IV. OPERATION
The general information below includes comments on sampling techniques, compensation for
temperature and pressure, number of passes required, and accuracy and speed.
I. Sampling Techniques
A. The operator should keep in mind that the smallest division of the burette is 0.1 ml. Therefore,
the resolution of the analyzer is 1 percent for a ten ml sample, but only 0.1 percent for a 100 ml
sample. For this reason, although smaller samples may be analyzed, it is strongly
recommended that a 100-ml sample volume be used for all analyses.
B. It is important to ensure that not only the proper gas be collected, but that the gas not be
contaminated, diluted with ambient air, depleted of certain components by dissolution in
confining liquids, or reacted with metal tubes or containers.
C. The sample may be delivered directly to the analyzer through a sampling tube, or it may be
collected and stored in a Tedlar bag and delivered later to the analyzer. The operator should
note that gases remaining in the manifold after an analysis are considered to be nitrogen. The
sampling tube and the connections must be well purged with the gas prior to making an
analysis.
D. The operator should also be cautioned that the volume marks on the burette do not include the
volume within the manifold. The error intr oduced by this volume will be negligible if a
sufficiently large sample is used. For this reason also, small samples should be avoided if
maximum accuracy is desired.
II. Compensation
A. Since the Orsat apparatus requires the measurement of gas volumes at constant temperature
and pressure, provisions must be made to ensure that changes of temperature and pressure do
not interfere during the course of an analysis. Since the apparatus is portable, with no
automatic pressure compensation, reference pressure is always that of the atmosphere. This is
compensated for, before measurements are taken, by specific procedures to produce identical
liquid levels in both burette and leveling bottle. Procedures are also included to adjust any gas
in the manifold to the same pressure as in the leveling bottle.
III. Number of Passes
A. It is difficult to state the number of passes required for the absorption of any particular
component since this varies with such factors as the design of the pipette, the reagent used, the
age of the reagent, etc.
B. Normally three or four passes will be sufficient for carbon dioxide. Six or more passes may be
required for absorption of oxygen and carbon monoxide. If more than five passes for complete
absorption of carbon dioxide, or more than twelve for oxygen, are required, the reagent should
be discarded and replaced with fresh solution.
C. NOTE: The only way to make certain that a particular component has been removed
completely is to pass the gas once more into the pipette, and note if any contraction in volume
takes place.
Accuracy and Speed
A. In analyzing gases, the operator should strive for accuracy and speed. To ensure accuracy the
operator must determine whether or not the components being tested for are present in the gas
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