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Teledyne Hastings Instruments Blog

FAQ Corner - Units for Vacuum Measurement

Posted by Doug Baker on Mon, Sep 22, 2014 @ 04:23 PM

Earlier this year, the applications engineers here at Teledyne Hastings discussed topics for our blog. We all agreed that one of the more frequent questions that we discuss with folks involve the units used to measure vacuum levels. We find the technicians who use their vacuum systems daily often seem to develop a sixth sense about the “health” of their systems. They know something isn’t quite right when the base pressure (or rate of pressure change) is not what they expect. So when pressure measurements are not consistent from batch to batch, that is the time when the user stops to ask the meaning behind the data that their vacuum measurement instrumentation is providing.

Now, most users know that vacuum is commonly measured using units of pressure. There are a few different sets of pressure units and this blog will discuss the more commonly used ones. In Armand Berman’s book, Total Pressure Measurements in Vacuum Technology, pressure unit systems are divided into two categories: “Coherent Systems” and “Other Systems”.

Coherent Systems of Units are based on the definition of pressure (P) as the force (F) exerted on a chamber wall per unit area (A). P = F/A.  The International System of Units, or SI units, is commonly used for pressure measurement. http://physics.nist.gov/cuu/Units/units.html  The SI unit for pressure is the Pascal (Pa). It interesting to note that at NIST (National Institute of Standards and Technology), published papers are always required to use the SI set of units. Again, the SI unit for pressure (force per unit area) is the Pascal. 1 Pa = 1 N /m2.

Now, the Pascal as a unit of pressure is not always the most convenient because vacuum systems are often operating in a range of pressures where we would need to collect data using large numbers. For example, near atmospheric pressure, we would measure approximately 100,000 Pa. So a more convenient unit, the bar, has been derived. (1 bar = 100,000 Pa)

Moving lower in pressure, it is very helpful to then use the mbar (1 mbar = 0.001 bar). So many vacuum users, especially in Europe, use the mbar as the basis for describing pressure levels. As a specific example, look at the base pressure specification of a turbo pump, it will be given in terms of mbar (e.g. Base Pressure < 1 x 10-10 mbar).

Another system of pressure units is based on the Torricelli experiment (shown in the diagram). In this experiment, the pressure exerted on the mercury can be shown to be P = hdg, where h is the height of the mercury column, d is the density, and g is the acceleration due to gravity.

 

Simple Barometer
 

 

By measuring the mercury column height, the user can determine the pressure. The Torr unit (named for the Italian scientist Torricelli) has been defined to be 1 millimeter of mercury (1 Torr = 1 mmHg). This unit is very common, especially in the United States. It is also common to use the mTorr (1 mTorr = 0.001 Torr). Many years ago, pressure was sometimes described in terms of “microns”, which simply meant a mercury column height of one micron (1x10-6 m). Note that the micron and the mTorr are the same.

One last word about the units used to measure vacuum: on occasion, there is confusion between pressure units. As we have seen above, the mbar and the mTorr are not the same. One mbar has the same order of magnitude as one Torr  (1 mbar ≈ 0.75 Torr).  The table below gives some approximate conversion values. A useful website for conversions:

 http://www.onlineconversion.com/pressure.htm

 

 

Pa

mbar

Torr

mTorr (micron)

Atm

1 Pa =

1

0.01

0.0075

7.50

~ 10-5

1 mbar =

100

1

0.75

750.06

~ 10-3

1 Torr =

133.3

1.333

1

1000.0

~ 10-3

1 mTorr (micron) =

0.1333

0.00133

0.001

1

~ 10-6

1 Atm =

101,325

1013.25

760

760,000

1

 

 

 

 

 

 

Douglas Baker is the Director of Sales & Business Development of Teledyne Hastings. Antonio Araiza prepared the Torricelli experiment drawing. Antonio is the head of Technical Documentation at Teledyne Hastings (and is among the best soccer referees in the Commonwealth of Virginia).

Tags: Teledyne Hastings Instruments, pressure, mTorr, mBar, micron, pascal, torr, vacuum pressure, units of measurement, vacuum gauges, vacuum meters, vacuum controllers

New! Free Teledyne Hastings Mass Flow Converter APP

Posted by Doug Baker on Tue, Aug 05, 2014 @ 02:56 PM

Teledyne Hastings is proud to offer our Mass Flow Converter app. We have created a version for iPhone, iPad, and Droid. We have also created a web-based version that you can find at www.massflowconverter.com

In this blog article, we will discuss the motivation to build the app, how it works, and how it can be used.

