【Introduction】Many industrial automation (IA) and manufacturing facilities frequently require the use of gases such as air, oxygen, nitrogen, hydrogen, helium and argon in a variety of processes and applications. Specific uses include cleaning, cutting, welding and chemical manufacturing. In many cases, delicate equipment and chemical processes require extremely fine control of gases to avoid difficult-to-diagnose equipment failures or process failures. Additionally, excess gas flow can result in loss of efficiency, as well as additional costs associated with gas container replacement.
Many industrial automation (IA) and manufacturing facilities frequently require the use of gases such as air, oxygen, nitrogen, hydrogen, helium and argon in a variety of processes and applications. Specific uses include cleaning, cutting, welding and chemical manufacturing. In many cases, delicate equipment and chemical processes require extremely fine control of gases to avoid difficult-to-diagnose equipment failures or process failures. Additionally, excess gas flow can result in loss of efficiency, as well as additional costs associated with gas container replacement.
Measuring precise gas flow in standard liters per minute (SLM) is an interesting problem because measurement accuracy is affected by pressure and temperature and the accuracy of the sensing mechanism. Standard mass flow controllers are often used to control gas flow, but these controllers lose accuracy over time and require periodic calibration during use, increasing lifetime costs. Advances in technology have led to the use of microthermal measurements of gas temperature to precisely determine specific SLM volume flow rates.
This article discusses the importance of industrial gases and the problems caused by inaccurate gas flow control. It then introduces Sensirion’s mass flow controllers with advanced gas flow sensing technology and explains how to set up and use them effectively to reduce overall cost while increasing efficiency, reliability and productivity.
Industrial gases require precise control
Industrial facilities use various gases for various purposes depending on their properties. Some systems, such as heating, ventilation, and air conditioning (HVAC) systems, can tolerate small errors in gas flow control, but precision equipment, such as chemical vapor deposition (CVD), gas and liquid chromatography, and mass spectrometers, require extremely precise gas control , to avoid equipment failure or process failure. These types of failures are difficult to diagnose and can result in lengthy and expensive downtime.
Flammable gases such as hydrogen, acetylene and butane are mixed with oxygen to produce heat, flames or controlled explosions. These gases must be mixed together at the appropriate concentration for the specific process. Just like in a car’s internal combustion engine, a too lean or too rich combustible gas mixture can create a flame that is not at the right temperature, resulting in inefficiency or failure.
Compressed gases such as oxygen, nitrous oxide and air are used as oxidants and also to assist combustion. Too little compressed gas can cause chemical processes to fail, while too much gas can lead to inefficiencies, wasted gas, and increased costs.
Inert gases, such as argon, carbon dioxide, and nitrogen, are commonly used in safety-critical operations, such as fire or oxidation control, and are also used to suppress some chemical reactions. Too little gas can cause a fire suppression campaign to fail, while too much can waste gas and increase associated costs.
Control Gas Flow with Industrial Mass Flow Controllers
Mass flow controllers are used to measure the proper amount of gas. In its simplest form, the mass flow controller is completely manual and requires no power. The volume of gas is adjusted by turning the dial to the appropriate set point. However, manual mass flow controllers can only measure volume at ambient temperature and cannot account for volume changes due to changes in gas pressure or temperature. For this reason, Electronic mass flow controllers are used for precise control of gases.
The unit of measure SLM for industrial gas volume flow is defined as the gas flow rate of 1 liter per minute at a standard gas temperature of 0°C/32°F and a standard gas absolute pressure of 1 bar. The volume of any gas changes based on temperature and pressure, so a mass flow controller needs to be able to account for changes in ambient conditions and change the flow accordingly. Most electronic mass flow controllers are calibrated against the target gas to provide accurate flow control over temperature and pressure changes, but this calibration tends to drift over time and requires periodic recalibration in use. This increases maintenance effort, while neglecting calibration reduces the efficiency of the system.
Precision mass flow controllers that require no in-service calibration
The solution in this regard is a family of precision mass flow controllers that do not require in-service calibration. Sensirion’s SFC5500 series of mass flow controllers is one such solution (Figure 1). The SFC5500 Series uses gas temperature microthermal measurement technology to accurately determine precise SLM volume flow independent of gas temperature and pressure variations.
Figure 1: The Sensirion SFC5500 series of mass flow controllers uses microthermal CMOSens technology to accurately measure the volume of gas passing through a gas flow channel, independent of temperature or pressure changes. (Image credit: Sensirion)
Sensirion’s gas volume flow technology, called CMOSens, accurately measures the volume of gas passing through a gas flow channel. CMOSens is a general term used by the Sensirion approach, which combines sensing, signal conditioning, and processing functions on a single CMOS device, enabling precise timing control in a small device (Figure 2, top).
Figure 2: CMOSens combine sensing, signal conditioning, and processing technologies on a single CMOS device (top). In gas flow measurement applications (below), temperature sensors and associated processing make microthermal measurements to ensure accuracy. (Image credit: Sensirion)
In the gas flow measurement implementation using CMOSens, temperature sensors are placed upstream and downstream with an adjustable heater mounted on a pressure-stabilizing membrane in between (Fig. 2, bottom). The third temperature sensor is used to detect the temperature of the gas.
