Dear All,
I want tomake a TDR based soil sensor to read water level detaction. if you have any suggestion or idea please share. I was finding more in google but we not get any good solution. I hope here we get somthing.

 

Just checking...what does TDR mean in this context?

 

Time Domain Reflectometry?

No, I don’t this would be the correct acronym.

 

That was my thought too but could not imagine how that would work. But you never know...

 

You're not wrong...


TDR moisture sensor - Wikipedia
A TDR moisture sensor employs time-domain reflectometry (TDR) to measure moisture content indirectly based on the correlation to electric and dielectric properties of materials, such as soil, agrarian products, snow, wood or concrete .

 

What is the exact problem to solve?

TDR can be used to sense moisture, but it would never be my first choice - given the complexity of the signal processing required.

Why do you think TDR is necessary?

 

I bought a couple of soil moisture probes off eBay that are resistance based - essentially two electrodes measuring the conductivity of the soil, but they never gave consistent readings. The probe part was just a 30cm stainless tube with an insulated section approx 2cm from the tip, to make the 2nd electrode. Inside the box up top was just a crude electromagnetic milliamp-meter and a AA battery, & no obvious means of calibration. So I repurposed the electrodes with an ESP32, a LiFe battery and solar cell/MPPT charger board to allow me to remotely monitor the moisture level under my new Japanese Maple. To calibrate I took 2kilo of soil from where the tree is planted and dried it in the oven at 120C for a few hours, then leave to cool in an airtight container. Then I packed it into a 50cm long 50mm dia plastic drain pipe and shoved the probe down the middle to a 25cm ldepth. 2kg of soil is approx 1000cc as is that pipe. I then poured 100cc of rainwater from my water-butt into the pipe and left it until the resistance stabilised for 10%, repeat for each 100cc/10% until a full litre (100% moisture by volume) has gone in (surprisingly it will). Now I know when to turn up, or down, the drip irrigation, though thats still manual for now. I have a DHT22 sensor in the box too so I get outside temp and humidity under the tree canopy too.

 

Did they use DC sensing instead of AC?
AC sensing avoids polarization.
Sometimes we use electrodes made of platinum or graphite.

 

Did they use DC sensing instead of AC?
AC sensing avoids polarization.
Sometimes we use electrodes made of platinum or graphite.

DC. The sense current was about 2mA from memory. I think my esp32 version is a bit more, maybe 10mA as it gave better results. I did wonder about using AC, or at least bidirectional DC, but the electrodes are so cheap I'm not worried about long-term corrosion. It's only needed for a year, after that the tree supplier says the tree will have established a good root system. At 1600GBP a mature tree isn't cheap, but it would take 10y to get to the size I wanted.

 

Hrrrm, Its Not the corrosion issue - its the electrochemical effect of DC on the electrodes ie building up material, increasing porosity ie if sacrificial, changes in capacitance too.

All the good moisture and air humidity sensors Ive ever seen that are consistent and stand the test of time are AC without any DC bias at all !

The better ones are pulsed AC with narrow +ve then -ve pulse widths and a good lag period so the electrode surfaces recover their original equilibrium conditions as far as possible - also good for checking capacitance at range of applied frequencies when not driven for AC conduction ie can offer a clue to changes in soil mineralisation too
;-)

 

In my travels through Google I came across this: https://vegetronix.com/Products/VH400/ which uses the TDR approach, and is under $50 one-off.

 

In my travels through Google I came across this: https://vegetronix.com/Products/VH400/ which uses the TDR approach, and is under $50 one-off.

That there is not a TDR (time domain response) that is a FDR (frequency domain response), it is a capacitive probe. It looks at frequency change, not reflections. It works better than the resistive probes, as it won't self destruct. However it is sensitive to soil type, placement, temperature, and salt content. If you just looking for a moisture sensor it is far better than the resistive types, but it isn't as good as the TDR sensors.

 

Welcome to AAC

That there is not a TDR (time domain response) that is a FDR (frequency domain response),

In the tech details it says (somewhat confusingly):

The VH400 moisture sensor uses superior transmission line techniques as does (TDR) to measure the water moisture in any soil regardless of soil salinity.