Teledyne Hastings Instruments Blog

This blog is the next installment in a series focusing on industrial gases. The first blog featured SF₆ and can be found here:   http://info.teledyne-hi.com/blog/sulfur-hexafluoride-gas-sf6 . The second blog focused on carbon dioxide (CO₂): https://info.teledyne-hi.com/blog/carbon-dioxide-co2 . Now we will take a look at Argon.

Argon (Ar) makes up just under 1% (0.93%) of the composition of air. Argon is odorless, tasteless, and has no color. It is a member of the noble gases. Science Notes (sciencenotes.org ) gives an interesting history of the term, “Noble gas”:

The term “noble gas” comes from a translation of the German word Edelgas, which means noble gas. German chemist Hugo Erdmann coined the phrase in 1898. Like a nobleman might consider it undignified to associate with commoners, noble gases tend not to react with other elements.

In any case, noble gases such as Argon are found in the right-hand column of the periodic table which means they have completed valence shells. The noble gases are generally monatomic and are mostly inert. The word “argon” comes from the Greek word “argos” which, according to Webster’s dictionary, means “idle, lazy.” This definition makes sense because Argon gas is quite unreactive and rarely forms compounds.

Because of its inert behavior, Argon gas has many uses. One of the most popular is the use of Argon as a cover gas when welding. A flow of Argon can provide an inert environment which prevents oxidation of welds and also allows the welder to have a more stable arc.

Argon is also used in the medical field. Argon plasma coagulation can be used to control tissue bleeding by injecting a jet of ionized argon gas. Also, since the physical probe does not have to actually touch the lesion, the procedure can be safer than other techniques. In ophthalmology, Argon lasers can be used to treat issues with the retina.

Many homes have double-pane windows filled with Argon gas. Argon provides better insulation than air because it allows less convection between the windowpanes. And because Argon is inert, it prevents deterioration of the window materials.

In lighting, an Argon glow discharge provides a pleasant purple-blue color. And in tungsten incandescent bulbs, a small amount of Argon is used to extend the bulb’s life.

Argon can also be used when making wine. In the wine’s casket or barrel, above the wine, is the headspace. Filling the headspace with Argon gas protects against oxidation and spoilage.

We could keep going in this blog and list many more applications. But we will stop this list with reference to a blog that we wrote back in 2018. The Emancipation Proclamation is stored in a double-paned encasement, designed by scientists at NIST, that that is mostly filled with Argon. You can read more here: https://info.teledyne-hi.com/blog/how-monitoring-instrumentation-is-helping-preserve-the-emancipation-proclamation

And, of course, when you need to measure Argon flow or vacuum levels with Argon, Teledyne Hastings is ready to help. Our flow instruments are able to measure and control flows from a few sccm (standard cubic centimeter per minute) up to several thousand slm (standard liters per minute).

Teledyne vacuum gauges are very good selection for use in Argon. The HVG-2020B (Click Here) is an excellent choice for measuring Ar from below 1 mTorr up to atmosphere. Convection driven pirani vacuum gauges, when used with gases other than N₂/air can have curious behavior as can be seen in the cartoon below.

The HVG-2020B vacuum gauge uses a gas-independent piezoresitive sensor that does not rely on convection affects and provides a more linear response to Argon across the entire measurement range.

If you would like more information about either the 300 Vue mass flow meters or controllers, or any of our vacuum gauges including the HVG-2020B, you can talk to any of our application engineers at 757-723-6531, email hastings_instruments@teledyne.com, or LiveChat with us at www.teledyne-hi.com

Special thanks to Lawrence Ferbee from the stockroom for his cartooning skills. If you would like to see Lawrence in action as he draws, check out our HVG-2020B video:

https://www.youtube.com/watch?v=_Bk3Q7SpSUc

A Pirani vacuum gauge is a type of thermal conductivity gauge that is also often referred to as a “thermal heat transfer gauge”. In a Pirani gauge, the rate of heat transfer is a function of the number of gas molecules present in the vacuum system. Pirani gauges are capable of providing reliable pressure measurement in a wide variety of vacuum technology applications.

What is the working principle of the Pirani Vacuum Gauge?

A Pirani vacuum gauge contains a heated component, such as a wire or thin-film membrane (see figure on right), which is brought to an elevated temperature, typically through the use of a bridge circuit. As changes in gas molecular density occur, the transfer of heat from the wire to the gas is affected. This heat loss is dependent on gas type and pressure, and the amount of energy required to keep the wire at temperature varies accordingly. Consequently, the amount of energy is dependent on vacuum pressure and can be converted to a pressure value.