The first question you might be asking is: Why do we need an app to convert from one set of mass flow units to another? For instance, if you want to convert from inches to centimeters, you would just multiply by 2.54. But, converting between mass flow units is not always that straight forward. So we have developed a tool that makes it easy.

We are going to look at some examples, but first let’s review what we mean by mass flow. When we think about mass flow, it can be helpful to think in terms of the flow of individual molecules. So while flow meters are often specified by units like sccm (standard cubic centimeters per minute) or scfm (standard cubic feet per minute), the mass flow rate is ultimately about the number of molecules (n) moving through a given cross sectional area per unit time (see figure below).

 

 Cross Section resized 600

 

 

 

So as our first example, let’s take a look at the conversion of 10,000 sccm (10,000 cm3/min) to a molecular flow rate. First, we need to ask, “How many molecules are in 10,000 sccm?” In the figure below, we show a container that is 10,000 cm3 in volume. Now, before we can calculate the number of gas molecules in a volume, we must know the pressure and temperature of the gas. We can use the ideal gas law:

n = (P * V) / R*T where n is the number of molecules, P is the pressure, V is the Volume, R is the Universal Gas Constant and T is the Temperature.

 

Framed Molecules per volume

Now we need to select some given pressure and temperature so that we can calculate the number of molecules – these are called the reference conditions or the STP (Standard Temperature and Pressure). In many cases, 0°C and 760 Torr are used for the STP. But this is not always the case. So it is always very important to specify the reference conditions (STP) any time you use a standardized mass flow unit like sccm, slm, scfh, etc (any mass flow unit that starts with “s” is going to need the reference conditions or STP specified). In our example, we are going to use STP of 0°C & 760 Torr.

OK, so here we go:     n = (1 atm) * (10,000 cc) / (82.053 cc * atm / K * mole) * (273 K)

Note that we have used a value of R in terms of pressure in Atmosphere (760 Torr = 1 atm), and Temperature in Kelvin (0°C = 273K). 

n = 0.45 mole

In other words, a flow rate of 10,000 sccm (0C, 760 Torr) is the same as a molecular flow rate of 0.45 Mole / minute.

OK that is the hard way. It’s much easier to use the mass flow converter app. In the example shown above, we would dial sccm on the left and Mole/Min on the right. Then to select the reference conditions, we use the menu in the center. See Fig. 3

 Framed screen shot mass flow converter app N2 resized 600

If you are like me, you will start to play with the App. And soon you will notice that the user can change the gas using the pull down menu at the top. But notice that in the case of our first example (converting from standardized mass flow units to molecular flow units), that the gas selection has no effect on the conversion.  This is because the standardized flow units (e.g. sccm, slm, scfh, etc.) are actually molar flow units based on reference conditions (STP) and the ideal gas law.

So, why do we allow the user to select gas? In the case of units like gm/sec, Kg/hr, or lb / min, we are going to need to know the gas so that we can calculate the mass. Let’s take a look at the case of converting from slm to grams/second. We will use as our same example of 10,000 slm (0°C & 760 Torr) and we will use methane (CH4) as our gas.

We showed earlier that 10,000 sccm is a molecular flow rate of 0.45 Mole / Min.  And since 1 slm = 1000 sccm, it is easy to see that 10,000 slm = 450 Mol/min. And since we know that our unit of choice (gm/sec) is in terms of seconds, let’s go ahead and convert our time units now:

10,000 slm = (450 Mole / Min) * (1 Min / 60 sec) = 7.5 Mole/ sec.

Now we need to know how much mass there is in a Mole of methane. Google is very nice for getting this number – just type, “Molecular weight of methane” and here is the result:

 

Framed mass flow converter app google screen shot resized 600 

 

By the way, Google will do this for almost all gases. So now we can finish our conversion and we get:

 

7.5 Mol / sec * (16.04 g/mol) = 120 g/sec

 


Framed mass flow converter app screen shot CH4 2 resized 600 

 

The mass flow converter app and website www.massflowconverter.com takes all the work out of these conversions and we hope that you will find this tool helpful. If you have any questions about mass flow meters and controllers, Application Engineers at Teledyne Hastings are always happy to help.

