APAS T1 User Manual
Thank you for choosing the APAS T1 volumetric water content sensor from DurUntash Lab. This cutting-edge sensor technology will enable you to accurately measure the moisture content of soil and soilless media alike. This online document will guide you through the most important features and specifications of the sensor, and explains how to correctly operate the sensor in a variety of applications. To understand the instrument and maximize its benefit, please make sure to read the contents of this user manual. We also recommend exploring the resources available on our website, and reading our blog articles.
The APAS T1 sensor measures the water content (moisture), and temperature of the soil or soilless media using the sensor blade, which is made of a 4-layer PCB. The traces (or electrodes) are in the two middle-layers of the PCB and completely covered. Thus, they will not be exposed to moisture or affected by soil EC. The temperature is measured with an embedded digital sensor, located right on the boarder of the blade and electronics. The electronics of the APAS T1 are protected by a plastic cover (ABS), which is filled with durable electronic-grade epoxy. The epoxy is resistant to moisture, humidity, corrosion, fungus, static discharge, etc and therefore the APAS T1 sensor. This forms the sensor head (black). The sensor has an overall low-power requirement, which makes it ideal for IoT applications, continuous monitoring using a data logger or occasional readings with a handheld reader.
Moisture Measurement Principle
The APAS T1 sensor falls in the category of so-called "capacitive" soil moisture sensors. The sensor creates an electromagnetic field around itself to measure the dielectric permittivity of the surrounding medium. This measurement is later calibrated and related to the medium water content (moisture). To do so, the sensor circuitry analyzes a 20-MHz wave that has gone through the soil and been modified mainly due to the water content. This creates an analog voltage and amplified using an op-amp.
EMI Noise Reduction
Noisy soil moisture readings due to cable interference or a bad power supply may appear mysterious and cause a lot of headaches. Good soil moisture sensors usually operate at a high frequency (MHz - GHz), and have a high sphere of influence (detect moisture in a larger volume of soil), which is at its peak when moisture levels are high. Such a soil moisture sensor can be assumed an antenna that is installed in the soil rather than in the air. The sensor cable can act as the extension of this antenna, interfere with the sphere of influence, and therefore cause fluctuations in sensor readings (noise). To reduce EMI for FCC regulatory compliance, and to minimize any noise caused by the aforementioned sources, we have added a high quality ferrite bead to the circuitry and a snap-on ferrite core to the sensor cable. Depending on the noise level, the ferrite core can be replaced by a larger one, and another one can be added to the other end of the cable.
Sphere of Influence
The APAS T1 has a very large volume of influence, making the sensor much less sensitive to air gaps during the installation process, and resulting in higher accuracy. The sensor blade is only 8.5 cm, while it delivers an impressive ~800 mL volume of influence. With the addition of the sensor head sensitivity to moisture, the volume of influence exceeds 1,000 mL.
The temperature is measured with an embedded DS18B20 sensor (digital), located right on the boarder of the sensor blade and electronics. For more information about the DS18B20, please refer to its datasheet. The temperature sensor response time is relatively fast and reflects the average temperature of the probe within minutes of installation. Considering the fact that the sensor head is black, the temperature measurements may show high if exposed to direct sunlight. Please also be aware that holding the sensor head in hand will increase the temperature readings and affect moisture readings. Please allow enough time for temperature equilibrium between the sensor and soil, especially when carrying out spot measurements of soil moisture and temperature.
The research-grade APAS T1 comes pre-programmed with sensor-specific calibration coefficients, and with a ferrite core on its cable. This makes the research-grade sensor great for both scientific research and irrigation scheduling, and allows it to work smoothly with DurUntash Lab data acquisition systems. The APAS T1 sensor is also well-suited for Arduino and Raspberry Pi projects.
Cable Length Options
The APAS T1 is available in three standard cable length options: 1.8 m (6 ft), 3.0 m (9.8 ft) and 4.5 m (14.8 ft). The cable length can also be easily extended using standard 1.8-m, 3-m, and 4.5-m extension adapter cables available from third-party sellers.
