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'[EE] flammable gas detector blues'
2019\08\08@103706 by Harold Hallikainen

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I have no experience with this, but I did just get a printed catalog from
Marlin P. Jones and Associates that had several gas sensors. Search for
Sensor at https://www.mpja.com/ .

Harold


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2019\08\18@025640 by Art

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Hi All,

I plan to convert to propane heat soon (live in Maine), but in
preparation, for the switch I seem to have hit a brick wall.

I want a decent flammable gas detector. It doesn't have to be extremely
high quality or highly accurate-but.....

Consumer grade flammable gas detectors on the market currently are only
good for 2 years, and they are grossly overpriced. And, they don't come
with schematics and the vendors for them seem to be generic no-name
companies.  I found a vendor (OEM) in the US, called them and they
didn't even know how it operated or the difference between oxygen and
propane. And, the detectors also seem to detect other flammable gasses,
though at different sensitivities!

I'd rather build my own I think-especially since it's the only way to
get the hardware that I want.

I'd like a propane detector that also measures oxygen content of the
atmosphere in the house as well. Propane is only flammable (explosive
merely means fast burning) between oxygen concentrations between 2 and
10 percent mass, (per
https://www.engineeringtoolbox.com/explosive-concentration-limits-d_423.html).


Youtube is full of Bozo's who don't have a clue about the need for
oxygen before the mixture becomes flammable/explosive! And the videos
they make just blow up a propane cylinder near an ignition source
outside in free air.....so, the the 'explosions' lead the non-saavy
types to believe propane safety is no big deal. But, houses that are
totally destroyed from propane leaks clearly must have had proper fuel
to air ratios-some are flattened right down to the foundation!

The issue is further complicated because propane is significantly
heavier than air. And, it's much colder than most air, so it sinks to
the lowest layer of gasses (usually in the basement or lowest level
floor of the building. SO, it doesn't mix with 'air' well. And, a layer
of propane isn't dangerous unless it's mixed with air (oxygen).

There was a time when I could research technical problems with search
engines-but these days all I get are ads from retailers who offer them
for sale! I use duckduckgo search engine, but even tried google-all the
search engines seem to be trending towards being nearly useless with
regard to technical issues.

When I was working, I used to deal with pyrophoric and toxic gasses,
including purge panels and vacuum systems needed for safety-so I'm not a
beginner::>

Can anyone suggest technical references on the web so I can evaluate
whether to build or buy?

GL to all and TIA!

Art









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2019\08\18@115404 by Isaac M. Bavaresco

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The way I understood it, it seems that you are only worried with gas
leakage if there is danger of explosion.

In my opinion, gas leaking and pooling in the basement is very worrying,
regardless of the O2 concentration.

I would install a sensor very close to the floor and replace it as often as
the manufacturer recommends.

Cheers,
Isaac

Em dom, 18 de ago de 2019 07:00, Art <spam_OUTky1kTakeThisOuTspammyfairpoint.net> escreveu:

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2019\08\18@153012 by Art

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On 8/18/19 11:56 AM, Isaac M. Bavaresco wrote:
> The way I understood it, it seems that you are only worried with gas
> leakage if there is danger of explosion.

Not true. Any amount of detectable propane is a problem. My point is
that I'd rather have a dual O2 and propane sensor unit-just for my own
info. The propane dealer who will install the system told me that the
odor causing compound they put into the tank will alert me because it is
such a strong odor.

> In my opinion, gas leaking and pooling in the basement is very worrying,
> regardless of the O2 concentration.

Yes!!

I find these O2 sensors based on the platinum all over the place and
there is some technical info on them. But,  they use so much power,
there's no chance of running the detector on batteries!! And the sensors
are physically large and they aren't cheap::> If i can't find a sensor
that doesn't require a lot of power, I will probably buy 2 of the
conventional sensors and put one at eye level near the furnace and on a
couple feet above the floor in the basement.

