Physics:SCCM (flow unit)

SCCM is standard cubic centimeters per minute, a flow measurement term indicating cm³/min in standard conditions for temperature and pressure of the fluid. These standard conditions vary according to different regulatory bodies. One example of standard conditions for the calculation of SCCM is [math]\displaystyle{ T_n }[/math] = 0  °C (273.15 K) and [math]\displaystyle{ p_n }[/math] = 1.01 bar (14.72 psia). SCCM is a measure of mass flow rate, [math]\displaystyle{ \dot m }[/math], and in spite of its name it must not be confused with a measure of volumetric flow rate, [math]\displaystyle{ \dot q }[/math]. One SCCM indicates the mass flow rate of one cubic centimeter per minute of a fluid, typically a gas, at a density defined at some standard temperature, [math]\displaystyle{ T_n }[/math], and pressure, [math]\displaystyle{ p_n }[/math].

To convert one SCCM to the measure of mass flow rate in the SI system, kg/s, one relies in the fundamental relationship between mass flow rate and volumetric flow rate (see volumetric flow rate) ,

[math]\displaystyle{ \dot m = \rho_n \dot q , }[/math]

where [math]\displaystyle{ \rho_n }[/math] is density at some standard conditions, and an equation of state such as

[math]\displaystyle{ \rho_n = \frac{p_n M}{Z_n R_u T_n } , }[/math]

with [math]\displaystyle{ M }[/math] being the fluid molecular weight, [math]\displaystyle{ Z_n }[/math] the fluid compressibility factor, and [math]\displaystyle{ R_u }[/math] the universal gas constant. By including units of measurement and their conversion between square brackets in the above relationship between [math]\displaystyle{ \dot m }[/math] and [math]\displaystyle{ \dot q }[/math] one obtains

[math]\displaystyle{ \dot m \left[\frac{kg}{s} \right] = \rho_n \left[\frac{kg}{m^3} \right] \dot q \left[\frac{cm^3}{min} \frac{1 \, min}{60 \, s} \frac{1 m^3}{10^6 \, cm^3} \right] , }[/math]

where [math]\displaystyle{ \dot m }[/math] is in [math]\displaystyle{ kg/s }[/math], [math]\displaystyle{ \rho_n }[/math] in [math]\displaystyle{ kg/m^3 }[/math], and [math]\displaystyle{ \dot q }[/math] is in [math]\displaystyle{ cm^3/min }[/math]. Thereafter, by replacing [math]\displaystyle{ \rho_n }[/math] with the above equation of state one obtains

[math]\displaystyle{ \dot m \left[\frac{kg}{s}\right] = \frac{p_n [Pa] M \left[ \frac{kg}{kmol}\right]}{Z_n R_u \left[\frac{J}{K kmol}\right] T_n [K]} \dot q \left[\frac{cm^3}{min} \frac{1 \, min}{60 \, s} \frac{1 m^3}{10^6 \, cm^3}\right] , }[/math]

where again units are kept inside square brackets.

Using this last relationship, one can convert a mass flow rate in the more familiar unit of kg/s to SCCM and vice versa with

[math]\displaystyle{ 1 \, \frac{kg}{s} = 6E7 \frac{Z_n R_u \left[\frac{J}{K kmol}\right] T_n [K]}{p_n [Pa] M \left[ \frac{kg}{kmol}\right] } SCCM , }[/math]

and

[math]\displaystyle{ 1 \, SCCM = 1.6667E-8 \frac{p_n [Pa] M \left[ \frac{kg}{kmol}\right] }{Z_n R_u \left[\frac{J}{K kmol}\right] T_n [K]} \frac{kg}{s} . }[/math]

With this conversion from SCCM to kg/s, one can then use available unit calculators to convert kg/s to other units, such as g/s of the CGS system, or slug/s.

To provide some usage examples, consider the conversion of 1 SCCM of nitrogen, which has a molecular weight of 28 kg/kmol, to kg/s. Consider standard conditions of 101325 Pa and 273.15 K, and assume the gas is an ideal gas (i.e., [math]\displaystyle{ Z_n=1 }[/math]). Using the unity bracket method (see conversion of units) one then obtains:

[math]\displaystyle{ 1 \, {\cancel \text{SCCM}} \cdot \frac{ 1.6667E-8 \frac{101325 [Pa] 28 \left[ \frac{kg}{kmol}\right] }{8314 \left[\frac{J}{K kmol}\right] 273.15 [K]} \frac{kg}{s} } { 1 \, {\cancel \text{SCCM}} } = 2.08E-8 \frac{kg}{s} . }[/math]

To convert 50 SCCM of nitrogen with the above considerations one does

[math]\displaystyle{ 50 \, {\cancel \text{SCCM}} \cdot \frac{ 1.6667E-8 \frac{101325 [Pa] 28 \left[ \frac{kg}{kmol}\right] }{8314 \left[\frac{J}{K kmol}\right] 273.15 [K]} \frac{kg}{s} } { 1 \, {\cancel \text{SCCM}} } = 1.04E-6 \frac{kg}{s} . }[/math]

As another example, to do the same for 1 SCCM of helium, which has a molecular weight of 4 kg/kmol, one obtains:

[math]\displaystyle{ 1 \, {\cancel \text{SCCM}} \cdot \frac{ 1.6667E-8 \frac{101325 [Pa] 4 \left[ \frac{kg}{kmol}\right] }{8314 \left[\frac{J}{K kmol}\right] 273.15 [K]} \frac{kg}{s} } { 1 \, {\cancel \text{SCCM}} } = 2.97E-9 \frac{kg}{s} , }[/math]

where note that 1 SCCM of helium is much less in kg/s than one SCCM of nitrogen.

Notice from these examples that, unlike measures such as kg/s or g/s, the SCCM is a mass flow rate measure that depends both on the type of fluid (e.g., nitrogen or helium in the above examples) and the standard conditions used.

A unit related to the SCCM is the SLM or SLPM which stands for Standard litre per minute. Their conversion is

[math]\displaystyle{ 1 \, SCCM = 1E-3 \, SLM , }[/math]

and

[math]\displaystyle{ 1 \, SLM = 1E3 \, SCCM . }[/math]

Another unit is the SCFM which stands for Standard cubic feet per minute.

Yet another unit related to SCCM (and SLM) is the PCCM (and PLM) which stands for Perfect Cubic Centimeter per Minute (Perfect Litre per Minute). One PCCM is one SCCM when the gas is ideal. In other words, one PCCM is exactly the same as one SCCM if and only if [math]\displaystyle{ Z_n =1 }[/math] in the above relationships.