Data loggers and field bus solutions for measuring stations
Outdoor enclosure with Lysimeter and Tensiometer sensor technique
For the layout of a measuring station a fundamental question is how to store data, or in general how to find the proper data acquisition system. Following thoughts should be considered right from the beginning:
- How many sensors and channels are planned?
- Which type of sensors are required?
- What is the measuring principle of these sensors, and what is their measuring technique?
- In which intervals should readings be taken?
- Should there be the possibility of a later extension of the site?
- Which data loggers are suitable for my needs?
- How should data be downloaded?
- Which requirements result for the current supply of the site?
Sensor depending requirements for the measuring technique
In general there are two groups of analog sensors.
- Passive sensors change their electrical property with the physical property (e.g. temperature). Normally these sensors are powered with an excitation current or voltage, and the resulting current or voltage drop is measured. For some sensor types the changes to be measured are really small, which means the requirements for the read-out technique increases. Thus, cable lengths are limited and interferences, like electromagnetic interferences, have to e considered much more.
- Sensors with an integrated amplifier normally have less demands regarding the data acquisition. Signal changes are amplified inside the sensor, and the sensor exitation is easier. Amplified output signals can be connected to a common ground. Thus, only one signal line per sensor is needed (= single-ended). Many loggers offer the possibility to connect two or more sensors to the same channel to which just one passive sensor could be connected. You save channels and reduce your costs by selecting a smaller logger. Although the integrated amplifier makes sensors more expensive, the advantages might compensate this. Therefore a cost calculation must be weighed out for the reespective case. Last but not least single-ended sensors are less sensitive to interferences or long cable lengths.
Connection of a full bridge, e. g. T4, to a measuring channel
Connection of 3 single ended signals, e. g. 3 x T8 to one channel
Modern sensors offer new read-out possibilities by integration of a serial interface. Such sensors with internal microcontroller can communicate with the data acquisition system through a serial line. A system of addressable sensors with bus-linking capability interface like RS485, is called a field bus system. Normally a master data logger addresses each sensor individually, and only this sensor reacts by sending its measuring data. Well-known bus protocols with standardised commands and signal levels are e.g. CAN, Profibus or Modbus. Often these are quite complex and too costly for low-cost systems. Therefore, many manufacturers offer their own protocols which are adapted to their certain type of sensors and demands. With the advantages of the new micro-controller electronics such "intelligent" sensor types more and more replace sensors with simple analog amplifiers.
The cabling of field bus sensors is very flexible - and cost-effective. Communication lines can be connected in a row, merged and diverged in many ways. Cable lengths of some kilometers are possible. Still attention should be paid to that a single interruption of broken sensors might cut off a complete branch of sensors.
The cabling of field bus sensors is very flexible - and cost-effective. Communication lines can be connected in a row, merged and diverged in many ways. Cable lengths of some kilometers are possible. Still attention should be paid to that a single interruption of broken sensors might cut off a complete branch of sensors.
Number of data logger channels
20 Tensiometers with power supply on a DL2e logger
The most important point for selecting the logger type is the number of channels, which depends on the type of sensors, and the diversity of sensor types. In many cases the capacity of a simple, low-cost logger is just too small. Some loggers like the DL2e can be extended with additional channel cards. But it gets complex, when many different sensor types with different requirements regarding read-out and supply should be combined. Still, data loggers like the Datataker DT80/800 offer very flexible configuration options.
Certain measures for future site extension should be considered from the beginning, as any unplanned extension of even one additional channel can cause high efforts and costs. In case, the best option might be a bus solution with easy and flexible extendibility. Simply add new sensors and distributor boxes to the network and to the data logger setup. For this, a logger needs a serial bus interface and compability with the used protocol, or capatibility for programming. A combination of analog inputs and a serial interface is ideal as then almost any type and number of sensors are connectable.
Certain measures for future site extension should be considered from the beginning, as any unplanned extension of even one additional channel can cause high efforts and costs. In case, the best option might be a bus solution with easy and flexible extendibility. Simply add new sensors and distributor boxes to the network and to the data logger setup. For this, a logger needs a serial bus interface and compability with the used protocol, or capatibility for programming. A combination of analog inputs and a serial interface is ideal as then almost any type and number of sensors are connectable.
tensioLINK sensors with DT80 logger
The well-priced data logger DT80 for example can read 5 to 15 analog weather sensors plus up to 64 Tensiometers type T8-2005 through the serial interface. Including the T8 sensors for pressure and temperature this is a total of 133 channels! Thus, a logger with a mixture of analog and serial channels offers highest extendibility.
Data read-out and transmission
Measuring station with GSM connection
Despite GSM and internet most measuring stations still are read out with notebook or memory card. This is because attending the station does not only mean to read out the data, but is essential for ensuring a failure-free operation. Some problems need service by a person. Therefore, the service intervals should be planned in consideration of the maintenance needs, but not in dependence of a probably much larger memory capacity.
Most loggers can communicate via GSM modem. Today, cost for wireless equipment are low. If you run several stations, the costs for managing the expected amount of data should be considered. Often modem transmission is used only for a status control. When planning a site, the power consumption of a modem transmission has to be regarded including the frequency of interrogation.
More and more stations are permanently connected to the internet, either through DSL technology if available close-by, or via wireless modem technology. Costs then do not depend on the time of connection, but on the transferred amount of data which normally is low.
Most loggers can communicate via GSM modem. Today, cost for wireless equipment are low. If you run several stations, the costs for managing the expected amount of data should be considered. Often modem transmission is used only for a status control. When planning a site, the power consumption of a modem transmission has to be regarded including the frequency of interrogation.
More and more stations are permanently connected to the internet, either through DSL technology if available close-by, or via wireless modem technology. Costs then do not depend on the time of connection, but on the transferred amount of data which normally is low.
Requirements for the power supply
Solar battery and controler with sufficient capacity
If no mains power is available the planning of the power supply is most significant. A wrong capacity will cause the complete failure of measurement. More information about this task will be published in a further article soon.
to be continued…
to be continued…
Examples for measuring stations
Agricultural lysimeter with T8, SIS and TRASE probes
Hydrolysimeter Wagna with weather station
Soil hydrologic station Wolfsbach
Mountain lysimeter Stoderzinken in May 2006
Contact
Georg von UnoldTel: +49 89 12 66 52 - 15
Fax: +49 89 12 66 52 - 20
E-Mail: gvu@ums-muc.de
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