Draught testing challenge

[thehungrytroglobite]

There are two caves a couple of kilometres apart. One draughts inwards in hot weather and the other (at lower level) draughts out. It would be useful to know if there is an air flow connection between the two caves.

Can anyone think of a way of labelling the inflowing air in such a way that it could be detected at the outwards draughting cave (in the manner of water tracing)?

eat a ton of baked beans, go inside, fart, then have a couple people on the outside sniffing for any bean-reminiscent odours

I just knew someone would make that suggestion sooner or later! 

As someone who has had to spend 10 minutes in a flat out crawl behind someone who had just scoffed two whole cans of baked beans I can testify to the accuracy of this method

 

[Pegasus]

Only in UKCaving are you likely to get two references to uranium hexafluoride in two threads within 24 hours.

...so proud 

 

[Fulk]

I think tht one was sulphur hexafluoride.

 

[Bob Mehew]

As was pointed out, uranium hexafluoride is notionally solid at room temperature.  More disconcertingly, it breaks down in contact with water to form hydrofluoric acid, lethal even in small doses, see https://www.cheshirewestandchester.gov.uk/your-council/emergency-planning/urenco-uk-ltd-capenhurst/safety-advice-for-residents.aspx for example.  Sulphur hexafluoride is stable but odourlessness see https://en.wikipedia.org/wiki/Sulfur_hexafluoride.  So I suggest neither are suitable for draft detection.

 

[Pitlamp]

I suspect that any method which will work will have to involve detection at sub-olfactory concentrations. We need a (non toxic) substance which can be captured by some sort of detector, similar to many water tracing techniques.

Thinking outside the proverbial box; spores have evolved to travel in air currents. (This is why mosses & ferns populate show caves with electric lamps, often a very long way in.) I wonder if releasing dry spores into air currents could be recovered in fine nets at the presumed destination cave?

 

[Mark]

I suspect that any method which will work will have to involve detection at sub-olfactory concentrations. We need a (non toxic) substance which can be captured by some sort of detector, similar to many water tracing techniques.

Thinking outside the proverbial box; spores have evolved to travel in air currents. (This is why mosses & ferns populate show caves with electric lamps, often a very long way in.) I wonder if releasing dry spores into air currents could be recovered in fine nets at the presumed destination cave?

Im sure there is an article about using spores for draught detecting in one of the early Descents or possibly SUSS Journals

 

[Pitlamp]

Aha - no point re-inventing the wheel, eh?

Is anyone familiar with this article?

 

[pwhole]

You can get lycopodium powder (club moss spores) quite easily online. It can be flammable in the right amount, but should be OK in a cave:

https://www.baldwins.co.uk/baldwins-lycopodium-club-moss-powder-lycopodium-clavatum?gclid=Cj0KCQiAys2MBhDOARIsAFf1D1dl3697M_774knBFuH4P1C_OaYVJ7Ql1YhfWT7cggabxvquqOQsKDwaAl6DEALw_wcB

 

[Mark]

You can get lycopodium powder (club moss spores) quite easily online.

That was the stuff the article mentioned

 

[Mark]

Just found this on google

https://books.google.co.uk/books?id=lbY5r7mXuSgC&pg=PA9&lpg=PA9&dq=lycopodium+spores+for+draught+detecting&source=bl&ots=UN0A2SNVqI&sig=ACfU3U3DrlsgARdFxVL_dWfktk3QX5MoNA&hl=en&sa=X&ved=2ahUKEwil57yI3530AhXOQvEDHdF-CcQQ6AF6BAgiEAM#v=onepage&q=lycopodium%20spores%20for%20draught%20detecting&f=false

 

[Pitlamp]

Some interesting info there. I wonder if anyone has a Hirst spore trap doing nothing for a while. It all looks rather bulky though.

 

[Bob Mehew]

The challenge with lycopodium spore is that each spore is heavier than air and can, when making contact with a surface (especially one which is damp or wet) stick to that surface.  So the spore will be 'filtered' out as the current of air wafts it way through the passage.  Maybe OK for a few tens of metres but hundreds of metres doubtful.  If one has access to a mass spectrometer, then one could use an inert gas.  Argon won't work since it naturally present at just under 1% in air. 

