Evaporation measurment 4

[JECCNC]

Hi Every one, First of all appologies if this is the wrong forum. But I have a large water basin 150 m x 50 M by 4 m deep made of concrete. This basin has to be tested for leaks and is filled with water. I am testing to BS 8009 which states that the max drop in waterlevel should be the lesseer of 10mm or depth/500 + evaporation, my problem is how to measure the evaporaion? any ideas?

 

[SlideRuleEra]

See this link for a scientific answer, but quite complex:

http://www.newton.dep.anl.gov/askasci/wea00/wea00146.htm

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[StructEIT]

I would suggest something more emperical. You're not going to get a very accurate evaporation figure for your short term test using a complex method that will likely require a number of assumptions, and it sounds like accuracy is fairly important. I spent some time trying to design an evaporation pond for disposal of potentially contaminated water a couple of years ago and ran into this same issue. We ended up going a different direction because of the environmental conditions of the area, but I did figure some things out with regards to evaporation rates.

If you're already doing field measurements, you might as well measure the evaporation as well. Get yourself a standard Evaporation Pan. They're basically big flat pans that you set outside. They're straightforward, in that you put water in, measure the depth, wait, and then measure the depth again. You obviously have to account for rain and things that may happen during the test. Set it up and run the evaporation test at the same time as your other test and you'll have a more accurate number for evaporation in the very specific conditions of your test than you'll get trying to calculate an estimate from multiple variables.

Note that the evaporation rate from a large body is less than from an evaporation pan. The body of the pan heats up and contributes to evaporation. There's a reasonable amount of literature about scaling factors to go from pans to lakes, though, and you should be able to come up with a good value. My notes say that 0.7 is commonly used for many applications, but you might be able to come up with a better value for your specific case. The term you'd want to look up is 'Pan Coefficient'

 

[hokie66]

A common way of checking for leaks in swimming pools is to set a weighted plastic bucket on a step in the pool, filled with water so that the level matches the pool level. If both go down at the same rate, no leak. If the pool goes down quicker, the rate of leakage can be measured. A variation of this would work with your basin. You would just need a support for the bucket.

 

[Compositepro]

Hokie that is so simple, logical, and elegant that your answer deserves a star. And I learned something.

 

[JECCNC]

I have no access to a standard pan, so I had one made of aluminum, i filled it with water, measured the depth and placed it inside the basin, so it is about what hokie66 suggested, do i still need to multiply by a coefficent, or does the fact that it is inside the test area give representative values?

 

[msquared48]

I had to simmer that one a while Hokie, but finally got it. Simple is good.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[hokie66]

Yep, simple works for me. I have had a couple of pools which leaked, and this method was suggested by an old-timer as a simple way to find out. My problem both times was a backflow preventer which needed replacing.