Frequently Asked Questions

SeaTag depth sensors can sense pressures anywhere from the vacuum of outer space (eg. 0 PSI) to the depth sensor pressure limit, such as 3000 PSI corresponding to ~2000 m ocean depth.  The typical surface atmospheric pressure of 14.7 PSI is thus merely a point on the pressure curve.  While some tags 'clamp' any negative readings to zero meters depth because negative depths are not possible for an ocean tag, this practice results in a loss of valuable information.   Expect to see depth readings at the surface of a few tens of cm to a few meters while at the surface, typically well less than +/- 1% of the full scale value of the sensor (such as < +/- 20m for a 2000m depth rated tag). For example,  an engineering packet indicating a depth of -2.43m indicates the depth sensor has a bias of -2.43m because an engineering packet reflects the tags current status, and can only be received by Argos if the tag is in fact at the surface.  An archived packet such as a daily summary of the same tag indicating that the minimum depth for a given day was -1m then really implies that the fish was at a depth of -1m - -2.43m = 1.43m.    In summary, make it a habit to check the engineering  depth reports to verify the validity of the depth readings in the archived packets.   If the tag is reporting 0 +/- a few meters while at the surface, .you can trust the archival data to be accurate to the same amount (and you might even subtract out that reported bias).  If however a tag at the surface reports 420m or -164m or the like, then the depth sensor is not reading correctly now and you should be sceptical of the depth reports in the archived packets.  

The "Turtle Tag" is a versatile low-weight tag designed to be mounted on shell of a sea-turtle.