Weighing method is the classic method of standard gas preparation

Weighing method is a classic method for international and domestic standard gas preparation. In the past, a precision mechanical balance was usually used as a preparation tool for standard gas, and a set of relatively mature methods for the evaluation and calculation of uncertainty in the weighing process of the balance has also been established. In recent years, with the development of electronic balance technology, more and more standard gases are prepared using precision electronic balances. Due to the difference in equipment principles and weighing procedures, the original uncertainty evaluation method is not fully applicable to the weighing process of electronic balances, and new evaluation methods must be studied to meet the needs of new technology applications.

1. Scope of application

      The weighing method is the method recommended by the International Organization for Standardization. It is only suitable for gases that do not react between components, components and the inner wall of the cylinder, and condensable components that are completely gaseous under experimental conditions. Any condensable component cannot be used when its partial pressure exceeds 70% of its saturated vapor pressure at the lowest service temperature.

2. Required equipment

      The standard gas preparation device consists of a gas filling device, a gas weighing device, a gas cylinder and a gas cylinder pretreatment device.

      1. Gas filling device and panel

      The gas filling device consists of a vacuum unit, an ionization vacuum gauge, a pressure gauge, a gas circuit system, and the gas circuit system consists of three parts: high pressure, medium pressure and low pressure vacuum system, so that the filling of the component gas and the dilution gas is independent of each other and avoids mutual contamination. . Use valves, pressure gauges and vacuum gauges with good performance to simplify the gas path as much as possible and reduce interfaces to ensure the airtight performance of the system. Specially designed cylinder connections are used to reduce wear and tear.

      2. Gas weighing device

      The weighing of the component gas is the key to the preparation of the standard gas. Since the gas cylinder itself has a large mass (usually 2-20kg), and the mass of the filled component gas is relatively small (2-20g), the balance is very demanding. high. It is necessary to use a high-precision balance with a large load (20kg) and a small sensitivity (10mg) to weigh the raw material gas of the inflated gas cylinder, and use an electronic balance [maximum load (16kg), sensitivity (0.1g)] to weigh the inflated gas cylinder. Diluent gas. In order to accurately weigh a small amount of gas filled in a gas cylinder with a large mass, in addition to the high requirements for the balance, it is also required to ensure a certain amount of weighing (for the component gas whose mass is too small, multiple prepared by dilution). Various measures must be taken during weighing operations to ensure a high degree of weighing accuracy.

      (1) Use reference gas cylinders with the same shape and similar quality for weighing (that is, place a reference gas cylinder on one side of the balance, and place the gas cylinder to be measured on the other side to balance it). The reference bottle weighing can offset the influence of the buoyancy of the cylinder, the adsorption of moisture on the surface of the cylinder, and the static electricity.

      (2) Carry out addition and subtraction operations on the side of the gas cylinder to be weighed to eliminate the unequal arm error of the balance.

      (3) Weighing after the cylinder is fully balanced.

      (4) Handle with care and keep the gas cylinder clean to avoid contamination and wear.

      (5) The weighing operation is carried out three times, and the average value is taken.

      3. Gas cylinder and gas cylinder pretreatment device

      Generally, 2L, 4L and 8L gas cylinders are used to fill standard gas. The gas cylinder pretreatment device is used for cleaning, heating and evacuation of gas cylinders. The heating temperature can be set arbitrarily within a certain range.