Alicat PCU Portable Calibration Unit - Manual (Page 21)

21
This reduces to: Pa Va / Za Ta = Ps Vs / Zs Ts , eliminating R and n.
Alicat mass flow meters model gas flows based upon the non-ideal gas characteristics of the calibrated
gas. The flow corrections are normally made to 25 C and 14.696 PSIA and the compressibility factor of
the gas under those conditions. This allows the user to multiply the mass flow rate by the density of the
real gas at those standard conditions to get the mass flow rate in grams per minute.
Because we incorporate the compressibility factor into our ‘full gas model’; attempts to manually
compute mass flows from only the P, V, and T values shown on the display will sometimes result in
modest errors.
Note: Although the correct units for mass are expressed in grams, kilograms, etc. it has become standard
that mass flow rate is specified in SLPM (standard liters / minute), SCCM (standard cubic centimeters
/ minute) or SmL/M (standard milliliters / minute).
This means that mass flow rate is calculated by normalizing the volumetric flow rate to some standard
temperature and pressure (STP). By knowing the density at that STP, one can determine the mass flow
rate in grams per minute, kilograms per hour, etc.
STP is usually specified as the sea level conditions; however, no single standard exists for this
convention. Examples of common reference conditions include:
0°C and 14.696 PSIA
25°C and 14.696 PSIA
0°C and 760 torr (mmHG)
70°F and 14.696 PSIA
68°F and 29.92 inHG
20°C and 760 torr (mmHG)
PCU Flow Meters reference 25ºC and14.696 PSIA (101.32kPa) - unless ordered otherwise. Refer
to the calibration sheet to confirm the reference point.
Standard Gas Data Tables: Those of you who have older Alicat products (manufactured before
October 2005) may notice small discrepancies between the gas property tables of your old and new
units. Alicat Scientific, Inc. has recently incorporated the latest data sets from NIST (including their
REFPROP 7 data) in our products’ built-in gas property models. Be aware that calibrators that you may
be spot checking against may be using older data sets such as the widely distributed Air Liquide data.
This may generate apparent calibration discrepancies of up to 0.6% of reading on well behaved gases
and as much as 3% of reading on some gases such as propane and butane, unless the standard was
directly calibrated on the gas in question. As the older standards are phased out of the industry, this
difference in readings will cease to be a problem. If you see a difference between the Alicat meter and
your in-house standard, in addition to calling Alicat Scientific at (520) 290-6060, call the manufacturer
of your standard for clarification as to which data set they used in their calibration. This comparison will
in all likelihood resolve the problem.