Hydrogen sniffers are a fabrication that is made from the life lines of a dog and several other unknown animals and insects. It has six insect-like legs, a dog like head, two mouths and two noses. Hydrogen sniffers are primarly used to detect hydrogen leaks in hydrogen breathers, such as the Leviathan. These creatures are usually under the control of a rigger, who takes them around the airship to look for leaks. Sniffers also assist in apprehending intruders on the ship.
In Leviathan, Newkirk and Deryn see some sniffers, and Newkirk squeaks when one of them sniffs his hand. Deryn sees a dead hydrogen sniffer when she is on the spine during the battle over the Swiss alps. The hydrogen sniffers frighten Alek when Deryn whistles an intruder alert and the sniffers try to apprehend him.