An analytical procedure to determine the concentration of carbonate species using the Garrett Gas Train (GGT) when performed to API specifications. A water mud filtrate sample is put into the GGT. N₂ or N₂O is the carrier gas. A CO₂ Drdger tube is used to measure the total carbonates released as CO₂ when sulfuric acid is added to the chamber containing the sample. Total carbonates are measured by the amount of CO₂ evolved in the test.

Reference:
Garrett RL: "A New Field Method for the Quantitative Determination of Carbonates in Water-Base Drilling Fluids," Journal of Petroleum Technology 30, no. 7 (July 1978): 860-868.