How to properly use a mass flow measurement / controller (MFM / MFC)

Mass Flow Meter / Controllere measures the flow rate of various gases accurately and quickly

It is a device that can measure / control and can be used in various environments using various gases.

In this article, Mass Flow Meter / Controller (hereafter, the name is unified as MFC)

We will explain the considerations for using it as accurately and safely as possible.

Cleanliness

Many users don't care about cleanliness when installing MFC.

However, contamination that occurs during installation can be prevented with a little care.

To understand this issue, it is helpful to have a preview of the room where the MFC detects and controls gas flow.

The figure below shows a typical thermal mass flow sensor and control valve.

Most of the gas passing through the MFC passes through the flow bypass, but some of the gas

It is passed through a small capillary tube (Mass Flow Sensor Tube).

The MFC measures the heat transfer rate of the gas flowing through the sensing tube using a thermocouple method.

If debris from the MFC installation enters this tube, the accuracy of flow detection may be compromised.

And the gas passes through the orifice in the control valve part,

These orifices are sized according to the flow rate and pressure conditions of the gas.

For example, the orifice of an MFC with a maximum flow of 5 sccm and a pressure of 30 psi would be

It can be as small as 0.01 mm , thinner than the thickness of a human hair.

​If foreign matter from the MFC installation gets into this orifice, the valve may not work properly.

Reason of Pollutions

What causes contamination while installing MFC?

The most frequent cause of problems is dust.

FMC must be installed in a dust-free environment, but when working on a panel or chassis

Before installing MFC, make sure to empty, wipe, and install the area.

For MFCs with pipe thread fittings, the user must be careful not to let the anti-leak tape enter the gas passage.

If the valve orifice is very small (less than 0.1 mm), dusted gloves (latex and rubber gloves) may be used during installation.

It is not recommended to use it.

Any contaminants, including moisture and impurities from the surrounding environment, can affect the accuracy of a thermal sensor.

Greases or lubricants used during installation may cause a chemical reaction with the gas used.

It must be carefully considered and monitored.

Any lubricant that coats the inside of the sensor capillary can distort or interrupt heat flow through the MFC.

The most common cause of FMC's continuous error is

The installation of a new MFM / MFC product on a line that has already been contaminated.

Here are some guidelines to avoid problems with it.

1. Prevention

The first thing to do is to use an inert gas to remove the defective MFC.

It is to remove impurities in products and lines.

Most systems use nitrogen (N2) or argon (Ar).

Traces of used gas and contaminants can be removed from the line.

After removing the device and checking all connections, including gas inlet and outlet of the product,

You will be able to see where the contaminants need further review.

2. Uninstall

The most common way is to use 'Spool' when installing MFC.

'Spool' is a kind of connection part used to connect MFC and pipe, and can be used to adjust the length of the entire line.

Clean through the part or remove contaminants   can be discharged.

3. Clean installation

MFC is shipped sealed in a clean bag.

As much as possible, take the product out of the clean bag immediately before installation.

Install a new FMC only on systems that have been determined to be free of contaminants.

Refer to the MFC manufacturer's specifications for the type of connection used by the system.

Filtration

Now let's talk about gas lines and filtration (filters).

Traces of oil and other contaminants and impurities may enter the gas stream under some conditions of use.

As mentioned earlier, if there are impurities in the components of the flow sensor and valve,

It can accumulate over time and affect MFCs.

One way to address this situation is to avoid contaminants or impurities.

The idea is to install an Inline Filter as shown in the photo below so that it does not come into contact with the internal components of the MFC.

When choosing a filter, it is helpful to know in advance what types of contaminants you need to prevent.

In the case of air produced by the Air Compressor, trace amounts of oil can be suspected.

If contamination by particulate impurities is suspected, a 15 micron filter can protect the MFC.

For a specific usage environment, we recommend that you test different filters to determine which one is best for your usage environment.

However, adding an in-line filter to an MFC system can cause a gas pressure drop.

For example, assuming that the gas is flowing at a high pressure of 500 psig at a flow rate of 20 slm at 0'C and 760 Torr,

A 15 micron filter drops pressure by about 0.5 psi.

