Section 1 Introduction
Reference Conditions
Due to effects of pressure and temperature on the compressibility of
gases, specific reference conditions must be used when reporting
volumetric flow rates in mass flow terms. For example, the unit of measure
SCCM (standard cubic centimeters per minute) refers to a volumetric gas
flow at a standard reference condition, NOT the actual volumetric gas flow
at the actual operating pressure and temperature. The key point is that the
MASS FLOW of the gas is fixed, but the reference volumetric flow can be
reported differently based upon the standard reference condition used in
the calculation.
Throughout the world, there are differences in terminology when describing
reference conditions for gases. The words “normal conditions” and
“standard conditions” are sometimes used interchangeably to describe the
reference STP (Standard Temperature and Pressure) for gases. Further
note that temperature and pressure values for standard or normal
reference conditions vary in countries and industries worldwide. For
example, the Semiconductor Equipment Manufacturing Industry (SEMI)
defines standard temperature and pressure conditions as 273.15 K (0 °C)
and 101,325 Pa (760 torr). The main concern is that no matter what words
are used for descriptive purposes, a gas mass flow rate must have a
defined standard pressure and temperature reference condition when
performing a volumetric conversion.
PC-based Support Tools
Brooks Instrument offers a variety of PC-based process control and service
tools to meet the needs of our customers. Smart Interface may be used with
any unit supporting RS485 S-P protocol in a multidrop configuration, thus
allowing users to control and monitor their Brooks devices. Brooks Expert Sup-
port Tool (BEST) may be used to monitor, diagnose, tune and calibrate Brooks
devices. BEST interfaces with Brooks products via a special service port.
6
