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In Reply to: Re: Leak detector - mini chem lesson posted by Steve on September 19, 2001 at 20:01:37:
ha, still like using a match for detecting gas leaks. The principle of halogen leak detectors is that the sensor consist of two aluminum plates shielded by a paper filter. The trick is to get these gases in between the plates that changes the dielectric constant of the capacitance of these two plates that form a high frequency oscillator circuit. The presence of halogen gases then changes the frequency of this oscillator, the frequency change is minimal, but it is hetrodyned with a constant frequency second oscillator to produce an audible tone.
Curious about the electronic ignition on my gas furnance, I traced part of the circuit out. When the thermostat calls for heat, the pilot shoots a small jet of HC's at an electrode, there was a small voltage impressed across this electrode and the base which acted like a ground, a one mega ohm resistor was in series with this electrode that permitted a small trigger current to flow. So apparently with just air surrounding the electrode, nothing happens, in the presence of gas, a trigger current flows that was amplified and hit a triac through a high voltage transformer causing a healthy spark igniting the pilot. The pilot gas flow was timed so if it didn't fire, the gas was turned off for safety. So apparently HC's have a degree of electrical conduction much greater than that of air. I would gather that this would tend to short out the two electrodes in a halogen leak detector, more like a leaky capacitor than one with a shifted dielectric so I don't think a halogen leak detector would detect HC type leaks.
On the other hand, if the current between the two plates were sensed, a unit could be designed to detect HC's, something to think about.
HC's like natural and propane gases have a distinct odor, in lieu of anything else, you could sniff around the system, if the odor is strong, wouldn't suggest lighting a match.