If MFC contamination is suspected while using the filter, the filter should be replaced.

If contaminants contaminate the MFC, it means that the filter is damaged.

A damaged filter must be replaced immediately as it continuously releases contaminants into the line.

​Pressure Regulation

Above we have explained how the valve orifice is selected according to the gas and flow and pressure conditions.

MFC's control feedback loop is designed to handle a variety of pressure conditions, but

If you use a pressure regulator like the picture below,

More accurate flow measurement / control is possible by flowing gas at a pressure appropriate to the pressure set in the MFC.

A properly selected pressure regulating valve can be a very good investment in your gas line.

The MFC label pictured below specifies the Up Stream and Down Stream pressures.

The pressure regulating valve should adjust the MFC to the maximum pressure that can be expected in the gas line.

A pressure regulating valve that is smaller than the flow rate of the MFC lowers the flow rate into the MFC.

It can interfere with accurate flow measurement/control.

When purchasing a pressure regulating valve, make sure the pressure regulating valve can properly handle the flow and pressure in the gas line.

Review the environment of use and ensure that there are no gas compatibility and use problems.

​Line Size

Another consideration when installing MFC is line size (diameter).

If the diameter of the line leading to the MFC is narrow, the pressure in the gas line will drop very significantly and

As with a small pressure regulating valve installed, the gas supply to the MFC may be less.

Also, because the MFC has to control the flow, the pressure drop between the MFC and the pressure regulator

The pressure at the top of the MFC can fluctuate and this can also prevent accurate measurement/control.

When multiple MFCs are installed in the gas line, the fluctuation of the upper end pressure by one MFC is

​May affect other MFCs.

​Straight Piping

As shown in the photo below, if an additional pipe is installed to measure a large flow rate of 50 slm or more,

It is important to install a straight pipe at the top of the flow meter.

This will allow the gas to enter the flow meter's gas bypass.

It helps to reduce the amount of turbulence, resulting in better accuracy and repeatability.

The upper part of the pipe needs a straight length that is at least 5 times longer than the diameter of the pipe installed in the MFC.

For example, when using a thermal mass flow meter with an additional 3 cm diameter tubing.

The minimum recommended straight length for MFC top pipe is 15 cm.

​Electrical Connections

There are a few things to keep in mind when wiring the MFC.

First, it is recommended to use a shielded cable in an environment with Electric Magnetic Interference.

Many MFCs reject electromagnetic interference to a certain level, but they can only reject electromagnetic interference if wired correctly.

In some cases, one of the MFC's connecting pins may need to be connected to ground.

Be sure to review the MFC manual for specific wiring requirements related to MFC.

You must also use a power supply that can provide sufficient power.

This is because if the output of the power supply drops, the flow signal output may become inaccurate.

4-20mA​

4-20mA input and output is a very important topic.

4-20mA input and output should be installed in an environment where the MFC is very noisy, or

This can be useful if you are away from the power supply.

0-5 VDC input and output may cause problems due to increased noise or signal loss depending on cable length.

If the MFC is more than 15m away from the power supply, you should consider using 4-20mA.

Calibration Interval

ISO 17025 sets out "General requirements for the competence of test and calibration laboratories"

It is an international standard for "General requirements for the competence of testing and calibration laboratories".

According to the standard, a calibration certificate or label contains recommendations for calibration intervals.

It is stated that it should not be included.

So, when should the MFC be calibrated?

MFC installed in a system using a chemically inert clean gas shall comply with the manufacturer's specifications.

Performance can be guaranteed for many years.

However, if the MFC is exposed to aggressive gases such as acids or corrosives, or if there is a possibility of contamination.

It is possible that the accuracy of the flow measurement will decrease over time.

In general, calibration is carried out every year at the beginning, and calibration data for accuracy after one year is not available.

If expectations are deviating, you should consider reducing the calibration interval.

After that, when the calibration data for accuracy comes in 2-3 times within the standard,

​You may want to increase the calibration interval again.

Additional Questions

Gas flow measurement can have a number of unique requirements depending on the environment in which it is used.

If you have any difficulties or questions about using MFC in various usage environments, please feel free to contact us.

We are always ready to think through and solve your difficulties together.