Developed in the 1960s, mass flow devices are important components in systems that generate the specialty gas standards needed to calibrate a wide range of instruments that analyze gases.
Mass flow devices create a signal that is directly proportional to the mass flow of a gas. The specific heat of a gas is unique to a specific gas and therefore, mass flow devices are essentially independent of temperature and pressure fluctuations. Hence, the signal generated by these devices is stable and very accurate.
If the signal from a mass flow device is used to indicate flow, it is grouped as a mass flowmeter (MFM). If the signal from a mass flow device is used along with a controlling valve and a reference signal, it is grouped as a mass flow controller (MFC).
When a number of MFCs are operated in parallel, and are electronically controlled within tight limits, the resultant gas mixtures are highly accurate. This is the principle behind a traditionally used filling technique known as “Dynamic Blending.”
Lead time and price are key considerations when a certified gas mixture is ordered from a calibration gas producer. These factors can influence whether or not the analytical instrument is properly calibrated.
Where urgency and volume are critical considerations in meeting calibration needs, the purchase of a dynamic gas blender may be more cost-effective than buying the calibration gas. Compared to other types of systems that create calibration standards, dynamic gas blenders are favored more because they can, in certain cases, produce the end product at a lower cost and more quickly, while ensuring high accuracy.
Some dynamic gas blenders, which are used to create mixtures for calibrating ambient air pollutant monitors, also create precision ozone levels. This ozone can be subsequently used to calibrate oxides of nitrogen (NOx) monitors and ambient ozone monitors that use gas phase titration with ozone.
There are many factors that can impact the overall accuracy of an MFC-based dynamic gas blender. These can range from choosing the MFC to the care taken by the end-user in operating and sustaining the equipment.
Qualifying and Selecting a Mass Flow Controller for a Dynamic Gas Blender
The flow sensor is the heart of a mass flow controller, just as the MFC is the heart of a high accuracy gas blender. Thermal mass flow controller is the most common and standard method for controlling and measuring mass flow. As observed before, this type of device can measure the temperature shift (or differential) that takes place in a small tube as heat transfers to and from the gas.
Figure 1 shows a thermal mass flow sensor. As gas flows through the sensor tube, it collects heat when it enters the tube and transfers some amount of heat back to the tube as it exits. Two independent temperature sensors, TC1 and TC2, measure the temperature differential in the tube. The sensors generate electrical signal that is amplified and converted into a linearized signal that is generally recognized by instrumentation.
When choosing an MFC for a gas blender, the sensor data provided by the MFC manufacturer should be reviewed. This data should include long-term repeatability and stability. In order to ensure that no potential inherent flaws are present, gas blender manufacturers need to have on hand all of the research data on the type of sensor used.
Whenever flow controllers are calibrated, they should be calibrated along with the process gas that is to be analyzed, but this is not always possible. If the process gas cannot be employed for calibration, a surrogate gas can be used as an alternative. During this calibration, a conversion factor (K) is applied to make sure that the flow through the MFC is accurate with regards to the process gas.
Figure 1. A thermal mass flow sensor.
It is necessary for the manufacturer of the dynamic gas blender to confirm that the MFC device supplier has committed adequate time for the proper development and evaluation of accurate K-factors for all of the gases that will be employed with the blender. K-factors should ideally be derived empirically. Shown in Figure 2 is a partial list of one company’s K-factors and the methods employed to derive them. (NOTE: A “K” factor is defined as the ratio of the actual gas flow rate to the equivalent nitrogen flow rate. To obtain the equivalent nitrogen flow rate, divide the actual gas flow rate by the K factor. Example: The K factor for argon= 1.45. QEQUIVALENT NITROGEN =(QACTUAL GAS / K) = 101.94 slpm /1.45 = 70.3 slpm.)
The most critical step that a gas blender manufacturer can take when choosing an MFC is to assess one set of MFCs, over a period of several months, from each qualifying manufacturer. Stability, repeatability, and short and long term drift are the most important characteristics to analyze. Figure 3 shows an evaluation that spanned seven months. On the day of each test, the MFC was flow-tested, using a primary flow standard at four points, across the full-scale range. In order to determine the average repeatability of each test, 10 repeatability points were taken at all four major flow rates.
Figure 2. Gas conversion factors for HFC-202.
