Just picked up two micrometers here last week [insurance replacement], and both were rated well..
the starrett dial type was rated 66 and the mitutoyo digital was rated 67...
basically first number [ie 6] means its sealed against dust or other dry particles..
second number [ie 6] means strong jets of water or spray, and 7 indicates protection against prolonged immersion at depths of up to 1 metre..
more detailed information available at
http://www.rlws.com/lit/load-cells/IP_ratings.html
in case you're interested, the Starrett 6" dial was $185, and the Mitutoyo 8" digital was $260.. Canadian funds.. 'bout $20. US it seems some days..
enjoy..