Soil Moisture Sensor Probes Frequently Asked Questions (FAQ)

VH400 Soil Moisture Sensor Probe Product Page

1. Is the VH400 Soil Moisture Sensor 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 soil moisture sensor probes do I need to measure an area?

The probe part of the sensor is approximately 4 inches long (for the VH400) 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 soil moisture sensor 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 soil moisture sensor 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. All soils are limited by a holding by their own holding capacity which is typically around 60%.

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 soil moisture sensor probe, or a customized interface. Can you customize the soil moisture sensor probe to my application?

In some high volume cases we are willing to do minor customizations to the soil moisture sensor, such as placing your companies logo and branding information on the silkscreen.

6. How long can the soil moisture sensor probe's cables 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.

The output resistance of the sensor is 10K ohms. Most wire cable is less than an ohm per meter, so the cables can be run very long lenths before there is any sort of reduction in signal.

7. What is the principle behind the soil moisture sensor probe's operation?

We get a lot of inquiries about how the probe works. All we can say is that it's - magic. ;)

8. What is the relationship between the voltage output of the probe and VWC?

See the VWC/Voltage curves.

9. Do I need to recalibrate the soil moisture sensor probe periodically?

No - the probe needs no calibration.

10. Does the soil moisture sensor probe use lead or other toxic chemicals?

The probe is ROHS compliant so it internally uses lead free parts and silver based solder.

11. Can the probe be powered down most of the time, and then turned on before taking a reading in order to conserve power?

Yes. For low power situations, you can turn off power to the sensor by removing power to the postitive terminal. The sensor requires a warm up time before taking a reading. Note that the ground (negative terminal of the sensor) must always be grounded when applying power to the sensor.

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

See the VWC/Voltage curves.If you are interfacing the probes to a microcontroller or computer, use a 10 point look up table. The conversion can be approximated by two different sloped lines.

13. What should I look for when choosing a soil moisture sensor probe?
  • Price - Let's face it, this is usually the most critical parameter when selecting anything. The VH400 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 VH400 series.
  • 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 VH400 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.
  • Form Factor - The VH400 probes are long and slender and so they can be used in more applications than their bulky competitors, including monitoring of smaller potted plants.
  • 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.
14. What are the A,B,C... or number markings on the side of the soil moisture sensor probe?

These serve as calibration markings. Under some applications, it is desirable to only place the probe partially into the soil. For example, if the probe isn't fully buried, depending upon how far it is inserted will change the output readings of the probe. If it is inserted half way it will deliver half the rated output voltage. These markings can also be used for calibrating water level measurements.

15. Can the soil moisture sensor probe be used to measure water level?

Yes, but the response is not linear. A better choice is the AquaPlumb liquid level sensor.

16. Can the soil moisture sensor probe be used to detect ice build up?

Yes. After all, isn't ice just frozen water. This means the moisture sensor can be used for refrigerator applications, and monitor ice build up on refrigerator coils and parts.

17. What if there is a gradient of moisture in the soil? For example, the lower part of the soil is wetter than the top.

The probe essentially returns an average over the whole length of the probe. The probe's sensitivity doesn't vary along it's length.

18. What cable do you recommend for extending the probes standard 2 meter length.

Use a 3 conductor cable that is shielded. For example you can purchase Belden part number: 1266A 0101000, from Newark.

19. How can you waterproof spliced cable connections?

We've created a cable splice kit that completely surrounds the wires and cable jacket making the splice water proof. Learn more

20. Do you have any reference designs, evaluations boards, or application notes for integrating the soil moisture sensor probe to larger systems?

For an evaluation board see our VG-RELAY boards. This simple board allows you to interface a single soil moisture sensor to valves, home automation systems, and sprinkler systems. It has a resistive thumb wheel so that you can easily set the trigger point. Boards are available which switch a relay on when the soil is too wet, or on when it is too dry.

See our Application Notes.

21. What forms of payment do you accept?

We accept checks and payment by credit card, Paypal, checks, wire transfer, and Western Untion. Extra fees may apply. Shipping and handling in the USA is typically $9.95 USD, $29.95 for Canada, $47.95 for Europe and Australia and Asia.

22. How can I use a microcontroller to control power to the sensor?

If you are using a third party controller, or your own microcontroller, and you want to conserve power, by turning off/on the power to a sensor, you can do this with an IO pin from your microcontroller, and a simple 3 part circuit using 2 MOSFETs, and a resistor. For a detailed explanation, see this article: "How to Power Sensors with a Microcontroller IO Pin".

23. Is there Arduino code available that will read the VH400?

Yes, one of our customers has posted his Arduino code to read a VH400 at VH400 Arduino Code at GitHub.

24. How can I get started, and try out a soil moisture sensor probe?

The best way to get started is to order a few sample probes and try them out in your application. Use our online ordering system to purchase samples. Each product page has an "Buy Now" button. We don't use distributors or other middle men, in order to keep our prices low. We ship our products world-wide directly to your door step from our factory. Yes, we even ship to your country!