The units require routine calibration I mailed mine to them to I have all 3 of my devices calibrated at the same rime. Unfortunately, I had waited to long and 2 of them could no longer be calibrated. So they offered me a refund or a discount on the purchase of a new device. I chose the discount and bought 2 new identical breathalyzers. They calibrated the 3ed one and sent it back to me. However then it would no longer work. I let them know and they kindly replaced the non functioning item with a new one that was the same model. I am very happy with their products, their maintenance of their products and their professional customer service. These are the ones I now own