Garrett gas train test is used to determine the content of soluble sulfide in drilling fluid.

    The instrument is mainly composed of a row of transparent plastic chambers, an inert gas source, a pressure regulator, a floating ball flowmeter and a disperser.

    The instrument can quickly and effectively separate soluble sulfide from drilling fluid, with high measurement accuracy, small repetition error, safety and reliability, simple operation and accurate test data.

Soluble sulfides include hydrogen sulfide（H₂S）, sulfur ions（S²¯ ）and hydrogen sulfide ions（HS¯ ）. Acidify the drilling fluid filtrate in the gas separator, so that all sulfides are converted into H2S, and it is carried out by carrier gas foamed by the sample. Garrett gas separator separates gas from liquid. The gas passes through a Dräger tube which can be prolonged and blackened by reacting with hydrogen sulfide. The length of blackering is proportional to sulfide in drilling fluid filtrate. Dräger tube for low concentration range was changed from white to black, and Dräger tube for high concentration range was changed from light blue to smoke black. Usually, pollutants in drilling fluid will not cause this color change.