Is it true that there is a known constant ratio between the amount of alcohol in breath and the amount of alcohol in blood?
No. Although this is an interesting idea and a convenient premise, it is a premise which is simply fictitious. The ratio between the amount of alcohol in one's blood and the amount of alcohol in one's breath is not a known constant. In fact, values in scientific literature range from about 1300:1 to 2700:1. Of course, in order to write a computer program to do the necessary calculations, a particular ratio had to be chosen and incorporated into the program which runs the machine. The ratio used in Virginia and the US is 2100:1 - meaning our machines are programmed to believe there is the same quantity of alcohol in 2100 parts of breath as 1 part of blood. Interestingly, the ratio used in Virginia and the US is different than the ratio used in other countries.