This is the one I have:
http://www.thealtimeterstore.com/highgear.html
I like it because it doubles as a watch. Also, the carabiner clip thing hinges so I can stick it in a dashboard vent and use the altimeter in the truck. Kind of fun when you're cruising up and down mountain roads. That's really the reason I wanted to get a dashboard altimeter. My wife got me this one for Christmas 2007 so I could also use it on backpacking and ski trips. Only later did I get the idea to use it in barometric caves.
Often, when we go out to the end of Jewel Cave and feel wind blowing, it leads to more cave. Jewel breathes barometrically, and there is only one known entrance. On a day trip, we know which way the wind is blowing at the entrance and that helps us to determine which leads to pursue. For example, if the cave is blowing out, wind blowing out of a lead should be coming from unknown cave. If wind is blowing into a lead when the entrance is blowing out, then that wind is probably going toward the entrance.
This sounds fairly simple except when you take into account that the cave wind can change direction at any moment. So, at the end of the cave you don't really know which direction the wind is going, unless you can correlate it with a known point in the cave. This gets particularly difficult on camp trips, when we are underground for 4 days. On the way to the southeast part of the cave from camp, we go through a place called The End, that is small and you can feel the wind there. That is the last place that we really can tell which way the wind is going until hours later when (hopefull) we start exploring windy leads. (and then you don't really know if it has flipped) We sometimes go beyond The End for 24 hours, and the wind direction can sometimes change several times in a day.
On the last camp trip in January, I took the altimeter and used the barometer setting to monitor pressure changes. A dropping barometer in the cave should mean outflow and a rising barometer should mean inflow. There are factors that complicate this, but generally it is true. Using the barometer on the survey trips we were able to make a more educated guess about which leads would take us farther from the entrance.
Last month, I took the barometer/altimeter to the Far East in Lechuguilla cave for 8 days. The barometric winds are much harder to feel in Lech than in Jewel. I don't know if this means it is a smaller cave, or just that the air behaves differently because of different cave geometry. At any rate, the barometer stayed fairly flat all week. Only on the last two days did it show the pressure dropping significantly. Interestingly enough, when I checked the surface pressure history after the trip, that is sort of what actually happened on the surface as well. Check out March 15-21 on this Carlsbad pressure graph:
While the instrument is not as useful for wind in Lech, it is very fun to use for depth. Like any altimeter it does need to be calibrated from time to time to account for pressure changes, but on a long trip I am not really using it for "to-the-foot" elevations above sea level. I do use it for relative depth from place to place and found that it is definitely accurate enough to gauge pit depth. I would trust a rope measurement derived from this thing. You can hold it in your hand and watch the numbers change as you raise or lower it one foot. I haven't checked it against a laser for pit depth but I plan on doing that soon.
On shorter trips, you can trust it to give absolute elevation. I recently took it to the bottom of Neff's Canyon Cave in Utah. Neff's is 1186' deep and the altimeter told us just where we were on the profile the whole trip. When we got to the Bedroom Complex the altimeter showed 600' below the entrance-halfway down. Looking at the map I saw that we were indeed halfway to the bottom. At the bottom it read 1191' below the entrance, so pretty close to spot on. It was also fun on the hike down from the cave to see when we were level with the elevation of the sump at the bottom of the cave.
Anyway, there are some of the things I have been playing with with the altimeter. It's another fun geeky device to carry around in a cave if anyone was looking for one. (I know there are no geeks that read this board
) On the next camp trip I plan on not being lazy and actually recording the pressure morning and night and seeing what I can learn from that, correlated with wind observations at the leads and pressure history of the surface.
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