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Teledyne Hastings Instruments Blog

John Glenn, NASA Langley, and Hampton, VA

Posted by Doug Baker on Mon, Dec 12, 2016 @ 04:22 PM

As we say good-bye to John Glenn, it is a good time for Teledyne Hastings to recall with pride our company’s and our city’s connection to this great American hero. Now, many people know that John Glenn was the first American to orbit the earth. But most people don’t know that the original seven Mercury astronauts, including John Glenn, received their original spaceflight training in 1959 at NASA-Langley in Hampton Virginia which is also our home for Teledyne Hastings.

 

The Hampton Roads area of Virginia has memorialized several landmarks to commemorate Project Mercury. There are several bridges in the city of Hampton which are named for the astronauts. “Military Highway” was renamed to Mercury Boulevard. And, in Newport News, the Denbigh branch of the Newport News Public Library System is the “Grissom Library”.   

NASA was formed in late 1958 when NACA operations were converted over. Previously, NACA (National Advisory Committee for Aeronautics) was established in 1915 and built Langley field in Hampton. Now in doing some background reading for this blog, I found it interesting to learn that NACA was created out of fear that the U.S.A. might be falling behind the Europeans in aeronautics and that NASA, in turn, was created out of fear that the U.S.A. was falling behind the Soviets in the Space Race.

 

In a book entitled The Story of Hastings-Raydist, Carol Saunders points out that NACA did not hire many engineers during the first part of the Great Depression. But, in 1935, NACA accelerated hiring and they brought on Charles Hastings as a “Junior Scientific Aide”. In 1939, a newly hired mathematician named Mary Comstock was hired and placed in an office across the hall. The two were married and together created Hastings Instruments in 1944.

 

And speaking of mathematicians at Langley, there is a movie “Hidden Figures” (released December 25, 2016), which tells the story of three female mathematicians who were part of the computer pool. Which brings us back to John Glenn. In the early days of computers, engineers did not always trust the results of the electronic data processors. The computer pool, in other words, human mathematicians, were used to crunch through complex calculations. Before his historic flight in 1962, Glenn requested that one of these computer pool women, Katherine Johnson, verify the results of the computer. The contributions of these women to the space program was remarkable.

For more information on Teledyne Hastings Instruments click the button below or visit www.teledynehastings.com

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The following books were referenced in the writing of this blog:

Hidden Figures by Margot Lee Shetterly

Hampton - From the Sea to the Stars edited by James T. Stensvaag

The Story of Hastings-Raydist by Carol Hastings Saunders  

Tags: NASA

300 Vue - Inputs and Outputs

Posted by Doug Baker on Thu, Nov 17, 2016 @ 08:52 AM

Vue_Touch_Screen.jpgTeledyne Hastings is proud to release our newest, most advanced, line of digital flow meters and flow controllers - the 300 Vue. In this blog, we will discuss the three types of Input/Output (I/O) that can be used with the 300 Vue. These are: Analog, Digital, and Touchscreen Display.

 

(1)   Analog

The 300 Vue is very flexible. The instrument can be configured to give and receive analog signals. For example, the 300 Vue can use 0-5 VDC, 0-10 VDC, 4-20 mA, or 0-20 mA.

Let’s take a look at a 300 Vue flow controller which has been setup to have a full scale flow rate of 100 sccm and has 0-5 VDC I/O. In the case of a flow controller, there are two analog voltage signals that we need to understand. The first is the flow output signal. In our example (0-5 VDC), 5 VDC corresponds to 100% of the full scale of the flow controller. The relationship between the voltage output signal and flow rate is linear. So, if we have an output of 1 VDC from the 300 Vue, then we would have a flow rate of 20% of full scale which corresponds to 20 sccm (20% * 100 sccm = 20 sccm).

Correspondingly, in our example, the 300 Vue will accept an analog command signal between 0 and 5 VDC. Again, 5VDC corresponds to 100% command signal. The command signal tells the flow controller how to set the flow rate. So, if we wanted the flow rate to be 75 sccm, we would provide a 3.75 VDC command voltage  (75 sccm* (5 VDC/100 SCCM) = 3.75 VDC).

