Air, nitrogen, oxygen and nitrousoxide (laughing gas) Argon, industrial helium, neon, krypton (inert gases) Hydrogen, methane (natural gas),ethylene, propane, butane Ammonia, carbon dioxide, carbonmonoxide, sulfur dioxide Mixtures of nitrogen and hydrogenor methane, endogas, exogas, city gas, mixtures of methane and carbon dioxide.

1.3. Calibration of thermal mass flow meters

T and R
S are wrapped, aswell as a laminar flow element. The laminar flow element generates a pressure drop proportionate to the flow, which drives the flow through the bypass capillary. The element mustbe designed so that both the flow inthe bypass and in the main channel is laminar and that the proportions re-main constant.Depending on the gas flow, the tem-perature conditions registered by the measuring resistors change. Differenttechnical designs for the bypass principle are possible. The thermal measurement is based on the thermal properties of the gas, the geometricdesign of the measuring body and theflow velocity of the gas. In thermal terms, gases differ by their specificheat capacity c In the calibration process, the measu-rement signal range of the sensor isclearly assigned to the flow control ormeasuring range. For this purpose, flow rates are set and the relevant sensor signals recorded on the basisof highly accurate flow normals (forexample, heating wire anemometers, which are regularly tested on a testbench with super-critical nozzles thathas been approved by the calibration authority). When the flow characteri-stic curve has been registered, theelectrical inputs and outputs can be calibrated. All the data are saved in digital form in an EEPROM. Mass flowcontrollers or meters generally containa calibration curve for a certain gas. They can only be used to control ormeasure a different gas if a secondcalibration curve has been stored. Exceptions to this rule are gases withvery similar properties (e.g. oxygenand nitrogen). In this case, a single conversion factor is sufficient for theentire flow range. In principle, everygas mixture can be measured, provi- ded its composition does not change. Mass flow controllers or meters areoften calibrated for the following gases:
p and heat conductivity
F . This means that, depending on themeasured gas, the measuring range ofa unit can be greater or smaller.
Figure 5: Calibration protocol 12/13 Bypass flow or bypass sensors (seeFigure 4) essentially have a bypass capillary around which the measuring resistors R