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Upon setting up I tested the unit and it worked but the numbers were lower than normal. I need this device to be accurate NOT Sorta Accurate. But accurate. Upon first use it was running about 5 to 6 digits lower on the oxygen side. After reading the instruction manual I figured out how to reset the unit to factory specs. After doing this once I used it and it was right back to normal and working just fine. So to anyone that buys this before you use it read the instruction manual and reset the unit back to factory specs. Other than that it takes a few seconds to get the reading for oxygen. Hope this helps