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“At what Centibar value should I be irrigating?”
This depends on the type of crop and the type of soil it is growing in. There are resources available for most commonly grown crops that can give detailed information regarding recommended tension levels.
Use the following readings as a general guideline:
• 0 – 10 centibars = Saturated soil (field capacity)
• 10 – 20 centibars = Soil is adequately wet (except coarse sands, which are beginning to lose water)
• 30 – 60 centibars = Usual range for irrigation (except heavy clay soils)
• 60 – 100 centibars = Usual range for irrigation in heavy clay soils
• 100 – 200 centibars = Soil is becoming dangerously dry for maximum production. Proceed with caution!
Your own situation may be unique because of differences in crop, soils and climate. Perhaps the most important soil moisture reading is the difference between today’s reading and that of 3 – 5 days ago. That is to say, how quickly is the reading going up. A slow increase means the soil is drying out slowly. But a big jump means the soil is losing water very rapidly. This tells you WHEN to irrigate. The trend can be more informative than the absolute values.
“I don’t believe the sensor”
As often as not, the truth is simply not what the user expects, but there are situations that can cause incorrect readings.
Some things to consider:
Are multiple sensors correlating with each other? One problematic sensor/ installation may be possible, multiple problems are unlikely.
Are the sensors responsive? If the sensors are responding well to irrigation and drying as expected, there is no reason not to believe the reading.
If the sensors are very unresponsive to expected changes in soil moisture, the sensor may need to be reinstalled. The WATERMARK sensor depends on a tight bond with the surrounding soil to absorb and release soil moisture. A poorly installed sensor can be just “sitting in the hole” with no ability to move water in and out of the granular matrix.
Are the sensors in the active root zone? If the sensor is sitting on top of a rock, below a hardpan, or outside of the area being accessed by the plant root systems, water movement can be impeded and the sensor will not accurately reflect the soil moisture available to the crop.
Is the sensor installed in a heavy soil that has recently been dried out (80cb +)? Heavy soils can actually pull away from the sensor when they become very dry. This can cause the loss of connectivity between soil and sensor. Typically an irrigation event will correct this problem, but reinstallation may be required in extreme cases.
“Sensor always reads 0”
A constant “0” reading can indicate a short in the sensor wiring. If the sensor is at the end of a spliced wire run, try disconnecting the sensor from the cable extension (at the field side) and see if the reading is still “0”. If so, there is a short in the wiring between the reading equipment and the sensor. Also see How can I check the sensors?
“Sensor always reads dry”
A maximum dry reading can indicate a broken or poor wire connection. Typical culprits include bad wire splices, chewed or cut wires, wires pulled out of the sensor, or poor connection to the reading equipment. A poorly installed sensor can also be constantly dry, with no bond to the soil allowing it to absorb soil moisture. Also see How can I check the sensors?
“Sensor is reading erratically”
Erratic readings are frequently the results of poor connections. Check all wire splices and connection points and ensure they are solid and waterproof.
Stray current from poorly grounded equipment in the field can also cause erratic readings. If it is possible to ground the reading equipment to a common ground with the suspect equipment, this may solve the problem. Also see How can I check the sensors?
“How long do the sensors last?”
The expected sensor life is 5+ years. At the five year point we recommend removing the sensors and checking them.
“How can I check the sensors?
The only way to thoroughly check a WATERMARK sensor is to remove it, soak it in water, and then hang it out to air dry.
A. With a sensor submerged in water, your meter reading should be from 0 to 5. If the sensor passes this test, go on to step B.
B. Let the sensor air dry for 30 to 48 hours. Depending on ambient temperature, humidity and air movement, you should see the reading go right up from zero to 150+
C. Put the sensor back in water. The reading should return to below 5 within 2 minutes. If the sensor passes these tests, it is O.K.
If the sensor has been installed on the end of PVC pipe, you can do a quick check on a sensor by pouring water down the pipe. The sensor should drop to below 5 within a few minutes.
“How do I install the sensors?”
Soak the sensors overnight in irrigation water. Always “plant” a wet sensor. If time permits, wet the sensor for 30 minutes in the morning and let dry until evening, wet for 30 minutes, let dry overnight, wet again for 30 minutes the next morning and let dry again until evening. Soak over the next night and install WET. This will improve the sensor response during the first few irrigations.
