EEVblog #603 - Gas Sensor Teardown - Drager Multiwarn II

[Stonent]

That yellow sensor I think is actually Hydrogen Sulfide, H2S.

And for comparison, here's Mike's teardown of a gas detector as well. Similar hardware, same processor, and the sensors are similarly designed.

[EEVblog]

That yellow sensor I think is actually Hydrogen Sulfide, H2S.

Yes it is, my bad.

[GBoos]

Hi Dave,

I've seen such an IR-Absorption Sensor open.
To detect multiple gases you use an Infrared Source with a wide spectrum (normally a simple Lamp).

Different gases absorb different spectral lines. To get an Information of the amount of a specific gas in the measuring chamber you have to ensure that you detect only at the specific spectral line. But you have a IR source with a wide spectrum.

So that thing which you can't pull out is a small motor with a filter-wheel attached. It's a small plastic wheel with different optical filters embbeded.
So you can turn the appropriate optical filter in front of the detector to detect only the one absorption line of the gas you wamt to detect.

[EEVblog]

So that thing which you can't pull out is a small motor with a filter-wheel attached. It's a small plastic wheel with different optical filters embbeded.
So you can turn the appropriate optical filter in front of the detector to detect only the one absorption line of the gas you wamt to detect.

I don't think that's the case here. The one I couldn't pull out would be the heater element. The TO92 packae is just that, and hte other 3 pin device would be the sensor. I think in this case it's a fixed wavelength device.

[JeanF]

The "4-ohms-across-device" looks like it is held in the hole by a threaded ring, IMHO. You would need a 2-pin spanner to unscrew that, but a pair of pliers with needle tips (or a pair of Circlip pliers) could also work...

[number33]

I wouldn't have thought you would need a hydrogen sulphide sensor, you've got one on the front of your face.

[Tac Eht Xilef]

I wouldn't have thought you would need a hydrogen sulphide sensor, you've got one on the front of your face.

You become desensitised to H2S real quick at extremely low concentrations - even below the threshold of smell - and you can become desensitised almost instantly at sub-lethal concentrations.

People who have been overcome & lived to talk about it quite frequently say they never smelt more than a quick whiff. It's also very common for someone to be overcome by it, then for their would-be rescuers to also collapse because they didn't smell it at all.

[electronics man]

nonane is just an alkane (no double bonds) like methane or propane

the word nonane comes from the fact that it has 9 carbon atoms the convention is

1 carbon = meth
2 carbon = eth
3 carbon = prop
4 carbon = bute
5 carbon = pent
6 carbon = hex
7 carbon = hept
8 carbon = oct
9 carbon = non

(these are prefixes)

and the fact that it has no double or tripple bonds makes in an alkane and the suffex for an alkane is -ane, an alkene (double bond) = ene and an alkyne (trippel bond) has the suffex -yne

the number of hydrogen atoms comes from the fact every carbon must make 4 bonds so they are there to make up the bonds that are not used for other atoms such as carbon, oxygen, nitrogen ect

this convention meens you can draw a molecule just from the name

edit: these molecules are just chains, whith no branches, that would complicate the name a bit. im not so sure why you would have a gas sensore for that, its a liquid at room temp, it has a boiling point much high than that of water (it boils at about 150c) so its no very volatile, strange ( however it does have a flash point of 31C)

[han]

H2S sensor = rotten egg detector...

[electronics man]

H2S sensor = rotten egg detector...

 

[Laertes]

I found a very short and unsatisfactory wikipedia article about an H2S sensor: http://en.wikipedia.org/wiki/Hydrogen_sulfide_sensor
Looks like maybe the wires in the H2S sensor are just varying their resistance according to the saturation of H2S in the weird gunk in the can? 

[Laertes]

Hm. I did a bit more research and found this: www.intlsensor.com/pdf/electrochemical.pdf
That looks like just the thing.So Dave was right about electrolytic material

[junkie_business]

As far as I know the cat sensor has no heating element - the methane decomposes on the platinum filament, hence 'catalytic'.

[tbscope]

Dave,

The Ex symbol does not mean it is intrinsic safe!
Actually, the red sensor isn't, so the complete system isn't. Only the other 3 sensors are intrinsically safe.

Look for the "i" symbol in the complete Atex discription lines. The "i" means intrinsic safe.

Ex devices do not always need to be intrinsic safe.

[mikeselectricstuff]

Dave,

The Ex symbol does not mean it is intrinsic safe!
Actually, the red sensor isn't, so the complete system isn't. Only the other 3 sensors are intrinsically safe.

Look for the "i" symbol in the complete Atex discription lines. The "i" means intrinsic safe.

Ex devices do not always need to be intrinsic safe.