Tags: Teledyne Hastings Instruments, Flow Controller, Flow Meter, mass flow conversion, Mass Flow Calculator, Mass Flow Range, Gas Flow Range, Mass Flow, units conversion

FAQ Corner – Specifying range, pressures on Mass Flow Instruments

Posted by Wayne Lewey on Wed, May 29, 2013 @ 10:01 AM

We often see the label “One Size Fits All”.  This may be fine for some consumer goods.  However, it can be quite problematic when applied to items like shirts, gloves, or even golf clubs.  “One Size Fits All” also does not work for Mass Flow Controllers (MFC).  Not all applications are alike.  Forcing a “One Size Fits All” MFC into an unsuitable application can squander accuracy and induce valve failure.

Why do you need to specify the gas flow range on a MFC?  The Full Scale (FS) Range and Gas on a MFC directly correlates to the transmitted output (analog or digital) of the device.  In an analog device, the maximum output value (such as 5 vdc or 20 mA) will be equivalent to the FS value.  The accuracy of most MFCs is a function of this FS Range.  For analog devices, it is commonly ±1% of FS.  For digital devices, it is commonly published to be ±(0.5% of Reading + 0.2% FS).  Selecting the FS Range close to an application’s maximum flow rate optimizes accuracy for that specific application.  This is a good practice.  In addition, the gas must be specified.  Most MFCs use thermal based sensors.  These sensors actually measure the molecular flow rate rather than the mass flow rate.  Various gas molecules transfer heat differently, and thus the gas must be known.

Why do you need to specify Upstream Pressure and Downstream Pressure on a MFC?  Again, not all applications are the same.  A “One Size Fits All” MFC is typically not set up for applications at high pressure, low pressure, high differential pressure, or low differential pressure.  The definition of high and low will also fluctuate from one user to another.  Teledyne Hastings Instruments selects and tests MFC valve components (orifice, spring, etc.) that optimize valve stability for the exact application pressure conditions.When using a Teledyne Hastings Instruments MFC, you will always find the FS Range / Gas and the Upstream / Downstream Pressures listed on the serial number label.

Sample Calibration Sticker for MFC

 

Wayne Lewey was first exposed to mass flow controllers while an undergraduate at North Carolina State University (Chemical Engineering).  Today, Wayne is the International Sales Manager at Teledyne Hastings Instruments and can be reached at wlewey@teledyne.com.

Tags: Teledyne Hastings Instruments, Flow Controller, Gas Flow Range, pressure, range, differential pressure, mass flow controller, mass flow meter

FAQ Corner – Teledyne Hastings Instruments at Pittcon 2013

Posted by Brandon Hafer on Wed, Mar 13, 2013 @ 03:20 PM

It’s hard to believe that it is now March, which means that Pittcon 2013 is right around the corner. Teledyne Hastings Instruments will have Applications Engineers and representatives in attendance to answer all of your mass flow and vacuum instrumentation questions.

 

PITTCON 2013 LOGOPittcon is an annual conference on laboratory science that is organized by The Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy. Pittcon started as a small technical conference held in 1950. The first 18 conferences were held in Pittsburgh, Pennsylvania, but the conference has since grown. Locations now vary from year to year with this year’s conference being held at the Pennsylvania Convention Center in Philadelphia, Pennsylvania from March 17-21.

 

There have been many changes over the 60 plus years of the Pittsburgh Conference, and remains a worthwhile event to attend. Teledyne has had a presence at the event for the past 35 years. Included in the weeks events are thousands of exhibitors, numerous technical programs and lectures, and short courses. It provides the opportunity to meet and interact with scientist from across the country and around the world. Papers and articles are presented daily, illustrating the advancements in science in the past year. And finally, it allows for a single location to walk around and see over 17,000 companies and exhibitors with their products and technologies.

