Standard gas setting method to determine the concentration of standard gas

Update:30-08-2019
Summary:

  In theory, there are two main methods for standa […]

 

In theory, there are two main methods for standard gas setting.

The first is the direct measurement method, that is, the determination method is used for the measurement. But so far, people have not found a definition method suitable for the standard gas setting, and many chemical analysis methods have large measurement errors when the gas concentration is lower than lo-4, and often cannot meet the requirements of the emblem.

The second is an indirect measurement method in which the concentration value of the standard gas is determined by physical measurement methods such as mass measurement (mass ratio mixing method), pressure measurement method (pressure ratio mixing method), and volume measurement method (volume ratio mixing method). In other words, the standard gas in the charging container (high pressure steel cylinder) is measured by the mass measurement method, and the standard gas prepared by the standard gas generation method is determined by the flow rate measurement method. However, this physical measurement method is based on the premise that the component gas and the diluent gas can be uniformly mixed and the different gases are mixed without generating a chemical reaction (composition change). Sometimes physical methods such as mass measurement cannot be used to determine the value. Therefore, in the standard gas setting, the measurement method and the chemical analysis method are simultaneously used.

The standard gas ratio medium-mass ratio mixing method is a method of measuring a high-purity component gas and a diluent gas by using a super large-scale high-precision balance, and quantitatively mixing to prepare a standard gas. Its characteristic is that its quality measurement value can be traced back to the international kilogram original device, and it does not need to correct the parameters such as pressure, temperature and viscosity of the gas during gas distribution. Therefore, the measured value is accurate and reliable. However, this method relies only on a method of mass determination to determine the concentration value, so it is based on the high precision of the balance and the high stability of the gas in the cylinder. In addition, the quality of the cylinder is much larger than the mass of the component gas or diluent gas. Moreover, weights and cylinders vary greatly in size, material and density, requiring high quality measurement techniques. Generally, a large-scale precision flat with a wide mass measurement range and measurement accuracy can be used, and a weight is used instead of the weighing method. Although quality is a problem compared to hybrid methods, it is much more accurate than other methods.

The procedure for determining the standard gas concentration value by mass spectrometry using standard gas settings is as follows:
(1) First clean the high-pressure cylinder with dilute gas, then vacuum it to below 10-2Pa, and accurately weigh the high-quality cylinder W0 with the balance.
(2) The high-pressure cylinder is filled with the component gas to be formulated and its mass W1 is measured.
(3) The high-pressure cylinder is again filled with the dilution gas and its mass W2 is weighed.

When measuring the mass of a large-capacity cylinder, problems such as the weight of the weight and the container on the surface (such as roughness, rust, paint detachment, etc.), average density, volume, etc., will bring errors to the quality measurement. In order to overcome the influence of the above factors on the measurement results, an alternative weighing method may be adopted, that is, selecting an alternative container which is very similar to the container filled with gas in the state of size, village material and outer surface state, and performs measurement after charging and tunneling. In order to offset the impact of the measurement environment. The standard value determined by mass spectrometry (mass ratio mixing method) is consistent with the measurement results of mass spectrometry, but this method is not suitable for a standard gas or a chemically reactive gas which is slightly mixed.

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