Ex covers a range of different methods to protect against nasty things happenning in flammable atmospheres - including intrinsic safety, flameproof enclosures (gas explodes but flame doesn't leave the box due to containment strength and controlled long flame paths, like a Davy safety lamp), sand filling etc. 
Intrinsic safety is generally the first choice as it imposes minimal requirements on enclosures - just that it doesn't become unsafe if dropped, but for systems that need to use more power, including mains, flameproof enclosures are used, which are typically precision machined metal castings. In these systems the main electrical constraint is that any capacitors discharge within 5 mins of powerdown so it's safe to open in a hazardous area.

[mikeselectricstuff]

A couple of other things  ;
Intrinsic safety does NOT cover any software aspects - the hardware must be safe regardless of what the software does. In fact it doesn't even cover anything to do with the device's actual functionality - it doesn't have to work, just not be capable of causing ignition.

However as the function of this device is to protect people, there may be some seperate requirements to show that it will actually do the job, though I don't think there are any standards covering this - at least when that device came out.

The sintered disc flame arrestor - this is made of metal granules that are fused together to form a sponge type structure with a very high surface area, so any flame will be cooled to extinction before it can make it through.  the same method is used for loudspeakers and sounders used in hazardous areas.

A while ago I did a teardown of a similar device made by MSA, and went into more detail about Intrinsic Safety design :

[Gribo]

An H2S sensor is similar to a battery, that is, an electrolyte reacts with the gas, creating some current.
The interesting part is the analog circuitry, amplifying those few nano amps while maintaining the chemical reaction. Alphasense has few appnotes
http://www.alphasense.com/index.php/safety/application-notes/

[han]

Maybe someone can make video about handling, and how to get data from the sensor...
i suggest from http://www.figarosensor.com/
i found the price is quite cheap about $20-30 in my place




My experience with sensor, whose ever handling humidity sensor that can damage by humidity...   ([/size][size=78%] [/size][size=78%]Phosphorous Pentoxide )[/size]

[Tothwolf]

Usually the failure point with these type of meters is the O2 sensor (the blue one). Most of the time that and/or the batteries are the only thing wrong with these meters when you get one second hand. The O2 sensor is humidity sensitive and ships in a sealed container. Once you open the container, the sensor usually lasts about a year in normal humidity. The H2S sensor generally seems to last indefinitely in normal conditions, although I think they rate those for a year as well.

I own a similar multi-gas meter and have replaced some of the sensors a number of times. The first time I bought a replacement sensor for it I was shocked at the high cost, but given what the entire meter costs brand new, they aren't all that expensive.

[Wim_L]

edit: these molecules are just chains, whith no branches, that would complicate the name a bit. im not so sure why you would have a gas sensore for that, its a liquid at room temp, it has a boiling point much high than that of water (it boils at about 150c) so its no very volatile, strange ( however it does have a flash point of 31C)

You've got to be careful with that assumption. Just because it happens to have a high boiling point, doesn't mean it's not volatile. Toluene has a higher boiling point than water, but can evaporate quite fast anyway. Similar for many of those simple hydrocarbons, they don't need all that much energy to become volatile, even if the boiling point seems a bit high.

Why specifically nonane, though? It's not the most common alkane in industry. There's not much that comes to mind that would be particularly interesting about it (and yes, I'm a chemist), just one of a long series of n-alkanes. They're all fairly similar chemically, so sensors made for nonane would surely also be sensitive to the others from that series. I did a bit of searching, and it turns out that it's a simple matter of practicality. The flame arrestors used in such devices slow the rate at which gases can move into the sensor. Nonane turns out to be the biggest one of the n-alkane series that's still small enough to get through those little flame arrestors in sufficient quantities to be detected reliably.

[xdever]

Hi Dave!

Can you please make some mesurements on battery pack and/or sensors to figure out something about pinout? I got the CPU and the sensor board (with sensors) of this device, and I'm interesed in getting data out of the sensors. The battery pack's pinout may help in powering up my own board and at least determine the power supply pins and voltages of the sensors.

[madires]

Maybe someone can make video about handling, and how to get data from the sensor...
i suggest from http://www.figarosensor.com/
i found the price is quite cheap about $20-30 in my place

I've built some gas detectors 20+ years ago based on the TGS822 before we got the cheap fire alarms with optical sensors. IIRC that sensor was quite sensitive.

[SeanB]

I bought a Velleman kit with the gas sensor in it, it is quite sensitive to both gas and alcohols. Interesting in that the "IC" they use is just a CA3046 transistor array with a custom Velleman part number on it.

[han]

Maybe someone can make video about handling, and how to get data from the sensor...
i suggest from http://www.figarosensor.com/
i found the price is quite cheap about $20-30 in my place

I've built some gas detectors 20+ years ago based on the TGS822 before we got the cheap fire alarms with optical sensors. IIRC that sensor was quite sensitive.

I wander is there anybody use TGS822  as drunk driver detector:D

[SeanB]

I have one with a gas sensor similar to that inside, they are quite common. At least they have the advantage over the chemical pipe units in that they can be reused. Good fun at a party to show that you will be over the drink driving limit with one beer, one shot or even a quarter glass of wine. 2 hours later you are still over in most cases.