Accurate pressure measurement depends on the ability of the bridge circuit to maintain very precise control of the sensor’s temperature. The most common technique is to use a Wheatstone bridge circuit. In one design of a Pirani bridge circuit, two legs of the bridge are controlled at the same voltage via an electronic feedback loop. The measurement technique then is to quantitate the power required to maintain the heated sensor at a given temperature above ambient. (See J. Vac. Sci. Technol. A 13(6), Nov/Dec 1995)

In the case of a gauge that employs a heated sensor wire, there are three main heat loss routes: bulk thermal conduction via the wire and its supports, radiation losses, and thermal conduction losses through the gas. It follows that a well-designed Pirani vacuum gauge minimizes loss of signal due to radiation and bulk thermal conduction. Radiation losses are strongly (fourth power) dependent on the temperature of the heated sensor. Consequently, when operating a Pirani gauge, it is important to keep the sensor at a low enough temperature such that the radiation loss mechanism does not dominate the overall signal. Similarly, a well-designed Pirani gauge minimizes bulk thermal conductivity via the sensor element. For both radiation and bulk thermal conductivity losses, the loss factors are dependent on temperature.

What are the advantages of the Pirani Vacuum Gauge?

The Pirani vacuum gauge offers numerous advantages. Foremost, the gauges are typically very easy to use and can provide years of trouble-free, reliable pressure measurement, because they have no moving parts. Additionally, they are highly economical, costing far less than capacitance manometers. Flexibility in output monitoring includes local displays, analog output, and/or digital communication. Pirani vacuum gauges are relatively fast and are very responsive to changes in the vacuum system’s pressure. Gauges with very small internal dimensions can offer increased response speed. It should be noted that convection-enhanced Pirani vacuum gauges require larger volumes to support convection currents and may therefore have slower response, simply due to the larger volume that must be evacuated inside the tube. Convection-enhanced Pirani vacuum gauges measure pressure up to atmospheric pressure, however, their accuracy in gases other than air or nitrogen at elevated pressure, can be very poor.

Combination gauges, such as Teledyne Hastings’ HVG-2020B, combine a piezo-based sensor with a conventional, thermal-based Pirani sensor to provide accurate pressure measurement from 10-4 Torr (0.1 mTorr) to 10+3 Torr.  Additionally, by combining the two technologies in one gauge, the errors associated with the gas composition specific Pirani sensor are eliminated at pressures above 10 Torr.

What are some of the applications for Pirani Vacuum Gauges?

Many Pirani vacuum gauges measure pressure from below 1 mTorr up to atmosphere and are a very good choice for a wide variety of applications:

• Pirani vacuum gauges are used to monitor the pump down from atmosphere to the start of the high vacuum region (where turbomolecular pumps or other high-vacuum pumps take over)
• In the vacuum metallurgy industry (high-purity alloys manufactured in a controlled environment) Pirani vacuum gauges ensure process consistency and optimization of different thin-film coating systems.
• Freeze dryers and vacuum dryers frequently operate in a vacuum region that is well-suited to Pirani vacuum gauges, as well as the manufacture and maintenance of cooling systems (air conditioning, ice machines, and refrigeration).
• Various analytical instrumentation requires vacuum levels in and below the mTorr region, and the Pirani gauge is often the most cost-effective instrument for these applications.
  

FAQs

What are some best practices for Pirani Vacuum Gauges?

Pirani gauges have no moving parts and generally do not require maintenance. It should be noted that this type of gauge is susceptible to contamination, especially in oil-based pumping systems. In systems with contamination, the gauge tube should be installed with the port facing downward to reduce oil accumulation inside the tube. 

Convection-driven Pirani vacuum gauges should always be installed with the cylindrical axis parallel to the floor. Failure to install this type of gauge properly will lead to erroneous readings above the rough vacuum level. 

If the vacuum system has excessive oil vapor or other contaminants, a molecular sieve filter can be used to extend the service life of the gauge.

For information on Teledyne Hastings Vacuum Gauges, visit https://www.teledyne-hi.com/products-services/vacuum-measurement-and-control or click the button below to contact us for more information.

What is a Thermocouple Gauge?

