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Department of Forest Ecology, University of Helsinki
Class leading ± 1% soil moisture accuracy- Can be buried or used as portable probe
- Easy data logger connection (0 – 1 V DC)
With its 1% accuracy the ThetaProbe has set the standard for soil moisture measurement for more than 20 years.
Highly accurate yet easy to use
With the ML3 ThetaProbe Soil Moisture Sensor it’s easy to make reliable, accurate soil moisture measurements. Simply insert the soil moisture probe into the soil, connect to your data logger or readout unit, provide 5-14 V DC at 18 mA, and within seconds you can be logging soil moisture.
The ML3 ThetaProbe is robust, buriable and maintenance free (environmentally protected to IP68). It can be easily installed at depth into augered holes using the optional extension tubes.
The ML3’s excellent salinity response has been characterised at EC values up to 2000 mS.m-1. It also has a wide operating temperature range, allowing it to be used at temperatures as low as -40 ºC (non-flexing cables).
The ML3 includes a thermistor sensor for the measurement of soil temperature (probe must be fully buried).
Data logging and readout
The ML3 can be logged by any Delta-T data logger; it is compatible with any logger that can provide 5 – 15 V DC excitation power and will accept the 0 – 1 V output signal. For portable applications the ML3 connects to the HH2 Moisture Meter and they can be ordered together as the convenient ThetaKit – see below (please note that the HH2 Meter does not provide temperature readings).
Easy installation
The ML3 ThetaProbe is buriable and its cylindrical shape makes it easy to install in augered holes (45 mm diameter). The tough stainless steel rods can be replaced if bent or damaged. The threaded base of the sensor can be connected to optional 50 cm and 100 cm extension tubes.
Applications
Typical applications include irrigation, hydrology, civil engineering, soil water profiling, pollution monitoring and forestry. Because the ThetaProbe has a predictable saline response (which actually declines as salinity increases) it can be used even in very saline soils – up to 2000 mS.m-1.
The ThetaProbe can also be used in non-soil media.
ThetaKit
Supplied in a convenient carry case, the ThetaKit contains everything needed to makes accurate spot readings of soil moisture content. The kit comes complete with a dedicated HH2 Moisture Meter for instant readout, an insertion tool for penetrating hard soils, and a set of spare replacement rods.
ThetaKits are an excellent choice for turf grass applications.
Please note that the HH2 Meter does not provide temperature readout or recording.
Specification
| Measurement | Volumetric water content and soil temperature |
| Accuracy | ± 0.01 m3.m-3 (1%) With soil-specific calibration |
| ± 0.5°C (0 to 40°C for temp sensor) ± 0.75°C (-20 to +60°C for temp sensor) | |
| Soil moisture measurement range | Full accuracy over: 0 to 0.5 m3.m-3 |
| Full range : 0 to 1.0 m3.m-3 | |
| Salinity range | 50 to 500 mS.m-1 |
| Salinity errors < 0.035 m3.m-3 from 0.05 to 0.4 m3.m-3. Can be calibrated up to 2000 mS.m-1 | |
| Temperature range | Full accuracy over: 0 to 40°C |
| Output | 0 to 1.0 V differential Corresponding to 0 to ~0.6 m3.m-3 |
| Resistance 5.8Ω to 28Ω for temp sensor | |
| Power requirement | 5-14 V, ~18 mA for 1s |
| Minimum 5.5 V with 100 m cable | |
| Environmental | IP68 (-40 to +70°C) |
| Sample volume | ~60 x 30 mm diameter |
| Sample volume is weighted towards soil immediately surrounding the rods | |
| Dimensions and weight | Overall: 158 x 40 mm diameter Rods: 60 mm x 3.2 mm diameter (Rods are replaceable) Weight: 0.1 kg (exc. cable) |
| Sensor calibrations | Individual sensors are interchangeable Recalibration advised every 3 years (depending on use) |
| Soil calibrations | Generalised Mineral and Organic soil calibrations are supplied |
Accessories
Hardware
Main components and software
Mountings and enclosures
Installation accessories
Cables and networks
Remote communications
Spares and consumables
Power options
Other
Product Resources
Data Sheets
- ML3 ThetaKit Data Sheet 2 MB
Filename: ml3-thetakit_v1l - Soil Moisture Measurement Catalogue 14 MB
Filename: Soil_Moisture_Measurement_Catalogue_ver_5 - ML3 ThetaKit User Guide for Sports Turf 823 KB
Filename: ml3_kit_user_guide_ver_1-0
Case Studies
- Seven brief Delta-T Devices soil moisture sensor case studiesDownloadFilename: →
Videos
Manuals
FAQs
- Setting up an M12 cable network to ensure a waterproof and secure connection.