Long vs. Standard Sensor Tip
The latest model of the research-grade APAS T1 comes with a longer and sharper tip, compared to the standard model, that makes it easier to insert the sensor into rockwool cubes. The latest model is about 20 mm longer than our standard APAS T1 model. The two models are identical in terms of performance and accuracy.
Volumetric Water Content Measurement
Range: 0.0 – 1.0 m³/m³ (soilless media calibration)
Note: This measurement range depends on the medium and sensor installation. A soil-specific calibration may change this range for other substrates.
Resolution: 0.001 m³/m³
Accuracy: ±0.05 m³/m³ with generic calibration
Frequency: 20 MHz (square wave)
Volume of Influence: ~800 ml
Duration: ~10 ms
Output Type: analog voltage
Range: −55 to +125 °C
Resolution: 0.125 °C (depends on DS18B20 settings)
Accuracy: ±0.5 °C from -10 to +85 °C
Note: For accurate temperature and water content measurements, the sensor must be given enough equilibration time.
Duration: 375 ms (typical)
Output Type: digital signal
Standard: 155 mm (L) x 31 mm (W) x 11 mm (H)
Long Tip: 175 mm (L) x 31 mm (W) x 11 mm (H)
Standard: 85 mm
Long Tip: 105 mm
Cable Length: 1.8 m | 3.0 m | 4.5 m
Note: Please read this section on wiring if different cable length is needed.
Connector Type: 3.5-mm 4-pole stereo plug connector
Operating Temperature Range: −10 to +60 °C
Note: The sensor performance will be optimum at a temperature range of 5 to 50 °C.
There are several simple methods for installing the APAS T1 for spot or continuous measurements in the field or plant pots. Most of installation methods are applicable to both soil and soilless media (e.g. rockwool, coco coir). The sensor can be oriented vertically or horizontally. The circuitry of the APAS T1 is protected by a durable epoxy fill and can handle the pressure applied to the head of the sensor during the installation process. In addition to the materials provided here on the APAS T1 installation, we strongly recommend that you visit our blog for articles that discuss this in more detail.
In all installation methods, please take the following into consideration:
Make sure the sensor body is not restricting the flow of water.
When installing the APAS T1 sensor in the soil or soilless media, be mindful not to reuse an opening/hole previously created by inserting the sensor. Repeated removing and reinserting the sensor into the substrate will widen the hole and create air gap. The air gap might result in inaccurate readings (increased or reduced readings).
Spot measurements using the APAS T1 sensor by inserting it into the substrate and removing it after taking a measurement is NOT recommended. This method is invasive, inaccurate and over time might damage both the sensor and plant roots. Please install enough number of sensors permanently in your spots of interest at the beginning of the growing season (before plant root is fully developed) and take readings using your handheld readout device or datalogger.
Avoid installing sensors too close (< 15 cm) to each other or to other types of sensors. They can interfere with each other's readings. If you need to carry out measurements at different depths close to each other, install the sensors in multiple trenches at a safe distance from each other. Please note this applies to All soil moisture sensors (from any manufacturer) with large sphere of influence that measure soil dielectric permittivity.
Choose a spot for soil moisture measurements that is well representative of the rest of the field or runs out of water faster. Sandy soils have the lowest water retention capacity and clay soils the highest (Sandy soil can be assumed as a small bucket of water). Please read this blog post for more information.
Avoid placing the sensor in proximity of metal objects. Metal object can interfere with the electromagnetic field around the sensor and change sensor readings.
Avoid installations too close to plant rooting system or too far from plants. The sensor should reflect soil moisture fluctuations caused by plant roots activity.
Do not leave sensor cable exposed to nature elements. Always use proper protection (e.g. PVC pipes).
Please read this blog post to know how to interpret your soil moisture readings.
Vertical Top-Down Pot Installation
A) Full Burial
The procedure for installing the sensor vertically in a pot (full burial) is illustrated in pictures below. Simply push the sensor with slight pressure into the substrate until the sensor head is about 1 cm under the soil surface. Cover it with soil and water the soil around the sensor to help with reforming the soil structure around the sensor. Make sure the sensor is installed close enough to the plant rooting system and is far enough that would not damage it. Sensor readings will reflect the average soil water content along its length.
