is it possible to calculate rate of evaporation ?

I am making a soil moisture sensor , I had a spare DHT11 , so I attached that too, I am wondering that soil will get drier faster if the rate of evaporation is high , is it possible to calculate the rate of evaporation when if we know the temperature/humidity and what else will be required ?

Search the internet for "evaporation calculator" There are lots of different ones for specific applications but the basic parameters are air temperature,humidity,water temperature and wind speed.

seaurchin:
I am making a soil moisture sensor , I had a spare DHT11 , so I attached that too, I am wondering that soil will get drier faster if the rate of evaporation is high , is it possible to calculate the rate of evaporation when if we know the temperature/humidity and what else will be required ?

Windspeed, surface boundary layer thickness - evaporation depends on the rate molecules escape
the surface, and also on the rate they diffuse back down again, which depends on transport of
humid air away from the surface, which in turns depends on the nature of the flow, turbulent or
otherwise of the air above.

Could you take a section of the soil you want to measure and build a pan that has a load sensor under it and record the weight loss over different time intervals?

seaurchin:
I am making a soil moisture sensor , I had a spare DHT11 , so I attached that too, I am wondering that soil will get drier faster if the rate of evaporation is high , is it possible to calculate the rate of evaporation when if we know the temperature/humidity and what else will be required ?

The DHT11 probably lacks of the precision need for that type of measurement.

The Davis Instruments Vantage Pro weather stations family is capable to offer this derived variable.
I would recommend have a look on this in page 48: Vantage Pro2 Console Manual

This document from Davis Instruments can be useful too:
DERIVED VARIABLES IN DAVIS WEATHER PRODUCTS

Also:
Measuring Evapotranspiration
Remote Sensing of Evapotranspiration

Good luck!

Do you want to calculate the measured rate of evaporation by measuring the humidity in the soil over time; or do you want to estimate what the rate of evaporation probably is given the current conditions?

Another good source of theory:

Rotronic Measurement Academy

other factor is going to be the nature of the surface from which the evaporation is taking place. A beach of pebbles, with water "under" them will lose water more slowly than a well plowed field with no plant life. And if you want to know how much water is being lost per horizontal square meter of surface, you will ALSO need to know what is growing there. Take the pebble beach: No plants, one rate of evaporation. Tall grasses: Very different.

Probably unknowable, in the end. But if the question is "when do I need to water", then other apporoaches may be valid.

OR, monitoring some of the things discussed above, and knowing that others aren't changing much (e.g. plant cover) can help you know when "more water" or "less water" will be right... even if you don't have an absolute "rate of evaporation" to use.

Just some random ideas.

Since the local conditions are relevant (e.g. type of soil, wind exposure etc) I think that the sensors output shall be referenced and calibrated locally.

This thread long dead I suspect...

Please have a look here:

http://forum.arduino.cc/index.php?topic=451151

Good luck!

MarkT:
This thread long dead I suspect...

Hehe..not yet :wink:

In my mind, ensuring accuracy and knowing probing errors are the challenges.

What is the value of soil moisture ? I mean, humidity is not H%, but is H% +- x. (either as a relative error or as an absolute error).

That x is more relevant than H%, as the magnitude of the error makes the H% more or less relevant.

I am not speaking about the precision of the sensor, but about of the error of the probing this time.

So, the first thing I would do (and I will do it asap) is to plug in a soil moisture sensor and a humidity sensor and see the collected values variation over time and different conditions.

In my view there are 2 distinct problems:
a) whether the collected values are accurate (i.e. reflect the real, actual values, as they were being measured by a high-end professional equipment, under known error statistics)
b) whether the collected values are useful (i.e. their variation can be used to make decisions, no matter how precise, accurate and real are - here the only point is the reproductibility of the values).

I believe that using a scale, oven dried soil and water you can get precise values so you can calibrate a moisture soil sensor.

I mean that during calibration there is not any plant because you have to calibrate the sensor only for water.
Once calibrated, the sensor is not sensible to weight variations.
I don't mean to use a scale as sensor. I mean to use a vase with only soil and water for calibration.