Are there other O2 sensors that are more suitable for powering them from
AA cells, or a LIPO cell?

TY all!

Art


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2019\08\18@172029 by David Robertson

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> The way I understood it, it seems that you are only worried with gas
> leakage if there is danger of explosion.
>
> In my opinion, gas leaking and pooling in the basement is very worrying,
> regardless of the O2 concentration.

Agreed. You want the detector to tell you there is a leak as soon as possible, before it becomes an explosion hazard.

I don’t really see the benefit of using an oxygen sensor in this application. However, you should also consider a carbon monoxide detector.


As for combustible gas sensors, there are a few variants, but these all operate by actually combusting small quantities of any combustible gas present.

In the simplest kind, the sensor element consists only of a coil of very fine wire made from (or coated with) platinum or another PGM. Current passing through the coil causes it to heat up to several hundred degrees C. If a combustible gas (and oxygen) are introduced, slow combustion occurs at the surface of the hot wire, assisted by the catalytic effects of the platinum. This, of course, causes the temperature of the wire to increase slightly, and with it, its resistance, allowing measurement of the gas concentration. Usually, the sensor is connected in a Wheatstone bridge arrangement.

In practice, the construction of the sensor element is usually a little more complex. Instead of a free-standing platinum coil, the coil is usually embedded in a bead/pellet of porous alumina doped with catalysts, creating a much higher surface area where the catalyst is active, and so increasing the sensitivity greatly. This is known as a “pellistor”. Also, it is common to have a reference coil/pellet which is catalytically-inactive, to allow compensation for ambient temperature and gas thermal conductivity variation.

Usually these sensors are housed inside a metal can with an opening to the atmosphere via a fine metal mesh or sintered disc, which serves as a flashback arrestor to prevent your detector from blowing the place up.

Because of the way these sensors work, they are not specific to any particular gas and instead will detect just about any combustible gas/vapours, which is arguably a benefit. For a constant molar concentration, sensitivity does of course vary between different gases, because the enthalpy of combustion varies for different compounds.

The caveat of using these sensors is that some vapours may poison the catalyst and therefore gradually decrease the sensitivity. This is the reason for the stated sensor lifetime.


David
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2019\08\18@191303 by David Robertson

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> Not true. Any amount of detectable propane is a problem. My point is
> that I'd rather have a dual O2 and propane sensor unit-just for my own
> info. The propane dealer who will install the system told me that the
> odor causing compound they put into the tank will alert me because it is
> such a strong odor.

It’s not safe to rely solely on the odourant, due to olfactory fatigue. There have been explosions after technicians, trying to purge air from newly-installed gas lines, assumed they would be able to smell when the air had been purged and gas was coming out.

David
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2019\08\18@192118 by Isaac M. Bavaresco

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I read somewhere that the odourant (mercaptan) may react with the metal
inside the cylinder and get degraded, losing its smell.

Em dom, 18 de ago de 2019 20:15, David Robertson <.....davidKILLspamspam@spam@robertson.yt>
escreveu:

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2019\08\18@202436 by David Robertson

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On a more practical note, take a look at the Figaro 2610-C/D  LP gas sensors. These work a little differently to my previous explanation in that there is a heating element that is not used for sensing, and a thick film sensing element made from tin dioxide. The -D version has a membrane filter of some kind to apparently increase selectivity by filtering out interfering combustible vapours. Power consumption is 280mW, which is manageable for intermittent operation on battery power.

https://static.rapidonline.com/pdf/50-9095.pdf <https://static.rapidonline.com/pdf/50-9095.pdf>

David
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2019\08\19@045604 by AB Pearce - UKRI STFC

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Thanks for that pointer. I'm in the thinking process of going caravanning, and such sensors are at the back of my mind.

As a side issue to this, but related to the above, I have heard of sensors for detecting the anaesthetic gasses purported to be used by people who break into caravans and motor homes. Does anyone know anything about these sensors?


