Volumetrically Based Mass Flow Units - Fluke MOLBOX RFM Operation And Maintenance Manual

molbox™ RFM™ OPERATION AND MAINTENANCE MANUAL
Volume flow is dependent on the actual temperature and pressure of the
flowing gas at the point where volume flow is to be measured. Generally, this
point is not at the molbloc, it is at another point in the flow system (e.g., at the
device under test). At another point in the flow system, it is quite likely that the gas
pressure and temperature are different from the gas pressure and temperature at
the molbloc. Then, even in steady state flow conditions, the volume flow at the
molbloc and the volume flow at another point in the system are likely to be different.
Therefore, to accurately predict volume flow at another point in the system,
molbox RFM must calculate volume flow based on the mass flow through the
molbloc and the gas pressure and temperature at that other point, not at the
molbloc. For this reason, molbox RFM requires that gas pressure and temperature
conditions at the DUT be specified for volume flow measurements.
Estimating the flowing gas pressure and temperature at the point at which
volume flow is to be measured may be difficult.
The relevant gas pressure when measuring volume flow is the gas's absolute pressure.
In cases in which the volume flow measurement is open to atmospheric
(ambient) pressure, the volume flow pressure is atmospheric pressure. In other
cases, there may be ways to estimate the pressure at the volume flow
measurement point but it probably should be measured.
For temperature, if the volume flow measurement point is very near the molbloc,
one possibility is to use the molbloc temperature measurement. By design, the
molbloc causes the temperature of the gas that flows through the molbloc to take
on the molbloc temperature. Therefore, the temperature of the gas as it exits the
molbloc is the same as the molbloc temperature. If the volume flow measurement
point is not immediately downstream of the molbloc, the best estimate of gas
temperature may be ambient temperature or the temperature of the device or
bath used to stabilize gas temperature if one is present.
3.4.3.2
molbox RFM supports a number of volumetrically based mass flow units of
measure. Volumetrically based mass flow units should not be confused with
volume or actual flow units (see Section 3.4.3.4). Volumetrically based mass
flow units define mass in terms of the quantity of gas that occupies a volume
under standard conditions of pressure and temperature.
universally accepted definition of standard conditions, molbox RFM supports the
three most common variances.
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© 1998-2007 DH Instruments, a Fluke Company
Because volume flow (sometimes called actual flow) is dependent on gas
pressure and temperature at the flow measurement point, gas pressure and
temperature must be specified by the user when selecting volume flow units
on molbox RFM. The measurement uncertainty (accuracy) in the volume flow
measurement is highly dependent on the measurement uncertainty in the
pressure and temperature specified. Typically, temperature errors have an
effect on flow of about 0.35 %/°C and pressure errors have an effect on flow
of about 1%/kPa (6.8%/psi) if the DUT is used near atmospheric pressure.

VOLUMETRICALLY BASED MASS FLOW UNITS

Standard units (sxxx): The "s" prefix indicates standard. Volumetrically
based mass flow units preceded with the letter "s" (i.e., sccm, slm, scfh)
define standard conditions as pressure of 101.325 kPa absolute (14.6959
psia) and temperature of 0 °C (32 °F) and take into account the true
compressibility of the flowed gas.
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