Heath Consultants Aqua-Scop - Manual (Page 35)

1. Nitrous Oxide with Infrared Detection: One of the advantages of
nitrous oxide is that it is water soluble and the line does not have
to be dewatered prior to the test. The infrared detector is very
specific for nitrous oxide so that there is no need to worry about
interfering gases. One of the real disadvantages of this system is
that nitrous oxide is heavier than air so that holes must be probed
to approximately the depth of the main. Two other disadvantages
are that heavier than air gases are more difficult to pinpoint than
lighter than air gases, and the infrared detection equipment is
very expensive.
2. 10% Helium/90% Air: With this system, the line must be dewatered
before the helium/air mixture is inserted. Helium has the advantage
of being an extremely small molecule and very light. Detection is
by Thermal Conductivity or Comparative Sonics.
3. Methane/Nitrogen: A mixture of 2 1/2% methane balance nitrogen,
utilizing flame-ionization as the detector, is an extremely effective
method. Methane, being lighter than air, requires no test hole
placement. However, the line does have to be dewatered.
4. Flame-ionization detection of methane is in the low parts per
million ranges, so that even small amounts of methane are readily
detectable.
5. Methane/Argon: This mixture takes advantage of both a heavier
and a lighter than air gas with the methane being detected by
flame-ionization and the argon acting as a confirming gas being
detected by comparative sonics. The helium and methane tracers
require a pre-survey of the area before insertion of the tracer.
They can all be interfered with by natural gas leaks or naturally
occurring methane.
Tracer Gas Surveys are extremely expensive compared to Sonic
Surveys and should only be considered when sonics are completely
impractical.
32