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HAZARDS OF PETROLEUM



                              2.4.4.3 Precision of Measurement
      ---ใช้เพื่อการศึกษาเท่านั้น---
                                        Correct response from these instruments is achieved only when
                                        measuring gas concentrations in mixtures for which the instrument has
                                        been calibrated and which remain gaseous at the temperature of the
                                        instrument.
                                        When selecting an instrument that uses an infra-red sensor for measuring์นาวี
                                        Relatively small deviations from normal atmospheric pressure in the
                                        instrument produce significant differences in the indicated gas
                                        concentration. If a space that is under elevated pressure is sampled, it
                                        may be necessary to detach the sampling line from the instrument and
                                        allow the sample pressure to equalise with the atmosphere pressure.
                                        care should be taken to ensure that the sensor will provide accurateย
                                        readings over the spectrum of gases likely to be present in theช
                              2.4.4.4 Instruments with Infra-red Sensors

               งานห้องสมุด ศูนย์ฝกพาณิ
                                        the percentage by volume of hydrocarbon in an inert gas atmosphere,


                                        atmosphere to be measured. It may be prudent to make comparison
                                        readings with a Tankscope to verify the acceptability of the readings
                                        provided by the instrument under consideration.


                           2.4.5 Inferometer (Refractive Index Meter)
                                                                          ึ

                              2.4.5.1 Operating Principle
                                        An inferometer is an optical device that utilises the difference between the
                                        refractive indices of the gas sample and air.

                                        In this type of instrument, a beam of light is divided into two and these
                                        are then recombined at the eyepiece. The recombined beams exhibit an
                                        interference pattern that appears to the observer as a number of dark
                                        lines in the eyepiece.

                                        One light path is via chambers filled with air. The other path is via
                                        chambers through which the sample gas is pumped. Initially, the latter
                                        chambers are filled with air and the instrument is adjusted so that one of
                                        the dark lines coincides with the zero line on the instrument scale. If a
                                        gas mixture is then pumped into the sample chambers, the dark lines are
                                        displaced across the scale by an amount proportional to the change of
                                        refractive index.

                                        The displacement is measured by noting the new position on the scale of
                                        the line that was used initially to zero the instrument. The scale may be
                                        calibrated in concentration units or it may be an arbitrary scale whose
                                        readings are converted to the required units by a table or graph.

                                        The response of the instrument is linear and a one-point test with a
                                        standard mixture at a known concentration is sufficient for checking
                                        purposes.

                                        The instrument is normally calibrated for a particular hydrocarbon gas
                                        mixture. As long as the use of the instrument is restricted to the
                                        calibration gas mixture, it provides accurate measurements of gas
                                        concentrations.

                                        The measurement of the concentration of hydrocarbon gas in an inerted
                                        atmosphere is affected by the carbon dioxide present when flue gas is



                                                   © ICS/OCIMF/IAPH 2006                                        25
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