Figure 3. An evaluation spanning seven months. On the day of each test, the MFC was flow-tested, using a primary flow standard at four points, throughout the full scale range; 10 repeatability points were taken at all four major flow rates to establish the average repeatability of each test.
Developing and Manufacturing an Accurate Dynamic Gas Blender
Beginning with a highly accurate MFC is just one part of developing an accurate gas blender. Another is how well a company develops the controls and software that will validate the accuracy chain.
If MFCs controlled by an analog voltage are used, then a minimum of 12-bit ADC and DAC should be designed into the instrument. Also important is the accuracy of the precision reference voltage applied to these devices. With 12-bit resolution, increments of 0.0012 VDC are applied to the MFC in order to obtain precision control. The MFCs as well as the equipment should be CE certified as this ensures that the instrument will not be vulnerable to electrical interference. The system should be designed using MFCs, 316 SS electropolished tubing, and fittings. Alternate seal and tubing materials may be compatible with different types of gases. Considerable costs can be saved through proper selection.
Improving the MFC specifications should be the main focus when it comes to developing software to control a gas blender. Some blender manufacturers, who use thermal mass flow controllers, have been known to install an MFC as it was received from the manufacturer without performing any additional calibration. If users depend on the accuracy of an MFC manufacturer’s specification without validating them with their own tests, they will end up with a blender with faulty specifications. A good gas blender manufacturer always uses a calibration standards lab to verify the factory specifications of an MFC.
Double Check the Flow Standard
An accurate flow measuring instrument, which has a specification of 0.2% of reading or better, should be used to calibrate each MFC installed in a gas blender. A minimum of 11 calibration readings should be collected on the flow standard in order to obtain the highest, most accurate results with a gas blender. The calibration data should be subsequently entered into the gas calibrator software and used as a reference table. Next, a flow-correcting algorithm, for example linear interpolation should be applied by the blender software to enhance the linearity and accuracy of the MFCs. Further, the gas blender should have an option that enables the entire gas path to be purged with an inert gas. The same principle should be applied to K-factor correction. After an MFC manufacturer is chosen, the blender control software should use that manufacturer’s library of K-factors to make corrections as alternate gases are employed.
One blender’s MAINTAIN PORTS mode is shown in Figure 4. This mode allows the operator to enter the gas types being fed into the MFCs and equipment’s input ports. The software features a built-in standard that allows it to automatically compare the process gas to the original calibration gas for each MFC installed (up to 9 MFCs). Each gas bottle can have up to (???) the individual component gases in a balance gas (usually nitrogen). For all the gases corresponding to the calibration gas of the MFC, the system automatically calculates and applies the K-factor. This figure also illustrates three component gases in a nitrogen balance.
Figure 4. Model 2000 application of K-factor correction.
The Responsibility of the End User to Maintain the Accuracy Chain
The end-user and owner of the gas blender are responsible for maintaining accuracy. It is important to ensure input gases, which are fed into any gas blender, are clean and free from moisture. Most anhydrous gases are inert and hence they are not affected by the standard materials used in a gas blender, but in the presence of moisture these gases can become corrosive.
Contamination can develop eventually and, if contaminants are carried downstream into the MFC sensor, they can make the MFCs useless. Moreover, contamination on the sensor tube walls reduces the effectiveness of the heat transfer between the sensor tube and gas and may even block the tube’s small inner diameter, usually rendering the MFC completely inoperable. NOTE: Ensure that all gas ports are capped when they are not in use. This makes sure that particulates, moisture and other airborne contaminants will not enter the system plumbing.
Cylinders with 100% pure gases should be used if possible. A pre-mixed gas that is not properly handled or checked can be the first source of contaminants that lead to errors in the gas blending system. If uncertified cylinders of premixed gases are used, they can introduce very large errors; these errors cannot be offset by a dynamic gas blender. If there is no choice other than to use a premixed gas, certified cylinders should be used for instrument calibration procedures, particularly when regulations or guidelines mandate it.
The most critical responsibility of the end-user is that they follow all manufacturer-prescribed calibration and maintenance precautions stipulated for that equipment. Even if regulations and guidelines do not mandate calibration of the gas blender used to create the calibration mixture, the dynamic blender should be calibrated periodically — usually once every year. Following all these basic guidelines will guarantee an accurate and long life of an MFC-based dynamic gas blender.
This information has been sourced, reviewed and adapted from materials provided by Environics, Inc.
For more information on this source, please visit Environics, Inc.