Dear Pratibha, your question does not mention which type of stratum are you using to estimate the GHG emission from. however, the flux chamber (as it is popularly known) can be built up with varied dimension and with different materials depending upon your frequency of sampling.

1. if you are using the material which is transparent, it will create a green house inside the chamber and ultimately it will increase the temperature inside the chamber compared to the ambient air temperature. To minimize these changes insulation is required. You may cover the chamber with a reflective material which will help in reduction in the temperature changes.

2. if you are not using a transparent material to built up the chamber, still chances of that material being heated up is more leading to increase in temperature inside the chamber because the air inside the chamber is consolidated devoid of any exchange of heat to the air outside the chamber. in this case also i would suggest you to use a reflective film to cover the outer surfaces of the chamber.

however, height of the chamber will again depend upon the duration of sampling period and expected emission of GHGs. if fan is not provided inside the chamber then there might be thermal stratification and different gases may form layers depending upon their densities, which will lead to inefficient sample collection. when you are using a fan, it will continuously circulate the gases inside the chamber which will ensure proper mixing of gases, hence you will be able to draw a representative sample.

insertion of vent tube might not be playing a major role unless and until it is improperly fixed air tight.

hope this will be of some help.

Regards.

I was interested in measuring GHG emissions from soil. and the chambers built from Acrylic sheet. I hope the vent tube used instead of the fan also can help in mixing of gasses. The vent tube diameter has to be increased if the chamber height is increased as the mixing of gasses would be better.

well, i think i didn't wholly understand vent tube will be helpful in mixing of gases. I have some doubts regarding the functioning (the mechanism) by which the gas circulation (proper mixing) takes place.. ! I would like to know more.. !

Thanks .. !

Commonly used chamber heights range from 15 to 40 cm. If the chamber is too low then measuring headspace volume accurately may be problematic. There has been discussion in the literature suggesting chamber height should not lower than 15 cm. Depending on the study, if plants are to be incorporated in the emission measurements, then this needs to be accounted for in the height of the chamber.

The ventilation ensures natural pressure fluctuations occur inside the chamber and prevent pressure gradients. If soils are very porous, then there may be a chance that gases are lost through the vent. This is not a problem in fine textured soils. A proper vent should be 10 cm in length with a 4.5 to 4.8 mm diameter

Chamber insulation will minimize an artificial increase in temperature inside the chamber during the time it is closed and samples are taken. Insulation or some sort of reflective material on the chamber should be used. This is an accepted standard.

Dear Pratibha, this is a very good question. There is still no consensus on some of the issues you touched in your post i.e. venting. I think that you will find a document published a few months ago by the Global Research Alliance on Nitrous Oxide Chamber Methodology useful in answering your questions. It points out the variability of chamber design and highlights the need for a common practice in using the methodology. We should really all aim for such a uniform approach. Therefore instead of me talking about my experience with chambers, I'd rather encourange you to read through these guidelines and decide what will work best for your site/ soil/ crop/ climate conditions.

Here is the link to the document http://www.globalresearchalliance.org/research/livestock/activities/nitrous-oxide-chamber-methodology-guidelines/

It covers areas you're interested in. In a nutshell, chambers should be prepared from non-reactive materials ensuring no heating is taking place inside the chamber due to the type of material used. Height depends on the chamber footprint and a certain range of height/area ratios is recommended. Venting/ fans have no clear recommendations as current literature has no clear answer.

Hope this helps. Happy sampling!

Thank you very much Dominika Krol for the answer and the literature provided by you. Still I have some questions regarding the use of fan or vent tube. and the second one is do we need to use manifolds or modify the sampling port if the chamber height is increased especially to have observations on CO2