Many industrial automation (AI) and manufacturing facilities often require the use of gases such as air, oxygen, nitrogen, hydrogen, helium, and argon for various processes and applications. These uses include cleaning, cutting, welding, and chemical manufacturing. In many cases, precision equipment and chemical processes require extremely fine control of the gas to avoid difficult-to-diagnose equipment breakdowns or process failures. Additionally, excess gas flow can cause a loss of efficiency, along with additional costs associated with gas container replacement.
The accuracy of the gas flow, measured in standard liters per minute (SLM), is an interesting problem as the accuracy of the measurement is affected by pressure and temperature, as well as the accuracy of the sensing mechanism. Standard mass flow controllers are commonly used to control gas flow, but they can lose accuracy over time and require periodic calibration while in service, increasing lifetime cost. Technological advances have led to the use of microthermal measurement of gas temperatures to accurately determine the precise volumetric flow of the SLM.
This article analyzes the importance of industrial gases and the problems derived from an inaccurate control of the gas flow. Next, he examines Sensirion's mass flow controllers with advanced gas flow sensing technology and explains how to configure and use them effectively to reduce total cost and improve efficiency, reliability and productivity.
Industrial gases need precise control
Industrial facilities use a variety of gases for various uses based on the properties of the individual gases. Some systems, such as heating, ventilation, and air-conditioning (HVAC) systems, can be forgiving of small errors in gas flow control, but precision equipment, such as chemical vapor deposition (CVD), gases and liquids and mass spectrometry, require extremely precise control of gases to avoid equipment breakdowns or failed processes. These types of faults are difficult to diagnose and can result in long and costly downtime.
Flammable gases such as hydrogen, acetylene, and butane mix with oxygen to create heat, flame, or a controlled explosion. The gases must be mixed in the proper concentration for the process. As in a car's internal combustion engine, a flammable gas mixture that is too lean or too rich can produce a flame of the wrong temperature, leading to an inefficient or failed process.
Compressed gases such as oxygen, nitric oxide, and air are used as oxidizing agents and also to aid combustion. Too little compressed gas can lead to a failed chemical process, while too much gas causes loss of efficiency, wasted gas, and increased costs.
Inert gases, such as argon, carbon dioxide, and nitrogen, are often used for safety-critical operations, such as fire control or oxidation, and also to suppress some chemical reactions. An insufficient amount of gas can lead to a failed fire suppression activity, while an excessive amount wastes gas and increases related costs.
Gas flow control with industrial mass flow controllers
Mass flow regulators are used to dose the proper volume of gas. In their simplest form, mass flow controllers are completely manual and do not require a power source. Gas volume is adjusted by turning a dial to the proper position. However, manual mass flow controllers only measure volume at room temperature and cannot account for changes in volume due to changes in gas pressure or temperature. For this reason, electronic mass flow controllers are used for precision control of gases.
The SLM unit of measure for volume flow of industrial gases is defined as one liter of gas flow for one minute at a standard gas temperature of 0°C/32°F and a standard absolute gas pressure of 1 bar. . The volume of any gas varies as a function of temperature and pressure, so the mass flow controller must be able to account for changes in ambient conditions and vary the flow volume accordingly. Most electronic mass flow controllers are calibrated for a target gas to provide accurate flow control over temperature and pressure variations, but this calibration often drifts over time, requiring periodic recalibration. while in service. This increases maintenance, while a missed calibration reduces the efficiency of the system.
Precision mass flow controllers without in-service calibration
The solution to this is a family of precision mass flow controllers that do not require in-service calibration. Sensirion has a solution with its SFC5500 series mass flow controllers (Figure 1). The SFC5500 series uses microthermal measurement of gas temperatures to accurately determine the precise volume measurement of the SLM, regardless of changes in gas temperature and pressure.
Called CMOSens, Sensirion's gas flow technology accurately measures the volume of gas through the gas flow channel. CMOSens is a general term for Sensirion's approach that combines detection, signal conditioning, and processing in a single CMOS device for precise control over time in a small device (Figure 2, shown above).
In the implementation of gas flow measurement using CMOSens, temperature sensors are placed upstream and downstream, with an adjustable heater mounted on a pressure-stabilized membrane in between (Figure 2, bottom). A third temperature sensor detects the temperature of the gas.
The gas flow over the two sensors and the heater create temperature readings at the two sensors. These two readings, along with the reading from the gas temperature sensor, are read by an integrated signal processor and combined with the stored calibration settings for the particular gas, producing an accurate volumetric flow reading regardless of pressure and temperature. temperature.
