Special Gas Transmission Testing Methods and Instrument Choice

June 26, 2009 - PRLog -- Abstract: gas transmission rate of special gases is very practical with great testing demands. However, owing to the different characteristics of different gases, how to test gas transmission rate of special gases, what are the relevant methodologies and how to select testing methods and instrument, are the experiences that needs to be collected in actual testing. This article combines gas characteristics and the relevant testing information, and makes introductions to special gas transmission rate methods with testing advices.
 Keywords: special gas, transmission rate, organic gas, differential pressure method, isostatic method

 Barrier property testing has not fully popularized in our country, but its importance and practicality are widely known. Moreover, the testing subjects of some researchers have come beyond the common transmission testing field, and into the special gas transmission rate testing. This shows that domestic barrier property testing has developed to a certain level, and has become a positive testing demand. Meanwhile, barrier property testing has led to more application fields, including aeronautic, aviation, special gas storage, medical device, building and many other fields, with great potentiality. As a new area in barrier property testing, how to special gas transmission rate, and the selection of testing methods and instruments, need to be summarized in actual testing. Labthink Lab has many experiences of special gas transmission rate testing and consulting services for her clients. Hence, in this article, the author, integrating gas characteristics and relevant testing information, gives an introduction to special gas transmission rate testing method and relevant testing suggestions.

1. Introduction to Testing Gases
 Oxygen and water vapor (moisture) are the most common testing gases in barrier property testing. Those gases are safe according to their characteristics. Though oxygen can help burning, it's safe and supportive for breath. Water vapor is also safe and harmless. But if the test gases changes to special gases, the testing process would be quite complicated owing to the great differences in their characteristics. For example, He is inert gas with stable characteristics, flameless, safe and no corrosive; H2 is safe, but explosive and flammable; CO can burn and poisonous; and CH4 is also explosive and flammable with a special note that it is an organic gas.
 The flammability and toxicity of test gas have direct relationship to the life safety of the test operator; but the corrosiveness of the test gas and whether the test gas is organic gas or not will directly affect the instrument structure and testing. As to specimen, there are obvious different influences of organic gases and other gases. Though the testing demands are from different fields, most specimens for gas transmission rate testing are high polymers or complex formed by high polymer and metal, paper, fiber, etc. When organic gas permeates into the high polymer, high polymer specimen would occur swelling which would greatly change its appearance and barrier property with increasing gas transmission rate and more difficulties to achieve transmission balance. Therefore, there are obvious differences between inorganic gas transmission testing and organic gas transmission testing.

2. Testing Methods for Special Gas Transmission Rate
 The author holds that special gases can be categorized into 3 kinds: the first kind is inflammable and non-toxic, including CO2, N2, and inert gases; the second kind is flammable or toxic inorganic gases, including H2, CO, NO, HCl, H2S, SO2 and Cl2, etc; the third kind is organic gas (vapor), such as methane and ethanol. The testing methods and instruments for the above three kinds are different and will be introduced. One thing needs to be specially pointed out is that if the test gas is of strong corrosiveness, testing instruments should be customized and its feasibility of testing methods should be evaluated.
2.1 inflammable and Non-toxic Inorganic Gas
 The testing for such gases can be accomplished with differential pressure gas transmission testing instruments with no instrument reconstruction and no accessories. The operator only needs to purchase testing gas and take care of the tail gas.
2.2 Flammable or Toxic Inorganic Gases
 This kind of gases is the so-called dangerous gas with certain dangers when testing. Since they are toxic, flammable and explosive or both, dangerous gas quantity of the whole lab (including those inside instruments) should be strictly controlled according to the characteristics of the gases. At the same time, the handling of tail gas should be strengthened so as to guarantee safety. The testing method for this kind of gases is also differential pressure method. But, proper adjustments are made in structural design according to different gas characteristics.
2.3 Organic Gas (Vapor)
 Only a few organics are gaseous under normal temperature, such as methane; most of organics are liquid with volatility, such as ethanol. Therefore, the testing gas mentioned here are organic vapor.
 Organic gases can lead to swelling of high polymer materials, and further affect their permeability obviously. Therefore, the testing methods of organic gases are quite different from that of inorganic gases. The biggest difference is the quite low gas concentration when testing. Considering the characteristics of organic gases, there should be an organic gas generating system to provide test gas of accurate concentration. For inflammable and non-toxic inorganic gas, the high concentration side should be of 100% concentration of testing gas; for flammable or toxic inorganic gases, the high concentration side should be controlled within the safe concentration scope; as to organic gas test, only when high concentration side is no higher than 100ppm, can we regard specimen swelling by organic gas as being no influence to specimen permeability.
Organic gas transmission rate can be tested with isostatic method. Labthink has developed professional isostatic method organic gas transmission rate testing device - PERMETM　OR2/410 Organic Gas Transmission Rate Testing System. This instrument is of patent design, and consists of transmission chamber, organic gas generator, timed sampling valve, isolation room and FID (flame ionization detector). It’s easy to operate, and it's the organic gas transmission rate tester with highest automation.  The drafting of relevant national standard of this method has begun.

 Though isostatic method should be applied for organic gas transmission rate testing, some applies differential pressure method (also widely applied in documentations) for some specific purposes. In fact, differential pressure method can be used for the organic gas transmission rate testing of certain organic gases. But, the testing instruments must be customized as that of flammable or toxic inorganic gases.

3. Conclusion
 The transmission rate of special gas are very practical and with great demands. In recent years, there are more and more testing methods and instruments for special gas transmission rate testing. And it can be predicted that, with the popularization of material barrier property testing, special gas transmission rate testing will be the important part in the future barrier property testing and with more applications.

Further information please feel free to contact Shirley Wu via trade2@labthink.cn, skype:shirley-labthink, MSN:shirleywu_labthink@hotmail.com or (tel)0086-531-85068709.

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