{Original Message removed}

2019\08\19@054645 by James Cameron

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Both the caravans and exploding house scenarios may benefit from heat
recovery ventilation, where a structure containing a heat exchanger is
used 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 to insert their attack.

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2019\08\19@081428 by Brent Brown

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There seem to be a few sensors around that give an "equivalent CO2" or "eCO2" output, though they are primarily a VOC sensor. Not sure how VOC and CO2 levels relate exactly and how exact "equivalent" is. Nevertheless such a sensor might well be suitable for the gas in question.An example is the one used in the TE AmbiMate module, though i can't immediately locate a VOC sensor part number...https://www.te.com/usa-en/products/sensors/multi-sensor-modules.html?tab=pgp-storyPerhaps a little slow at 60s.Whatever you do, don't use relay contacts to alert that flammable gas is present~!
-------- Original message --------From: James Cameron <quozlspamKILLspamlaptop.org> Date: 8/19/19  9:46 PM  (GMT+12:00) To: "Microcontroller discussion list - Public." <.....piclistKILLspamspam.....mit.edu> Subject: Re: [EE] flammable gas detector blues Both the caravans and exploding house scenarios may benefit from heatrecovery ventilation, where a structure containing a heat exchanger isused to ensure the inside of the box never has an atmospheric mixunlike the standard planetary atmosphere.On the other hand, if it is standard an attacker could figure outwhere to insert their attack.-- James Cameronhttp://quozl.netrek.org/-- http://www.piclist.com/techref/piclist PIC/SX FAQ & list archiveView/change your membership options atmailman.mit.edu/mailman/listinfo/piclist
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2019\08\19@095859 by RussellMc

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>
> On the other hand, if it is standard an attacker could figure out
> where to insert their attack.
>
> We had an exploding house in NZ within the last month.


If ventilation is ongoing then most leaks would be well below the level
where the output gas was flammable.

A "Davies lamp" arrangement could be employed where a flame inside a mesh
barrier allowed flammable gas mixtures to enter and be combusted but did
not allow flame to propagate beyond the enclosed space.


              Russell


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2019\08\19@101842 by AB Pearce - UKRI STFC

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Further to that, maybe take a look at the IDT rage of gas sensors.

They have one they list as suitable for detecting flammable gasses. They have an evaluation kit for it which may provide you with all you need in the way of hardware.

https://www.idt.com/products/sensor-products/gas-sensors?utm_source=google&utm_medium=cpc&utm_campaign=sensors&utm_content=gas&gclid=EAIaIQobChMIpJDdqMiO5AIVRvlRCh2FJwQ8EAMYASAAEgIRFvD_BwE



{Original Message removed}

2019\08\19@112358 by AB Pearce - UKRI STFC

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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-attacks-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 compartment 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 removed}

2019\08\20@033039 by Sean Breheny

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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 <
EraseMEalan.b.pearcespam_OUTspamTakeThisOuTstfc.ac.uk> wrote:

{Quote hidden}

> {Original Message removed}

2019\08\20@080454 by RussellMc

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On Tue, 20 Aug 2019 at 19:31, Sean Breheny <shb7spamspam_OUTcornell.edu> wrote:

> My $0.02 on a few quick items:
>
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.

My experience of pre-meds was stunning.
C4-5 spinal fusion.
As we were about to enter the operating room.
"I'll just give you a shot of premeds ..."
I have no recollection of anything from that moment until waking up in
intensive care.
He was very proud latterly. I was annoyed. I value (no doubt strangely) the
rigmarole of preparation for operation (not that I've had overly many).
To (presumably) experience that, but to lose it entirely from memory, was
disturbing.


