The boiloff rate of liquid helium of the model SR& 2 is less than 4. 2 liters/ day. A valve of low melting point alloy is used for vacuum and seal. It is vacuumed easily and sealed reliably. SR&2的液氮日蒸发率小于4.2升/日,容器夹层抽空密封采用低熔点合金阀,抽空容易、密封可靠,经七年使用,未发现漏气现象。