The GPSMAP 76S has the barometric sensor, at least OLC thinks so,
according to the online flight display.

Or is it the straight 76 that you bought?

I bought the GPSMAP 76 (there's also a straight GPS 76 but it's not
suitable for our purposes: not enough trackpoints, for one thing). The
GPSMAP 76S does have the baro sensor (as well as an electronic compass
sensor) but I'm not sure whether it's evident from the track log
whether the user locked out the auto-calibrate feature that slowly
adjusts the altimeter to GPS altitude to account for pressure changes.
I'm also not sure whether the track log records both pressure and GPS
altitudes. If the answer to these questions is yes (i.e., it's evident
if auto calibration is turned off and it records pressure altitude),
then the extra money for the GPSMAP 76C might be worth it. It's still
not an IGC $ecure flight recorder but it's the next best thing...at 1/5
or 1/6 the price.

Erik Mann (Papa3) was a great help during my evaluation of backup
flight recorders and is a lot more conversant with this subject (he's
even interacted with Garmin). As I mentioned earlier, I'm really
impressed with how much functionality you get for $150.

Chip Bearden
ASW 24 "JB"