Hydrogen Leak Detetor
Development of the Electronic Halogen Leak Detector
For many years the most effective, and only, method for finding
gas leaks of any type was the use of a soap or "bubble" solution.
For substantial leaks this is still a very effective "detector"
and is, in fact, the method most commonly used to this day in
high pressure gas production such as compressed air, oxygen, nitrogen,
small capacity refrigeration or air conditioning systems, however,
significant leaks of a few ounces a year could not be easily found
with bubbles, and the "halide torch" was introduced. This tool
uses an open flame to heat a small copper plate, and any halogen
gas introduced causes a change in the flame color. It is a reliable
tool but has several disadvantages: small color changes, especially
in bright sunlight, are very difficult to see; large doses of
chlorinated refrigerant can produce phosgene gas (COCI2), a poison;
the inability to detect non-chlorine based refrigerants (E.G.
HFC 134a), and the fact that in some locations an open flame is
The invention, in 1963, of the Corona Discharge Detection Method
introduced the age of the electronic detector. Over the years
this concept was refined and improved with extensive research
into electrode metals, tip shell materials and finishing procedures
as well as circuitry changes and features to improve response
time and "clearing" time. From a user point of view, one of the
most significant of these features was the introduction of the
"Micro-pump" probe assembly. This small motor-driven fan assembly,
mounted in the probe handle, actively draws air into the sensing
tip and vents it through the back of the handle.
gives appreciably quicker response time than a system which relies
only on diffusion for a gas leak to penetrate the tip, and very
much quicker clearing time, allowing the user to move quickly
to the next suspect leak point. This mechanical pump is far superior
to any so-called ion-pump method. Ion-pumping, or electron-wind,
is a phenomenon readily demonstrable in a laboratory at 50 or
100,000 volts but is negligible to nonexistent at the voltages
used in electronic leak detectors.
The growing concern over ozone-depletion, the Montreal Protocol
and the U.S. Clean Air Act led to the introduction world-wide
of chlorine-free refrigerants (HFC's) and an immediate rush by
detector manufacturers to produce a tool able to respond, at appropriate
levels, to a gas with no chlorine content. All previous halogen
refrigerants had contained chlorine (CFC's and HCFC's), and that
is the most easily detected component using Corona Discharge Technology.
The response to fluorine is some 20 to 100 times less, and this
requires major changes in tip sensitivity and circuit gain.
Such changes include: new tip shell and electrode (not interchangeable
with previous styles), modified voltage pulse to the tip to change
the duty cycle and separate (switchable) hi-gain circuitry to
handle the very low signal input from fluorine detection. The
inherent problem with hi-gain amplifiers is their susceptibility
to electronic "noise" and their amplification of unwanted signals,
which is why a separate circuit was developed for HFC signal handling,
rather than one circuit, with variable gain, to handle everything.
Reprinted with the permission of TIF Instruments, Inc.Technical
The GHH5000 Series (right) halogen leak detectors operate upon a principle
known as "negative corona discharge," which works due to the phenomenon
of "electron affinity," the ability of a given compound to "steal"
or capture free electrons. Detection occurs as a result of this
effect, causing a change in the current flow within a sensing
tip designed to take advantage of the phenomenon. The sensing
tip is composed of two electrodes, an anode and cathode, exposed
to the atmosphere. A stepped-up voltage with a potential up to
2000 volts is applied to the electrodes causing a corona, or "cloud"
of electrons, between the electrodes. This corona causes a current
flow with a given value in any given atmosphere.
a halogen or halogenated compound enters the tip, and displaces
the existing atmosphere (or part of it), electron affinity comes
into play and electrons are "stolen" from the corona. This causes
the current flow to decrease. The greater the concentration of
halogens within the tip, the lesser the current flow.
circuit is designed to convert the tip current flow into a proportional
voltage potential, and monitor such potential. Upon initial power
up, the circuit reads this potential and stores its value for
reference. When tip current decreases the corresponding voltage
potential also decreases. If the potential falls below the stored
value an audible (and visual on the GHH-5650) signal is generated
to alert the operator to detection. At any point the reference
(stored) value may be reset to prevent ambient or "background"
contamination from generating new alarms or to return to maximum
sensitivity if previous levels have been set.