Teledyne Hastings was founded in 1944 as “The Hastings Instrument Company” by Charles and Mary Hastings. The introduction of thermal sensing technology in late 1940s at the young company offered potential for a variety of new technologies and quickly became the foundation for many early Hastings’ instruments. Early products included air velocity indicators, thermal mass flow meters, stack emission monitors and, of course, the thermocouple vacuum gauge tube.

By 1964, Hastings Instruments had grown into one of the leading vacuum and thermal mass flow instrument companies in America. Today, our popular Thermocouple Vacuum Gauge Tube product lines, including the DV-4, DV-5, and DV-6, are trusted globally where repeatable, rugged, and dependable pressure measurement is needed. Based in Hampton, Virginia for over 75 years, the highly-skilled employees at Teledyne Hastings are dedicated to delivering thermocouple vacuum gauges of exceptional quality.

How does a thermocouple gauge tube work?

The physics behind how a thermocouple vacuum gauge tube functions is simple. For example, let’s look at the DV-6R thermocouple vacuum gauge tube, which is capable of pressure measurements within the range of 0 – 1000 mTorr. The DV-6R can be seen in the image below, along with the representative schematic.

A thermocouple consists of a junction of two dissimilar filaments that are soldered together. Three thermocouples are shown (A, B, and C) in the DV-6R diagram, and many thermocouple gauge tubes use multiple thermocouples to form a thermocouple array. At each junction, there is a small voltage (on the order of a mv) which is a function of temperature. When the gauge tube is in operation, thermocouples A and B are resistively heated inside the gauge tube’s housing. Because each filament connected to both A and B thermocouples is exposed to the gas in the vacuum, thermal energy is transferred away from the array at a rate which is dependent on the number of collisions between the gas molecules and the filament wires.  This transfer of thermal energy is, in turn, dependent on the pressure inside the tube. In essence, by measuring the rate of thermal energy transfer, the pressure inside of the tube is indirectly determined!

To understand the concept more fully, let’s consider two extremes. In high vacuum, where the pressure is very low, there will be fewer gas molecules to collide with each filament and the tube’s voltage output will be relatively high. Higher temperature corresponds to higher thermocouple output. As we approach near atmospheric pressure (760 torr), there are more gas molecules, resulting in more thermal energy transferred away from the filament. Consequently the thermocouples run at a lower temperature resulting in a relatively low output. The varying output produced by the thermocouple gauge is dependent on the thermal conductivity of the gas in the vacuum system, which is then used to measure pressure.

Thermocouple vacuum gauges are “indirect gauges”, meaning they accomplish pressure measurement by measuring a physical property, such as thermal conductivity or ionization rate of gas molecules, to determine the pressure in a vacuum system.

How to use a thermocouple vacuum gauge tube:

Teledyne Hastings’ vacuum gauge tubes are manufactured to the highest quality and tested to be extremely repeatable from one gauge tube to another to ensure accurate process control. Thermocouple vacuum gauge tubes are physically installed on a vacuum system and then used to measure various pressures across the tube’s full scale range. When the filament inside the tube is excited, the output is then converted to a pressure measurement.

An example gauge tube installation would consist of a DV-6R thermocouple vacuum gauge tube connected to a DCVT-6 panel meter or HPM-4/5/6 handheld meter. Both the DCVT-6 and HPM-4/5/6, are factory configured to work seamlessly with DV-6R gauge tubes, out of the box. The output from the gauge tube is then monitored by the DCVT-6 or HPM-4/5/6 and the level of vacuum displayed.

In the case of the DCVT, two contact relays are available for process control, and the dual relays will independently toggle according to user-defined setpoints, when the vacuum setpoint has been crossed. This demonstrates how the factory-tested repeatability of our gauge tubes results in consistent and repeatable process control! The DCVT also provides serial communication (RS232), LabVIEW™ drivers and linearized analog output options (0-1 VDC, 0-5 VDC, 0-10 VDC, 4-20 mA).

Related Products

Teledyne Hastings' Vacuum Products
DCVT-6	HPM 4/5/6	DV-6R	DV-6S

Application Example:

There are numerous, diverse vacuum applications and different vacuum systems can require specific thermocouple vacuum gauge tubes. At the most basic level, vacuum systems can be installed in a variety of environments including outdoor with exposure to weather elements, indoor industrial and laboratory conditions in which exceptionally high cleanliness standards are required.