The flow of gas and heater gas through both sensors produces temperature readings on both sensors. These two readings, along with the gas temperature sensor reading, are taken by an integrated signal processor and combined with stored gas-specific calibration settings to produce an accurate volume flow reading independent of pressure and temperature .
The typical settling time of the SFC5500 mass flow controller is less than 100 milliseconds (ms), allowing accurate readings to be obtained under rapidly changing temperature, pressure and flow conditions. Since CMOSens technology can compensate for temperature and pressure, this configuration has zero drift over time, so unless the target gas changes, the SFC5500 never needs to be recalibrated in the field.
CMOSens based mass flow controller
An example of the SFC5500 mass flow controller is the SFC5500-200SLM. It is a high capacity flow controller designed for use with air, nitrogen and oxygen and calibrated for these gases. Nitrogen and air gases are supported, with a maximum full-scale flow rate of 200 SLM and a specified control accuracy of 0.10% of full-scale flow rate or 0.20 SLM. Oxygen flow is supported, with a maximum full scale flow of 160 SLM and a specified control accuracy of 0.20% of full scale flow or 0.32 SLM. Sensirion states that the accuracy of the unit may decrease slightly when the gas flow exceeds 100 SLM. The SFC5500-200SLM is designed in such a way that it allows precise control of air or oxygen without the need for in-service calibration.
The Sensirion SFC5500-200SLM connects to the host computer via a common RS-485 DB-9 connector. Additionally, DeviceNet and IO-Link communications are supported. Legris compression fittings are used for gas inlet and outlet connections, with an outside diameter of 10 millimeters (mm). This is compatible with standard 10 mm gas fittings.
To support other gases, Sensirion has introduced the SFC5500-10SLM multi-gas mass flow meter. In addition to air, nitrogen and oxygen, this controller also supports hydrogen, helium, argon, carbon dioxide, nitrous oxide and methane. It supports a maximum full-scale flow of 10 SLM for all gases except nitrous oxide, argon, and carbon dioxide, which are 5.0 SLM full-scale. Worst case accuracy is 0.30% of full scale flow. It supports the same communication interface as the SFC5500-200SLM. Gas inlet and outlet connections use Legris compression fittings with a 6 mm OD, compatible with standard 6 mm gas fittings.
The SFC5500-10SLM provides the flexibility to support multiple gases with one controller, simplifying inventory. The controller must be configured and pre-calibrated for the target gas it controls before it is put into operation. It cannot be used for other gases without reconfiguration.
Configure and develop
The SFC5500 mass flow controller must be preconfigured with the target gas before it is put into operation. Since different gases have different densities and properties, each gas requires different settings and calibrations. To aid in configuration, calibration and evaluation, Sensirion provides the EK-F5X evaluation kit for the SFC5500 series (Figure 3). Note that this kit does not include a mass flow controller.
Figure 3: The Sensirion EK-F5X evaluation kit allows developers to configure, calibrate, and evaluate the SFC5500 mass flow controller (not included in the kit) before putting it into use. (Image credit: Sensirion)
To configure the SFC5500 for service, it must first be connected to the gas to be controlled. The EK-F5X evaluation kit comes with a custom DB-9 cable that plugs into the DB-9 connector on the top of the SFC5500. The DB-9 cable splits into an AC adapter for powering the SFC5500 during operation, and a USB connector for connection to a host computer. The SFC5500 device driver for the host computer and the SFC5000 viewer software are provided on the included USB flash drive, both of which must be loaded on the host computer before connecting via USB. Plug the SFC5500 into power first, then connect the USB connector to the host computer. After the computer becomes familiar with the USB-connected SFC5500 with the usual beeping sound, the SFC5xxx viewer software starts and asks for configuration of the COM port. The software then displays all available calibrations for each gas supported by the specific SFC5500, as well as calibrations in use (Figure 4).
Figure 4: The Sensirion SFC5500 Viewer software provides numerous calibration options for each gas supported by the connected unit. (Image credit: Sensirion)
The SFC5xxx Viewer software displays the connected SFC5500 variant with its serial number and firmware version, as well as the COM port configuration. The System tab is selected at startup and displays available flow calibrations highlighted in green and calibrations in use highlighted in red. To change a calibration, right-click the calibration for the target gas and select “Load Calibration”. The connected SFC5500 is now calibrated for the selected gas. The calibration is stored in EEPROM, so it does not need to be recalibrated after a power cycle. Recalibration is only required if the device is used for a different gas.
After calibration, select the Data Display tab. This tab is used to set and control the gas flow, either for a constant flow rate or for generating a custom waveform to vary the flow. The SFC5500 is now calibrated and configured for automatic operation.
For more complex applications where the flow must be changed programmatically, the SFC5500 can be controlled by DeviceNet. The DeviceNet tab is used to configure the DeviceNet MAC ID and baud rate. Sending 0x0000 for no flow, 0xFFFF for full-scale flow, or any value in between to the unit makes it easy to control flow remotely via DeviceNet. This allows for complex flow control operations and for quick and easy remote shut-off of airflow, which is useful in emergency situations.
Epilogue
Precise control of industrial gases is critical in industrial processes. While calibration drift may require periodic recalibration to maintain accuracy, new gas measurement techniques can eliminate this need, resulting in increased efficiency, reduced maintenance, and overall cost savings in the long run.
(Source: Digi-Key, by Bill Giovino)
The Links: SEMIX603GB12E41P G242CX5R1RC