Brooks Instrument Blog

Thermal mass flow controllers are traditionally calibrated for a specific gas, a desired flow range, and a set of operating conditions. Over time, the use of conversion factors based on a ratio of specific heats between gases came into use as a way for users to configure a single mass flow controller for multiple gases. This method of configuring a mass flow controller for multiple gases is still common today – you’re using it if your device lets you select a gas by: rotating a knob, pressing a button on a display, or sending an RS232 command to the device.

Accuracy is the primary issue with this method of conversion. Converting flow rates between the calibration gas and another gas based on a ratio of specific heats can result in a mass flow control error of 5-6%. This error is the result of the conversion method because it ignores other property differences that exist between gases in the real world. If you’re changing the gas on your device with one of the actions above, ask the manufacturer of your mass flow controller what the accuracy of the device is for a gas other than the calibration gas.

P.S. If you’re told that such a device is linear in all the available gases and thus the mass flow accuracy doesn’t change when the gas is changed, RUN! This is not physically possible. Feel free to contact us for comparison data.

Multiflo by Brooks Instrument is a leap forward in configuring a mass flow controller for multiple gases because it converts based on gas differences in specific heats, densities, and viscosities. Multiflo-Capable mass flow controllers cut the conversion flow control error in half compared to devices that convert gases based on a specific heat ratio alone.

This video demonstrates how a Multiflo configuration is performed on a mass flow controller. We welcome your thoughts or questions in the comments below.

You can find more information on Multiflo-Capable mass flow controllers on the Brooks Instrument LinkedIn company page. Your local Brooks product expert would also be happy to help you configure a Multiflo-Capable mass flow controller for your applications using the information entered on this form.

If you’d like to read a bit more about instrumentation and process control, feel free to check out more of my contributions summarized on my Google Plus profile.

In my last post, we discussed several applications for mass flow controllers where precise flow control is needed despite backpressure changes. I introduced a flow effect called choked flow, which many of our customers use in these applications to ignore downstream pressure changes. This is also referred to as sonic flow or critical flow.

To my flow-savvy readers: You’ll notice that I’m discussing choked flow in conceptual terms rather than demonstrating complex formulas and calculations. Don’t be alarmed! Feel free to post any additional thoughts you have on this topic in the comments below.

The sketch to the right shows gas flow through a typical orifice. The green shaded areas are high pressure, low velocity flow areas, and the blue area is a low pressure, high velocity flow area. Inlet gas flow speeds up as it compresses to pass through the orifice, then re-expands and slows back down on the outlet side. The flow rate through the orifice is primarily set by the inlet and outlet pressures, as well as the diameter of the orifice opening. Gas temperature also plays a part.

A gas expands in a space as its molecules collide with whatever else is present. (pipe walls, other gas molecules, etc.) Every gas expands at its own rate, and pressure increases in a gas are a result of squeezing more gas molecules into the same amount of space. Applying these factors to the orifice flow pictured, gas expansion causes some of the molecules that are expanding in the green area on the outlet side to collide with and deflect the “fast” molecules in the blue area. If the pressure rises in the green area on the outlet side, it’s because there are more gas molecules present in the same amount of space.

More molecules in the green outlet area mean that more molecules deflect the “fast” molecules in the blue area. This reduces flow velocity in the blue area, which is what reduces the flow rate passing through an orifice at higher backpressures. If the pressure drops in the green outlet area, it means that fewer molecules are present in that space, which results in fewer deflections of “fast” blue molecules. This causes a higher velocity in the blue area, and thus a higher flow rate when the backpressure drops.

Choked flow occurs when the flow velocity in the blue area reaches the speed of sound. At this velocity, the molecules in the blue area are essentially traveling faster than the molecules in the green outlet area are expanding. So deflection between molecules at the blue/green border doesn’t reduce velocity in the blue area. With a fixed inlet pressure, the outlet pressure can change over a wide range without changing the mass flow rate as long as the conditions to maintain choked flow remain in place.

So how can we reach the conditions needed to maintain choked flow? We’ll cover that in our final post in this series.

