Teledyne Hastings Instruments Blog

Thermal mass flow measurement instruments are typically separated into two main categories: Mass Flow Meters and Mass Flow Controllers. It’s estimated that over 90% of mass flow measurement instruments sold are mass flow controllers. So, it’s no surprise that most mass flow application conversations focus on mass flow controllers. Notwithstanding, there are still numerous applications for which mass flow meters are critical.

The following are common applications for mass flow meters (also known as thermal mass flow meters):

One popular application for mass flow meters is leak testing. It’s often used on products where failure is not an option. For that reason, it is widely accepted in the aerospace, automotive, and medical industries. High throughput leak detection using mass flow meters is a nondestructive testing method that offers a fast, clean, precise, and affordable solution for parts manufacturers seeking to perform leak testing on 100% of their products.

A typical test stand is shown in Figure 1. For high throughput leak detection, a pressurized manifold has multiple connections for mass flow meters. On the outlet side of each flow meter is an isolation valve and the test part. When the isolation valve is opened, test part leaks will be noted by the movement of clean dry air across the flow meter. The test part is only exposed to clean dry air.

How does leak detection with mass flow meters compare to alternative methods? It’s faster than pressure decay testing and helium leak detectors. It’s also cleaner than hydrotesting, bubble testing, and liquid penetrate testing.

To learn more, visit our Application Note:

https://www.teledyne-hi.com/resourcecenter/Application%20Notes/PB-172-High_Throughput_Leak_Testing.pdf

Common products for which high throughput leak detection has been implemented include:

• Automotive Parts such as fuel injectors, brake components, exhaust systems, emission components, fuel pumps, fuel tanks, power steering systems, air conditioning systems, engine blocks, cylinder heads, transmission castings
• Aerospace Parts such as jet engine components, heat exchangers, cabin valves, oxygen delivery systems, hydraulic systems, air conditioning systems, brake systems
• Other: medical devices, filters, space suits, and much more
  
  

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Frequently Asked Questions (FAQS)

Q: What’s the difference between a thermal mass flow meter and mass flow controller?

A: A thermal mass flow meter measures flow using a thermal flow sensor but does not include a proportional control valve and does not control gas flow. While a thermal mass flow controller will both measure and control gas flow, according to a command signal (setpoint). For accurate flow control, the controller measures and adjusts the gas flow using a precise, proportional control valve to maintain stable control of the flow rate based on the readings from the heated sensor.

Q: Are thermal flow meters gas specific?

A: A thermal mass flow meter is built for a user’s specific operating conditions. Teledyne Hastings' thermal flow meters are thermal based, so measurements are gas specific. For example, there would be a difference in heat conductivity between natural gas vs compressed air. Our thermal mass flow meters are built to cover a user's flow range and gas type. 

​Q: What does SCCM stand for?

A: SCCM stands for Standard Cubic Centimeters per Minute. This is a unit of measure for gas molecular flow rate (often referred to as mass flow). In standardized volumetric flow units, the reference conditions, or “STP" temperature and pressure, define the amount of gas by determining the number of gas molecules using the Ideal Gas Law.

In this blog, we will discuss various system connections, or fittings, that are available for both our mass flow and vacuum products. We will briefly explore why you might select a particular family of fittings for your system. Also, we will touch on some basic installation Dos and Don’ts.

Mass Flow Meters and Mass Flow Controllers

Many users of low flow (0-5 sccm up to 0-25 slm) instruments appreciate the convenience of compression fittings. The Swagelok ™ brand of compression fittings is very popular, and many users ask for these by name. Compression fittings can be very reliable; also, they can be quickly uninstalled and reinstalled as needed. And, unlike VCR ™ and VCO ™ fittings (we will talk more about these in a minute), compression fittings do not require a separate o-ring or single-use gasket. Our flow products are offered with various size Swagelok™ brand compression fittings. While the 1/4” size is the most popular for many low flow applications, we also offer, even smaller, 1/8” size as well. Metric sizes, including 10 mm and 12 mm, are also available. Then, for even higher flow applications, we also offer 1/2”, 3/4”, 1”, 1.5” and even 2” Swagelok ™ fittings. Note that the largest compression fittings require a swaging tool. (Link?)

Per the “Tube Fitter’s Manual” published by Swagelok®, here are the steps for manual installation of Swagelok Tube Fittings up to 1 in. or 25 mm.

