# Measuring and calculating rate of evaporation of water from a surface

I am looking to use a relative humidity/temperature sensor with an arduino Uno to measure the rate of evaporation of water from a surface (unit = g/m2/hour).
Can anyone tell me how I'd go about this - I am currently able to get temperature and RH measurements from my sensor but need to calculate rate of evaporation from them.

Don't know your sensor, but the easiest way is to measure weight of water column in given depth.

zwieblum:
Don't know your sensor, but the easiest way is to measure weight of water column in given depth.

I'm looking to make something similar to this device (VapoMeter - Delfin Technologies) - they measure change in RH and convert to g/m2/h

why donâ€™t you do exactly the same as described on that page?

An accurate digital scale supporting a shallow pan of water comes to mind.

RH only makes sense if you have a closed volume of air. Any exchange with the outside environment messes up the measurement. Of course the evaporation rate of water is dependent on the RH itself, plus a whole host of other factors.

Evaporation rate is very sensitive to ventilation/airflow over the surface. Undisturbed air over water has a
gradient of relative humidity since water molecules are diffusing through the air. This can choke off
the evaporation at the surface.

Disturbing the air brings lower RH air in contact with the water, causing the evaporation rate to increase
markedly (you experience this, its called wind-chill, forced-air hand-driers use it, blowing on your soup spoon
to cool it down etc etc).

There's no convenient way to measure the RH of the air really close to the water, but you can try to fix the
airflow rate to be fairly constant with a fan. Then you may be able to estimate evaporation rates (but not
measure them, though) from RH + temp.

To actually measure evaporation you do need to weigh the water....

Agree with the above - also would of thought the relative humidity of the air just above water to always be 100%?

Hi Bluestar.

From RH, volume and temperature you can calculate partial pressure and thus the mass of water.

Relative Humidity, Temperature & Pressure tutorial - YouTube explains the principles, you can find formulae in engineering toolboxes.

By measuring RH as a function of time, you can thus calculate mass as a function of time.

If the volume of gas is closed, then humidity will increase as water pervaporates through the skin into the air. After a while it will reach equilibrium. You can calculate from the gradient at time 0 what was the rate of pervaporation in g/h, and knowing the area of skin, you can derive g/m2.h.

You might need to circulate the air as noted by Mark T

Hope this helps