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hydrocarbon detection 

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 THE GAS CHROMATOGRAPH

 Gas-chromatograph - the main device for analysis of the mud-logging unit

   Sample (gas-air mixture) is brought in from degasifier, mounted at flow-line. Is quickly obtained so called Total Gas (total hydrocarbons) and some more slowly, other strengths for alkanes: methane, ethane, propane, isobutane, n-butane, isopentane and n-pentane.

    What is it? Why Croma? And why graf? The name of the device can induce wrong ideas. It is "Gas" because the analysis is done using a gas who carries the sample through a column. It is "Chroma" (color in Greek) since it first appeared for the analysis of liquids by their migration at different speeds on a filter paper impregnated with specific indicators. After a while, appear colored rings or bands depending on the composition of the liquid. Various substances migrates at different speeds and gives different tint. It is used today in quick medical tests (provisional). In our case is not shows out any color but is only one similarity technique. Finally is "graph" because it is recorded some curves and from processing of a graph, the concentrations is extracted by various methods for calculating.

Flame Ionisation Detector - FID

    Through a thin tube of stainless steel flowing hydrogen in an atmosphere of air. It is ignited by a spark, match or lighter. On the top of the tube (nozzle) a small blue flame appear, little light and not too extensive. Area where the high temperature flame keep ionized atoms is just a few millimeters from tube. It do not grow too much because the reaction is fast. At the close of an centimeter from flame is put a stainless steel cylinder electrically isolated from the nozzle and the combustion chamber walls. This cylinder is connected to a picoammeter. Very few ions will arrive on cylinder when only the hydrogen burn in flame. The ensemble nozzle-cylinder has a resistance gived by air resistance (thousands of GigaOhms) at humidity, pressure, temperature and cleanliness there were. If in the flame emerging other substances, ionizing easier, then ionized area extends in space and to the colder edges. These ions survive at lower temperatures in the area of the cylinder. So the ions reach cylinder. In this case the nozzle-cylinder ensemble has a lower electrical resistance (tens of GigaOhms). A weak current can flow between the nozzle and cylinder. Current obtained is amplified and recorded in the computer. Hydrocarbons generates ions in this detection ensemble. The more carbon atoms in the molecule (heavy molecule) will be even more ions in the flame. For this reason, the detector is called "mass" detector. The signal will be greater for the heavier molecules. The current is in proportion to the mass concentration (derived from the masses ratio of substances) and NOT with the volume concentration (derived from the volumes ratio of substances) or molar concentration (derived from the number ratio of molecules).

Technical specifications

   Our gas-chromatograph is in a continuous improving process of analysis length, accuracy and stability.