EEVblog #603 - Gas Sensor Teardown - Dräger Multiwarn II

What's inside a Dräger Multiwarn II gas detection warning system used in mining, oil, chemical, and emergency services etc to detect hazardous or explosive atmospheres.
Intrinsic safety design and how both catalytic bead and infrared IR gas detectors work, and pellistors.
And a teardown of a Hydrogen Sulfide (H2S) sensor.

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[faxezu]

Nonane is a linear alkane hydrocarbon,  it´s used to calibrate the meter for use in explosive atmospheres. 

Greetings from a german firefighter

[nbq]

Great tear down.   I used to work on these.    Here is some more info about this old unit from Drager (pronouced Dre-Grr).   Upto 28 different types of chemical sensors were available for detecting various gases and chemicals as needs require, and upto 5 can be installed at once for a configuration of multiple gas detection. All of the other sensors are "electro-chemical", which means they will expire within 6 months to a year typically and with usage and air exposure, and also  "poisoning" (being used and exposed to the gases they detect over time), they will not be so responsive and will have to be replaced.    The silver mystery chamber next to the pump is a Mirror and an Infared Sensor  which is used in conjunction with the
"Catalyic Sensor". (the silver flame arrestor one). It's also sometimes called
an LEL Sensor, which means (Lower Explosive Limit) for Combustible Gas detection,   The IR  sensor is immune to the poisoning, and operates like a backup for detecting explosive gases.  Mainly  for CH4/Methane, Propane and Natural gas detection, but other options are available. Those sensors typical are Percentage or Percentage Volume, while the rest typically show in Parts Per Million.  They have adjustable alarm set points and the sensors are rated for certain concentrations and default part per million calibration setup ranges, which can be changed.

The typical "4 gas" configuration is "CO" (Carbon Monoxide), "H2S" (Hydrogen Sulfide),   "O2" (Oxygen) and Catalyst "LEL" Combustible Gas.  A square plastic chamber plate with a thumb screw is attached to the red part of the detector and screwed down, draws the air in through a 3 to 6 ft usually a PVC  tube connected to a 1 foot probe wand with a replaceable water-filter on the base end of it (the filter will absorb water, encase water is pulled into it to prevent the pump or the sensors and components from getting damaged).   The pump is pretty quiet other than a small hum and draws a constant small amount of air pressure over the sensors to the outlet. The black sensor next to the pump is the pressure sensor, and it will stop with the pump blockage alarm if anything blocks the pump, and can be reset from the panel, with a button to continue after the blockage is removed.

It's usually calibrated using a gas cylinder with regulator and feeds the gas mixture
for the sensors installed and all of the sensor range concentrations to set and test the sensors, then is zeroed out in fresh air.   These units typically last for a full day on battery and record Time Weighted Averages, Short Term and long term exposure limtis and datalog.

[DejanTesic]

"Dräger Multiwarn II" - what a killer name for a metal band. ;)

[petermarczinowski]

Hi Dave, the infrared sensor with the mirrors is a "white-cell", a multipass (multiple reflexion) absorption gas sensor for very low concentration in a narrow spectral range. Many years ago I was partly involved in the developement in Lübeck.

Thanks a lot for your very interesting videos!
Peter

[FabianThoma]

I work in a chemical Company and I'm pretty sure that the EX-Sensor is a general purpose combustable Gas sensor like we use in our newer Dräger devices to detect if a explosive atmosphere is present. That's needed anytime you open up non intrinsically safe measurement equipment or ofc control cabinets.

We usually have a Oxygen Sensor as a addition so you get alerted if there is something leaking and displacing the oxygen, even if it's not directly detected, or a whole device dedicated to a specific Tasks that has the needed Sensors ready.

And yeah the plant Fire Department has a ton of Sensors, they dedicate a whole Vehicle to it here, so no need to replace the sensors on route to the emergency, there's no time for that anyway!

[mikeselectricstuff]

Intrinsic safety approval does NOT look at any software aspects at all. The hardware must be safe regardless of anything the software can do. 

[Oshbotscom]

I can clearly see why the repair shop didn't want anything to do with that thing. Very interesting video, Dave.

[SimoWill]

The 'Ex' symbol does not mean Intrinsically Safe, it only shows that the product is ATEX certified.
The codes determine the protection level. Ex'i is Intrinsically Safe, but there's also Ex'd (Explosion Proof) Ex'e (Increased Safety) Ex'n (Non-Sparking) Ex'p (Pressurised Enclosure) and so on.

[hubertrybak]

PA6 GF25 -> Polyamid with 25% glass fiber

[Darkfuturee]

I like the old smd resistors form factor, they are beauty. 
regards Dave.

[francistheodorecatte]

99% sure the yellow banded sensor pack is a not a hydrogen sensor, but a hydrogen sulfide (H2S) sensor.

[Aussie]

Very interesting bit of kit!, glad you referred to the manual ;), i know its more fun to speculate how things work. but the manuals are great to really see what goes on :D
I have a few NIB Danfoss Ammonia sensor boards with those catalytic sensors on them, they are used to detect ammonia leaks in large Nh4 based HVAC-R system rooms.

[krawutzimon]

22:55 nonane is used for calibration purposes, from a chemist's perspective it's very similar to the octane you can find in your car's fuel.

[RoughriderUT]

Didn't see anyone mention this, but that CAT sensor, the red one with the rough "Solder looking" intake surface, that is probably granular platinum, hope you didn't toss it in the bin.

[mikeselectricstuff]

I wonder if that IR sensor is a pyroelectric type as used in PIR detectors - pinout and wavelength range look right

[jean-bernardfullenwarth]

PA6 GF25 indicates the plastic type : Polyamides PA 6 (Nylon 6) + 25% Glass Fiber

[MrRoyzalis]

I like that you speculate about the nature of parts and what the function of parts may be when you first look at them.  Many times you're correct, but I especially like that you don't edit out when you guessed wrong - it really adds to the human part of the story and I would suspect just one of the many reasons you are so popular. 

[frankschieschke]

Hi Dave,
here you have the missing "ä".
Frannk

[gamccoy]

Perhaps the IR / Mirror / Sensor is for counting particulate matter?

[rohitkhanna]

The mysterious complex optical sensor is NDIR (non Dispersive Infrared sensor) this is used for detecting CO2 gas.