Thats why they should be calibrated at least yearly. They should have come with a calibration certificate. Think non armed services the tolerance is 5% No way to tell unless you have a testing bench which are few and far between. Then you get into the stupid side also they have to be tem soaked for a day at around 70degrees less that 30% humidity before they can be calibrated. OK Thats what we did but what do you do when it is 10 degrees out and break it out of the tool box to use?????? Did it every day along with the rest of the USAF. The ones that never ever went out of calibration were the beam type.