AGA Report No. 9 (also known as AGA 9), flow calibration has been required for all fiscal applications when using gas ultrasonic meters (USMs). Traditionally the meter, along with the associated piping and flow conditioner (called the Metering Package in AGA 9), has been sent to the laboratory to be calibrated as a single unit. For small line sizes, the added cost of shipping all the piping components with the meter isn’t significant. However, when larger line sizes are involved, the additional cost becomes very substantial. It also increases the complexity of the shipment since many of these complete assemblies are very heavy and quite long. The latest version of AGA 9, Fourth Edition, dated January 2022 , now specifically addresses the issue of calibrating the USM with laboratory piping, also referred to as “surrogate spools” by AGA 9, in lieu of the field piping.