Eletronic Hydrogen Leak Detetor
The 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, etc.
In 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 obviously dangerous.
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.
This 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 Description
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.
As 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.
The 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.