Note 1: This method of installation can be used for both spot and continuous (permanent) measurements. In the case of a spot measurement, you can remove the sensor by pulling the head out of the soil.
Note 2: The sensor head is also sensitive to moisture. By default, the sensor is calibrated only for the sensor blade sensitivity to soil moisture to facilitate measurements in rockwool. Adding about 20% to the measurements and scaling back to 0 - 100% can account for the sensor head sensitivity. In irrigation scheduling applications, where absolute values are not critical, the math may be skipped.
Note 3: According to our research findings, automatic temperature-compensation is a bit less effective with this installation type. This is because of the temperature gradient in the vertical soil profile and cable noise.
B) Partial Burial
The video and pictures below explain another method of installing the APAS T1 soil moisture sensor, which can be used for measurements in soil and soiless media (e.g. rockwool). In this method, the sensor is pushed into the substrate until the green sensor blade disappears and the black sensor head touches the surface.
Note 1: As discussed before, the sensor head is also sensitive to soil water content. In wet soils or high soil water content (right after an irrigation event), the measurements may appear slightly fluctuating, which is normal.
Please watch the video to learn how to install the APAS T1 in rockwool. Installing the APAS T1 sensor into rockwool is very easy. Take the following steps to install the sensor:
Pick a corner of the Rockwool and insert the sensor at a 90 degree angle.
Gently, push the sensor into the Rockwool until all of the green blade disappears.
Note: When installing the APAS T1 sensor in rockwool, be mindful not to reuse an opening/hole previously created by inserting the sensor. Repeated removing and reinserting the sensor into rockwool will widen the hole and create air gap. The air gap might result in inaccurate readings (increased or reduced readings).
Horizontal Trench / Hole Installation
The horizontal installation is best suited in applications where multiple sensors are installed at different depths or soil moisture measurements in deep soil is desired. Soil moisture measurements in coco coir can also be carried out this way.
To install the APAS T1 horizontally, digging a trench may be necessary. Push the sensor carefully into the wall of the trench and return the soil back once done. Water the installation site afterwards.
Readout Device Setting Depending on Installation
Due to the sensitivity of both the sensor head and blade to soil water content, a different calibration setting is required for complete sensor burial compared to spot soil moisture measurements with only the green blade in the substrate. The soil moisture measurement range is directly related to how much of the sensor is exposed to the substrate. Be mindful of some degree of error in spot measurements where the sensor head is partially in contact with the soil surface, but not completely buried.
Depending on the media and sensor installation type, DurUntash readout devices apply two different default calibration coefficients to soil moisture readings by the APAS T1. By default, DurUntash readout devices are programmed for vertical top-down pot or rockwool installation, which is simply shortened to "rockwool (Partial Burial)" media type. You can change this to "Soil (Full Burial)" if that is your media type (and method of installation) of interest. If you have berried the sensor in any other media such as coco coir, you also need to select the latter setting.
Please refer to the SHUSHAN CVI-Wireless user manual (this section) to learn how to implement this.
The APAS T1 sensor comes with a 3.5-mm 4-pole stereo connector, which can be plugged into any DurUntash sensor interface (e.g. data loggers, reader). If the user intends to integrate the APAS T1 into third-party loggers or a custom-made board, either the connector should be clipped off (followed by stripping and tinning the wires); or an appropriate 3.5-mm 4-conductor stereo jack or extension cable (See here and here for examples) used. To order sensors with pre-stripped wires, please contact us directly.
The APAS T1 has an embedded voltage regulator that accepts an excitation voltage in the range of 3.3 to 15.0 VDC. To avoid internal heating, however, we recommend a maximum voltage of 5.5 VDC. The use of the regulator means, longer cables can be used without worrying about the voltage loss.
Connect to DurUntash Readout Devices
We have also developed a free-of-charge Windows-based graphical user interface (SHUSHAN CVI), which can be used for data visualization, device/sensor configuration, firmware updates, and real-time data logging and monitoring.