Mercapten is reported to have an odour at a concentration of down to 1 ppb (part per billon or 1 in 1,000,000,000) see https://en.wikipedia.org/wiki/Methanethiol. If I have it correct that is equivalent to 1 gram in 1,000,000 metre cubed or roughly the volume of 23 Gapping Gill Main Chambers (Descent 229 p26).  So it has potential.  But given it is also toxic above 10 ppm (parts per million), safely releasing it presents a challenge.  (Oh and it is also explosive up in the % range.)  One potential problem is that it is soluble in water, so could be 'washed' out.

 

[shotlighter]

Mercapten is reported to have an odour at a concentration of down to 1 ppb (part per billon or 1 in 1,000,000,000) see https://en.wikipedia.org/wiki/Methanethiol. If I have it correct that is equivalent to 1 gram in 1,000,000 metre cubed or roughly the volume of 23 Gapping Gill Main Chambers (Descent 229 p26).  So it has potential.  But given it is also toxic above 10 ppm (parts per million), safely releasing it presents a challenge.  (Oh and it is also explosive up in the % range.)  One potential problem is that it is soluble in water, so could be 'washed' out.

You really don?t want mercaptan hanging round. Apart from being pretty foul stuff, it?s also a bloody nuisance.
True story, in the late 70?s the analytical instrument company that I worked for had in office on a small town High St. One day we noticed a hole outside with ?West Midlands Gas Board? barriers round it. Apparently they were looking for a reported gas leak & over the next few weeks, the hole & workmen came and went. The hole was in a slightly different place each time, as they couldn?t locate the leak. Then one of my colleagues had a moment of enlightenment. The hole digging had started about the same time that we received a customers sample. He'd sent it so that we could do a HPLC quantitive analysis method for him. The compound of interest in the sample was mercaptan, a tiny amount of which is added to natural gas to make it smell.....
We quietly returned the sample to the customer and shortly after, the holes & workmen went away.

 

[Pitlamp]

The challenge with lycopodium spore is that each spore is heavier than air and can, when making contact with a surface (especially one which is damp or wet) stick to that surface.  So the spore will be 'filtered' out as the current of air wafts it way through the passage.  Maybe OK for a few tens of metres but hundreds of metres doubtful.  If one has access to a mass spectrometer, then one could use an inert gas.  Argon won't work since it naturally present at just under 1% in air. 

Hm; thanks Bob. The distance in question is a couple of kilometres in a straight line, so not sure the spore idea is practicable.

Do we have an aerobiologist in the caving community who might be able to advise?

 

[T pot 2]

During copper and lead mining when compressed air was used to operate the drills in deep mines the alarm used to vacate mines was the smell of eucalyptus, which was drawn into the compressed airlines on the surface and delivered deep underground

 

[Chocolate fireguard]

How about using some poly sheet and a large fan at one end or the other to try to engineer a draught reversal? Only a small change in pressure may be needed and no fumes to worry about other than from your generator for the fan.

No one has commented on this, but if it was my project and I really wanted to test a link between caves that far apart I would give it serious thought.
It would cost a bob or two and need some organising, but could work.
The main advantage is that the stuff you are putting in at the top end (air) doesn't need to travel all the way to the other end, it just has to pressurise the system so the flow rate out of the bottom end increases measurably.

 

[Pitlamp]

That's a really interesting suggestion Chocolate Fireguard - but we suspect the system in question has a significant number of entrances, so it may be too leaky for that to work effectively enough. If it was only a single conduit that'd be a cracking idea.

 

[Bob Mehew]

So if we are thinking of a 1,000,000 metre cubed volume of passage way, one would only need to pump in an extra 1,000 m3 of air to get a 1 milli bar pressure rise.  Air fans moving 4000 m3 per hour are available.  The BMP 085 pressure sensor has a noise level of down to 0.03 mBar so 1 mBar should be easily discernable.  The major assumption would be there are no leaks between the two ends which are sealed.  But I would expect the pressure change with respect to time at the other end of the system away from the fan would be spread out compared to that by the fan.  What I don't have data on is the rate of natural pressure change.  Is all that correct?

 

[Pitlamp]

That all sounds sensible to me Bob.

 

[Steve Clark]

From my limited experience of pressure testing buildings, the external factors can be quite large. Wind being the worst. If the two entrances (and unknowns in between) are in different locations, the wind gusting over the fell will affect each at different times.

Temperature is also a factor. Obviously limited solar gain, but there may well be convection dependant on the average cave temp vs. external temp. This varies with time and location.

I think probably the best way would be to run the fan with a pre-determined sequence (eg 1min on, 1min off, repeat x times, then reverse the direction and do it again). Then you could data log the digital manometer on the other end and see if you can pick out the signal by Fourier or similar.