One last comment before we move on. Analog I/O is still used in many applications. Older flow power supplies and PLC’s often utilize analog I/O. The 300 Vue flow instrument makes it easy to integrate into these systems.

Interested in more information on the Vue? click here

(2)      Digital

The 300 Vue can provide digital I/O via RS232 or RS485. Connection to the digital port is made via the micro USB connector or the small bayonet-style connector. Let’s take a quick look at an RS232 command for the 300 Vue. If we send “F”, the 300 Vue will respond with the flow rate.

f <cr> <lf>

25.889 sccm

 

Simple - right? Now in the case of a flow controller, we will want to be able to send a command signal to tell the flow controller how to set the flow rate. One way to do this is to use V5, the “Setpoint”. The Setpoint Command is simply the flow rate expressed as a percent of the flow controller’s full scale. So, “V5=100” will set the flow rate to 100% of full scale. You can also use V4 which sends the command in the given units, as opposed to % of full scale.

Digital communication with the 300 Vue can be utilized in a few different ways. First, you can use our free user software which can be obtained from our website. If you want to see all of the capability (including flow data logging) you can watch this short “How To” video.

Next, many of our digital flow controller users write their own code using LabView. By working with the “F” and the “V5=  “, or “V4=  “ commands, the user can easily read and control the flow rate in their application.

Here at Teledyne, we often use TeraTerm for communicating with our digital flow instruments. Click to visit TeraTerm website for more information.   

TeraTerm is nice because it is open source (free) yet it is very powerful. I also like the fact that TeraTerm allows the user to save and restore a communication set up file. In other words, once you have a TeraTerm “ini” file working, you can save it so that you don’t have to reconfigure the settings each time you start up the program. If you have TeraTerm and would like a copy of my setup file for the 300 Vue, just send me an email

Interested in more information on the Vue? click here

(3)      Touchscreen Display

Ok, we’ve talked a little bit about analog I/O and digital communications. Now, let’s explore the coolest feature of the 300 Vue – the color touchscreen display. With the touchscreen display, it is very easy to see and control the flow rate. First, we should point out that the 300 Vue flow instrument is very easy to power up; you just plug in the connector and you are in control.

Top View with Plug.jpg 

Once the flow instrument is powered, the flow rate is observed as shown in the picture below:

DSC_0155m.jpg 

Now, to change the flow setpoint or command signal we touch Setpoint and we see the numeric keypad screen as shown below.

DSC_0161m.jpg 

Changing the setpoint is easy… you just type the value you want and hit ENTER. The display then returns to showing the flow rate.

 

The 300 Vue is very flexible with respect to Inputs and Outputs. If you have questions about I/O, our applications engineers are always standing by and ready to help. You can reach us at hastings_instruments@teledyne.com or by calling 1-800-950-2468.

Interested in more information  on the 300 Vue Series

Tags: Digital Flow Meter, Flow Controller, 300 Vue

Benefits of a Flow Service Plan for Mass Flow Meters/Controllers

Posted by Stuart Taliaferro on Tue, Jul 05, 2016 @ 02:09 PM

Flow_Service_Plan.jpgThere are many benefits for having a Flow Service Plan for your Mass Flow Meters/Controllers.  This blog touches on just a few of them.

Maintaining calibration on measurement instrumentation is essential to minimizing uncertainties and ensuring accurate readings. Hastings Instruments offers its flow calibra­tion services featuring deeply discounted pricing. The Flow Service Plan allows the user to integrate high-quality calibrations into metrology schedules for Hastings’ 200 Series, 300 Series, and digital 300 series flow meters and controllers. The Flow Service Plan may be purchased for either new or recently reconditioned instruments.

HFC-D-302A.jpgEach instrument under the plan is eligible for three calibrations anytime within a 36-month period. Under the discount Flow Service Plan, the user purchases two calibrations and receives a third at no cost. At the time of purchase, the user may specify a calibration interval; Hastings Service will track the unit’s history and provide advance notice (four weeks) of the next scheduled calibration.

200_series_of_flow_instruments.jpgThe calibration and service department will clean, recalibrate, and ship the instrument back to the user within 5 working days or less per instrument.  The Flow Service Plan will improve up-time at the user’s facility while ensuring compliance to metrology requirements. All calibration performed at Teledyne Hastings is traceable to the National Institute of Standards and Technology (NIST).  In addition to this, calibrations are compliant to ISO 17025 requirements. 

