Common mass flow meters and mass flow controllers work with thermal measuring principles (with bypass sensor or according the CTA principle (Constant Temperature Anemometry). Thermal sensors operate on the principle of heat transport in the sensor element. This method depends on the type of gas, since the heat transport depends directly on the heat capacity and the thermal conductivity of the gas to be metered.
Since NO2 has a temperature and pressure-dependent equilibrium with N2O4, the parameters in the sensor element can change constantly. Consideration of the equilibrium using a single conversion factor to a reference gas is not sufficient, especially for pure NO2 or N2O4. Through gravimetric tests, we have determined that massive under-dosing can occur at a dosage of pure NO2 (approx. 10 % of the target value).
A further challenge with a thermal mass flow controller in the closed state, corresponding to a flow rate of 0 ml/min, is that it can produce pseudo signals of up to 10% of the maximum dosing range. The reason for this is that the sensor element contains a mixture of NO2 and N2O4, which is constantly influenced by the active heating of the sensor element. Thus, a heat transport in the device is faked and a flow rate is indicated.