Question
What is the best practice for setting up a M12 cable network to ensure a waterproof and secure connection..
Answer
It applies to:
GP2 for SDI-12 and logger networks.
Celora for SDI-12 networks.
All sensors with M12 connector cables.Examples of SDI-12 network and GP2 logger networks:
DO
When connecting multiple cables and T-piece together, it is essential that any M12 connector is tightened sufficiently:
• Ensure internal connector O-ring and opposite sealing face are free from dirt.
• Tighten the connector thumbscrews as much as you can by hand or use a torque driver set to 1Nm to be sure. See video:We recommend the Weidmuller “1900021000” torque driver:
• This will save lots of painstaking troubleshooting hours if you do have a loose connection and give you piece of mind that all connectors are correctly tightened.
• Untightened connectors can come undone and let water into the cable network which can damage wire cores and cause erroneous readings.
Connect in the office first and then transfer the network to the field. This way, you’re not trying to do it in unpleasant weather and the time pressures are off.
DON’T
Use duct tape. This will hold water against the connector. If tightened correctly, the connector will seal and hold. - Are Delta-T soil moisture sensors waterproof? Can they be buried? (SM150T ML3 ThetaProbe)
Question
I see that the environmental protection rating for Delta-T soil moisture sensors is IP68. In practical terms what does this mean for the performance of the sensors and their cable connections, and can they be buried?
Answer
The IP68 Standard
The IP standard defines the degree of protection against the entry of water and dust that an instrument has. The first digit corresponds to performance for dust protection, the second figure for the level of water resistance. A rating of IP68 means that a device is completely sealed against dust and that it can withstand complete and continuous submersion in water. More information on the standard can be found here: https://en.wikipedia.org/wiki/IP_Code.
When Delta-T develop products for use in the field, we not only aim to meet the requirements of relevant IP standards, but, where possible, to surpass them. Our product design and testing aims to establish the highest performance we can achieve for the sensor and the cable connection.Sensor sealing
During the design of the *SM150, SM150T, *SM300 and ML3 sensors, extensive water resistance tests were performed with reference to IP68 standards. Thermal shock tests were also carried out, which involved continuously monitoring the humidity inside the sensor as it was subjected to temperatures above 100OC before being plunged into ice cold water. It was demonstrated that, even under these harsh ‘shock’ regimes, the sensors showed no signs of water ingress.M12 connector sealing
During the design stage of the ML3 ThetaProbe (our most recently released sensor) we completed a comprehensive range of tests on the M12 cable connector (also fitted to several other Delta-T soil moisture sensors) to ensure satisfactory water resistance when submerged in water at depth.These tests will continue and results made available. Test results so far indicate that the M12 connection with the soil sensor can withstand burial to a depth of 2m – providing care is taken to ensure the connectors are clean and dry when assembled, and that they are hand-tightened securely.Routine production testing
*SM300s, *SM150, SM150T and ML3 sensors are batch tested using the hot water leak test – which involves submerging a proportion of sensors from a manufacturing batch in a 60°C water bath for a fixed time whilst checking for bubbles emerging from the sensor or connector. In terms of pressure, this treatment is comparable to an immersion in 1.4m depth of water. We have determined that this an acceptable test to discover assembly errors.
5 year warranty
All Delta-T products will benefit from a 2 year warranty. However, for additional peace of mind, once you have registered your SM150T or ML3 soil moisture sensor with us, the warranty increases from 2 years to 5 years.*SM300 and SM150 are now discontinued