These oximeters also continuously record oxygen, heart rate, motion and other values in their internal memory as files. Files are stored in a binary format that can be decoded as described in this note.

The first 40 bytes are the header:

00 int16   version (2 bytes)
02 int16   year
04 byte    month
05 byte    day
06 byte    hour
07 byte    minute
08 byte    second
09 int16   file size   
13 int16   duration in seconds
17 byte    average O2
18 byte    minimum O2
19 byte    3pct O2
20 byte    4pct O2
22 int16   O2 < 90% seconds
24 byte    events under 90%
25 byte    score

Take the total length of the file (number of bytes), remove 40 (i.e. the length of the header). Divide by 5 (i.e. the length of a record). You’ll get the total number of records in the file.

You can get the interval between each record by dividing the duration (bytes 13 and 14 of the header) by the number of records.

Reading Records

Skip 40 bytes (the header), you’ll arrive at the beginning of the first record. Take 5 bytes:

00 O2 %
01 Heart Rate bpm
02 ??
03 Motion
04 Vibration

As you already have the interval, you can estimate each record’s timestamp by multiplying record ID by interval, plus the start time (bytes 02 to 08 inclusive of the header).

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