I don't have a GLL 3-50 but plenty of experience with the GLL 3-80. You can get up to 30 m on rocksalt. Slate will reduce the range 15 m. You'll usually get a much better range underground than what the manufacturer says. I used ISO 800 F 4.5 and 1 second exposure for the photo.
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The downside of theses lasers is that you need a line of sight and enough space to move around with the equipment without destroying any stations.
Another option could be a hose scale. A lot simpler than using a hose an trying to measure the pressure.
I'd still go for the altimeter. It's probably not as cheap as you expect as you'll need a fairly accurate device. I use a Greisinger GTD 1100 which sells at £ 130. I also know of at least two of these being used in the Peak District.
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The resolution of the altimeter should be ok:
-1640 … -655 ft ~5ft
-654 … +1999 ft ~2ft
2000 … 19999 ft ~5ft
The big advantages are that you don't need a line of sight between start and end point and there is no need for intermediate stations. You can actually cover quite long distances between the stations. Small pressure differences due to ventilation can be more or less neglected. You'll have to compensate atmospheric pressure changes during your survey for a better accuracy unless the time between the readings is very short.