Project guidance on soil moisture measurement

Hi,
i have to measure soil moisture using arduino and show that my measurements are true. i have this soil moisture sensor. sensor. And i have this

and i know how tu use it. But i have a soil moisture meter

My problem is, soil moisture meter is 20 cm and my sensor is far shorter than that: it is about 5 cm. So they measure different parts. And they give different results. I want get the same readings. What can i do?

If this is a school project- say so. You will get more respect from the forum members.

You could waterproof the sensor connector and bury the sensor to the same depth as the reference probe. Hot-melt glue would work for this. Don't bury the amplifier board.

You will also have to calibrate your sensor by making some tests. If you are lucky, the offset needed will be close to linear, so simply adding an offset to your reading should get you close to the reference.

SteveMann:
If this is a school project- say so. You will get more respect from the forum members.

No, most replies then will be of the "Do you really expect us to do your homework for you?" and "Can I give you my address so you can send me your diploma" variety.

squalo89:
My problem is, soil moisture meter is 20 cm and my sensor is far shorter than that: it is about 5 cm. So they measure different parts. And they give different results.

Well, of course.

What can i do?

Before we can get to that, you first have to answer this question: what do you want?
Get the same readings off of them? Measure the same part of the soil? Something else entirely?

sayHovis:
No, most replies then will be of the “Do you really expect us to do your homework for you?” and “Can I give you my address so you can send me your diploma” variety.

That depends on the question, and to what extent the poster shows they have done their homework. But that’s not limited to school projects, that applies to all posters.

SteveMann:
If this is a school project- say so. You will get more respect from the forum members.

You could waterproof the sensor connector and bury the sensor to the same depth as the reference probe. Hot-melt glue would work for this. Don't bury the amplifier board.

You will also have to calibrate your sensor by making some tests. If you are lucky, the offset needed will be close to linear, so simply adding an offset to your reading should get you close to the reference.

I will try to calibrate it, thanks. But i'm wondering while one of them measures entire pot, sensor just measures the top of the pot. Isn't it a little bit problem?

wvmarle:
what do you want?
Get the same readings off of them?

Yes.

squalo89:
i have to measure soil moisture using Arduino and show that my measurements are true.

Well, I think you have lost the game to start with.!

You can expect no repeatability or accuracy whatsoever from either of the devices you cite, so "measurements are true" becomes meaningless.

The materials used for the sensor for the first one will degrade quite rapidly with continued use and much the same for the second which is intended to be inserted and removed after making an individual measurement.

This is the result of electrolysis of the materials. To minimise if not prevent this, you would need to use resistant materials for the electrodes, and make the measurement using low-frequency alternating current. A capacitor in series with at least one of the terminals (but preferably both) can mostly prevent electrolysis.

squalo89:
I will try to calibrate it, thanks. But i’m wondering while one of them measures entire pot, sensor just measures the top of the pot. Isn’t it a little bit problem?

The long sensor probably just measures the moisture at the tip of the probe, where the isolated tip is.
So if you place the Arduino sensor at the same depth and near location as the long sensor you may be able to “calibrate” it.
Tom… :slight_smile:

Paul__B:
To minimise if not prevent this, you would need to use resistant materials for the electrodes, and make the measurement using low-frequency alternating current.

Make that high frequency... 10 kHz or so would be a good number. That can also prevent the migration of minerals in soil.

OP: you will see that when you power up the sensor you get different values for the first 0.5-1 seconds, or even longer before it stabilises. That's the result of electrolyses.

thanks for answers.

TomGeorge:
The long sensor probably just measures the moisture at the tip of the probe, where the isolated tip is.
So if you place the Arduino sensor at the same depth and near location as the long sensor you may be able to "calibrate" it.
Tom.... :slight_smile:

i tried to calibrate like what you said. but soil moisture meter shows dry and my sensor output is around 350 (it is 10 bit value.(0 wet - 1024 dry) . also my soil looks like wet, not dry. what could be the reason of this?

squalo89:
also my soil looks like wet, not dry. what could be the reason of this?

See reply #5 !