A good example of how a thermocouple vacuum gauge tube meets the requirements of a specific application, is presented in the vacuum insulation or vacuum jacketing field. The technique of using a vacuum guard or barrier to thermally insulate a cryogenic or refrigerant tank is straightforward: by removing the air from around an object, conductive heat transfer is eliminated. The better the vacuum attained, the higher the level of vacuum insulation. To effectively measure pressure below the 1 Torr range, a vacuum dial gauge is simply not sensitive enough; a thermocouple vacuum gauge tube is required.

To obtain vacuum readings from DV-6 gauge tubes, a dedicated electronic display or handheld battery-operated readout is used. The Teledyne Hastings’ DCVT provides continuous monitoring via an easy to read LED display. For periodic vacuum monitoring of one or more tubes, the hand-held HPM 4/5/6 is recommended because of its portability (9V battery power) and its ability to be quickly connected to a DV-6 tube and obtain an accurate reading with fast response time. To ensure the most accurate measurement, the DB-20 Reference Tube can be used to validate the electronic calibration. This can assist in determining if the thermocouple vacuum gauge tube requires replacement by ensuring that it is producing an accurate reading.

Teledyne Hastings has created a detailed application note on vacuum jacketing insulation at this link.

For more information about any of our thermocouple sensor series or vacuum gauges, we are here to help. In addition to LiveChat on our website, you can contact us at hastings_instruments@teledyne.com or call 757-723-6531 (800-950-2468) or click the button below.

March is Women’s History Month and this year we’d like to focus on Mary Hastings, one of the key founders of Hastings Instruments. Mary Comstock graduated from William & Mary with a degree in physics with minors in math and chemistry. She was truly a pioneer in many ways. After college, she took a job as a “computer” at NACA (National Advisory Committee for Aeronautics) in Hampton, Virginia. There, Mary met Charles Hastings, a young engineer who had his office across the hall from her. The two were married in 1940.

In a business era that was almost thoroughly dominated by men, Mary was an invaluable contributor to the success of Hastings Instruments. She promoted the fledgling company through press releases that she prepared for local papers and thereby helped to secure critical financial support. She attended shareholder meetings of other companies to learn how they conducted their annual meetings. For many years, Mary prepared the annual report for Hastings shareholders. In short, she was not afraid to tackle any challenge that would help grow the company. Moreover, Mary was a constant voice of wisdom to her husband with respect to company decisions and policy. She accomplished all of these things while the couple raised three children.

It is reasonable to assume that without Mary Hastings, the company would not have been nearly as successful. So during Women’s History Month, we want to celebrate her many accomplishments. You can hear more about Mary from her daughter Carol Hastings Sanders in this video:

https://www.youtube.com/watch?v=AxdFQQXEEzQ

Happy Valentine’s Day 2021! Time for me to run out and order some freeze-dried flowers for my wife. What?... yes, it’s true! In case you have not heard of this, freeze-dried flowers can make a beautiful gift.

According to Flowers Forever – Bellabeads of Columbia SC, https://myflowersforeverjewelry.com/pages/freeze-dried-flowers, freeze-dried flowers will, “retain their beauty as if frozen in time as a lasting memento.” They can be placed in shadow boxes or frames and stored for many years.

So this is probably the best time to tell you about this fun application of vacuum technology. I recently worked with one of our vacuum customers who services freeze driers for florists, so this topic is “fresh” in my mind. Freeze drying is well-known for use in the manufacturing of food and drugs. Most likely, you have something in your house that has been processed with freeze-drying. So, let’s take a closer look.

Freeze drying, also known as lyophilization, is a process in which water molecules are removed from biological cells without damaging the cell structure. For starters, the product to be freeze-dried is chilled and the water inside is completely frozen (i.e. placed in the solid state). Next, the pressure is reduced using vacuum pumps and the water molecules sublimate – that is, water goes from the solid phase directly to the gas phase.

The gas load due to the water vapor is usually very high and requires condensers to trap the liberated water and stop it from overwhelming the pumping system. Commercial freeze driers (see image below) include a heat exchanger which serves two purposes: first, the heat exchanger is used to cool the product and later, it is used to gently heat the product to drive the water sublimation.

View inside commercial freeze-drier

Do you know of some other cool applications of vacuum technology, we would “love” to hear from you! Please visit Live chat with us at www.teledyne-hi.com or call 1-800-950-2468.

Special thanks to Flowers Forever – Bellabeads for the use of flower image from their beautiful website.

This blog is the next installment in a series of blogs focusing on industrial gases. The first blog featured SF₆ and can be found here:   https://info.teledyne-hi.com/blog/sulfur-hexafluoride-gas-sf6 In this blog, we will explore one of the most useful, and well-known, of the industrial gases, carbon dioxide (CO₂).

