Calibrating Soil Watch 10 Sensor and accuracy of the readings

[aylashiv]

Hi,

I have ordered your soil watch 10 sensor, and waiting for the shipment, before getting it, I'm having some questions can any one clarify it?

How shall i get convinced that what ever readings from soil watch 10 is generated are appropriate ?

how to calibrate Soil watch 10 sensor? could you provide any mechanism?

Is this sensor is based on volumetric Water content (VWC)?

How long this sensor is durable while placing in the soil?

Kindly don't take it in a negative way, these are the sensors which are of urgent need for me?
These are the questions raised by our Team.

Thanks,
Shiva.

[pinolec]

Hi,

Thanks for good questions.
The readings you get are not absolute. To get close approximation VWC you need to calibrate the sensor for specific soil. We are working on preparing calibration curves.
We don't have a specific mechanism to calibrate sensor at the moment.
Our sensor gives repeatable readings (good observation is needed to spot the best levels for the application even with known VWC).
We estimate that our sensor will last 5 years+ but we don't have enough evidence to be certain.

If you have any further questions please don't hesitate to ask.

Thanks,
Piotr

[aylashiv]

Thanks for the reply.

Please let me understand ,how to measure VWC?

Also, you mean to say that we have to fix the VWC depending on the soil type, so this differs from type of soil.

Is this sensor VWC type?

Can i bury this sensor around 3-5 feet into the soil? For measuring the moisture content.

[pinolec]

Hi,

Sorry if I confused you.

The only method known to me to measure with 100% accuracy is to dry sample of soil.

Code: [Select]

VWC = (Wet [g] - Dry [g]) / Volume [cm3]

But this is of curse time consuming and very slow. Capacitive sensors or TDR give approximate water content instead.
Depending on calibration results may be quite off. Best results are achieved when the sensor is calibrated in the soil which will be used.

If the sensor is to be used in a typical farming situation the best idea is to determine desired moisture levels (min-max) get the readings straight from the sensor and use it without converting to VWC.
For eg. sandy soil can keep around 15-25% water by volume, on the other hand, clay can keep around 50-60% water by volume. Applying VWC determined in one type of soil to a different type of soil may be disastrous.

To estimate VWC you can use the equation:

Code: [Select]

y = 15.5x + 2.58
Please mind, that the equation it is work in progress.

The sensor can be buried in soil.

Regards,
Piotr

[aylashiv]

Hi,

Finally we have received Soil watch 10, Very Excited to test and calibrate with it..

we have tested this sensor, its pretty straight forward,Thanks for so user friendliness, and got the readings for the soil with different moist levels.

we have also submerged it fully into the water, it shows value 700 for the same.

There are few Questions and Doubts arised while testiing this one,

1) Will this sensor readings will have effect due to mineral and fertilizer composition present in the soil,
As the resistive ones vary significantly due to the presence of these fertilizers present in the soil and have a great effect on readings, irrespective of water content.

2)What is the length of the sensor that we need to bury inside the soil, I mean to ask,the sensor portion right from tip to the black waterproof casing?


Thanks,
Shiva.

[pinolec]

Hi Shiva,

ad 1. The fertilizer and minerals in the soil will have very little influence on the output of soil moisture sensor.
ad 2. To use a full potential of the sensor it is best to insert whole green part into the measured soil. The sensor also can be buried in the soil completely. To measure average moisture over the depths insert it vertically. To measure water content at depth insert it horizontally.

Thanks,
Piotr

[aylashiv]

hi,

can you elaborate this point,

what is the difference between soil  moisture and soil water content?

As i previously said the ADC value is crossing even value 700 when present in soil, and pouring water into the soil, what could be the possible solution to this issue?

The condition that we have tested earlier, is placing the soil watch 10 sensor into a complete bowl of water and dipped the whole green part upto black casing,there we have got value 700.

can we add a Pull up Resistor(a 1k ohms tied to a 3.3v supply ) at the signal output of the soil watch 10 sensor, will this have any effect on readings of the sensor.

Thanks,
Shiva.

[pinolec]

Hi Shiva,

I did use soil moisture and water content as a synonym.

To be sure we are on the same page, the typical output when submerged in water is around 2.7-2.8V
Your readings will depend on reference voltage used in ADC and the resolution of ADC. For 3.3V reference and 8bit resolution ADC reading of 700 mean 2.25V.

As i previously said the ADC value is crossing even value 700 when present in soil, and pouring water into the soil, what could be the possible solution to this issue?

Sorry but I don't understand. When you pour water into the soil next to sensor the readings will be close to readings in water and may go over 700. 2.8V is close to 850 on 3.3V reference and 8bit resolution.
I don't know what you want to achieve with the pullup but it is no problem to add it to the output. The output will not change. I would suggest bigger value like 10k - less power consumption.

PS Not sure if I was clear about the formula to estimate VWC. You need to insert as x the output from the sensor in volts.

Regards,
Piotr

[aylashiv]

I don't know what you want to achieve with the pullup but it is no problem to add it to the output. The output will not change. I would suggest bigger value like 10k - less power consumption.

Actually, we want to sense whether soil watch sensor is attached or removed from the micro controller ADC, Will this pull up additon works? Any ideal solution for this condition.

And also, we are mapping the generated ADC value to a percentage of 0-100, Actually as you said, the soil watch sensor can reach a max value of 850, in that case, if i feed max value as 700 in my program then it may become a problem.

[pinolec]

Hi Shiva,

I've done some tests with a 3.3Meg pull-up resistor. Checked all failure possibilities:
disconnected:

• vcc - gives readings of 0.05V. Anything below around 0.15 we should consider as reading in the air (sensor not fully inserted) or in this case disconnected vcc
• gnd - will give vcc - 0.3V. When you power sensor from 3.3V you will get 3V output.
• out - will give vcc

I do understand that you map the values as 0-100. Just set max value as maximum readings you get.
For example, if you get max value 850, just set maxADC to 850. That way you will get the readings within 0-100.

Code: [Select]

mappedValue = map(moistureValue,minADC,maxADC, 0, 100);

You may determine min and max values by getting readings in the air and in the water and use it as a minADC and maxADC.

Best,
Piotr

[aylashiv]

Hi,

Thanks for the reply and for your effort for the research you have done , what I've understood from the post you've written is when I disconnect that particular line i'd get the readings as mentioned, Actually we are using a Multiplexer

To minimize the ADC pins being used for various sensors,I've tried pulling up the ADC channel using a 1M resistor but it has some stray on it and giving some unknown values.

 After viewing the multiplexer data sheet , it is recommended to use a 10k pull down. Resistor on channels
Being used, I've implemented it and no stray is coming now.

Now the problem is ,when  the sensor when kept in only water it  is showing a value of 700 and putting up in soil and pouring water ,it is touching 850, how to over come this scenario? What should we consider as ideal 100 % water condition value of soil.Also salt(common salt)  has some effect on readings when I put some salt into the soil/water  a slight variation in the readings are being obsrverd.

And as you said in the previous post that the water holding capacity would be different for types of soil, but we need to monitor a remote field which is very far away from our location,and we cannot detemine the water holding capacity of that particular soil.how to ovrcome this problem? any suggestions?

Thanks,
Shiva.

[pinolec]

Hi,

Now the problem is ,when  the sensor when kept in only water it  is showing a value of 700 and putting up in soil and pouring water ,it is touching 850, how to over come this scenario? What should we consider as ideal 100 % water condition value of soil.Also salt(common salt)  has some effect on readings when I put some salt into the soil/water  a slight variation in the readings are being obsrverd.

The salts in the soil most likely play a role in here. Personally, I would discard the readings above maxADC (700) value measured in water and display 100%. Unless you would like to monitor some very wet field then this may be no go.

Why not to convert ADC readings to volts.

Code: [Select]

x = 3.3 / 1023 * adcValue
And use formula where y will be your aproximated VWC:

Code: [Select]

y = 15.5x + 2.58
That way you will have no problem with values over 100%

Best,
Piotr

[aylashiv]

Hi,
 

The salts in the soil most likely play a role in here. Personally, I would discard the readings above maxADC (700) value measured in water and display 100%. Unless you would like to monitor some very wet field then this may be no go.

Now, totally I'm confused, As the soil in which we where to deploy this sensor will have the fertilizer and salts present, and again I cannot understand this math,

y = 15.5x + 2.58.

The VWC never crosses 50%,provided the voltage 3.3volts converted from analog readings measured,though i inserted the sensor in a jar full of water as seen in picture.

So Please justify us how this VWC functions?

[pinolec]

Hi,

It may be confusing at first but it's really simple. VWC stands for Volumetric Water Content. It indicates how much water is in measured soil by volume. The data from your measurement indicate that in your sample 43% of the volume is water remaining percentage is either soil or air (between soil particles). So if we measure VWC in the soil, there is no way to have 100%. Most mineral soils don't cross 45% VWC.

https://en.wikipedia.org/wiki/Water_content - more on VWC and GWC (Gravimetric water content)

Please have in mind that calibration formula is for mineral soil only.

The salts and fertiliser effect on reading is most prominent in saturated soil. Usual moisture levels in the soil will show less influence on readings.

Thanks,
Piotr

[aylashiv]

Hi,

Thanks for the reply.

The math provided for calculating VWC does not need the volume of soil, so the math applies for all types of soil, and the maximum VWC could be assumed at 45%, correct me if I'm wrong.

 Here is another scenario what we have observed.

Actually we are running our system on a  rechargeable battery, so what is happening is,the battery voltge level varies from max to min,

So also the voltage feeding to the  soil watch also gets varied and also there is a variation in readings we observed,

What could be the best alternative to this situation?
We are considering an LDO regulator such that it provides a fixed voltage and the drop out voltage would be minimum, for the current it sources.