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Part 3. Measuring flow rates Flow and Pressure Control
Interpreting flow meter measurements
Interpreting flow meter measurements
Bubble meter measurements yield flow rates at the local temperature and local
atmospheric pressure. Electronic flow meters may be calibrated for
temperatures other than 25°C or for pressures other than 1 atm. However, the
GC display shows values corrected to Normal Temperature and Pressure (NTP)
conditions. If you do not correct your meter’s flow rate to NTP, the readings will
not agree with the GC.
To convert meter flow rate measurements to NTP (25°C and
1 atmosphere), you must know:
• The local atmospheric pressure or the electronic meter calibrated pressure
• The bubble meter temperature at the time of measurement or the electronic
meter’s calibration temperature.
The conversion is:
where:
Flow rate at NTP is the flow rate in mL/min corrected to Normal
Temperature (25°) and Pressure (1 atmosphere)
Flow rate
local
is the flow rate in mL/min measured by the bubble
meter
Temperature
local
is the temperature of the bubble meter at the time
of measurement or the meter’s calibration
temperature. This number is in Kelvins
(Kelvin = Centigrade + 273).
Pressure
local
is the local atmospheric pressure at the time of
measurement or the meter’s calibration
temperature. This number is in atmospheres
(1 atm = 1.01325 bars = 760 Torr = 760 mm Hg (at
0°C) = 101.325 kPa = 14.7 psi).
Flow rate at NTP =
Flow rate
local
x 298 x Pressure
local
Temperature
local
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