Underwriters Laboratories (UL) calibration requirements stipulate that inspection, measuring and test equipment (including flow meter) must be calibrated at least annually for their intended function and use or whenever the flow meter has been subject to some form of abuse that may affect the measurement device fitness for use.
The manufacturer’s most commonly recommended calibration interval for precision test equipment is also one year. The one year interval optimizes the cost of calibration versus the cost of an out-of-tolerance for many users. This interval achieves the desired reliability target and helps to resolve early minor problems that affect process consistency and repeatability for most industries.
However, if in practice a response to an out-of-tolerance measurement leads to significant costs or if you are using a new meter or one located in an hostile environment, or if you are requiring FDA approval, it is recommend to calibrate on shorter calibration schedule in order to investigate early minor deviation from the standards until you get an accurate “footprint” of the meter and then switch to longer interval of calibration.