• Unit of gas flow rate (SLM)
  • We usually use SLM, Standard Liter per Minute, where the "standard" condition is 0 °C and 1 atm (in most cases).  This definition can be found at many websites for example at Wikipedia.
  • Most of our flow controllers/monitors are of Teledyne Hastings.  This Teledyne Hastings page mentioned "Teledyne Hastings Instruments assumes STP of 0 °C and 760 Torr".
  • Note that the flow rates recorded in our system (in unit of SLM) have already been converted to the volumes at the standard condition.  They don't vary with the condition (i.e. temperature and pressure) of the gas flowing was.

Then about the volume conversion, I cannot find any handy webpage written in English. But using this NIST Chemistry WebBook: https://webbook.nist.gov/chemistry/fluid/ The density of gHe is 22.426 L/mol at 0 degC and 760 Torr, which is almost the ideal gas. On the other hand, LHe: 1 L --> 125 g --> 125/4 = 31.25 mol. Thus the volume of gHe made from 1 L of LHe is 31.25 mol * 22.426 L/mol = 700.8 L (at 0 degC and 760 Torr).