A simple method to identify the vacuum insulation performance of thermos

Pour the boiled water into the thermos cup and tighten the stopper or lid clockwise for 2 to 3 minutes, then touch the outer surface of the cup body. If the cup body is obviously hot, it indicates that the product has lost vacuum degree and can not achieve good thermal insulation effect.  And if only the mouth of the cup is a little hot, the bottom temperature of the cup body does not change, indicating that such a vacuum cup is effective, good insulation performance.