Sunday, November 30, 2014

Selection Criteria for Mass Flow Meters

mass flow controller
Brooks Mass Flow Controller
There are four primary areas to consider when specifying a thermal mass flow controller (MFC).
  • Flow range
  • Gas conditions
  • Desired accuracy
  • Communications
Additionally, you must take in to consideration the interior finish for high and ultra high purity applications, materials used in the flow path, the environment (high temp ambient conditions, water-tight, dust-tight, etc …), mounting position and serviceability.

Introduction

Mass flow controllers are used to measure and control the flow of gasses in a process. The overall flow rate must be considered since some processes require very low flows measured in SCCM (cubic centimeters per minute). Larger flows are usually expressed in SLPM (standard liters per minute) or SCFM (standard cubic feet per minute). Many devices able to measure flows of 200 SCCM up to 30 SLPM using the same body size, however, the internal components are changed to allow a more specific flow range. In most cases, once you establish your maximum flow, you have the ability to measure and control down within a certain band. A 10 to 1 turndown ratio is fairly common, and 50:1 or 100:1 is available from some of the leading MFC manufactures. Setting a maximum flow very low such as 10 SCCM or 3 SCCM while being able to measure and control down to 2% is remarkable. When desired flow reaches more than 30 SLPM (or 1 scfm), the physical size of the MFC body usually changes to allow for greater flow through larger passageways inside the MFC. When flow becomes higher than  200 SLPM, the MFC body size increases significantly to allow for controllable flows up to 2500 SLPM  and measured flows up to 9000 SLPM, without the use of a control valve.

The gas being controlled/ measured by the mass flow controller is an important consideration in applying the right MFC. If you are flowing one simple, common gas such as air, oxygen, nitrogen, helium, hydrogen, argon, CO or CO2, a more simple or basic design MFC can be used. If the process gas is very reactive, volatile, corrosive or erosive, your selection of an MFC is more involved. Modifications such as interior polishing, and VCR or VCO end connections can be specified to prevent process gas contamination or reactions with moisture.

Process gas pressure also needs to be considered. Some manufacturer’s MFCs are limited to 100 psi or 500 psi maximum working pressures. Industrial MFCs are usually standard at 1500 psi (100 bar) and can be made to withstand 4500 psi (300 bar) pressures. These requirements are common in MFC applications found in catalyst research, hydrogenation of food or drugs, or many petrochemical processes.

Likewise, if your process operates under vacuum, an MFC designed for vacuum services is required. Gases behave differently at positive pressures versus high vacuum, and calibrating a mass flow controller for vacuum service requires precision. The temperatures of the gas is also very important. Most gases are measured and controlled at relatively mild temperatures, typically, 140-160°F, and, if required by the process, elevated after the MFC because of the temperature limits of the measuring technology inside the MFC.

Accuracy is an important term and can be easily misunderstood. Keep in mind the difference between “percent of full scale” accuracy statements and “percent of rate” accuracy. Some processes require less precision, but excellent repeatability. Also keep in mind different designs of mass flow controllers carry with them different error factors.

mass flow controller flow chart
Accuracy comparison % full scalle vs. % rate
(Courtesy of Brooks Instrument)
Accuracy claims of +/- 1 % of full scale on a 100 scam MFC has a error of 1 SCCM at any flow. If the flow were to be set at 50%, a 1% FS error would actually be 2% of that rate. If the flow were at 20%, the same 1 SCCM deviation would now be a 5% error. Devices with error statements given in % fs are not as accurate as those given in % rate. Let us look at a device with a stated accuracy of +/- 1% of rate from 100% flow down to 20% flow and +/- 0.2% of full scale below 20 percent flow. Using the same example, if the 100 SCCM MFC were flowing at 50% the maximum error would be 1⁄2 SCCM or 1 % of rate.

If the flow were set at 20%, the error would be 0.2 SCCM or 1% of that rate. Please see Figure 1 which shows the error bands for % fs and % of rate.

Communicating electrically with an MFC can be done in a number of ways. Most common are an analog signal of 0-5 volts DC, a current of 4-20 mA, or an RS 485 protocol connected to DeviceNet, ProfiBus, or Foundation FieldBus.

At the heart of the most accurate MFCs is a Hastelloy® sensor to protect against aggressive gases. A feature called MultiFlo by Brooks Instrument overcomes the viscosity and density change limitation of many thermal MFCs. The Brooks MultiFlo uses a database built from thousands of native gas runs to establish correction functions that account for both thermal and physical differences among gases making the Brooks GF Series among the most accurate and flexible MFCs/ MFMs available today. Its programming is simple and fast and can be reprogrammed without being taken out of service. This technology provides great value to:

  • OEMs will reduce the number of gas and range specific MFCs that they inventory.
  • Solar, biotech, CVD, plasma, glass, web coating, nano- technology, vacuum processing and similar large users of mass flow meters and mass flow controllers will greatly reduce their gas- and range-specific spares inventory.
  • R&D, research, and laboratory users can quickly change experiment conditions and achieve much better actual process gas accuracy vs. traditional mass flow devices.

Summary

Before choosing a mass flow meter always carefully consider the flow range, the gas conditions, the desired accuracy, and what type of communication or process signal you’ll require.

This blog entry is excerpted from Brooks Instrument "Choosing the Right Mass Flow Controller for your Application" written by N.K. Glover, Brooks Instrument, Snellville, GA. For a copy of the entire document, visit this link.