DATA COLLECTION AND HANDLING - Wind Resource Assessment: A Practical Guide to Developing a Wind Project - page 98

original record of the conversion constants applied to the raw sensor readings (this is
of particular importance for those data loggers that only provide the converted data
rather than the raw outputs of the sensors). Without access to these files, an independent
reviewer may have doubts about the reliability and accuracy of the measurements.
7.1.1 Data Storage Types
The following is a list of common raw data storage media.
• Data Card. These are small, removable storage devices widely used in digital
cameras, camcorders, and similar applications under brand names such as Sony
Memory Stick, MultiMediaCard (MMC), and SecureDigital (SD) card. Many
laptops are equipped to read such data cards directly. The data can also be
imported into a laptop or desktop computer through a data card reader.
• Solid-State Modules (SSMs). These nonvolatile devices, also called internal mem-
ory , are hardwired into the logger. The data are read through a direct cable
connection from the logging system to a laptop.
• EEPROM Data Chip. This is an older integrated-circuit technology that served
as internal memory for earlier loggers. The manufacturer's software and an EEP-
ROM reading device are required for data transfer.
A laptop computer is needed if the data are to be transferred on-site. This is the
recommended method for manual data retrieval since it allows the integrity of the data
to be verified during the site visit. If no laptop is available, then the data card must be
replaced with a fresh card and brought back to the company office. Depending on the
storage type, special cabling, interface hardware, external power supply, and software
may be required, along with portable drives or USB drives.
7.1.2 Data Storage Capacity
The minimum required storage capacity of the logger depends on several factors,
including how often the data are retrieved (once every 2 weeks or more often is
recommended), the data recording interval (typically 10 min), the number of sensors
being monitored (typically 8-12 on a 60-m tower), and the number of parameters
calculated and stored by the logger. The capacity of the data storage devices commonly
used today, at least 16 MB, is ample for most situations. One exception may be when a
data recording interval shorter than 10 min, such as 2 s or 1 min, is desired. Then larger
data storage or more frequent data retrieval may be necessary. Another exception may
be if the tower is likely to be inaccessible for months at a time (e.g., because of winter
snow and ice). Then if the telecommunications uplink fails, the logger may be called
upon to store data for up to several months.
Manufacturers usually provide tables or methods to calculate the approximate avail-
able storage capacity (in days) for various memory configurations. Capacity estimates
should also allow for delays in retrieving the data.