The APAS T1 comes with either 1.8-m (APAS T1-1.8C) or 4.5-m (APAS T1-4.5C) standard cable lengths. Standard 1.8-m, 3-m, and 4.5-m extension adapter cables are also available from DigiKey to extend the cable length. An adapter cable has a connector for the stereo connector on one end and a stereo jack (or four wires) on the other end. Please note that this extension solution is not waterproof and additional work may be required to seal the joint (Adhesive-lined heatshrink tubing can be an economical sealing solution). The cable length can be extended beyond 4.5 m to a standard or non-standard length for an additional [customization] fee. Due to signal attenuation, risk of noise, as well as the limitations of DS18B20 communication protocol, among others, we recommend caution with cable length that is too long (usually more than 20 m). Please contact us for a customized cable length.
You can connect the APAS T1 wires to your custom-designed board, with the power supply wire (red) connected to the excitation (e.g. digital pin of an Arduino), the moisture wire (white) to a an analog input, the temp wire (green) to a digital input, and the ground wire (black) to the ground.
Soil moisture and temperature changes happen very slowly. In most applications, measurements once or twice a day will provide enough resolution. DurUntash data acquisition equipment are programmed to measure moisture every 5 (if single-channel) or 10 s (if 4-channel). DurUntash data loggers will average these high frequency readings at the specified logging intervals. If the APAS T1 is integrated into an existing project, an excitation time of less than 1 s (duration of 10 ms is required for an isolated moisture measurement, and 375 ms for temperature measurement with 12-bit temp resolution) per measurement and the sampling frequency of at least 5 s is strongly recommended.
The DS18B20 has two bytes of on-board programmable ROM. Sensor-specific calibration coefficients (varies from one sensor to another) and single-byte sensor ID ("100"; the same for all APAS T1 sensors) are pre-programmed in these bytes. DurUntash data acquisition equipment reads the memory of the DS18B20, every time before carrying out a moisture measurement, to determine whether a sensor is connected or not. Right after a sensor is connected, the sensor 64-bit digital ID (read-only; set by the factory) is also read and reported to the user. The short measurement time of ~10 ms, low-power requirements (<15 mA @ 3.3 V), and the unique 64-bit digital ID make the APAS T1 an excellent choice for Internet of Things applications. In applications, where small battery is the only source of power, and hundreds of sensors may operate on the same network, our APAS T1 can save a lot of headaches.
Connecting APAS T1 to Arduino
To learn how to connect the APAS T1 sensor to an Arduino board, please read the following tutorials:
The raw soil moisture readings depend on the readout device that the APAS T1 is connected to. DurUntash equipment scale and report the readings to values from 0 to 100% (Depending on the installation method and other factors the readings might exceed 100% or be slightly negative). If connected to third-party or custom-made readout devices, the raw values will depend on the resolution of the analog to digital convertor. As an example, the ADS1115 ADC (16-bit resolution) will report numbers that may range from 4,500 to 12,000 when set for 1x gain.
To minimize sensor-to-sensor variability, individual research-grade APAS T1 sensors are tuned to match each other's readings under similar conditions in the air, water and different substrates. This ensures consistent readings specially when a large network of them is installed. Each APAS T1 sensor has non-volatile memory (DS18B20 ROM), which allows for storing sensor-specific calibration coefficients. This stage adds to the flexibility of sensor application, completes the previous hardware tuning, and allows for individual sensor calibration for specific substrate, soil or condition. Every time that a sensor is read by DurUntash data acquisition devices, these sensor-specific calibration coefficients are automatically applied to moisture readings. The sensor-specific calibration is carried out by us at DurUntash and the user should not worry about the details, unless the APAS T1 is integrated into their existing system. The required code (in C/C++) to read and apply these coefficients will be provided to customers for easy integration.
Sensor water content readings are, by default, scaled based on readings in the air and water (0-100%), which works great for irrigation scheduling in homegardens, urban farms, greenhouses, etc. There is a large variation, however, in terms of soil and soilless media. That is why we recommend medium-specific (also called soil-specific) calibration for sensors used in research to make sure volumetric water content (VWC) is measured with the highest accuracy in a given medium. Please contact us to discuss your specific calibration needs.
