How large is the impact, when the operating conditions change? (pressure and temperature)

The thermal mass flow meter directly measures the number of gas molecules (=mass), which flows past the sensor. The thermal flow measurement principle is therefore insensitive to changes in pressure and temperature. Vögtlin calibrates each unit on two temperatures and a temperature sensor in the gas stream compensates any minor temperature error due to temperature effects in the electronics. Due to this compensation there are almost no measurable temperature effects in the range of between 0…50 °C.