Teledyne Technologies Incorporated will have 4 companies in attendance at Pittcon this year. In addition to Teledyne Hastings Instruments, Teledyne Tekmar, Teledyne Leeman Labs, and Teledyne Judson will be exhibiting. Teledyne Tekmar is a leader in the design and manufacturing of analytical instrumentation including products for gas chromatography sample introduction, total organic carbon (TOC) and total nitrogen (TN) analyzers. Teledyne Leeman Labs is a producer of world-class instruments for elemental analysis including ICP spectrometers, atomic absorption spectrometers and mercury analyzers. Teledyne Judson is a leading designer and manufacturer of high performance infrared detectors and accessory products. The Teledyne family of companies will be located in booths 916 and 917, which are located near Entrance D to the Pennsylvania Convention Center. Teledyne employees will be giving presentations on a variety of topics while at Pittcon. If you would like more information on the schedule or the topics to be covered please contact us or stop by our booth and we can provide that information.

Teledyne Hastings Instruments has a great deal of experience with the analytical instrumentation industry. We are always interested in new applications even if they do not exactly fit into the standard product design for mass flow or vacuum instrumentation. We are very willing to examine possible custom designs to meet the requirements of your system. Some examples of previous custom applications include a variety of non-standard packages for both our mass flow and vacuum products, modified electronics, high pressure designs, and even custom designed flow and vacuum sensors.

 

We welcome your comments and your questions and look forward to seeing you at Pittcon 2013. Please stop by our booth and discuss your projects with either Vikki Jewel or Brandon Hafer. You can also email your questions to Victoria.Jewell@Teledyne.com or Brandon.Hafer@Teledyne.comand we’ll be happy to respond and work with you. 

Brandon Hafer is an Application Engineer with Teledyne Hastings Instruments. He was raised in Pottsville in Eastern Pennsylvania and is a fan of the Philadelphia Phillies and Philadelphia Eagles. He is looking forward to returning to the Philadelphia area for Pittcon 2013. If you would like to contact him, he can be reached at brandon.hafer@teledyne.com.

 

 

 

Tags: Teledyne Hastings Instruments, vacuum instruments, mass flow instruments

FAQ Corner – What is the Importance of STP Conditions on Mass Flow

Posted by Brandon Hafer on Thu, Mar 07, 2013 @ 03:19 PM

As I go through the day looking at various mass flow applications, I often notice that it is very easy for users to overlook one of the crucial items required for calculating mass flow. Looking at an application with its established requirements, we often jump right to determining “what flow rate is required?” However, it is important to remember that mass flow applications using volumetric units must reference a standard temperature and pressure. But why is this the case?

When examining liquid flow instruments, we know that liquids are incompressible and thus the amount of a substance present is determined by the volume being used. This leads to a simple calculation using density with the already determined volume to find the mass present in the volume or the volumetric flow.

GAs molecules @ STP Gases, however, ARE compressible and so the volume is only one factor in determining the amount of material being measured. If we look at the ideal gas law that you may remember from a chemistry class school (PV = nRT), we understand that temperature (T) and pressure (P) must also be considered in the equation. Otherwise it is impossible for us to know “how much” of the substance (n) there is in the space (V) or flowing through the system.

But given all of this information do we actually end up with the mass flow? The actual quantification of this “how much” calculation is expressed in moles (n), which is an extremely large number of molecules of a gas stated as Avogadro’s number, equal to 6.02x1023 (Don’t be scared by this value, though. A mole is a number, just like one dozen is 12, so one mole is 6.02x1023 molecules). Since the number of molecules of a gas and the mass are directly related for each gas type (i.e. molar mass), we are able to calculate the mass of the volume or volumetric flow based on the number of moles present. This is based on the assumption that the measured gas is pure and not contaminated with any other gases.

We’ll look at an example of the difference of STP conditions in a mass flow meter.  Teledyne Hastings Instruments assumes STP of 0°C and 760 Torr, but would prefer the customer to specify their STP conditions for the application. We will use the frequently referenced STP of 20°C and 760 Torr for the second part of this example. Suppose that we are looking to  measure 1 SLM (Standard Liter per Minute) of Nitrogen gas. As I’ve discussed earlier, the 1 SLM must be referenced to an STP value, so we will use our assumed conditions of 0°C and 760 Torr. If we were to change to the second set of conditions, the number of moles present in the flow (Molar Flow Rate) would change, and our mass flow rate would thus change (based on the direct relationship between mass and moles). Our initial mass flow rate of 1 SLM of Nitrogen at 0°C and 760 Torr would now be 1.074 SLM of Nitrogen at 20°C and 760 Torr.