The sensor measurement frequency is relatively high and has no exposed electrodes, which minimizes the sensitivity to soil texture and EC. Some dry soils, however, can have a high dielectric permittivity of up to 6 (compared to 80 for water at 20 °C). This means sensor readings are affected by both the components of the dry soil and its water content. For substrates with high dielectric permittivity, an optional substrate-specific calibration is recommended to achieve a higher measurement accuracy. The coefficients of a custom calibration equation (linear or polynomial) can be stored on the memory of any DurUntash dedicated reader or data logger using the SHUSHAN CVI software, and automatically applied to sensor readings.
To learn how temperature (Ts) fluctuations affect moisture measurements, read our blog article below:
Briefly, temperature fluctuations can affect moisture readings by affecting the sensor circuitry and by changing the soil (particles and water) dielectric permittivity. We have minimized the effect of Ts on the electronics in the circuit design and also by calculating the temperature sensitivity of the circuitry after the minimization. In addition, we have developed a transparent two-stage temperature compensation algorithm based on multiple regression analysis that takes both temperature and moisture readings and compensates water content readings for fluctuations that are caused by temperature change:
Comp1 = m x (Tref - Ts), where m (constant) is determined as a function of circuitry temperature sensitivity (mV/°C) and ADC resolution, and Tref is the reference temperature (25 °C).
Comp2 = b x (Tref - Ts), where b is determined as a function of water (and to some extent substrate) temperature sensitivity (mV/°C) and ADC resolution.
The values calculated in the above stages are added to water content readings for compensation. The effect of temperature on soil moisture readings after the temperature compensation is negligible, and the results may differ slightly from substrate to substrate, sensor to sensor, and depending on the temperature range. The algorithm resides on the dedicated reader or data logger, and the whole process is worry-free, real-time and automatic. The algorithm is transparent, and the user has full control over it. The parameters can be adjusted, enabled/disabled to suit specific soil, substrate or condition.
Under normal conditions the APAS T1 sensor is maintenance-free, and does not require cleaning. However, if cleaning is desired please rinse the sensor head and blade with tap water and dry the sensor using a piece of cloth or paper towel.
There are three main sources of problem:
Unusually high or low moisture/temperature readings
It is always a great idea to start by checking the connection from the reader (DurUntash reader, data logger or a custom readout board), to make sure that there is no wiring issues. If the stereo plug connector or the sensor cable is damaged please contact us on how to fix it.
Unless there are obvious signs of damage, unusual sensor readings could be related to the installation rather than the failure of the sensor head (circuitry) or blade. Unusual sensor readings can be caused by a heavily compacted soil. If you have carried out a custom soil-specific calibration, please make sure that the math is correct. If you are using the generic calibration, you may want to consider developing your own substrate-specific calibration equation to account for variations among different soil types.
Any soil moisture reading exceeding ±1 m³/m³ with a random nature (Gaussian distribution) can be considered unusual noise. Noise in continuous soil moisture readings may be due to a variety of reasons. Cable noise (the cable of sensor being in its sphere of infuence), followed by a noisy power supply are the main reasons for soil moisture value spikes. We have dealt with these sources of noise by adding a ferrite core (clammed on) and ferrite bead (embedded), as discuuse earlier. If you are not using battery to power your readout device or data acquisition system, please make sure that you have a clean (noise-free) power supply. Another way to reduce noise is to use logging intervals that are at least 15 min (60 min recommended) and average all the readings during this time.
We are available for questions, problems, or feedback Monday through Friday, 8:00 to 17:00 (Los Angeles Time).
Note: If you purchased your instrument through a distributor, please contact them directly for assistance.
You can reach us via our contact page here.
Please make sure to include the following information in your message:
Instrument serial number (SN)
Description of the problem
Note: Instrument SN is normally attached to the enclosure or cable. If the SN is missing for any reason, use the SHUSHAN CVI software and follow the instructions in the user manual to retrieve the SN.
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This product may contain chemicals, which are known to the State of California to cause cancer, birth defects or other reproductive harm. Please go to the proposition 65 warnings website for more information.