An instrument used for quantitative analyses of sulfides and carbonates. Specific test methods have been published by API. The oil-mud procedure analyzes active sulfides and uses whole mud samples, whereas the water-base drilling fluid procedure tests filtrate. The GGT unit is a clear, plastic block (2. 5 in. X 4 in. X 6 in. ) that contains three interconnected chambers. A carrier gas is used to flow an inert gas through the chambers. The sample is placed in chamber #1 and is acidified to release sulfides as H₂S and carbonates as CO₂. The appropriate Drdger tube is used to measure the effluent gas that is evolved from the sample. The device is named after Bob Garrett, who invented it while at Exxon Production Research.

Reference:
Garrett RL: "A New Field Method for the Quantitative Determination of Sulfides in Water-Based Drilling Fluids," Journal of Petroleum Technology 29, no. 9 (September 1977): 1195-1201.

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.

Garrett RL, Carlton LA and Denekas MO: "Methods for Field Monitoring of Oil-Based Drilling Fluids for Hydrogen Sulfide and Water Intrusions," SPE Drilling Engineering 3, no. 3 (September 1988): 296-302.