Most of the machines are simply looking for a ratio of nitrates and phosphates above certain background limits. They're not really terribly complicated machines. The complexity comes in with having them test quickly, consistently, and making the protein sack users run them correctly.

And talking of using them correctly, the TSA drone at DCA this morning was using a single swab to run across groups of 5+ people at a time "because the line was busy". Adding to the hilarity, in between groups she was running her hands through her hair (to keep it from her eyes) which obviously had styling product in it. She gave me a really dirty look when I said to the group (none of whom knew each other) as she swabbed us that if it came back positive we should all point at the 5 year old in the group.