent relative humidity is defined as the ratio of the prevailing water vapor pressure c . to the water vapor pressure if the air were saturated, t, multiplied by 100 ... 

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