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. This is not a problem, provided standard temperature and pressure (STP)conditions under which the mass is converted to volume are agreed.
Normal conditions (
ln/min): reference conditions are temperature of
0 °Cand a pressure of
1.013 bar.
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): reference conditions are temperature of
20 °C (instead of 0 °C) and a pressure of
1.103 bar.
These reference conditions are indicated by the underlying letter "s" in the unit of volume used.
When a user says his reference conditions are "normal" he may very well mean that they are room temperature and one atmosphere, which could be 20° C and 1.013 bar (that is normal to him). However an instrument ordered at Normal (ln) reference conditions would use reference of 0° C and 1.013 bar. Why is this important?
With the capillary bypass thermal mass flow technology a portion of the gas flow is shunted through a capillary measurement sensor with that gas rejoining the main flow before exiting the meter or entering the control valve. Advantages of this technology in measuring mass flow include accuracy and speed of response, but may not be the best choice for a moist or dirty gas. The 
In the through flow thermal mass flow technology instruments offered by Bronkhorst USA the gas moves straight through the flow body of the meter with mass flow being measured using the constant temperature anemometer (CTA)principle. These instruments can be advantageous when working with moist or dirty gases, but do not allow for accuracy as high as the capillary bypass instruments. The new 
MEMS thermal mass flow technology is also offered by Bronkhorst USA. In instruments using this technology the gas flows directly over the MEMS sensor. Very low flows of clean, dry gases can be measured and controlled with this type of instrument. The 
Bronkhorst USA also offer coriolis mass flow technology wherein the gas flows through oscillating tubes and causes a variable phase shift between the tubes which is strictly proportional to the mass flow rate. Coriolis instruments offer high accuracy, excellent repeatability, and fast response time. The 