Mass Flow Meter  Mass flow controllerAn important item to note is that the STP conditions are not actually present during the calibration of mass flow meters and mass flow controllers. Gas conditions are not brought to 0°C and 760 Torr prior to running calibration of equipment. The substance may not even be in gas phase at 0°C. The STP conditions are simply stated to define the standard volumetric flow rates of a substance IF it were an ideal gas at standard conditions.

This is also the reasoning for the addition of the “S” or “Standard” at the start of the stated volumetric flow rate (e.g. Standard Liters per Minute (SLM) or Standard Cubic Centimeters per Minute (SCCM)). We are stating the volumetric flow that would be present using standard conditions. So, using the information that we learned earlier, by stating the units in Standard Volumetric Flow Rate we are actually stating the Molar Flow Rate. This information changes based on the standards we are referencing and emphasizes the importance of stating the required STP conditions.

We welcome your comments and your questions about mass flow. Please complete the form below:

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Brandon Hafer is an Application Engineer with Teledyne Hastings Instruments. He completed his undergraduate degree studying meteorology at the Pennsylvania State University before serving as an officer in the United States Navy. He received his master’s degree in Systems Engineering from George Washington University and has been with Teledyne Hastings Instruments for two years. If you would like to contact him, he can be reached at brandon.hafer@teledyne.com.

Tags: Teledyne Hastings Instruments, Flow Controller, Flow Meter, STP, Thermal Flow, Standard Temperature and Pressure

Happy 45th Birthday Teledyne Hastings Instruments

Posted by Doug Baker on Tue, Feb 26, 2013 @ 03:17 PM

 

The entries in these blog pages are intended to provide helpful knowledge regarding vacuum gauges, vacuum instruments, gas mass flow meters, and flow controllers. But we could not pass an opportunity to celebrate an anniversary of sorts – on January 30th, 1968, Teledyne and Hastings - Raydist, Inc. announced that Teledyne would acquire the Hastings - Raydist company. According to the announcement in the Wall Street Journal, Hastings shareholders would receive one share of Teledyne stock for each 2.98 shares of Hastings – Raydist stock. So Hastings has been a part of Teledyne for 45 years…


Happy 45th birthday Teledyne Hastings Instruments!

 

Teledyne Hastings Vacuum gauge Apollo 11The history of the Hastings Instruments Company stretches all the way back to 1944. Next year, Hastings will celebrate its 70th birthday. But while we are in a corporate history mood, it might be fun to recall everybody’s favorite Hastings’ story:  In 1967, Hastings vacuum sensors were designed to travel to the moon and back. One of the objectives of the Apollo missions was to bring lunar samples back to earth. Special boxes, fitted with Hastings vacuum thermocouples were designed and built by Oak Ridge National Labs. Each box was required to be vacuum sealed; the Hastings thermocouple ensured that the seal was good before launch, and after splash down. The box and sensor worked perfectly.  Today, the thermopiles from the Apollo 14 mission are on display on a wall between one of the company’s conference rooms and a hallway. A magnifying lens and lamp installed in the display allows visitors to see the vacuum sensor.

Carol Hastings Saunders, daughter of Charles and Mary Hastings, recounts an interesting story in her book, “The Story of Hastings Raydist”. Two years prior to the acquisition of Hastings by Teledyne, Hastings was looking for an acquisition of its own to handle military contracts. The company considered Automated Specialties in Charlottesville Virginia. In 1965, Hastings began to acquire Automated Specialties by investing $100,000.But before the year was over, Automated Specialties was itself acquired by Teledyne. As a result, Hastings then held 11,948 shares of Teledyne. In late 1966, Hastings sold the shares and recognized $800,000 after taxes. Not bad on a $100K investment.


Today, Hastings Instruments is part of the Instrumentation Segment of Teledyne Technologies Incorporated (NYSE: TDY). The Instrumentation Segment provides measurement, monitoring and control instruments for marine, environmental, scientific and industrial applications. The Segment also provides power and communications connectivity devices for distributed instrumentation systems and sensor networks deployed in mission critical, harsh environments.  A complete history of Teledyne is given in Dr. George A. Robert’s book, “Distant Force – A Memoir of the Teledyne Corporation and the Man Who Created It”.


