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5- Sensors and actuators
The databases can accommodate up to 50 values per row. The number of rows, and therefore the
stored history, is limited by the available memory. Because the available memory is constant, more
values per row mean less space for rows. In other words you need to make choices about what you
want to store, so that the databases cover the period you are interested in.
To save space, the database contains nothing but the values – the only way of knowing what these
values are is by consulting the denition. In other words, if the denition is changed, there is no way
of interpreting the existing information in the database. For this reason the databases are deleted
automatically when changes are made to the denition.
5�8�1 Statistical functions
The short time database can only contain momentary values. In the hour and day databases, we are
usually more interested in statistical values that tell us something about the period since the last
update. The table shows the various options.
Name Comments
Momentary value An instantaneous value.
Mean value The mean value over the period.
Max value The highest value during the period.
Min value The lowest value during the period.
Variance A statistical measure of how much the value has
varied over the period.
Standard deviation Square root of the variance. The standard deviation
is better than the variance when comparing two
sets of data.
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