Bad Air, Bottom of Dig.

Tommy

Active member
Me and the gang are currently working on a dig, it's at the bottom of a vertical tube.

At the bottom of this tube we are noticing we are getting short of breath, presumably this is settling CO2. Could we lift out kibbles of CO2? The hauler has enough spare time between full kibbles to take a few up, so even if it barely helps at least it's keeping them warm.

Also there is a slightly sulphurous smell from the soil. It seems that every now and then we disturb some gas, what is this likely to be? I have found reference to Hydrogen Sulphide with a brief search of this forum. Is this the culprit?

New to digging, I need to call on your collective experience!

We didn't feel too bad down there, just short of breath like being at altitude or similar, we'll just crack on for the next few weeks but thought I may as well ask on here.
 

Maj

Active member
How deep is the shaft?
You could try a 12v x 100mm diameter in line extractor fan with 100mm flexible ducting (or any 4 inch pipe). Run the fan off either a car battery or a burglar alarm battery.

Maj.
 

bograt

Active member
Maj said:
How deep is the shaft?
You could try a 12v x 100mm diameter in line extractor fan with 100mm flexible ducting (or any 4 inch pipe). Run the fan off either a car battery or a burglar alarm battery.

Maj.

Surely better to pump fresh air in via a compressor than to extract bad air out not knowing where new air will enter?
 

Mark Wright

Active member
bograt said:
Maj said:
How deep is the shaft?
You could try a 12v x 100mm diameter in line extractor fan with 100mm flexible ducting (or any 4 inch pipe). Run the fan off either a car battery or a burglar alarm battery.

Maj.

Surely better to pump fresh air in via a compressor than to extract bad air out not knowing where new air will enter?

That method didn't work very well for Neil Moss.

Mark
 

cap n chris

Well-known member
Poor air in pits can take weeks to clear; the deeper the pit, the greater the problem. It can put paid to digging.
 

PeteHall

Moderator
bograt said:
Surely better to pump fresh air in via a compressor than to extract bad air out not knowing where new air will enter?

I think bad air can stagnate in pockets if you just pump in clean, so sucking it out from the lowest point tends to be the favoured option, particularly if you are using a small battery powered fan.

If you can get power in (generator on surface), you can do what we did once and have bouncy castle inflater blowing a hurricane down the shaft to you dig, which not only keeps if fresh, but keeps you cool digging  ;)
 

bograt

Active member
Mark Wright said:
bograt said:
Maj said:
How deep is the shaft?
You could try a 12v x 100mm diameter in line extractor fan with 100mm flexible ducting (or any 4 inch pipe). Run the fan off either a car battery or a burglar alarm battery.

Maj.

Surely better to pump fresh air in via a compressor than to extract bad air out not knowing where new air will enter?

That method didn't work very well for Neil Moss.

Mark

Lot more science to that one Mark, I dare say an extractor fan wouldn't have worked either --. 
 

Maj

Active member
When we were out of sinc with our clear, drill, bang routine and needed on the odd occasion to drill, bang & clear, we were reasonably successful in sucking bang fumes out from our dig with a 12v extractor fan set up. But as the worst of the bang fumes were extracted, so the extracted air/fume mix became less concentrated, but it still did a good job of extracting the fumes such that we could get back down to clear in the same evening session.
CO2 being heavier, I would have thought it would have worked better than on bang fumes.
I believe the diggers over at Limekiln near the Wessex extracted CO2.

Maj.
 

bograt

Active member
I admit that could work, but where is the fresh air coming from? professional way is to pump fresh air in to rid the bad air.
 

Bob Smith

Member
I'd have to disagree there, professionally, we preferred extraction over fresh air introduction to any tunnelling work. In my current work with submarine construction we rely mainly on extraction at the point of work in any of our confined spaces.


Pumped in fresh air usually ends up sitting on top a CO2 pocket, whilst extraction will dilute it.
 

Pitlamp

Well-known member
I think the blowing air in method is only more successful if a very powerful system is available. Extraction is probably better in many caving situations where more lightweight (less efficient) gear is used.

Extraction has the disadvantage of needing a non collapsing ducting set up whereas blowing air in gives more options for (scrounged?) ducting - such as lay flat polythene tube (or running a pressurised hose right to the dig face). The big advantage of extraction is that it doesn't simply blow the contaminant back up the passage, with implications for those not working at the actual dig face.

Agree with someone above; if in any doubt, always analyse the air properly.

If carbon dioxide is in high concentration then maybe consider absorbents. (These only work in certain situations and would become expensive if used over an extended period). This approach should only be used in conjunction with regular air analyses, to ensure that it is actually working.

Was it the Orpheus, many years ago, who hauled someone up in front of the committee for taking the club's Hoover and extension lead underground to snort out contaminated air from a dig? Or was that an apocryphal tale? (I always thought they should have been congratulated for demonstrating exemplary cavers ingenuity!)

 

Maj

Active member
bograt said:
I admit that could work, but where is the fresh air coming from? professional way is to pump fresh air in to rid the bad air.

If you just pump fresh air in, you need to pump at a high enough rate to push the heavier CO2 out of the shaft, otherwise the CO2 may just settle back down the shaft.

If you suck out the heavier CO2 from the bottom, the fresh air from above will replace the displaced CO2.
In a very simplified way you can kind of think of CO2 like muddy water settling at the bottom of a tank of water, you pump the muddy water out from the bottom and the cleaner water above it will take its place. But obviously gases (air etc) move much more freely and so can be more easily mixed when disturbed.

Maj.
 

SamT

Moderator
Think of CO2 as though it is water.  Its heavier than air and will settle/pool/sump at the bottom of your dig.

If you blow fresh air in, just like in water, it will 'bubble' up back to the surface and the CO2 will remain at your shaft base.  Think of blowing bubbles gently through a straw into you glass of water.

If you extract, the CO2, fresh air should replace it from above.  Ideally, it wants to be extracted over a point at which the CO2 will 'flow' away from the dig to a less confined area, otherwise it will just fall back down into the base of the dig (but at least it will be less concentrated and you'll have a bit of circulation)
 
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