We welcome your comments on this history topic. Please complete the form below:

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Douglas Baker used his first vacuum gauge while an undergraduate physics major at Indiana University of Pennsylvania. In graduate school at William and Mary, Teledyne Hastings vacuum gauges monitored the forelines in the vacuum systems in the atomic and molecular lab where he worked. Today, Doug is the Director of Sales & Business Development at Teledyne Hastings Instruments and he can be reached at dbaker@teledyne.com

Tags: Teledyne Hastings Instruments, Vacuum gauge, Sensor

FAQ Corner – How Accurate is My Thermocouple Vacuum Gauge

Posted by Vikki Jewell on Tue, Feb 05, 2013 @ 08:24 AM

 

Recently, I found myself pouring through dozens of website posts comparing watches used by long distance runners.  This year I am pursuing, perhaps in vain, a new PB at my favorite 10K; this will require a boost from technology.  Now, if I have learned nothing in the last decade working with measurement instrumentation, it is that if a process can be measured, it can be improved.  Understanding those measurements can be a challenge.    Repeatedly, bloggers on running sites have been asking if the race course mileage should closely match the mileage on a GPS watch.  Reminding me, no matter the application, a key selection criterion for a measurement instrument is accuracy.

 Accuracy is the deviation of a reading when compared to a standard.  In general, better accuracy requires higher instrument costs.   However, too little accuracy and process efficiency, production costs, and user satisfaction may suffer. So many users want to know, “What is the accuracy of my thermocouple vacuum gauge”?   In this blog, I will discuss the accuracy of thermocouple vacuum gauges.

In the low to medium vacuum range, many users select thermocouple vacuum gauges for their long-life, rugged performance, and low cost. Typical applications in the low to medium vacuum range include lighting, monitoring cryogenic jackets, vacuum pump performance, and HVAC/refrigeration.   

Vacuum Gauge OutputIn  order to better understand the accuracy of a thermocouple vacuum gauge, it is helpful to review the response curve of these vacuum gauges. In the accompanying figure, we show the output of three of Teledyne Hastings most popular vacuum gauges. Note that each vacuum gauge tube family (DV-4, DV-5, and DV-6) has a range of pressures where the sensitivity, defined as the change in output as a function of pressure is very good. In this pressure region, the output is very repeatable and gives the best accuracy. Note that at the far ends of the curves, the sensitivity flattens out which in turn causes more uncertainty in the pressure measurement. So in general, the best accuracy of the thermocouple gauge is found in the middle of the curve. This fact can help the user select the best vacuum gauge tube family for a given application.  Note that the measurement accuracy reflects the gauge as a whole system (meter, cable, and thermocouple gauge tube) and not the individual components.  (So, it does not make sense to ask, what the accuracy of a thermocouple gauge tube is.)   Users can look up their pressures by reading their output voltages.  The voltage shown here is an amplified signal derived from the thermocouple output. 

In Nitrogen with a new vacuum gauge tube and 8 feet of meter cable, the anticipated accuracies* of the Teledyne Hastings Vacuum products are over some given range of pressures  However, as we discussed previously, in the middle of the response curve, the user can expect to have better accuracy:

VT-4 Series ± (20% of reading + 0.01 Torr) (max)

VT-5 Series ± (20% of reading + 0.2 mTorr) (max)

VT-6 Series ± (15% of reading + 1 mTorr) (max)             

*Data collected with digital meters

 It might be helpful at this point to review some of the factors that can affect thermocouple gauge accuracy. The accuracy statements shown in the chart above are for nitrogen. Since thermocouple gauges are gas species dependent (in other words, the output that the gauge will give will depend on the composition of the gas for a given pressure), the use of the gauge in gases other than nitrogen will affect the accuracy.  Also, the condition of the gauge tube is important. A gauge tube that is contaminated with pump oil and/or process material may not provide the expected accuracy. Temperature variations can also cause changes in thermocouple gauge output. Applications engineers at Teledyne Hastings are available to help you understand these effects.

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                  We would like to hear from you.  Contact us with your vacuum experiences.

 Vikki Jewell is a part time 10K runner and a full time applications engineer at Teledyne Hastings. She has been helping users of scientific instrumentation for twenty years.  Vikki can be reached at victoria.jewell@teledyne.com or hastings_instruments@teledyne.com.

Tags: Teledyne Hastings Instruments, Vacuum gauge, accuracy