The 753a features 400 hour sensor life under normal use a spare sensor in included with each instrument for a total of 800 hours of continuous use and can detect leaks as small as 0.1 ounces per year (R134a / R22) 0.1 ounce per year sensitivity Detect all existing refrigerants and blends including 404A, PURON ® and R-507 (AZ50®) Pump driven for increased sensitivity with blended refrigerants