1. Fully insert the tube into the fitting and against the shoulder; rotate the nut finger tight.
   
   
   
2. Mark the nut at the 6 o’clock position.
   
   
   
   
3. While holding the fitting body steady, tighten the nut 1 ¼ turns to the 9 o’clock position.
   For 1/16, 1/8, and 3/16 in. or 2, 3, and 4 mm tube fittings, tighten the nut ¾ turn to the 3 o’clock position.
   
   

Reassembly Procedure is as follows:

1. Prior to disassembly, mark the tubing at the back of the nut, mark a line along the nut and body flats. Use these marks to ensure that you return the nut to the previously pulled-up position.
   
   
   
2. Insert the tubing or tube adapter end connection with pre-swaged ferrules into the fitting until the front ferrule seats against the fitting body.
   
   
3. While holding the fitting body steady, rotate the nut with a wrench to the previously pulled-up position, as indicated by the marks on the tubing and flats. At this point, you will feel a significant increase in resistance. Tighten the nut slightly.
   
   

The VCR ™ system from Swagelok™ is very popular with users who need high purity, all-metal, reliable sealing for either positive pressure or vacuum applications. For these fittings, a gasket, usually metal, is used to seal between the two symmetric sealing faces. In some cases, an elastomeric or PTFE gasket can be used. Metal gaskets (e.g. copper, nickel, or stainless steel) in VCR ™ connections should only be used once. Metal gaskets can be purchased with a retainer to hold the gasket in place when installing. The gasket is secured between the mating surfaces and the nut is drawn finger tight. Then, to finish installation, two wrenches are used to tighten the connection and create the leak-free seal. Note that copper gaskets require a 1/4 turn (90°) beyond finger tight while nickel and stainless steel only require an 1/8 (45°) of a turn.

HFC-302 with VCR fittings

The VCO™ system is convenient when the user wants to have fast make and break connections. It is also handy when space is limited. One part of the VCO™ connection includes an o-ring while the mating connection has a flat smooth finish. Installation is easy. A nut is made finger tight and then a wrench is used to tighten by 1/8 (45°) of a turn.

Usually, your fitting selection and piping are going to be a function of the flow rate. Our application engineers are available via email, phone or LiveChat to help you.

Vacuum Gauges

There are several popular systems of connections for vacuum gauges. Selection of a system should be driven by base pressure, outgassing load, and of course, cost.

For many users who just need to reach the mTorr range of pressures, tapered pipe thread (NPT: National Pipe Tapered) connections are simple, require no external clamps or bolts, and can be assembled quickly. However, PTFE tape or some other sealant should be used on the threads for two reasons. First, the tape/sealant fills the void between the mating thread surfaces and second, the tape/sealant acts as an anti-galling lubricant between the threads.

When wrapping PTFE tape onto NPT threads, start with clean surfaces and a clean cut of the tape. Make sure the tape is flat as it is wound onto the sealing surfaces and wrap in the direction of the threads. Two to three wraps is adequate. End the wrap with a clean cut of the tape. Tighten the connection with a wrench. How tight? Well, there is no right answer except to say that you want the system to seal, but you don’t want the threads to strip. So, use a wrench until tight, but do not try to force and overtighten.

The KF system is convenient for users who need a fast leak tight system connection for their vacuum gauges. “KF” is short for Klein Flansche which is German for small flanges. Vacuum systems with KF flanges can reach into the 10^-8 Torr range. KF sizes, such as KF-16 and KF-25, are related to the maximum nominal inner diameter tubing in millimeters that can be attached to the flange.

Teledyne DV-6-KF-16 (Shown with o-ring assembly and clamp)

And lastly, ConFlat hardware is ideal for high vacuum and ultrahigh vacuum systems. ConFlat flanges have a knife-edge that seals against a gasket, usually copper. The connection is made by tightening a series of bolts; the number of bolts is a function of the size of the flange. Clean, baked, suitably pumped systems using ConFlat hardware have been known to reach pressures below 10^-13 Torr.

DV-6 Gauge Tube with ConFlat Flange

If you would like to discuss your application for vacuum gauges, mass flow meters, or mass flow controllers, we are standing by. You can reach us by phone (1-800-950-2468), email (hastings_instruments@teledyne.com) , or by using our LiveChat box at www.teledyne-hi.com or clicking on the box below.

Note: All photos of Swagelok fittings in this blog are used with their written permission.

Using thermal mass flow instruments by Teledyne Hastings is an easy way to quickly and accurately measure gas flow. And in some cases, a mass flow instrument may be calibrated for one gas, but then the user may want to use the instrument in another gas. In this blog, we will show how to use GCFs (Gas Conversion Factors) when using flow instruments in different gases.

