Salinity FAQ

User Manuals

Download the current Soil Salinity Sensor User Manual (version 1)

How many Soil Salinity Sensors can the Smart Logger read?

The Smart logger has 250 channels as standard. It does not need any extension. The first 62 channels are identified as letters or numbers such as A, a or 1, 2, 3. From channel 63 onwards the identification is a Unicode character entered in hexadecimal as opposed to a common number or letter. Expandable to 250 channels means that when you are programming the sensor interface (hence sensor) with its identification using HyperTerminal, you will use a Unicode character for channel 63 and above as opposed to a common (ASCII) letter or number for the first 62 channels.

The Smart logger can take 250 sensors as it is delivered to you standard. For example, it could record 250 soil temperature sensors simultaneously. However in the case of the salinity sensor there is both a conductivity measurement and a temperature measurement being made by the sensor and the sensor interface in order to determine the soil salinity value corrected to 25 degrees C. If you wish to store and display only the salinity value on the Smart Logger you could have 250 salinity sensors connected. If you wish to store and display the salinity value and the temperature value, this would require 2 channels for each salinity sensor. This means that you would be able to connect 125 salinity sensors with salinity and temperature values being stored and displayed at the same time.

Simply have the option to record and display the temperature data if you wish. It will have no effect on the salinity value as the measurement of temperature is made at the same time as conductivity and the salinity value corrected to 25 degrees is the stored and displayed value.

What is the purpose and advantage of the sensor interface?

The purpose of the sensor interface is to supply the correct power requirements to the sensor and to convert the returning signal to a serial output. All of the interfaces connected to the sensors are giving serial output. There is no limit to the length of the cable with this serial output. The length of the cable could be several kilometres if required as serial data can be transferred over much larger distances than analogue voltages. If you wish to keep adding to the system, it's very easy. You can purchase an additional Interface Hub (IB8, IB12 etc.), connect it to an existing Interface Hub, connect your sensors, press the two buttons on the front of the Smart Logger and you will immediately be logging from the new sensors connected in and displaying the values in engineering units, e.g. dS/m. Then you can simply keep on expanding the systems as you require.

For example: initially you purchased the system with salinity sensors (Salinity Station), later you decide to convert it to a weather station. By buying a six channel DataBus Hub (IB6), wind speed, wind direction, temperature, humidity and rain gauge sensors and connecting the sensors to the IB6 and connecting the IB6 to an existing Interface Hub, you will then have a weather station + salinity station. As you can see there is no wiring required and no programming required. The sensor interface is smart providing the serial output which eliminates the need for programming and wiring.

How are the Salinity Sensors installed?

Placement of sensors in the soil requires that you take soil cores, place the sensors in the soil with the ceramic having good contact with the soil such that the soil solution can be in equilibrium with the solution in the ceramic. This is clearly shown in the Installation of Soil Salinity Station pdf which can be downloaded.

Can extension cables be multi-functional?

The extension cables can be used three ways. They can increase the distance of an individual sensor from an DataBus connection or they could increase the distance from the Smart Logger to an DataBus connection which then has a large number of sensors connected to it at one location or increase the distance between 2 DataBus connections. The system is completely flexible to add the extension cables as you see fit to expand the system.

How do the Salinity Sensors measure soil salinity?

The sensors measure EC in dS/m directly. The measurement process is as follows:
There is an AC impedance bridge in the sensor interface which necessarily generates a capacitance signal, this is cancelled out to give a resistance value. Then this resistance value has the sensor calibration applied to it for the individual sensor and the value in dS/m is stored and displayed. It is not possible to measure the salinity of the soil accurately using Direct Current (DC) resistance. If you apply a direct DC voltage you would polarise the chemicals in the soil solution and an incorrect reading would result. In the process of connecting an ohm meter you would see a drift in the DC resistance as you are making the measurement. It's for this reason that an AC impedance bridge is used and then the capacitance correction is made to get the true resistance value. This is the reason these sensors cannot be connected to a common, analogue data logger.

What is the maximum EC that can be measured?

The maximum EC that can be measured is 50 dS/m , which is equivalent to sea water.

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