The role of the data logger

Most modern automatic weather stations are designed around a configuration that links outside sensors to an indoor console on which all the weather readings are collated and displayed. With this design of station, the console will typically possess a data output port. Although a few of the cheapest designs allow this data port to link directly to a PC, in almost all professional weather station designs the console data output passes first into a data logger and only then onward to the PC. The great advantage of this buffered approach is that the dedicated data logger is always active and data will only be lost in the event of a major malfunction. On the other hand, direct linkage to the PC relies on the PC being always switched on, not being too busy with other applications and so on. Using a data logger is therefore a much more reliable and robust approach.

In practice, the data logger has several modes of operation, with two modes being especially important:

• Archive mode;
• Current data pass-through;

Archive mode is the data logger mode that maintains the store of long-term weather data awaiting download to the PC. This mode is always active unless it has been explicitly turned off by the controlling PC. The current data pass-through mode allows data to pass directly through the logger to the PC – if the PC is switched on and listening – and to update the PC’s display every few seconds.

It is worth being clear about these two main modes of operation because the functioning of the Weatherlink software is substantially easier to understand when one appreciates the different sources of data. The Weatherlink module generates a new set of data every 2 seconds, containing the latest available reading from every installed sensor. (NB Some weather parameters change only slowly, so their latest reading will only change in value eg every 10 or 60 seconds). However, if all this generated data was stored long-term then a quite unnecessarily detailed weather record with considerable repetition of identical values would result, overwhelming the data logger’s memory and requiring large amounts of storage capacity on the PC. On the other hand, someone watching the PC display during a storm might wish to see an updated reading of wind speed every few seconds.

The answer to these conflicting needs is that the current data mode passes each new data record through to the PC when it is generated (assuming that the PC is switched on and listening). The PC software can then display a short-term trend graph covering perhaps one or a few hours of wind speed data but with every single data point included.

At the same time, the archive mode of the Weatherlink module calculates a summary or archive data record covering a longer, preset period of time, varying between 1 and 120 minutes; this archive record contains representative values for all sensors for the period in question and in addition includes, for example, the highest wind gust in that period. Each new 2-second data record is included in the calculation of the archive data record for the current archive period, but the 2-second record is then discarded as a separate entity (unless it has been separately captured by the PC). Consequently, the data logger only has to store the archive records. It has capacity for 2500 such records and if say the archive period was set to 60 minutes, the logger can retain data for over 100 days before filling up.

Thus, when the Weatherlink software is looking at current data, say in ‘Bulletin’ mode, it is accepting the 2-second records from the logger’s Current Data mode, but won’t store this highly detailed data. For its long-term database, the software will request from the logger – a process that can be initiated either manually or automatically in a number of ways – a download of the latest set of archive data generated by the archive mode.

Programmers may be interested to note that much of the technical detail for communicating directly with the Weatherlink data logger is published by Davis, who also make available free of charge a Windows DLL to handle low level communication with the logger. Those who wish can therefore write their own programs to run quite independently of the Weatherlink software (though the standard Weatherlink pack still needs to be bought to source the data logger). More details will be found in the other pages of this Advanced Topics area.