Theta Probe FAQ
On this page, you will find answers to a number of commonly asked questions. If you cannot find the answer to your problem here, then send us an e-mail and we will try to help, (support@mea.com.au). We'll even post the answer here.
| General |
|---|
| What volume of soil does the Theta Probe read in ? |
| How does the Theta Probe work ? |
| Software |
| Which software do I use with the Theta Probe ? |
What volume of soil does the Theta Probe read in ?
The accuracy of the Theta Probe comes from two main areas: its use of a single frequency (rather than a range of frequencies) and the confining of the measurement area to the body of soil captured between its pins. The design thus means that the probe is reading moisture in a body of soil approximately 70mm long and 40mm across.
How does the Theta Probe work ?
The Theta Probe acts like a low power radio transmitter, sending a signal down its centre pin. The outer pins may be likened to the outer screen on a coaxial cable (the style used for a television or radio antenna). When the signal reaches the end of the pins some of the energy is transmitted into the soil and some of it is reflected back down the pin to the transmitter. How much energy is transmitted and how much is reflected can be accurately measured (in radio terms it is known as the SWR or standing wave ratio). When the moisture content of soil changes a property known as its dielectric constant changes (The relationship between the two has been extensively studied and can be mathematically modelled). As the dielectric constant changes, the ratio between the amount of signal which is transmitted into the soil versus what is reflected back changes. The Theta Probe measures this change and, by use of the relationship between soil moisture content and the dielectric properties of the soil, converts it into a measure of soil moisture content.
The fact that the probe operates at a single frequency and that the measurement can be confined to the area of soil enclosed by the pins means that it can give accurate, repeatable measurements. Capacitance probes by comparison operate over a range of frequencies and the amount of soil into which the signal carries varies with soil type and moisture content. Unless such a device is calibrated on site, it can at best give relative rather than absolute readings. Further, changing a sensor or using a different access tube can cause step changes in the readings.
Using factory supplied calibrations, the Theta Probe will give 3 to 5% accuracy across the range of soil types. By performing a simple two point calibration, accuracies of 1% can be achieved. Calibration is simple - take a sample of the soil, wet it up and take a reading with the Theta Probe. Oven dry the soil and take another reading. The two readings are then used to generate two coefficients which are fed into the calibration equation for the probe.
Which software do I use with the Theta Probe ?
The software used with the Theta Probe will depend on the application.
When used as a portable probe in conjunction with the HH2 Hand Held Reader, soil moisture data will be displayed using a spreadsheet program. Readings saved in the HH2 are downloaded into a computer using the software supplied with the HH2. A Data-set Import Wizard is then used to format the data for display in spreadsheet format.
When used in a continuously logged system, the Theta Probe data will be displayed using the Magpie software.