Apologies that I haven't been around for a while, folks. A lot of Real Life® stuff has been happening.
As with essentially all thermal cameras, the various Therm-App devices require a set of calibration files. Most cameras have this data permanently stored onboard but Opgal, for whatever reason, decided to hold the calibration files on a central server and download them, just once, the first time a camera is used on a particular Android device.
Briefly, no two pixels respond the same - adjacent pixels can have wildly different responses, 'cold' temperatures, non-linearities and so on. The calibration data is effectively a map of all these foibles, for that one specific sensor at one specific frame rate.
If you go looking round the Therm-App folder on your phone you'll find a sub-folder that matches your camera serial number, and inside that are a dozen or so files named things like 1.bin, 2.bin, 3.bin and so on. These hold the calibration data for your particular camera's sensor at its specified frame rate. The only thing that really ever needs changing is the dead pixel map, and that's done via a 'dead pixel repair' utility built into the Therm-App software.
(Better cameras have bigger and better calibration files to cope with a wider range of, for example, ambient temperature, frame rate, particular lens or whatever, but calibration takes time and money which is one reason the Therm-App TH - the 'thermography version' is more expensive than the standard Therm-App).
Whether you think it's a good idea that these files are stored centrally is a matter of opinion. At least in theory, if Opgal's server is switched off you won't be able to install the camera on a new phone - but your existing installation(s) will continue to work just fine. I don't know whether it's possible to fool the software into not running its 'first time, run once' bits, but I expect some clever person will figure it out.
If you upgrade a 9Hz Therm-App camera to 25Hz, all the calibration goes screwy and you don't get a usable picture. However, the amazingly clever Jinhua (of ThermViewer fame) has written a utility that can produce new calibration files from scratch. Broadly, you have to put your camera in a freezer to get it good and cold, then point it at a wall and run the utility whilst the camera returns to room temperature. Then you have to heat it (eg with a hair drier) and again let it return to ambient. All the while, the utility is capturing response data. ThermViewer then re-calculates the calibration files based on the data recorded by the utility and produces new .bin files. However, if you install the camera on another Android device the original calibration files will get downloaded to the new installation, so you have to copy the calibration files from an existing installation and overwrite the downloaded 'stock' data.
Most people will never have to do any of this. Opgal's calibration is fairly good - albeit not on a par with a research-grade FLIR or similar costing ten or a hundred times as much.
Hope this explains what's going on.
I think I should point out that I'm a 'self-appointed' Therm-App 'ambassador': Opgal does not pay me or give me free goodies (with the exception of a couple of spare USB cables that I asked for). However, if someone would like to give me a nice new XGA+ resolution camera and a selection of lenses in exchange for a few nice words, I'm open to offers!