Before we get into GCFs, let’s quickly review the operation of one of our flow sensors. Below, we show a diagram of the 200 Series flow sensor. In this sensor, gas flows through a capillary tube which is heated in the middle to a temperature which is approximately 130°C. Two thermocouples, one upstream (TC-1) and one downstream (TC-2), measure the temperature. The temperature difference between the two thermocouples is proportional to the heat flow through the capillary tube. The heat flow, in turn, is proportional to the mass flow times the specific heat Cp of the gas. So, to first order, if we want to use a thermal mass flow meter that has been set up for one gas, and use it with another gas, we will multiply the output of the meter by the ratio of the specific heats. GCF ~ Cp1 / Cp2

There are a couple of things we need to point out. First, the ratio shown above is a simple approximation and does not tell the whole story. Next, the best GCFs are those that have been measured experimentally. However, in the case of dangerous gases, we use the best thermodynamic data available.

Here is a table of some common GCFs.

Next, we will discuss how we apply GCFs in practice. Let’s take an example of a flow meter that is calibrated for nitrogen. If we wanted to use the flowmeter in argon, we would take the output and multiply by the GCF for Argon.

Here is another example; suppose we have a meter that is calibrated in helium and we want to use it in hydrogen. You would start by dividing the output by the GCF for helium (think of it as converting to the nitrogen equivalent), and then multiplying by the GCF for hydrogen.

Remember, always use the appropriate set of GCFs for the flow series that you are using. In other words, if you are using our Digital 300 Series, don’t apply GCFs from a 200 Series manual – they are not the same. And certainly don’t use non-Teledyne table of GCFs for use with Teledyne flow products. They might get you in the ballpark, but they will not be your best conversion.

One other quick note about applying GCFs. Our line of flow power supplies, the THCD-101 (single channel) and the THCD-401 (four channel), can be used to quickly scale the analog input which is equivalent to applying a conversion factor. Let’s take another look at the Argon example. If we used the THCD-101 power supply with the nitrogen flow meter as shown below, at the nominal full scale of the flow meter, we will have a 5 VDC signal. If we want to use this same meter and power supply with Argon, we just need to “tell” the THCD-101 what value to display when it receives 5 VDC. So, if our flow meter was calibrated for nitrogen to give 5 VDC at 250 sccm, then the same flow meter will give 5 VDC in argon at 350 sccm. (250 * 1.4 = 350). So, we would then range the THCD-101 for 350 sccm. This can be done from the front panel or via the internal webserver.   

Now let’s make things a little more interesting and discuss a flow controller example. Analog flow controllers work by receiving a command signal (usually 0-5 VDC, or 4-20 mA) and then they adjust their control valve such that the flow, and thus the analog signal output, matches the command signal input. (You can think of it like the cruise control in your car – you tell it you want to go 78 miles per hour, and then the engine does what it needs to do to maintain that speed). In the case of a 0-5 VDC flow controller, a 5-volt setpoint command is instructing the flow controller to set the flow to 100% of full scale. The relationship between flow rate and command signal is linear, so if the user wanted to control at 25% of full scale, then they would send a 1.25 VDC command signal (0.25 * 5 VDC = 1.25 VDC).

Now, suppose we had an HFC-202 flow controller (200 Series) that was calibrated for 200 sccm of methane and we wanted to use it to control the flow of argon. What voltage level would we need on the command signal to have a flow rate of 100 sccm of argon? Let’s first determine the full-scale flow rate (5 VDC) when using argon:

Flow (Ar) = Flow (CH4)/GCF (CH4) * GCF (Ar) = (200 sccm / 0.77) * 1.401 = 363.9

So, a 5 VDC command signal will give us 363.9 sccm of argon. If we want 100 sccm, we would send:

Command Voltage = 100 sccm (5 VDC / 363.9 sccm) = 1.374 VDC.

Now, one important note about using flow controllers in different gases. Just because we can apply GCFs does not mean that a flow controller’s valve will work properly when switching from one gas to another. As an extreme example, a flow controller valve that has an orifice sized to handle hydrogen will have a hard time handling significant flows of large polyatomic molecules like C2H6.

Teledyne flow products are easy to install and use. And our application engineers are standing by to help. We can be reached by email (hastings_instruments@teledyne.com), by phone 757-723-6531, or via LiveChat on our website www.teledyne-hi.com or by clicking the contact us button below.