        Russell
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2019\08\20@212425 by Sean Breheny

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One of the qualities of sedatives usually considered valuable for medical
procedures is anterograde amnesia (temporarily blocking the ability to form
new memories) - which is what you experienced, Russell. There are some
sedatives which produce this effect at doses where you still remain
somewhat conscious - which enables the patient to follow simple
instructions like "turn on your side". Some members of the benzodiazepine
family (same family as diazepam/Valium and alprazolam/Xanax) do this well
(e.g. midazolam aka Versed). Ethanol can also do this sometimes (often
called a blackout). I think their thinking is that if you do experience
something unpleasant at least you won't be traumatized by it. It can be
rather mind-twisting to consider what happens if you are able to feel pain
but don't remember it later. Anesthetists are concerned mostly with
anesthesia recall (ability to remember painful events) since that is the
most directly observable event in terms of what the patient can report
afterwards, but they do concern themselves with preventing the conscious or
even unconscious experience of pain to begin with.

Modern anesthesia practice uses a layered approach - they sometimes use
local anesthetic to reduce or eliminate pain at the operation site. They
also sometimes use an opioid analgesic to reduce the sensation of pain. On
top of these is the general anesthetic (usually inhaled for major surgery
but sometimes could be purely IV-delivered). The patient is also usually
sedated prior to receiving the general anesthetic (as I already mentioned)
and sometimes a muscle relaxant is given to prevent involuntary movement or
airway reflexes (like gagging/coughing/larynx spasm) while the patient is
intubated. Since sedation and muscle paralysis do not produce
unconsciousness or pain relief, great care is used to ensure proper pain
relief and depth of anesthesia (lack of consciousness) - especially if
muscle relaxants are used.

Without muscle relaxants, painful stimulus will sometimes cause jerking or
twitching of limbs by the patient which is thought to be reflex-based and
not conscious action. If you induce profound general anesthesia, this
doesn't usually happen but it's considered safer to maintain a slightly
lighter depth (still sufficient to prevent consciousness but not deep
enough to suppress all reflexes) and use other means of pain relief (like
local anesthetic or opioids). Anesthetists can use these reflexive
reactions to painful stimuli as a guide to achieving the proper depth of
anesthesia and pain relief.

Secondarily, and especially if muscle relaxants have been used, they use
respiration rate, heart rate, blood pressure, and brain waves to determine
proper depth, pain relief, and whether the patient might be aware or in
pain. Someone who is experiencing great stress will have an elevated
respiration, heart rate, and blood pressure even if skeletal muscles cannot
move. They are supposed to watch for this and provide more pain
relief/deeper anesthesia if it happens. (Even though they make every effort
to prevent it from happening in the first place and it usually doesn't).
There are also various algorithms which produce an anesthesia depth "score"
or figure-of-merit from analysis of brain waves. Bispectral Index is one.

I'm not a doctor or a nurse but medicine is one of my interests and I've
read quite a bit about anesthesia.

I've had "conscious sedation" several times for endoscopy procedures. My
amnesia was not as profound as Russell's - I do remember a bit of each
instance but definitely less than if I were fully awake. Meperidine/Demerol
or fentanyl were used for pain relief.

I've only had general anesthesia once - propofol - and beforehand I was a
bit worried about the philosophical question I mentioned above (What if I
feel pain but don't remember it afterwards?). I suppose that question is
not possible for me to answer, still, of course, but my experience was very
good and perhaps as close to answering that question as possible: about 20
seconds after being given the injection, I became very dizzy (I was of
course already lying down) and then suddenly blacked out. The memory of
that moment of going out is quite vivid and strong. I do have memories then
from before waking but they are memories of a dream - just like normal
sleep. They are very vague but it was definitely not a nightmare - it was a
dream about the procedure I was having but it was neither super pleasant
nor horrible, just like waiting for something to be finished. Then I also
remember the moment of waking up at the end - just like normal sleep.

Sean


On Tue, Aug 20, 2019 at 8:10 AM RussellMc <@spam@apptechnzKILLspamspamgmail.com> wrote:

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