I have to discuss an issue regarding relative humidity in Radon monitoring from soil gas. We are continuously monitoring Radon from soil (24/7) for the whole year. With the passage of time, our instruments are reading irregular values because of increasing levels of humidity. Desiccant like Anhydrous Calcium Sulphate (provide with RAD-7 instrument) is useful if we have to do sampling for a limited time and for long term monitoring this method is very expensive. A device like Drystik from Durridge Company is also available but its pump life is of one year. So, for a reliable and long-lasting solution what should have to do to improve our data acquisition and data quality?
There are several possible solutions. The choice of design will depend upon intent of measurement and instrument enclosure.
It appears you are using the RAD-7. Are you pumping from a given volume: depth? Could you use a passive instrument?
What is the necessary time resolution? Hours, minutes, daily?
Where is the instrument placed with respect to the volume sampled? How is it protected?
Could you pass the gas through a cold trap to condense most of the water before using a desiccant?
Joseph L Alvarez I am using SARAD RTM-2200 for measurement of radon from soil and AlphaGurad DF-2000 for measurement from water. RTM2200 intake a sample of air from the ground at one-meter depth after every 15 minutes. This air coming from the ground is full of moisture that causes the instrument to read irregular values and with the passage of time instrument become useless. For protection normally people use desiccants but for continuous monitoring, it is not affordable to use this so the only solution is to use an electronics-based mechanism to remove moisture from the air without affecting results.
Paul Milham Thanks for your reply, Sir.
You are right the desiccants can be dried and reused but the level of moisture from soil is very high that causing an increase in the cost of desiccants. So we need a solution that has low cost and long-lasting.
I am afraid that if we do condensation, then maybe we will not get original results and maybe our parameters will affect with the cooling process.
What about a cold trap in line? Ice would do to remove most of the water
The moisture in the instrument can be controlled several ways that could be compatible with your measurement requirements.
- Place the instrument in a controlled environment (a small enclosure that is exhausted and held at a temperature that ensures no condensation within the instrument)
- Cool the sampling tube for moisture removal, then warm for analysis
- Pull the sample through a porous tube that is annularly placed within a tube containing rock salt. Change out the annular tube on a regular basis so the salt can dry for reuse. Very simple and inexpensive.
- Use a submersible diffusion barrier type detector as used for continuous water monitoring.
Any further suggestions requires knowing your sampling requirements and the intended accuracy to meet your experimental needs.
Since the climate in Syria is moderate or Mediterranean, so the winter season, where high humidity occurred, is limited to almost three months. Therefore, we stop carrying out soil gas radon measurements, and most of the field geological work also stops during the winter season. Accordingly, during our career we have not encountered any problems such as the effect of humidity factor on soil radon concentration. However, a common solution to reduce the effect of moisture on radon measurements in soil air is (as you mentioned) to use some moisture absorbent materials such as dehydrated crystals. In your case, I suggest that you measure the relative humidity in parallel with the radon measurements and try to establish a relationship that shows the correlation coefficient between the two parameters. So, you can mathematically show the influence of relative humidity on the monitored value of radon concentration, and take this into account while interpreting the results.
This is what came to my mind at the moment, but if I have any other ideas related to this issue, I will send it to you in time, with my best wishes for success in your field work, which includes continuous radon monitoring.
I think the only way is to use multiple dry stick in the line of air succession. It is possible to design a cold trap using electric cooler but the condensed water adjacent to air affects the radon concentration as radon is soluble in water. so designing cold trap in a way that condensed water does't affect the radon is so complex.
Paul Milham Cold trap is a good option but maybe it will affect the concentration of radon too.
Mohamed Al-Hilal Thanks for your informative answer, Sir. I will try to find the correlation coefficient and I think for different locations there will be a different value of this coeffient.
Mansour Jafarizadeh Thank you for your informative answer. I agreed with your answer that condensed water adjacent to air affects the radon concentration as radon is soluble in water. The way of finding the correlation of humidity with radon is the way to play with data.
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