What does mass flow mean?
Although it means mass flow, it is generally used as an abbreviation for flow control devices and measuring instruments such as mass flow controllers and mass flow meters. To distinguish between mass flow controllers and mass flow meters, mass flow controllers are sometimes abbreviated to MFC and mass flow meters are abbreviated to MFM, which are sometimes used in flow diagrams.
What is the difference between mass flow and volumetric flow?
Mass flow and volumetric flow both express the same flow rate (the amount of flow in unit time), but volumetric flow requires consideration of density factors that vary with temperature and pressure. Mass flow, on the other hand, uses mass as the unit of measure, and thus can accurately measure flow rate without considering density.
In mass flow, the value sought is mass, or weight. It expresses the weight of material flowing per unit time. For gases, SCCM or SLM units are used, while g/min is used for liquids. Volumetric flow rate, on the other hand, is the value sought in volume, or volumetric volume. It expresses the volume of material flowed per unit time. Regardless of whether it is a gas or a liquid, the general unit of volume flow is the volume at a specified temperature and pressure, such as ℓ/min.
What are the units of mass flow?
The units of mass flow are generally SCCM or SLM for gas mass flow. These are units that represent the volume at which the instantaneous flow rate continues for one minute, converted to 0°C and 1 atmospheric pressure. There are also units such as NCCM, NLM, CCM, and LM, where the temperature to be converted is 20°C or 25°C. On the other hand, g/min is generally used as the unit for liquid mass flow. This unit represents the weight (g) when the instantaneous flow rate continues for one minute. Unlike gases, it is not expressed in terms of volume, but in terms of weight.
SCCM
It stands for standard cubic centimetre/min. In the semiconductor industry, the ideal state of a gas is considered standard at 0°C and 1 atmospheric pressure. At Lintec, it represents the volume (㏄) of a gas that continues for one minute at 0°C and 1 atmospheric pressure. However, please note that the standard may differ depending on the industry.
SLM
Abbreviation for standard liter/min, the volume (in ℓ) of an instantaneous flow rate under 0°C, 1 atm equivalent conditions for one minute continuous. 1000 times the flow rate of SCCM. Note, however, that temperatures considered standard may differ.
NCCM
It stands for normal cubic centimetre/min and represents the volume (㏄) that would result if the instantaneous flow rate continued for one minute at 20°C and 1 atm equivalent.
NLM
Abbreviation for normal liter/min, the volume (ℓ) of flow that occurs when the instantaneous flow rate continues for one minute at 20°C and 1 atm equivalent. 1000 times the flow rate of NCCM.
CCM
It stands for cubic centimetre/min and represents the volume (㏄) that would result if the instantaneous flow rate continued for one minute under conditions of 25°C and 1 atm equivalent.
LM
Abbreviation for liter/min, the volume (ℓ) of an instantaneous flow rate under conditions of 25°C and 1 atm equivalent for 1 minute. 1000 times the flow rate of CCM.
g/min
A unit of mass flow rate of a liquid, expressing the mass (g) of the liquid when the instantaneous flow rate continues for one minute.
What is operating differential pressure?
Operating differential pressure refers to the differential pressure between primary and secondary pressure required for a mass flow controller or mass flow meter to operate properly. The internal structure of a mass flow controller or meter contains minute flow measurement and flow control sections, which generate a certain amount of pressure loss. Therefore, a pressure differential is required according to the specifications. Generally, the operating pressure of mass flow controllers is in the range of 50 to 300 kPa. However, care must be taken in the selection process because it varies depending on the specifications and flow rate, such as low differential pressure specification, liquefied gas specification, and high pressure specification.
What is the operating differential pressure of the mass flow controller?
Typical mass flow controllers have an operating differential pressure of 50 to 300 kPa. At high flow rates, the required differential pressure tends to be large.
In addition, when liquefied gas is controlled by a general mass flow controller with a large differential pressure, the temperature may drop and re-liquefy due to the Joule-Thomson effect. Therefore, a mass flow controller with a low differential pressure specification should be selected for liquefied gas.
What is the control range of the mass flow controller?
In general, the control range of mass flow controllers is 2 to 100% F.S. (full scale). Therefore, if a set value of less than 2% is entered, the control valve will be fully closed, resulting in 0% control. However, depending on the model, options, and specifications, Lintec can control the flow rate to achieve the set flow rate even with a set value of less than 2%. Please contact us for details.
What is the conversion factor?
Mass flow controllers control the mass flow rate of various fluids. In thermal mass flow controllers, the ease of warming (specific heat) varies with the type of fluid, and must be compensated for by a fluid-specific coefficient. This coefficient is called the conversion factor.
Typical Conversion Factors
| gas type | C.F. | gas type | C.F. |
|---|---|---|---|
| nitrogen (N2) | 1.000 | Air | 1.000 |
| Helium (He) | 1.400 | Carbon monoxide (CO) | 1.000 |
| Hydrogen (H2) | 1.010 | Carbon dioxide (CO2) | 0.740 |
| Oxygen (O2) | 0.990 | Nitric oxide (NO) | 0.990 |
| Argon (Ar) | 1.400 | Acetylene (C2H2) | 0.630 |
Conversion Factor Measurement
Conversion factor measurements include the build-up method using Boyle-Charles’ law。
C.F. measurement procedure by build-up method
- Vacuum the vessel whose volume is being monitored on the secondary side of the mass flow controller for N2 calibration.
- Shut off the valve on the secondary side of the vessel that has been evacuated.
- The mass flow controller controls the gas to be measured at a constant flow rate.
- Calculate the flow rate from the pressure rise and temperature of the vessel on the secondary side of the mass flow controller.
- Calculate the ratio between the calculated flow rate and the flow rate set by the mass flow controller for N2 calibration.
Conversion Factor Calculation
If the constant pressure specific heat and molecular weight are known, the conversion factor can be calculated. For gas flow control where the conversion factor is unknown, please contact Lintec directly.
Mixed gas conversion factor
For mixtures of two or more gases with different conversion factors, the conversion factor varies depending on the gas ratio. Lintec’s mass flow controllers can calculate the conversion factor when controlling gas mixtures. Please contact us directly.
What mass flow measurement methods are available?
The main mass flow measurement methods include the following
- Thermal Mass Flow: This method uses the specific heat of a fluid to measure mass flow rate. Heat is generated to measure the temperature change of the fluid, and the mass is estimated from the amount of change. Generally, the mass is calculated from the amount of heat lost by the fluid using a heating resistor.
- Coriolis Mass Flow: This method uses the inertia of the fluid to measure mass flow rate. An oscillating sensor is inserted into the fluid, and the degree of oscillation varies depending on the mass of the fluid. The mass is estimated by detecting this change in vibration. Coriolis mass flow meters are highly accurate and can be used with a wide range of fluids.
- Differential pressure type mass flow: This method measures the pressure difference generated when fluid passes through a pipe to estimate the mass flow rate. Typically, a constriction in the pipe or a specially shaped device is used to create a pressure differential proportional to the velocity of the flowing fluid. The mass flow rate is calculated from this pressure difference. Orifice plates, Venturi meters, and pitot tubes are examples of differential pressure type mass flow meters.
Each of these methods has its own features and advantages and is selected according to the intended use and the nature of the fluid. It is important to select the most appropriate method depending on detailed performance requirements and applications.