CO₂ is an odorless and colorless gas. As a matter of fact, you exhale about a quarter liter of CO₂ every minute (https://www.nytimes.com/1990/08/14/science/q-a-burden-of-breathing.html). And while we are on the subject of breathing, it is interesting to note that it is usually the buildup of CO₂ that triggers a breath, not the lack of oxygen. A recent special feature in Gasworld US Edition noted that “low concentrations are not harmful, higher concentrations can affect respiratory function, cause excitation and depression of the central nervous system.”

The structure of CO₂ is linear. The electronic structure of the molecule features bonding pairs of electrons around the central carbon atom. These electron pairs repel equally while bonding to oxygen atoms. The equal repulsion is what gives CO₂ its linear arrangement. A diagram of the molecule is shown on right.

There are many fascinating uses of CO₂, some are well-known while others may be new to the reader.

• Used in industry to produce chemicals and as feedstock
• Used in the metals industry to improve the hardness of castings
• Often used as a carrier for spraying of paints or for spraying vegetable oils in cooking
• Liquid CO₂ can be used as a solvent in eco-friendly dry cleaning
• Solid CO₂ can be used for cold storage
• Jets of CO₂ are used for special effects in movies, live shows, and amusement parks
• Gas phase CO₂ is used for fire extinguishers
• Carbonated beverages

A quick note on purity... CO₂ used for fire extinguishers can be ~ 95% pure. However, the purity of CO₂ in carbonated beverages can be as high as 99.9995%.

A word on explosive decompression of elastomers when using CO_2. CO₂ is known to more easily penetrate elastomeric seals such as o-rings and diaphragms at pressures significantly higher than atmosphere which in turn can cause the host material to swell. If the pressure is suddenly decreased (i.e rapid decompression), the internal gas can rupture the elastomer and the seal can be compromised. There are a few things the user can try to reduce risk. First, select materials that are less susceptible. Viton® is often not a good choice for CO₂ applications. Second, if the application will allow, try to reduce the amount of time the elastomeric seal is held at elevated pressure. And third, when the pressure is reduced, allow more time for the CO₂ to exit the material.

Flow controllers from Teledyne Hastings are used to measure and control CO₂ flow in many of the aforementioned applications. The 300 Vue can provide all-metal seals and Kalrez® valve seat. (see diagram below). For users of the elastomeric-sealed 200 Series, we recommend Buna-N seals which are less susceptible to explosive decompression.

Teledyne vacuum gauges can be used with CO₂. The HVG-2020B (Click Here) is an excellent choice for measuring CO₂ from below 1 mTorr up to atmosphere. Convection driven pirani vacuum gauges, when used with gases other than N₂/air can have curious behavior as can be seen in the cartoon below.

The HVG-2020B vacuum gauge uses a gas-independent piezoresitive sensor that does not rely on convection affects and provides a more liner response to CO₂ across the entire measurement range.

If you would like more information about either the 300 Vue mass flow meters or controllers, or any of our vacuum gauges including the HVG-2020B, you can talk to any of our application engineers at 757-723-6531, or email hastings_instruments@teledyne.com or LiveChat with us at www.teledyne-hi.com

Special thanks to Lawrence Ferbee from the stockroom for his cartooning skills. If you would like to see Lawrence in action as he draws these, check out our HVG-2020B video:

https://www.youtube.com/watch?v=_Bk3Q7SpSUc

Viton® is a registered trademark of DuPont Performance Elastomers

Kalrez® is a registered trademark of DuPont Dow Elastomers

The use of Liquefied N₂ (LN₂) has revolutionized aluminum extrusion mold performance. CVS Corporation based in Gimpo, South Korea has developed a proprietary LN₂ cooling process that allows production output to more than double for some extrusion profiles. Production times vary based on the geometry of the extrusion profile. Ram speed test data can be found in Figure 1 below.

Figure 1: Extrusion Profile Table

The Aluminum Extrusion Mold Liquid Nitrogen Cooling System from CVS is designed to control the temperature of both the profile and the mold during the extrusion process. As the billet passes through the extrusion mold, the friction between the billet and the mold creates excess heat. This excess heat limits the extrusion ram speed and is the main cause of reduced productivity in the extrusion process. The CVS process maintains optimum profile and mold temperatures by using automated cryogenic proportional control valves to provide precision dosage of Liquefied N₂.

