Principle of MFC/MFM

Principle of Mass Flow Instruments:

Our mass flow measuring/control technology based on mass flow meters and mass flow controllers has been used widely for the manufacture of semiconductors, liquid crystals, optical fibers, and other electronic devices; gas supply for fuel cells; combustion gas control for burners and the like; and for test, production, and inspection quipment in the food industry, biotechnology, and many other industries.

In comparison with conventional mechanical products, mass flow measuring instruments offer more sophisticated flow measurement because they are not susceptible to temperature and pressure and they can pick up electric signals from the flow.

1.Volume flow and mass flow: 

Gas flow meters can be roughly divided into volume flow meters and mass flow meters. Volume flow meters include area flow meters, positive displacement flow meters, and differential pressure flow meters, while mass flow meters include coriolis flow meters, vortex flow meters, and thermal flow meters.

The float type flow meters are classified as area flow meters in the category of volume flow meters, while the mass flow instruments  are classified as thermal flow meters in the category of mass flow meters.

n terms of classifi cation, the terms, "thermal mass flow controllers and thermal mass flow meters," are used according to the basic principle.

2. Principle of mass flow sensor: 

The flow sensor used in mass flow is called a thermal flow sensor in general. The principle of detection is as follows.

Structure of sensor section

A resistive element with a large temperature coeffi cient of resistanceis wound on the upstream side (Rus) and the downstream side (Rds), respectively, around the capillary tube that is a sensor as shown in Figure 3.

When electric current flows through these sections, the two resistive elements generate heat. When no fluid flows in the capillary tube at that time, the temperature of the upstream side is the same as that of the downstream side, matching each other.
(The solid line in Figure 3: Zero fl ow = Position of the zero point used for mass fl ow instruments.)

When the fluid begins to flow in this state, the temperature distribution changes as shown by the broken line in Figure 3.

The heat of the upstream side is drawn at that time, and the heat is transferred by the fl ow to the downstream side conversely. In other words, a temperature difference (∆T) arises between the upstream and downstream sides.

Structural chart of mass flow meter

As the temperature difference (∆T) has a functional relation to the mass flow of fluid, mass flow instruments pick up electric signals that represent the change in respective resistance values and amplify and correct the signal to permit the mass flow to be measured under a certain condition.

This is the function of the mass flow meter shown in Figure 4.

Structural chart of mass flow controller

In the mass flow controller shown inFigure 5, the opening of the flow control valve is controlled by a high-velocity, high-resolution piezo or solenoid actuator based on the comparison between the external flow setting signal and the flow signal output from the sensor.

This system permits stable mass flow control, which will hardly be affected by changes of various conditions such as temperature and pressure.

3. Measuring Unit of Flow Rate: 

A mass flow meter measures the mass flow irrespective of pressure and temperature. When representing mass by the flow, it is necessary to use units such as g/min and kg/min which are different from the familiar units used for general fluid measurement.

Therefore, it is common to use volume fl ow under predetermined standard conditions of pressure and temperature. At present, Pa·m3/s is used in conformance with the SI units, but SCCM and SLM which have long been used for mass flow instruments are still used as principal units.

With respect to the definition of the standard unit, we adopted the definition based on the SEMI standard in October 1998.

SCCM is an abbreviation of Standard Cubic Centimeter per Minute, indicating cc/min at 0°C at 1 atmospheric ressure, while SLM is an abbreviation of Standard Liter per Minute, indicating L/min under the same conditions. Other units of fl ow, if they are recognized as units of measurement at present, can be used for calibration and manufacture of our products.

In some industries other than the semiconductor industry, SCCM and SLM are defi ned as the units at 20°C at 1 atmospheric pressure and NCCM and NLM as the units at 0°C at 1 atmospheric pressure.

Concerning the flow indication of our mass flow instruments, the standard temperature and pressure in units of SCCM (0°C, 1 atm) and NLM (0°C, 1 atm) are shown on our products and in test reports.

4. Calibration with actual gas and conversion factor method: 

mass flow instruments are calibrated with N₂ gas in principle before shipment. The accuracy of thermal sensors cannot be guaranteed unless they are calibrated with actual gas.

The actual gases used for calibration at our company are N₂, O₂, H₂, He, CO₂, and Ar. Concerning other gases, certain conversion factors (CF) are used for correction after calibration with N₂ gas.

For example, when Ar is fl owed through a mass fl ow instrument that was calibrated with N₂ gas, a 1.4 times larger quantity of Ar than the reading of the mass fl ow instrument will fl ow, because the CF of Ar is 1.4. In other words, the fl ow of Ar = 1.4 x Reading of N₂mass
fl ow instrument. The CF is calculated for various gases based on calculation and the accumulation of data obtained through measurement with actual gases.

However, the CF of one gas may not be exactly the same depending on the condition of the actual gas (temperature and pressure), the type of sensor of the mass flow instrument, and combination of the bypass (laminar fl ow element). The public standard CF should be used just as a standard value.

If you desire calibration with actual gas without using a CF, please provide us with the actual gas, and we will use it for calibration (this will incur a separate fee for gas calibration).

However, we cannot accept some dangerous gases in view of the safety of products and facilities. Please contact us in advance for details.

5. Definition of specifi cations:
 

The indication of mass flow instrument specifi cations in this taxt basically conforms to the SEMI standard. The defi nitions of representative specifi cations are explained below.

(1) Accuracy

The accuracy is indicated in the form of"Full scale ±
%."This is the % value with respect to the full-scale value of the error when the calibration standard gas (N₂, for example) is used for our standard fl ow meters. Therefore, when the accuracy is ±1% in the range of Full-scale 50 SCCM, the flow rate will be guaranteed with the uncertainty" of 50 x (1/100) = ±0.5 SCCM with respect to our standard flow rate.

(2) Repeatability

The form of"Full scale ±
%"is the same as the accuracy.

This value indicates the deviation of the value obtained by measuring the flow, which is set under the same environmental conditions,with our standard flow meters.

This definition is different from the reproducibility that shows the deviation of the value after the environmental condition is changed.

(3) Response

The response is indicated by the time taken for the output of mass flow instruments to stabilize at 98% of the full scale after starting control from zero flow.
Such indication is adopted usually because it is diffi cult to analyze 100% in the case of an asymptotic line.

Note

The value used to indicate the fl ow range is the full-scale
(100%) value only when N₂ (or air) is made to flow.

Therefore,when the type of gas and pressure conditions are different, even if the fl ow is the same, we may not be able to manufacture products according to the desired specifi cations; please contact us in advance.