I have an Brunton 8099 Eclipse. No bubble when I'm home at 5400 feet elevation. As I go on hikes and gain elevation a bubble developers, but does not appear to affect accuracy AFAIK. I have not checked the exact altitude where the bubble just starts appearing ... but I'd guess around 10,000 feet, plus/minus. It's definitely there when I'm at 12,000+ feet, as I've seen it at that altitude positively for sure.