The external audit module allows the client to check the performance of the analyzers by inserting a module containing a flow-through cell and gas-in/gas-out ports. The gas-in port is meant to be connected to a gas cylinder containing known levels of the target gas. Cylinder standard gas, flow control and sampling lines to the audit cell from the cylinder, and exhaust disposal are the responsibility of the client. The audit module inserts the fiber optic cabling (in series with the optics) at the analyzer. The user simply flows calibration gas from a standard source (cylinder, permeation device) through the cell and determines the equivalent concentrations from the ratio of the measurement path length to the audit cell path length times the calibration gas concentration.