Expert: Daniel Mazur - 10/2/2007

Question
QUESTION: Hey, I'm an undergrad and designing an evaporimeter to measure the rate of evaporation from a wall. I'm having difficulty coming up with a good design - a pan evaporimeter will be too vague and not specific to a point close to the wall so I was thinking of adapting a piche evaporimeter so that this attaches to the wall, with the filter paper vertical on the tube rather than horizontal as the evaporation from the wall would be horizontal. Would this work? Have you any ideas for such an evaporimeter? Thank you, any help greatly appreciated.

ANSWER: Hello Charlotte,

It's not my field but it seems to me that an adjusted Piche's evaporimeter ought to work. You get evaporation from both sides of the filter paper in the classical one and you will probably want just one side.

As I understand your problem, you would like to measure with paper vertical. You can do that by implementing an L-bend between the calibrated tube and the filter paper. The paper should than be parallel to the wall. I don't see any problem with the proper soaking in the vertical configuration, if you use enough water.

The evaporation from the double-sided filter paper will probably be asymmetrical and you will want the part away from the wall (as opposed to "towards the wall"). Perhaps if you use a doublesided tape to attach a plastic disc (cover the whole one paper surface with the tape and the plastic disc) to the filter paper on the side towards the wall, that should measure the wall evaporation rate most precisely.

Personally, I would question a little if the filter paper surface is representative of the wall surface, it seems the evaporimeters ignore difference of evaporation from different surfaces. Nevertheless, seeing that people use them I guess it does not make a significant error.

Cheers,
Daniel

---------- FOLLOW-UP ----------

QUESTION: Hey Daniel, that's fantastic, thanks - I hadn't even thought of the L shaped design.  This is definitely not my field either so your help was greatly appreciated.
Slightly confused however by your section on attaching a plastic disc to the filter paper? Do you mean so the filter paper is attached to the disc by double sided tape and this is then attached to the entrace of the L-shape (at the bottom)? Won't this then stop any evaporation from the tube as it'll stop the water from getting to the filter paper?
Thanks again,
Charlotte

Answer
Hello Charlotte,
I am glad I could help. Let me try to show without a picture, what I meant by the plastic disc...

As I understand Piche's evaporimeter, the filter paper is clamped to the bottom of the tube just so that no water leaks on the sides. However, as the water soaks the complete paper and not just the bit that's in a direct contact with it, the evaporation surface is approximately the 2 times the area of the paper *minus* the cross section of the tube (and minus the area covered by the clamp).

I suggested to put a plastic disc between the clamp and the paper, so that the paper is *still in contact* with the water body. It really should not stop water from soaking the paper and from evaporating from the still-free surface of the paper. The evaporation area would be just the area of one side of the paper (the other is covered by the disc) minus the cross section of the tube. It is not much but it ought to work.

The clamping force and the water pressure must be just right, of course, so that the paper wets completely, but does not drip. My suggestion to use a double-sided tape for attaching the disc was aimed at eliminating one surface completely, but without disturbing the soaking qualities of the filter paper (as opposed to some liquid glue).

If you want to compare measurements with this adjusted evaporimeter to the standard, you must multiply your result with the ratio of the evaporation surfaces (i.e. times standardArea/adjustedArea).

Let me know, if I can help some more.
Daniel