The WT1800R can save measurement data to a USB memory stick in ASCII CSV (.csv) format. There are two routes depending on whether you want a one-shot capture or continuous logging.
Before you start: use a USB memory stick that is USB Mass Storage Class Ver. 1.1 compliant, and connect it directly to a peripheral type-A USB port on the instrument (not through a hub).
One-shot: save the displayed numeric data
Continuous logging: use STORE
Stored data is written as binary .wts files (with a .hds header). The CSV is produced automatically by Auto CSV Conversion, or after the fact by selecting the file and choosing CSV Convert in the File List.
Limits: the maximum .wts stored-data file size on a USB device is 1 GB, and a converted CSV file cannot exceed 2 GB.