ThermViewer appears to have manual setting of upper and lower span temperatures but I have never got this to work properly (= the way I'd expect) on the ThermApp cameras. It may work better on other cameras that the app supports.
I would expect to be able to lock (and adjust) the lower and upper temperatures (and probably the linearity of the span) but it looks as though ThermViewer only sort-of locks one or the other and still auto-adjusts. As far as I am concerned this is by far the biggest - indeed only - failing in an otherwise superb piece of software, but it makes it essentially unusable for panoramas. Therm-App Plus by digitalbiology is much better - it locks properly - but it doesn't support the 640x480 Pro nor provide superresolution. (I've been in discussion recently with digitalbiology and he tells me Opgal are currently acting a bit odd about their SDK, though I'm not sure of the details, but I understand the bottom line to be that he can't compile any new versions of the software without it).