Tool mount for soil moisture sensor SMT100 or SMT50 available?

it may be unlikely but I wonder if anyone has designed a mounting tool for the SMT50 or SMT100 professional soil moisture sensors from Truebner which are pretty popular in Europe?
They are more expensive than the simple soil moisture pcbs but fully waterproof and much more accurate.


I’m not aware of any, but it would be possible adapt one of the freely available CAD models if you are familiar with CAD modelling and 3D printing.

Hi Rick,

thanks for the quick response, sounds great. I do have access to SolidWorks, but I am not very familiar with CAD modelling. Onshape may be better for beginners with its clean interface. I will give it a try.

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Hey Maria,

I am using the STM 50 but I have it stationary covered below soil level and read it out with a Lolin Board. I think this way you’re getting definitely more precise measurements. My experience shows that reading out single values is not really working because of the measurement error/ noise. Same will be here with the bot I guess, remember that the output level is only 3V of the Truebner sensor…

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Good point, I have both SMT50 and SMT100 sensors permanently installed under my lawn already for a couple of years and that works very well (SMT100 is more precise but SMT50 is fine for irrigation with a simple threshold setting). The good contact between sensor and soil is very important. So it may happen that pushing a sensor in and out of the soil will not guarantee a good contact between sensor and soil. I will try it first by hand to see the variations and confirm your experience which seems very reasonable. The SMT50 has a max. output of 3V, the SMT100 is available with different output voltage ranges, mine has 0-10V which is perfectly suited for my Siemens PLC. I may consider to burry the AquaFlex sensor which has a lenght of 3 m and could cover the whole area with good mean value, though my inital idea was to get a fine spatial resolved moisture distribution.

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Excellent to speak to someone who has some experience with these sensors :slight_smile:
Thanks for making me aware of the aquaflex sensor, I might look into this in future!

I burried mine SMT50 approx. 10-15 cm below soil in my raised bed. One needs to learn how to read the values. I’d expected far more higher values as it has rained almost all week but it never seems to go above 30% and mostly stays below 25%.

I still need to adjust and fine tune my soil moisture threshold for watering. As I only needed to water the bot once or twice this year, I couldn’t really experiment with it.

Any more ideas about these kind of sensors that you can share?


Maybe you need to perform an empirical measurement of soil moisture, so that you can accurately calibrate your moisture sensor?
I am unable to try this at present, being away from home, but my thoughts are:
Take 100g of wet soil from the Farmbot, then dry it thoroughly in the oven. Weighing this dried soil again will enable you to get a definitive measure of soil moisture.
Use this value to calibrate your sensor, then repeat the test when the moisture content changes to make sure that your readings from the moisture sensor are linear.

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