700M HFID
6-2-15
7
Operating Principle
The CAI 700M HFID methane/non-methane hydrocarbon analyzer uses the principle of Flame
Ionization Detection (FID) to determine the hydrocarbons within a gaseous sample.
For methane analysis, a solenoid valve diverts the sample through a cutter assembly that removes
all the hydrocarbons from the sample except methane. The removal efficiency is 98% for ethane
and heavier hydrocarbons, with less than a 10% loss of methane. The subtraction of the methane-
only concentration from the total hydrocarbon concentration provides the concentration of non-
methane hydrocarbons in the sample.
A heated oven (191
°
C) contains a burner and an optional heated pump. The small flame of the
burner is elevated and sustained by the regulated flows of air and either pure hydrogen or a 40/60
mixture of hydrogen and helium or nitrogen.
The split-ring detector contains two electrodes. One electrode is negatively polarized using a
precision power supply, and the other electrode (known as the collector) is connected to a high-
impedance, low-noise electronic amplifier. The two electrodes establish an electrostatic field.
When a gaseous sample is introduced to the burner, it is ionized in the flame, and the
electrostatic field causes the charged particles (ions) to migrate to their respective electrodes. The
ion migration creates a small current between the electrodes. This current is measured by the
precision electrometer amplifier and is directly proportional to the hydrocarbon concentration of
the sample.
Heated Oven
The heated sample gas is maintained above its dew point by a self-contained internal oven. The
oven temperature is adjusted at the factory to be controlled at 191°C. The sample gas is
maintained at this elevated temperature until it exits the analyzer’s bypass outlet, preventing any
loss of hydrocarbon concentration in the sample due to condensation.
