Sensor to detect micro temp variation for thermal flying (typical 0,03 °C) this device beeps if you comes nearby a thermal column (rise in temperature). Previous devices in the 80's didn't compensate for temperature variations with pressure. Sniffer was beeping in heavy sink (not intended if searching for a thermal because you would be looking for the heaviest sink) They didn't store unlike traditional altivarios a reference (like 0 m/s) and gave you just a sound in function of the temp variation (a bit like if your altivario gave you audio-output only if rise in thermal was increasing or decreasing not if it was stable) therefore you need a reference temperature (like 18,7 °C was the last recorded temp before temperature increase or decrease) The device will beep in function of this reference.