What does Bronkhorst mean by ln/min or ls/min?
Mass flow should actually be expressed in units of weight such as g/h, mg/s, etc. Most users, however, think and work in units of volume. No problem, provided conditions are agreed upon, under which the mass is converted to volume.
Mass Flow vs Volume Flow
Normal conditions (ln/min): a temperature of 0 °C and a pressure of 1013.25 hPa(a) are selected, and these reference conditions are indicated by the underlying letter “n” in the unit of volume used. The direct thermal mass flow measurement method is always based on these reference conditions unless otherwise requested.
Standard conditions (ls/min): here the reference conditions are based on 20 °C instead of 0 °C. Please be aware of this difference, because mixing up these reference conditions causes an error of 7%!