A number of factors should be considered when extracting gas from a cylinder through a delivery system.

Minimum operating pressure

The information attached to the cylinder will give a pressure value below which the gas should not be used. It has been reported that at a certain pressure, gas molecules attached to the cylinder wall will desorb when the pressure in the cylinder drops, resulting in a higher mole fraction.

In addition to the impact on the stability of the gas mixture, from the manufacturer's point of view, the gas in the cylinder cannot be completely emptied, otherwise it may be contaminated by the ambient air. This is especially important for cylinders that need special treatment for stability.

Temperature

Manufacturers usually limit the storage and use of gas mixtures containing easily condensing components to a certain temperature range. In the calculation of such a gas mixture, the manufacturer will give the allowable temperature and calculate the vapor pressure at that temperature. Usually, a safe temperature range is given for use. However, when used outside this temperature range, condensation of the easily condensing components may occur, thus changing the concentration of the components of the gas mixture. When analyzing the gas mixture, each cylinder should be kept at the same temperature. According to the relevant gas law (Boyle,Gay-Lussac), the temperature difference between different gas mixtures will affect the response value of the gas analyzer to these gas mixtures.

Reduced pressure and flow

Typically, the extraction of air from a cylinder is regulated by a pressure relief valve or a flow controller (needle valve, mass flow controller, capillary device, etc.). Gas cooling may occur in the cylinder due to adiabatic expansion when taking gas from the cylinder. The throttling effect may also change the temperature of the gas itself in the delivery. This temperature effect may cause condensation, especially for gas mixtures containing condensable components. Therefore, the pressure drop of the flow controller should be minimized. The flow characteristics of the flow controller are usually given by the manufacturer and are sufficient to determine whether the selected flow controller can control the required flow. When calculating the pressure drop, the pipe size (inner diameter and length) should also be considered.

External HEATING of the GAS in front of OR over THE pressure RELIEF VALVE, or the use of several pressure relief valves for stepwise pressure reduction help minimize the risk of condensation.

Displacement or change of cylinder position

Reconnecting cylinders may cause leakage, which can be avoided by leak detection. In an optimal setup, the pressure relief valve should always be attached to the cylinder with some gas retained in the cylinder. This helps reduce the number of times the pressure relief valve has to be replaced before analysis. However, EVEN IN THIS OPTIMAL SETTING, ambient air may diffuse into the pressure relief valve after a period of pressure retention.

Note: IN THE long TERM, EVEN if the pressure is maintained, there is usually oxygen and water from the environment diffused into the pressure relief valve.