Equivalent circuit and connection type with Teros 11/12 sensors from Metergroup

Hi Everybody,

I am new in this forum. I am a french agronomist working on monitoring of physical parameters in agrosystems. Congratulations for the library you developed, it is really helpful. I am developping a station based on your SDI-12 library with Teros sensors from Decagon.

I am using the same schema you give in your documentation :

I read in the SDI-12 Documentation (page 4) and in the Teros 11/12 datasheet (page 3) that a minimal impedance was required when the transmiter is on or when the low power mode is activated.

When an SDI-12 device has its transmitter on, its direct current (DC) source resistance
must be greater than 1000 ohms and less than 2000 ohms

When any SDI-12
device’s transmitter is off, including during a low-power standby mode, the DC resistance to
ground must be within 160K to 360K ohms.

And especially (I am using a 3V alimentation and a 5V booster to supply sensors) :

If an SDI-12 sensor does not use the 12-volt line for
power, its data line resistance to ground while powered down must be within 160K to 360K ohms

My questions is :
– Are the components required in the circuit of the SDI-12 documentation/Teros data sheet (capacitors, resistance, …) already present in the sensor or is it compulsory and must be added to the circuit to supply and make the sensors working well?

For the moment, I have only one sensor using your schema (it’s working well) and that’s why I want to know if your second schema is sufficient to use properly multiple sensors or if SDI-12 documentation modified circuit (resistance, capacitor, inductor) is needed to use them?

Second, using your examples to get measurements from the sensor, I sometimes (1 for 20 or 25) have some “bad” values returning -68 for soil moisture and 0 for temperature. Is it usual to have some “errors” during measurement with Decagon Sensor like Teros 11-12/GS3? Is it a bad initialization? I could repeat measurement until a value is different from -68 in my code but this is a trick and it will just mask the real problem.

Finally, it would be a pleasure for me to buy and test your “Mayfly” Datalogger but it is unfortunately not available in France from Amazon. Would it be available in the future? I am really interested in.

Thank you in advance for your advices.

Kind regards