Plasma cutter potential dangerous fumes and gases
Most of us use plasma cutters without really thinking about possible dangerous fumes and particles.
A couple of days ago I was cutting some 6mm Al plate and unusually for me I forgot to turn on the extractor in my welding bay fume hood. Within 30 seconds of cutting an alarm went off and at first I thought it was my smoke alarm as it has done that before when I forgot to turn the extractor on while welding although usually I take the smoke alarm off the ceiling when welding. However as I approached the alarm I realised it was not the smoke alarm but my new CO alarm.
That's strange I thought it has never gone off before but then I realised I recently reset the threshold alarm level to 50 ppm after I learned that long term exposure to 35ppm was considered dangerous (I used to have it set at 100 ppm). The reason for me setting it higher than 35ppm is that the sensor is at ceiling level and it would be lower than this around my head.
Anyway I just let it sit to see how high it got and it eventually got to 300 ppb! Nothing was on fire and I could not see where the carbon was coming from but then I thought the atmosphere has plenty of CO2 in it, and indoors, especially sheds with stale air, can have several thousand ppm of CO2. My guess is that plasma cutters probably dissociate CO2 into CO as this is common process in plasmas. FWIW I have worked on an analytical instrument called an ICPMS ( Inductively Coupled Plasma Mass Spectrometer) where CO from CO2 in air is commonly detected and a damned analytical nuisance.
There is also the possibility that the MQ7 sensor used to detect the CO is also sensitive to other gasses so I looked into this and found it was sensitive to molecular Hydrogen (H2), Alcohol, methane and LPG. Of these, teh last 3 are not likely and while dissociation of H2O could produce hydrogen it would be fully ionised H+ and not H2.
I then got to thinking what else could the plasma make so I checked out what research had been done, and found this excellent article for 2017 .
https://academic.oup.com/annweh/arti.../3/311/2877141
Extract from the conclusions.
Quote:
In the present study, we found that stainless steel plasma cutting fume contained high amounts of toxic metals, such as Cr6+.
The fume generation rates and oxidation levels were directly associated with the arc current.
Higher arc current introduced more vaporization and fume formation, as well as oxidation.
These findings underscore the need for workers to exercise caution when selecting arc current to balance the job duty with fume exposure.
It was worth noting that the high positive correlation between NOx and Cr6+emission under the same arc current provides an alternative way to quickly determine Cr6+ concentration using a low-cost NOx sensor and monitor in the workplace.
Particulate concentrations of both fine and coarse fractions were impacted differentially by arc currents.
The high current arc created more fine particles, although it shifted the GMD to a larger one that is less favorable for respiratory deposition.
[insert by OP]Around 1 billion fine particles per cubic metre are produced using a 50A current[ends insert]
In the coarse particle fraction, medium arc current produced fewer particles, since the excessive energy was avoided.
So why haven't we heard about much this before?
I know the instruction manuals suggest the possibilities of dangerous fumes but it's nice to know whats going on.
One of the next things I will do is build an NOx gas sensor since NOx levels were shown to be proportional to Cr6+ levels when cutting SS. I already have an NOx sensor (MQ135) in my stash so no waiting on the slow boat from China.
In a related way I was TIG welding some short strips of mild steel recently without the extractor on. Ten minutes or so after I finished I could see a fine haze in the air and I walked past my particle counter located about 5m away and noticed the dust levels were 50X higher than usual. Remember this is TiG - no flux and therefore one would think no dust - but it was clearly making plenty of fine dust especially to be able to reach 5m away. Next time I run the TIG I will bring the sensor much closer to see what it does.
Meanwhile I guess everyone that doesn't have any should seriously think about setting up some extraction or ventilation for this sort of thing. Don't rely on just opening a door or window or a whirly bird get something that is forced at doesn't rely on the vagaries of the weather.