Where did the names choked flow, sonic flow, and/or critical flow come from? Please post where you think one of these names came from in the comments. The first poster that correctly lists the reason for each of the names will win a 4 GB jump drive in the shape of a mass flow controller.

If you’d like to read a bit more about instrumentation and process control, feel free to check out more of my contributions summarized on my Google Plus profile.

While we were cleaning out our offices a few weeks ago a co-worker of mine handed me a great Norman Rockwell oil painting that we used to have hanging in one of our hallways. It was taken down when we were updating our facility. He wasn’t sure what to do with it. The reason we used to have it hanging up in the walls at Brooks (and the reason I’m going to find it another nice home on our walls) is because in the background of the picture you can see some Brooks Sho-Rate variable area flow meters between the two men on the left. Brooks provided Sho-Rate flow meters to  NASA for some of the first missions into space. Read more…

We always like to take the opportunity to share our customer success stories. Here’s one from the land Down Under where Brooks worked with Measurement Plus Pty. Ltd. to install a new gas mixing system at a research institute in Melbourne.

Researchers at the institute were using N2, CO, CO2 and Ar to stabilize a reaction chamber. However, at some point during the process, they wanted the ability to flow in a mixture of these gases. At the time, the institute was using older Brooks thermal mass flow meters inside a self-designed panel. The panel and its components were about 30 years old, rusted and looked like a bird’s nest. Read more…

This has been a product that we have been working on and perfecting on over the past several months and I’m extremely excited to finally formally launch the new GF40 and GF80 thermal mass flow controller and flow meter! These mass flow controllers provide outstanding performance, reliability and flexibility in many gas flow measurement and control applications. One of the best features of this new mass flow controller is the patented 4th generation MultiFlo gas and range configurability. MultiFlo programming is simple and fast – a new gas and range can be programmed under 60 seconds plus the device can be programmed without removing it from service or disconnecting the device from any process or tool control system.

The GF40/GF80 Series mass flow controllers features fast sub one second settling times and corrosion-resistant Hastalloy sensor tube for long-term stability. The superior valve technology provides minimum leak-by, maximum turndown and fast response which reduces overall gas panel cost and increases throughput. Additionally, the measurement accuracy of every device is verified using traceable primary calibration standards.

Want to learn more? Download the datasheet or contact your local sales engineer.

Yesterday, during the Chem Show in New York City, our very own Steve Kannengieszer was interviewed by Chemical Processing magazine‘s Senior Digital Editor, Traci Purdum. We’re still waiting for the video but I thought I would share some of the Q&A’s that were covered. If you are at the Chem Show, stop by and see us at booth 303! Read more…

During the same exact week that we’ll be at the Chem Show, we’ll also being exhibiting at the Fuel Cell Seminar, dividing and conquering! You’ll see our industry experts Jan Christensen and Nigel Glover hanging out and ready to talk flowmeters with you at our booth, which is booth #412.

We have a lot of great new products to showcase at this years Fuel Cell Seminar like the SLA5800 Series digital thermal mass flow controllers and mass flow meters. The SLA5800 Series elastomer sealed digital thermal mass flow meters and mass flow controllers offer unparalleled flexibility and performance for the fuel cell industry. They offer faster response, better accuracy and improved control over traditional analog devices. Industry leading repeatability ensures a stable process even under changing conditions. The devices’ self diagnostics and alarms eliminate downtime. These devices have a proven track record in many fuel cell test stands, so stop by and ask Nigel or Jan about them. Read more…

It’s time for the Chem Show again. For Brooks, it’s nice that this show is right around the corner from our headquarters in Hatfield, PA … just a simple train ride up to New York. I’m looking forward to the Chem Show and if you are going, be sure to stop by our booth, booth #303.

We have a lot of new stuff to share with everyone at the Chem Show this year. We have our new vacuum capacitance manometers, the CMC and XacTorr Series vacuum capacitance manometers. These capacitance diaphragm gauges incorporate industry-leading features to improve measurement reliability, minimize drift, resist diaphragm contamination and minimize thermal effects in vacuum measurement applications. Read more…