The tube generates a known concentration (2ppm) of the formaldehyde fume, which can be measured by the instrument in order to check accuracy and function fo the device. Limited usage time:The standard has a capacity of about 100 tests or a useful lifetime of 6 months (whatever will happen earlier of both). These 6 months are valid from the date of manufacture/delivery and not from "first use".After 6 months (or 100 uses) the formaldehyde will begin to drop quite quickly - causing lower levels. After a while bacteria and fungi, previously killed off by the formaldehyde, will start to grow in the standard and give off ethanol and other similar gasses which cause the instrument readings to go back up and beyond the original formaldehyde levels, due to cross-sensitivity.This is why it's very important to observe the date on the tube and to keep a track of the usage. Some users mark off each sample on the label to remind them of their usage count.
