FPMAN-1-190214
Page 7
Since the temperature sensor is embedded inside the sensor block, if ambient temperature is
different from gas temperature, there would be a discrepancy between what the sensor reads and
the true gas temperature. The flowmeter body would track ambient temperature while gas
temperature would heat/cool the body at a different rate.
Likewise, if temperature variation is rapid, the flowmeter body may not follow it quickly enough
due to the mass of the metal flow chamber, which in turn would result in inaccurate
measurement of gas temperature.
For optimal performance, always allow two to four hours from the time the ambient and gas
temperatures are stabilized to when the first flowmeter reading is taken.
Reference Conditions for Mass Flow Measurement
Although the correct units for mass are expressed in grams, kilograms, etc., it has become
somewhat standard that mass flow rate is specified in SLPM (standard liters per minute), SCFH
(standard cubic feet per hour) or other similar units.
This means that the mass flow rate is calculated by normalizing the volumetric flow rate to some
standard temperature and pressure (STP). By knowing the gas density at that STP, one can
determine the mass flow rate in grams per minute, kilograms per hour, etc. STP is usually
specified at sea level conditions; however, no single standard exists for this convention. UFM
uses STP of 70° F and 14.7 PSIA.
Note: If used outside the parameters specified in this manual, the proper operation of the
flowmeter cannot be guaranteed.
