My $0.02 on a few quick items: 1) David did a good job of describing how typical flammable gas sensors work. They operate at a temperature below the usual combustion temperature (by means of their catalytic effect) and this combined with a flame arrestor (usually just a few layers of fine mesh which cools any escaping gas well below the autoignition temperature) prevent them from setting off an explosion. They do usually need an oxygen supply (from the air) to work. 2) There are alternative sensor technologies (such as non-dispersive infra-red sensors) which last longer but they are far more expensive and they are only slightly more selective than the pellistors. Neither the NDIR nor the pellistors are good for sensing O2 concentration as far as I know. 3) There is also a chemiresistance effect where a heated porous material varies its electrical resistance depending on how many of a certain type of molecule are adsorbed into its pores. These can be pretty selective to gas type and cheap but do not have a long life. 4) Some gasses (like CO and I think O2) can participate in electrochemical reactions and have detectors which form a little electrochemical cell where the potential depends on the concentration of the gas in question. CO detectors usually use this type and achieve about a 10 year life. 5) Lower flammable limit/lower explosive limit and upper flammable limit/upper explosive limit are almost always expressed in VOLUME percent, NOT mass percent. For gasses, volume percent is also very close to molecule percent (# of specific gas molecules/total # of molecules in an air sample) also known as mole fraction. There is sometimes a minor distinction made between flammable limits and explosive limits - the latter being the more narrow range where the combustion has enough energy to generate an over-pressure rather than just a risk of setting other stuff on fire - but the difference is usually small and the terms get used interchangeably. 6) You are correct that LEL and UEL assume there is the typical amount of oxygen present. They are in fact even more specific than that - they also vary with temperature and pressure, with the usual values being given for 1 atmosphere, 25 deg C, in air with 20 to 21% oxygen and the remainder nitrogen. 7) Do not rely too much on gravity-induced concentration gradients. While it is true that denser gasses tend to sink, there is also a lot of diffusion going on, so that an equilibrium is reached where there is considerable mixing even when there is a big density difference. 8) I second others recommendation that the most important sensors for you would be first a CO detector and then a flammable gas detector. If there is enough flammable gas accumulating to fall within the LEL-UEL range, it will most likely have enough pre-mixed volume with air to have enough oxygen to burn. 9) The methyl mercaptan or other odorant added to the gas helps but there are situations where the propane can become separated from the odorant (like if it bubbles through water or soil to escape). This scenario is probably not very likely in a properly-installed home propane tank. 10) I find the stories of people being robbed by anesthetic gasses very hard to believe. At the end of the article linked here, there is a statement from the Royal College of Anaesthetists which backs me up on this. Most inhaled anesthetics have an induction phase where there is airway irritation and agitation which would wake-up most people. In a medical setting, this is why they typically sedate you with some other agent first (usually intravenous although it could be nitrous oxide) and then begin administering the inhaled anesthetic. They'd have to be pretty sophisticated to make this work right by feeding N2O first through a pipe and then some other anesthetic (N2O doesn't render you completely unconscious and you recover very quickly when exposed to clean air). Large quantities of these expensive agents would be needed and there would be a lingering smell. Sean On Mon, Aug 19, 2019 at 11:25 AM AB Pearce - UKRI STFC < alan.b.pearce@stfc.ac.uk> wrote: > The problem with a caravan or motorhome is that often the usage is in > summer and you have a roof vent open rather than use any form of aircon. > > Typical scenarios detailed in this article ... > > https://www.telegraph.co.uk/news/uknews/crime/11068184/New-spate-of-attac= ks-by-sleeping-gas-gang-caravanners-warned.html > > There have also been reports of persons of 'high net worth' waking up in > their hotel rooms to find a similar groggy feeling and finding their > valuables have been stolen. > > Before we moved to the UK in '97 there were reports of similar things > happening on trains in eastern Europe and Russia. When we travelled > overnight between Moscow and St Petersburg the train attendant issued us > with a plastic 'lock' that fitted over the door knob inside the compartme= nt > to prevent someone from using a security key to unlock the door from > outside, as similar attacks using gas had been known to happen on this > route. > > > > -----Original Message----- > From: piclist-bounces@mit.edu On Behalf Of > James Cameron > Sent: 19 August 2019 10:47 > To: Microcontroller discussion list - Public. > Subject: Re: [EE] flammable gas detector blues > > Both the caravans and exploding house scenarios may benefit from heat > recovery ventilation, where a structure containing a heat exchanger is us= ed > to ensure the inside of the box never has an atmospheric mix unlike the > standard planetary atmosphere. > > On the other hand, if it is standard an attacker could figure out where t= o > insert their attack. > > -- > James Cameron > http://quozl.netrek.org/ > -- > http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive > View/change your membership options at > http://mailman.mit.edu/mailman/listinfo/piclist > > -- > http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive > View/change your membership options at > http://mailman.mit.edu/mailman/listinfo/piclist > --=20 http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .