STEMMA Soil Moisture Max Reading

[dbrazil]

Good Afternoon,

I'm using the Adafruit STEMMA Soil Capacitive Sensor.
When I run it in Arduino my maximum measurement is 1015, It differs from the specs given by Adafruit, which should be in the 200-2000 range. In an average moist soil gives a reading of 1014. In contact with pure water, it gives a 1015 reading. So I have barely any span to measure the moisture of the soil.
Anyone had this issue?

I just connected it through I2C and I'm using the Example from the Seesaw library for the Arduino (v1.2.0). Tried in 3.3V and 5V, same problem happens.
Later on I will use the sensor with a TI microcontroller.

Screen Shot 2019-10-27 at 3.00.39 PM.png (184.08 KiB) Viewed 1003 times

[adafruit_support_bill]

The Arduino version of the library returns a raw value between 0 and 1023. We have seen some sensitivity issues similar to what you are reporting and we are looking into them. I'll try to get back when we have a better understanding of them.

[faithraven]

I'm experimenting the same issues. I'm very eager for an update.

[adafruit_support_bill]

Based on testing, we have found that the first version of the firmware did tend to saturate quickly in some conditions. We have revised the firmware to provide usable readings in a wider variety of conditions. All new Stemma soil sensors will be shipping with the new firmware.

[faithraven]

How do we go about updating the firmware on the sensors that we've already purchased?

[adafruit_support_bill]

Re-flashing the firmware requires a JTAG programmer. But we can just replace the sensors.

Please contact [email protected] with a link to this thread. And let us know how the new firmware works for you.

[faithraven]

I've got a replacement, thank you.

Sadly there's not much of a difference. The new one "saturates" just as quickly, but mostly because of touch, not actual moisture.
Air value: ~320.
Dipped in water: ~750.
Slightly squeezed between fingers: 1015 (max).
Packed soil: 1015 (max).

[adafruit_support_bill]

What kind of soil are you testing with?

[faithraven]

There's a picture attached below with the soil I'm using.
However, I don't think the soil can be the issue. The main red flag is that the sensor produces a higher value when held between fingers than when dipped into water. That shouldn't happen, right?

[adafruit_support_bill]

The main red flag is that the sensor produces a higher value when held between fingers than when dipped into water. That shouldn't happen, right?

The sensor is not measuring moisture directly. It is measuring capacitance. Since the permittivity of water is much higher than air, the capacitance changes when the air intermixed with the soil particles is replaced by water.

Human tissue is mostly water, but it also contains other compounds which affect the permittivity. So it does not behave the same as plain water.

If you are not able to use these for your project, we can refund your purchase. Please contact [email protected] with a link to this thread.

[faithraven]

Thank you for the refund offer, but that's not what I'm after. Debugging the sensor was a good learning experience and I'm grateful for it.

I understand that it can't measure water levels quantitatively, but surely it must be erroneous to give a maximum reading for human tissue that's 50-75% water, and half of that in water which is well... 100% water.

There seems to be other capacitive moisture sensors that can give more accurate readings. One of them is sold by Adafruit, https://www.adafruit.com/product/1965.

It feels like this STEMMA sensor is just not on par with the otherwise excellent quality of Adafruit products.

[adafruit_support_bill]

surely it must be erroneous to give a maximum reading for human tissue that's 50-75% water, and half of that in water which is well... 100% water.

It is not a simple matter of percentages. The water in human tissue is not pure water. And the dielectric properties of water vary depending on what is dissolved in it.

I've ordered one of the Stemma sensors with the newer firmware installed and plan to do some testing on it in some different soil types.

I'm not sure what conditions were used to test the original firmware. I was able to reproduce the saturation problem using the original firmware on a Stemma sensor in a peat-mix potting soil similar to what you are using. On a prototype sensor with the firmware modifications I was seeing values peaking in the 950 range. I will check to see how the production units compare with my prototype.

[faithraven]

Please also look into the evolution of the readings over time (preferably a few days) for an undisturbed sensor. One would expect the values to slowly drop as the soil dries up. That didn't happen for me, the readings remained constant. This is another reason why I think that the sensor measures mostly touch and not much moisture.

[karlwachs]

I have I think the same problem.

got 3 of these .

on an RPI, measuring every 10 secs,

in air ~ 300
in water ~ 600
in between fingers ~ 1015
in bone dry soil: 1015
in completely wet soil ~ 1015 - no change

I understand the capacity differences with finger and water..

===> BUT no change in soil w dry and completely wet soil? <===

when putting the sensor into the soil ~ 1/3 (= not up to the marking) the range between bone-dry and wet is ~ 560..1015

any advice?

Thanks

Karl

[adafruit_support_bill]

@karlwachs - what type of soil are you using?