Verifying the correct operation and accuracy of the Gas Chromatograph (GC) is an integral part of a custody transfer metering system, and involves ensuring the accuracy of the analyzer at the time of testing, as well as confirming that the GC performed properly during the periods between validations and assessing the likelihood of continued proper functioning until the next validation. Because the GC will be offline during much of a validation procedure, the validation should only be performed at a time when the composition of the gas flowing through the metering station is relatively stable. Since periods of stable gas composition can be hard to predict, stations with Ultrasonic Meters should measure the speed of sound and compare the result with the calculated speed of sound for a fixed composition to monitor the validity of the fixed composition during the validation. If the variation in the speed of sound varies by more than 2%, the validation procedure should be halted, and the GC should be returned to service until the gas composition stabilizes