What do ln/min, Is/min, slm and sccm stand for?
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.
Our STP or standard temperature and pressure conditions used are stated here:
Normal conditions (ln/min): a temperature of 0 °C (or 32°F) and a pressure of 1.013 bar (or 14.69 psi) 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 (68°F) instead of 0 °C (32°F).
According to the ‘American’ definition the prefix ‘s’ in sccm or slm refers to ‘standard’ conditions, 101.325 kPa absolute (14.6959 psia) and temperature of 0°C (32°F).
Please be aware of the reference conditions when ordering an instrument. ‘Normal’ and ‘Standard’ can be relative to each customer, because mixing up these reference conditions causes an error of 7%!
| Typical flow unit | Reference conditions for gas temperature | Reference conditions for gas pressure |
|---|---|---|
| mln/min (milliliter normal per minute) | 0 °C / 32 °F | 1.013 bar / 1 atm / 14.69 psi |
| ln/min (liter normal per minute) | 0 °C / 32 °F | 1.013 bar / 1 atm / 14.69 psi |
| sccm (standard cubic centimeter per minute) | 0 °C / 32 °F | 1.013 bar / 1 atm / 14.69 psi |
| slm (standard liter per minute) | 0 °C / 32 °F | 1.013 bar / 1 atm / 14.69 psi |
| mls/min (milliliter standard per minute) | 20 °C / 68 °F | 1.013 bar / 1 atm / 14.69 psi |
| ls/min (liter standard per minute) | 20 °C / 68 °F | 1.013 bar / 1 atm / 14.69 psi |