My simple observation of a single HB-100 module is that the frequency increases as temperature decreases.
I think it has gained 150 kHz while cooling of 15 °C, so -10 kHz/°C. That's just one test of one specimen, YMMV, even MMMV. In any case the temperature should be stabilized as much as possible, at least for short term variations, by adding thermal insulation.
I wonder if the relationship between temperature and frequency is deterministic, therefore either parameter can be inferred if the other is known: frequency-based thermometer, or temperature-based frequency estimation.
