Weighing method is a classic method of standard gas preparation

The weighing method is a classic method for preparing international and domestic standard gases. In the past, precision mechanical balances were usually used as standard gas preparation tools, and there has been a set of relatively mature methods for evaluating and calculating the uncertainty of the balance weighing process. In recent years, with the development of electronic balance technology, more and more precision electronic balances are used to prepare standard gases. Due to the difference in equipment principles and weighing procedures, the original uncertainty evaluation method is not completely suitable for 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 the components, the components and the inner wall of the gas cylinder, and the condensable components that are completely in the gaseous state under the experimental conditions. Any condensable component cannot be used when its partial pressure exceeds 70% of its saturated vapor pressure at the lowest use temperature.

2. Required equipment

      The standard gas preparation equipment is composed 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 is composed of a vacuum unit, an ionization vacuum gauge, a pressure gauge, a gas circuit system, and a gas circuit system consisting of a high-pressure, medium-pressure and low-pressure vacuum system, so that the filling of component gas and diluent gas are independent of each other to avoid 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 gas cylinder connectors are used to reduce wear.

      2. Gas weighing device

      The weighing of the component gas is the key to preparing the standard gas. Because the mass of the gas cylinder itself is relatively large (generally 2-20kg), and the mass of the component gas filled is relatively small (2-20g), the balance is very demanding. high. A high-precision balance with a large load (20kg) and a small sensing volume (10mg) is required to weigh the raw material gas of the refillable bottle, and an electronic balance [maximum load (16kg), sensing volume (0.1g)] is used to weigh the refillable bottle. Dilution gas. In order to accurately weigh a small amount of gas filled in a gas cylinder with a large mass, in addition to high requirements on the balance, it is also required to ensure a certain amount of weighing (for the component gas mass is too small, use multiple times Dilution method preparation). Various measures must be taken in the weighing operation to ensure that the weighing reaches high accuracy.

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

      (2) Add and subtract missing codes on the side of the cylinder to be weighed to eliminate the unequal arm error of the balance.

      (3) Weigh after the cylinder is fully balanced.

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

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

      3. Gas cylinder and gas cylinder pretreatment device

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