CFC decay process - stratospheric balloon experiment

[andrzej]

Hello,
I take part in student's experiment. Our goal is to study CFC decay process in stratospheric conditions. In a ballon we have to build a system which could measure level of CFC.
Because it's student's experiment we have limited budget. And because it's balloon experiment we have limited power budget and weight.

We were advised to use PDECD detector (Valco), however we cannot take the controller. We would have to create all electronics and pneumatics system from scratch.
Do you think it's doable?
Are there other ways to perform the experiment?

Andrzej

[rb6banjo]

Fascinating! We need some more details on your experiment. Is the balloon manned - will you be going up? Are you trying to collect real-time data or could you just get a sample of the air (at whatever altitude) and bring it back to your lab for analysis? There are lots of good trapping-type sampling modes/devices out there for this type of thing.

[andrzej]

Unfortunately the balloon is unmanned The project is part of REXUS/BEXUS programme organised by European Space Agency.
We are going to send mixture of CFC and an inert gas, which would be exposed to UV light (which causes photodissociation of CFC). Then we'd like to measure how level of CFC changes.
We are somewhat stuck in choosing a detector. PDECD seems to be most adequate, but also most difficult in handling.
Other detectors we consider are Plasmadetek-2 or even simple miniPID.
We need to find trade-off between precision and ease of handling.

[rb6banjo]

Pretty tricky experiment. I just looked up the Plasmadetek-2. Are you going to send an entire chromatograph up there? Is a simple electron-capture detector (no pulsed-discharge) out of the question for you?

I'm sorry but I don't have any "ah-ha!" ideas for you. That's a tough problem - especially without the ability to have someone up there to attend to it.

[andrzej]

Yeah, the experiment is even more complicated if you think about pumps and valves in low pressure and temperature.
We can't send an entire chromatograph, so we try to build a portable version
We were thinking about ECD, but because it contains radioactive material we rejected it (license problems). However if you think it is easier to control, we could reconsider it. This type of detector was also used in similar experiments like ACATS-IV or LACE, so it might be good choice.

[rb6banjo]

I would think that a good old ECD would be less complicated from an electrical standpoint. I figured the radioactivity might be a problem and that's why you discounted it in the first place.

[AICMM]

andrzej,

I too make a non-radioactive ECD but the issue is the same, namely the electronics. If you can live with an FID type board modified for very high standing current then maybe this path will work. Then you would watch the decline in high standing current. Problem is still that you have to supply reaction gas plus dopant plus electronics. Linear range might suffer but if you are starting with low concentration to begin with then this might not be an issues in this mode.

However, must still chromatograph because you will have to separate O2 from CFC since ECD will respond to both. If no O2 present, then GC-less might work.

PID will probably not work since most of the CFC will have too high an IP for PID to work. Argon mode might work since the working IP for this mode is ~ 11.5 and below. However some CFC will have IP above that so if you are trying to see all, then not possible, but only looking for some, maybe. So it would depend on your choice of CFC. Would require FID type electrometer, argon mode not sensitive to most inert gases so column could be somewhat short.

Happy to discuss it further with you if you would like. Aicmm at flash dot net.

Best regards,

AICMM

[GasMan]

Why must you send the mixture of CFC and the inert gas up in a baloon. Is it not possible to reproduce the UV, pressure and temperature conditions that you have at altitude in the lab?

Gasman

[andrzej]

Actually we plan to do such an experiment in a lab - with artificial source of UV in a thermal-vacuum chamber.
However we don't know how reliably we can simulate stratospheric conditions in the lab. So the balloon experiment can be a verification of the ground experiment.

[huamincai]

Hi

The PDECD is not that difficulty to use. The detector includes all the electronics needed, so you don't need to redesign them. For CFC the ECD is the best choice because it has enough sensitivities for these compounds.

Cai