Make a sensor access hole to the desired depth with an IRROMETER installing tool or a 7/8” O.D. rod. Fill the hole with water and push the sensor down into the hole so it “bottoms out”. A length of ½” Class 315 PVC pipe will fit snugly over the sensor’s collar and can be used to push in the sensor. A good snug fit in the soil is important. This PVC can be solvent welded to the sensor collar with a PVC/ABS cement (IPS Weld-On #795 or equal). If the PVC is solvent welded to the sensor, drill a small (1/8”) vent hole in the pipe just above the sensor.
If the PVC pipe is not left on the sensor, then backfill the hole so the sensor is buried (see Fig. 1). The sensor’s wires can easily be staked up for easy access. If PVC is left on, then compact the soil around the surface to seal off the hole (see Fig. 2). The PVC acts as a conduit for the sensor’s wires. Be sure to cap off or tape the top of the pipe, so surface water will not infiltrate to the sensor and give a false reading.
For very coarse or gravelly soils, an oversized hole (1” – 1-1/4”) may be needed to prevent abrasion damage to the sensor membrane. In this case, auger a hole to the desired depth and make a thick slurry with the soil and some water. Fill the hole with this slurry and then install the sensor. This will “grout in” the sensor to ensure a snug fit.
Another method of installing sensors in difficult gravelly soils, or at deeper settings is to use a “stepped” installing tool (see Fig. 3). This makes an oversized hole for the upper portion and an exact size hole (sensor is 7/8” O.D.) for the lower portion where the sensor is located. The hole must be carefully backfilled and tamped down to prevent air pockets, which could allow water to channel down to the sensor.
“How does soil temperature affect the reading?”
Soil temperature affects WATERMARK sensor readings by 1% of the measured resistance per 1 degree(F) temperature. If the reading equipment does not use some form of temperature compensation, it is not unusual to see some movement in the sensor reading from day to night due to temperature effects. Soil temperature changes are typically mild at depths below 12”. Accurate temperature compensation requires that the temperature sensor be located at a depth that represents the temperature around the WATERMARK sensors.
“What types of soil can I use WATERMARK sensors in?”
WATERMARK sensors are designed for use in typical soil conditions, from sandy loam to heavy clay. Exceptionally coarse or loose soils like sand or potting mixes do not present good conditions for WATERMARK sensors, potentially leading to very slow sensor response. Consider IRROMETER “LT” tensiometers for these applications.
“Can I read a WATERMARK sensor with a digital multi-meter?”
No, WATERMARK sensors require specific reading circuitry to read properly.
“What does (insert your volumetric measurement here) equal in Centibars?”
WATERMARK sensors are calibrated to represent soil moisture in Centibars (or kPa) of soil water tension, equivalent to the reading given by a tensiometer in the same soil. There is no direct comparison with volumetric measurements possible without creating a site specific calibration, as different soil types and conditions create different tensions for the same volumetric contents in different soils.
“Can I read a WATERMARK sensor with my device?”
The WATERMARK sensor is read as a resistance. Excitation for the sensor must be of a specific nature with very precise timing in order to produce accurate results, and the circuit must be designed to account for other factors that will affect the lifespan of the sensor. Contact IRROMETER for more information.
IRROMETER offers a voltage output version of the sensor, the 200SS-V. This version of the sensor outputs a simple 0-2.8 volt scale representing 0-239 Centibars, and can be read by most devices with analog inputs.
200SS-V Datasheet
“Is the WATERMARK sensor a gypsum block?”
The WATERMARK sensor is a granular matrix sensor (GMS) which in operating principle is similar to a gypsum block, but is constructed to last considerably longer, maintain contact with surrounding soil, and provide better response at lower tension levels.
"What soil characteristics affect the Watermark Sensor readings?"
Watermark Sensors, like all electronic type moisture sensors, are affected by things that will change the conductivity of the soil water. Salinity is the typical concern and the Watermark has enough internal gypsum to buffer the effects of "typically salinity levels found in irrigated agriculture." We publish no reference as to how much salinity is too much. Certainly a newer sensor can buffer for more than an older one because gypsum does go away over time. Researchers using them for different applications have told us that effects from salinity are not usually noticed unless it is >3 dS/m. Sometimes after a fertilization event the reading may appear artificially wetter from the added salts if high but after a flush with the next irrigation cycle they will return to normal.
“How many records will the 900M, or 950R Data Logger hold?”
The 900M Data Loggers will hold 4094 records, it will start deleting the oldest records, then add new records, when full.
The 950R Wireless Data Loggers will hold 8094 records, it will start deleting the oldest records, then add new records, when full.
"How can I update the firmware on my 900M Data Logger?"
Firmware update instructions if you have the firmware file from within WaterGraph here
Firmware update instructions if we sent you the firmware file here