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How the Measurements are Made
OverviewModified gypsum blocks were installed at 5 depths (from 0.3 to 1.5 m) at each measurement location. These were connected to a buried CSIRO Wireless micrologger located 1 m from the sensors, the connecting cables being buried in a trench 0.4 m deep. At pre-programmed intervals (every 12 hours in this case), each micrologger reads each sensor and radios the results to a receiver located within range of both paddocks on each farm. The receiver is connected to a small computer with a built-in cdma telephone, called a pc-ephone. The computer stores the sensor results. Once each day connects to the internet via the cdma telephone and sends the day's data file to a CSIRO computer. Before 6 am each day, the previous day's data is automatically added to an excel spreadsheet, the charts are updated, and copied to the computer hosting this website. If all goes well, the previous day's data is available for viewing by 6 am each day. Occasional glitches occur when the radio link fails because of excessive interference, or the cdma signal is not strong enough for the call to be made. Built-in fail-safe procedures keep these to a minimum. and prevent data loss when it does happen. Gypsum blocksGypsum blocks are porous blocks that wet and dry as the soil they are in contact with wets and dries. The water content of the block is measured by measuring its resistance. The gypsum in the block provides a buffer against background soil salinity so that it does not affect the resistance measurement. The water content of the block unfortunately does not equal the water content of the soil, but is related to the 'soil water potential' of the soil with which the block is in contact. Soil water potential provides an absolute measure of how wet or dry the soil is; a value of zero indicates that the soil is 'saturated', a value of ~10 kPa indicates that the soil is at 'field capacity' or 'drained upper limit', while a value of 1500 kPa indicates that the soil is at 'wilting point' - ie so dry that plants cannot extract more water from it. [In contrast the water content at these limits needs to be determined independently for a measured soil water content value to be interpreted in this way.] Sensors such as gypsum blocks, which measure soil water potential, require less care to be taken during installation. They do not need to be in intimate contact with the soil, as is required for most sensors that measure water content. Instead, the sensor can be bedded in a contact material (usually diatomaceous earth, better known as swimming pool filter material) that ensures good contact between the sensor and the soil and allow it to equilibrate with the soil's water potential. [In contrast, if a soil water content sensor is bedded in a contact material instead of being perfectly in contact with the soil, it measures the water content of the contact material instead of the water content of the soil This means that it is relatively straightforward to install several sensors in one narrow (~ 50 mm diameter) hole, bedding each in a layer of contact material and separating them with a layer of bentonite that ensures that water dos not move down the installation hole between the sensors. More information about interpreting soil water potential measurements can be found at How to interpret the measurements. Further details about the Watermark and about gypsum blocks in general, can be found at the following websites:
Wireless microloggers
The micrologger has been designed to be buried and left unattended for long periods of time. It has a low power requirement, and its high capacity batteries will last several years at daily or sub-daily measurement intervals The data are stored in memory (up to 5,000 individual measurements can be stored), so that if radio transmissions are interrupted, the data can be recovered by manually interrogating the logger with a PC. Each logger is sealed in a watertight PVC housing and lowered down a PVC casing. While the casing extends to the soil surface, the top of the logger is at least 30 cm deep. Thus, although the top of the casing may occasionally be damaged by, for example, tillage operations, the logger remains protected. However, to transmit the radio signal, an antenna needs to protrude above the soil surface. This is designed to be easily disconnected and re-connected at sowing and harvest, and easily laid down on the soil surface to permit spraying operations.
Loading the data to the web pageEach morning, software running on a CSIRO server automatically copies any incoming data files from the ftp site, processes them, and adds them to an excel spreadsheet. This spreadsheet updates charts of the data and saves them as "gif" images, which are uploaded to the web page and are immediately available for viewing. While the quality of the charts is not extremely high, they are small files (< 15 kB) so that can be downloaded quickly even on slow dial-up lines. Occasional glitches occur when the radio link fails because of excessive interference, or the cdma signal is not strong enough for the call to be made. Built-in fail-safe procedures keep these to a minimum. If the cdma link fails repeatedly, then providing the radio communication has taken place, the data is stored on the pc-ephone computer and can be retrieved manually. Similarly, if the radio communication fails, the data is not lost as it is still stored in the data logger, from where it can also be retrieved manually if necessary. |
The Wireless micrologger
has been custom-designed by CSIRO Land and
Water. Each logger can measure up to 8 sensors. At
a pre-programmed time (or times) each day,
the logger automatically turns on, measures
the sensors and stores
the data. Every time
measurements are made, the logger has the
capability to radio the results to the receiver,
as described below. Alternatively, to save
power, results from several
measurement times can be radioed together.