The typical settling time of SFC5500 mass flow controllers is less than 100 milliseconds (ms), allowing accurate readings during rapid changes in temperature, pressure, and flow conditions. Because CMOSens technology compensates for temperature and pressure, this setting has zero drift over time, so an SFC5500 never needs recalibration in the field unless the target gas is changed.
Mass flow controller based on CMOSens
An example of an SFC5500 mass flow controller is the SFC5500-200SLM. It is a high volume flow controller designed and calibrated for air, nitrogen and oxygen only. Nitrogen and air gases are supported with a maximum full scale volumetric flow rate of 200 SLM and a specified control accuracy of 0,10% of full scale flow rate or 0,20 SLM. Oxygen gas flow is supported with a maximum full scale flow rate of 160 SLM, with a specified control accuracy of 0,20% of full scale flow rate or 0,32 SLM. Sensirion specifies that the accuracy of this unit may deteriorate slightly when the gas flow is greater than 100 SLM. The design of the SFC5500-200SLM is such that it allows precise control of air or oxygen without the need for in-service calibration.
The Sensirion SFC5500-200SLM connects to a host computer via a common RS-485 DB-9 connector. DeviceNet and IO-Link communications are also supported. The gas inlet and outlet connections are Legris compression fittings with an outer diameter of 10 millimeters (mm). It is compatible with standard 10mm gas fittings.
To accommodate other gases, Sensirion offers the SFC5500-10SLM multigas mass flowmeter. In addition to air, nitrogen, and oxygen, this controller also supports hydrogen, helium, argon, carbon dioxide, nitrous oxide, and methane. Supports a maximum full scale flow of 10 SLM for all gases except nitrous oxide, argon, and carbon dioxide with a full scale flow of 5.0 SLM. Worst case accuracy is 0.30% of full scale flow. It supports the same communication interfaces as the SFC5500-200SLM. The gas inlet and outlet connections are Legris compression fittings with an outer diameter of 6mm, compatible with standard 6mm gas fittings.
The SFC5500-10SLM offers the flexibility to support multiple gases with a single controller, simplifying inventory. The controller must be configured and pre-calibrated before being put into operation for the target gas being controlled. It cannot be used for another gas without being reconfigured.
Configuration and development
SFC5500 Mass Flow Controllers must be preconfigured for the target gas before being put into operation. Since different gases have different densities and properties, each gas requires different setup and calibration. To aid configuration, calibration and evaluation, Sensirion offers the EK-F5X evaluation kit for the SFC5500 series (Figure 3). Please note that the kit does not include a mass flow controller.
To configure an SFC5500 for service, it must first be connected to the gas being controlled. The EK-F5X evaluation kit includes a custom DB-9 cable that connects to the DB-9 connector on the top of the SFC5500. The DB-9 cable splits into an AC adapter to power the SFC5500 while it is running, and a USB connector for interfacing with a host computer. A USB flash drive with the SFC5500 device driver for the host computer is included, along with the SFC5000 viewer software, both of which must be loaded on the host computer before connecting via USB. The SFC5500 is first plugged into power and then the USB connector is connected to the host computer. After the usual beeps as the computer becomes familiar with the USB connected SFC5500, the SFC5xxx viewer software starts and asks to configure the COM port. The software then displays all available calibrations for each gas supported by the particular SFC5500, along with the available calibrations (Figure 4).
The SFC5xxx viewer software displays the connected SFC5500 variation with its serial number and firmware version, along with the COM port settings. The eyelash System is selected at startup and displays the available flow calibrations highlighted in green, with the active calibration highlighted in red. To change a calibration, right-click the target gas calibration and select “Load Calibration”. The connected SFC5500 is now calibrated for the selected gas. Calibration is stored in EEPROM, so there is no need to recalibrate after a power cycle. Recalibration is only necessary if the unit is used for a different gas.
After calibration, the tab for calibration is selected. data visualization. This tab sets and controls the gas flow, which can be set to a constant flow rate or a custom waveform can be generated to vary the flow. The SFC5500 is already calibrated and configured for automatic operation.
For more complex applications where the flow must be varied by programming, the SFC5500 can be controlled via DeviceNet. The eyelash DeviceNet configures the MAC ID and transmission speed of DeviceNet. Flow is easily controlled remotely over DeviceNet by sending 0x0000 to unity for null flow, 0xFFFF for full scale flow, or anything in between. This allows complex flow control operations to be performed, and allows quick and easy remote shutdown of gas flow, useful in emergency situations.
Conclusion
Precise control of industrial gases is vital in industrial processes. While calibration drift may require periodic recalibration to maintain accuracy, new gas measurement technologies can eliminate this need, resulting in greater efficiency, less maintenance and total cost savings in the long run. .