Figure 2: Aluminum Extrusion Mold Liquid Nitrogen Cooling System from CVS

Mr. Ko Hwa-Jin is the President of CVS Corporation and can be reached by email or website: www.ln2doser.com

Mr. Yoonk Min is the President of Inforad Corp. which is the distributor for Teledyne Hastings Instruments for South Korea. Mr. Min can be reached by email or website: www.inforad.co.kr

Wayne Lewey is the International Sales Manager at Teledyne Hastings Instruments and can be reached by email. 

Did you know? Even if your flow controller does not have the color touchscreen display, the 300 Vue flow line still allows you to switch between analog & digital control by using the zero button. (See the diagram below). You can also toggle between RS232 & RS485. This is a convenient feature when users want to switch their setup. Instructions can be found in the manual (pg. 14).

And, of course, we have our free Windows™ software. With the software, you can quickly configure and control your 300 Vue. And you can also record flow data and store to a file If you would like us to send a secure transfer download link for the free software, click here: https://www.teledyne-hi.com/resource-center/software

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.

Over the next several blogs, we will be discussing various industrial gases. While some of these (carbon dioxide, argon, methane, and hydrogen) may be very familiar to our readers, other gases may not be as well known. In this blog, we will take a look at sulfur hexafluoride (SF₆), a gas that is one of the most important today in the utility industry.

SF₆ is an interesting gas primarily because of its electrical properties. Certain neutral gas molecules can easily capture free electrons and form stable negative ions. The efficiency of negative ion formation in a gas is determined by its electron affinity. SF₆ , it turns out, has a very high electron affinity and therefore has excellent electrical insulating strength. So, in an electrical discharge inside a volume containing SF₆ gas, the free electrons generated by the discharge are captured by neutral SF₆ to form negative ions. These large negative ions are not able to travel quickly and so the discharge is usually quickly extinguished. One other note about SF₆, the insulating property of the gas improves with increasing pressure. SF₆ is colorless, odorless, non-toxic, and non-flammable. As you can see, these properties make it very useful to the generation, transmission, and distribution of electricity. 80% of the world’s SF₆ gas is used by electrical utilities in circuit breakers, transformers, and gas insulted switches.

SF₆ is arranged in a hexagonal structure. Each of the six fluorine atoms shares two its electrons with the outer shell of the sulfur atom in the middle. This structure gives SF₆ its stability over a broad range of temperatures; the gas is thermally stable up to 500°C.

SF₆ is often used in high voltage breakers. One example is the so-called dead tank breaker. In a dead tank breaker, the tank is electrically tied to earth/ground. In the live tank version, the tank is floating at a higher voltage.

The “make/break” mechanism of the breaker is shown in the diagram below. As noted above, the insulating properties of SF₆ are improved with increasing pressure. So one of the jobs of the breaker’s piston actuation is to compress the SF₆ gas and force it to flow into the arc region. As the contacts are moved apart, current will try to continue to flow as an arc. Any resulting arc is quickly extinguished by the pressurized SF₆ flowing into the region. Incidentally, during breaker manufacturing, vacuum gauges from Teledyne Hastings are used to measure vacuum levels inside the vessel during pump down as the air is removed. After evacuation, the region can be filled with SF₆.  

OK, one last note about SF₆ to conclude this blog and this falls under the category of “Don’t Try This at Home.” Just like Helium will make your voice sound higher if inhaled, SF₆ will make your voice sound lower. You can find many demonstrations of this on YouTube. The most famous example is probably the demonstration on “The Big Bang Theory.” However, SF₆ is one of the most powerful greenhouse gases and its release into the atmosphere should be minimized.

Teledyne Hastings builds both vacuum and flow instrumentation which can easily work with SF₆. Note that SF₆ has a very high thermal conductivity. Conceptually, this makes sense because the gas molecule has many degrees of freedom – translational, rotational, and vibrational.  The GCF (gas conversion factor) for SF₆ use with the 300 Vue line of flow controllers is 0.27. In other words, if you wanted to use a 300 Vue mass flow meter that had been set up for nitrogen, you would need to multiply the output by the 0.27 GCF. The good news for you is that with the 300 Vue, you can just select the gas from the front panel as shown in the photo below. Just keep in mind that if you wanted to do this, the required pressures for the valve are going to be different. You will likely need a higher pressure drop. But as always, our application engineers can be reached by email, phone, or Live Chat on our website: www.teledyne-hi.com