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Vegetronix Soil Moisture Sensor Probes Frequently Asked Questions

 

1. Is the probe waterproof, and can I bury it?

Yes, the probe has been designed and waterproofed such that it can be completely immersed into water for durations of time, and left buried in soil.  After all, this probe is intended for sprinkler irrigation systems. 

2. How many probes do I need to measure an area?

The probe part of the sensor is approximately 4 inches long (for the VG400) and .3 inches wide, and provides accurate readings for the medium with which it is in direct contact. If the soil is homogeneous and receives similar watering over an area then a single probe will give a representative reading for that area.  If regions of soil are different in composition, for example sandy or loamy, then you may want to use a probe for each type of soil region, and for areas that receive different watering.

3. I don't have a data logger, how can I try out the probe? 

The probe can be powered with an DC supply or batteries in the range of 3.5 to 20 volts.  The output is a voltage in the range of 0 to 3V, so any multimeter can be used to measure the moisture level.  Merely power the probe, and place the negative lead of the multimeter to the ground of the probe (bare wire), and place the positive lead to the output wire of the sensor.  Set the multimeter to measure DC voltage.   Dip the probe into water or a damp cloth to see changes voltage level, which is proportional to moisture content. 

4. How sensitive is the probe and what is it's range?

The probe is very sensitive. It gives a full range from completely dry to completely immersed in water.  It can measure the full range of VWC from 0 to 60, depending on the water hold capacity of the soil. 

In fact it is so sensitive, it can be used to measure the moisture content of your hands.  

5. I have a application where I need a customized probe, or a customized interface. Can you customize the probe to my application?

Yes, if your application requires a sufficient quantity of probes, we can customize the probe to your application. 

6. How long can the cables can be? 

The probe puts out a DC voltage representing the soil moisture and the cables are shielded so the cables can be very long.  If the cable is subject to common mode EMI then averaging techniques or common mode rejection techniques can be used to overcome EMI noise.

7. How linear is the probe?

The probe is completely  linear with water content.  The following graph shows the voltage output of the VG400 probe tested with various values of VWC in Miracle Grow Brand Potting Soil.  Results may vary from the graph below depending on soil type and soil compaction. 

 

8. Do I need to recalibrate the probe periodically?

No the probe needs no calibration.

9. How do I convert from the voltage output to GVC or VWC?

Since the output is linear, a simple linear equation can be used to calculate volumetric water content (VWC):

VWC= V*21.18644 -10.3814,

where V is the output voltage of the VG400 probe. 

10. What should I look for when choosing a moisture probe?

  • Price - Let's face it, this is usually the most critical parameter when selecting anything.  The VG400 series of probes leads the pack in pricing.  If you are the first person to find a comparable probe for a lower price let us know, and we'll send you a gift certificate to the Olive Garden!
  • Power - For battery applications power efficiency is critical.  We have yet to find a lower power probe in  the same class as the VG400 series.  It VG400-LP can run for over 5 years from a coin battery, with soil samplings of once per minute.
  • Technology - Many "cheap" probes use what is called resistive or conductive sensing of the soil.  The problem with these sorts of probes is that they corrode and rust over time, since bare metal of the probe is exposed to the soil.  If this weren't bad enough, they also give inaccurate results since soils with different salinity levels give different readings.  These sorts of low tech probes should be avoided because of their short life span and inaccurate results.  The VG400 series use superior sensing technology and will never corrode, and is insensitive to salinity. 
  • Durability - Avoid any probes that measure conductivity or resistance, since they will wear out over time.  
  • Accuracy and Linearity -  A quality probe should give an output which is proportional to water content over the full output range.  In addition, the probe should have a good output range to reduce sensitivity to noise.  
  • Supply Voltage Range - Many inferior probes must be powered with a specific voltage, otherwise the will be damaged or give inconsistent results. The VG400 series probes have voltage ranges that span 2V to over 20VDC.  In addition, their outputs are consistent over the full supply voltage range.

11. Do you have any reference designs or application notes for integrating the probe to larger systems?

See our Application Notes.

12. How can I get started, and try out a probe?

The best way to get started is to order a few sample probes and try them out in your application.  Contact us now to purchase samples.

 

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