23
This reduces to: P
a
V
a
/
Z
a
T
a
= P
s
V
s
/
Z
s
T
s
, eliminating R and n.
Alicat mass fl ow meters model gas fl ows based upon the non-ideal gas characteristics of the calibrated
gas. The fl ow 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 fl ow rate by the density of
the real gas at those standard conditions to get the mass fl ow rate in grams per minute.
Because we incorporate the compressibility factor into our ‘full gas model’; attempts to manually
compute mass fl ows 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 fl ow rate is specifi ed in SLPM (standard liters / minute), SCCM (standard cubic centimeters
/ minute) or SmL/M (standard milliliters / minute).
This means that mass fl ow rate is calculated by normalizing the volumetric fl ow rate to some standard
temperature and pressure (STP). By knowing the density at that STP, one can determine the mass fl ow
rate in grams per minute, kilograms per hour, etc.
STP is usually specifi ed 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)
M Series Flow Meters reference 25ºC and14.696 PSIA (101.32kPa) - unless ordered otherwise.
Refer to the calibration sheet to confi rm 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 Scientifi c, Inc. has incorporated the latest data sets from NIST (including their REFPROP
7 data) in our products’ built-in gas property models. Be aware that the calibrators that you may be
using may be checking against 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 Scientifi c at (520) 290-6060, call the manufacturer of your
standard for clarifi cation as to which data set they used in their calibration. This comparison will in all
likelihood resolve the problem.
To Next Page
Loading...