Customized Vacuum Flask From Nowest Houseware Our practical vacuum flask made of high-quality stainless steel or robust plastic feature excellent insulating performance and clever closures. They are the ideal companions for on the go and are designed for an extra helping of coffee or tea. The process of making thermos cups is much more complicated than other water cups, and the technical requirements are also very high. Vacuum flasks have been used to house standard cells and ovenized Zener diodes, along with their printed circuit board, in precision voltage-regulating devices used as electrical standards. The flask helped with controlling the Zener temperature over a long time span and was used to reduce variations of the output voltage of the Zener standard owing to temperature fluctuation to within a few parts per million.