Hello Clare,
A lot depends on how the readings are obtained. If from standpipes
I would normally expect a peak reading followed by a drop off,
particularly if monitoring is quite frequent. If you monitor every hour
for example then the volume of gas exhausted from the standpipe
every hour may be more than the volume of carbon dioxide flowing
back into the monitoring tube from the surrounding soil. Thus the
concentration in the standpipe will be lower than if you monitored
once per week. Is there a consistent approach from ALL
consultants to allow you some comparison - I would guess the
answer is NO !! (one consultant might literally throw a short pipe
down the standpipe, where another might have a 6m length of tube
attached to the pump. What is the flow rate of the pump, how long
will the pump be switched on for, and what is the permeability of
the surrounding soil?? - all of these factors will affect the
CONCENTRATION you measure because they are all FLOW
RELATED and are a function of gas PRESSURE!! Ten years ago
this was an issue - it still seems to be the case today!! (Of course
other factors such as ground-water fluctuation, barometric
pressure, wind speed, temperature, relative buoyancy of gas
mixture, diffusion, etc will all have an effect on the concentration
recorded from a standpipe, since various fluxes are established
which generate gas movement!).
What are we really trying to measure here?? - this has always
been my question. Surely it must relate to the rational behind the
risk assessment and the potential impact on the 'receptor' (I
think!!). It could well be appropriate to use the steady state values
rather then peak values if you feel that steady state conditions will
prevail in practice, although the extremely cautious person might
also have an eye on the peak value (Risk assessment is the
essential requirement here rather than the absolute values!! - and
the construction stage could well prove most critical in confined
space working rather than the final structure which should be well
capable of adequate design precautions against gas).
I hope these comments help and am sure others will have views on
this subject. The comments apply as much to methane as carbon
dioxide.
Regards,
Paul
> Hello everyone,
> can anyone profer any advice on carbon dioxide analysis.
> There are two sets of results, one which are absolute readings and the
> other are steady state readings. The steady state readings are lower
> than the absolute readings and the consultant is using them to negate
> the need to carry out remedial measures at a housing development on an
> old fertiliser works. I hope all goes well with everyone.
>
> Many thanks
>
> Clare
> Clare Horton
> Authorised Officer (Contaminated Land), Environment & Property
> Department 4th Floor St. Nicholas House Broad Street Aberdeen AB10 1BX
> 01224 523781 fax. 01224 647333 This message is not intended to have
> contractural effect.
>
_________________________________________
Dr. Paul H. McMahon
Civil and Environmental Engineering
School of Aeronautical, Civil and Mechanical Engineering
Faculty of Science, Engineering and Environment
Room LG9, Newton Building
University of Salford, Manchester, M5 4WT
Tel. 0161 295 3458 (Direct Line with Voicemail Facility)
Fax. 0161 295 5060
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http://www.acme.salford.ac.uk/
http://www.acme.salford.ac.uk/civ/dept/staffpages/mcmahon/dr_mac.htm
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