Hastings offers a complete service department dedicated to recalibration, repair, and service for all of our mass flow and vacuum products.

If you have any questions or would like a Flow Service Plan quoted for your new or recently purchased mass flow instrument, click the button below

Interested in Service Plan

or contact your local Teledyne Hastings representative or the factory (757-723-6531 or HASTINGS_INSTRUMENTS@teledyne.com)

Tags: Flow Controller, Flow Meter

Fundamentals Vacuum and Mass Flow Technology

Posted by Doug Baker on Wed, Jun 22, 2016 @ 10:27 AM

One of the goals of these blog postings is to give readers knowledge about vacuum and mass flow technology. The Society of Vacuum Coaters has established a foundation (SVCF) with a similar goal. Dr. Don McClure (Acuity Consulting & Training) has created “The Vacuum Wizard Video”. Dr. McClure worked at both IBM & 3M and has been teaching for over 20 years about vacuum coating onto flexible substrates.

Generic_Roll_Coater_Designs.jpg

As stated on the SVCF website, “The Vacuum Wizard Video brings to life the fundamentals of vacuum and vacuum coating technology through an informal and thought provoking presentation using non-technical jargon and filled with live demonstrations.

The Vacuum Wizard Video seeks to raise awareness of students and educators about the fascinating world of vacuum and vacuum coating technology. The only prerequisite is a curiosity about this amazing technology.

The Vacuum Wizard Video can be a useful training tool in the corporate world for personnel who require a basic understanding of vacuum technology. Sales representatives, customer service personnel, field service and maintenance technicians, lab technicians, and engineers with no vacuum technology background, can all benefit from the Vacuum Wizard Video.”

Vacuum_Model_2002_Gauge.jpg

(Check out the Teledyne Hastings’ Vacuum Model 2002 Vacuum Gauge on the table)  Click the button below to request an evaluation sample of the 2002 Vacuum Gauge

 Request   Evaluation Sample

You can get more information about the SVC Foundation and the video series by visiting:

http://svcfoundation.org

Click to see a sample of the Vacuum Wizard Video 

 

Tags: Mass Flow, vacuum gauges

FAQ Corner – What is turndown ratio?

Posted by Wayne Lewey on Mon, Jul 27, 2015 @ 04:03 PM

We are occasionally asked for the turndown ratios of our flow meters and flow controllers.  There are varying perceptions as to what this term actually means.

The turndown ratio of a Mass Flow Meter (MFM) or Mass Flow Controller (MFC) defines the usable range for which it can operate while maintaining its published accuracy.  It can be expressed using the following formula:

Turndown_ratio

Teledyne Hastings Flow Meter HFM-200-202A flow meter with a large turndown ratio will have a large operating range.  This can also be indicative of the flow meter’s cost.  For example, variable area flow meters (rotameters) typically have lower turndown ratios compared to thermal mass flow meters.

Most analog mass flow meters have an accuracy of ± 1% of Full Scale (FS) and have resolution better than 1%.  The usable range is from 1% to 100%.  They will have a turndown ratio of 100/1 or more commonly expressed as 100:1.  Digital flow meters will have an even greater turndown ratio due to their higher accuracy.

HFC-D-308Most analog mass flow controllers also have an accuracy of ± 1% FS.  However, they typically have an automatic valve shut circuit that closes the valve at flow rates below 2% of FS.  This is to ensure full valve closure in the event of a small zero offset.  The usable range is from 2% to 100%.  Since measurement is not possible below 2%, these will have a turndown ratio of 100/2 = 50/1 or 50:1.

 For more information on Turndown Ratio or our Flow Meters, please contact Wayne Lewey 

 

 

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Tags: Flow Meter

Desired Characteristics of a Thermal Mass Flow Sensor - Part 2 of 2

Posted by Doug Baker on Tue, Jun 09, 2015 @ 11:58 AM

This is part two of a two-part blog on Thermal Mass Flow sensors.  In part one, we described the desired characteristics of a thermal mass flow sensor.  In part two, we will discuss the operation of the 300 series flow sensor (Patent #6,125,695) and how its design addresses the desired traits.

300_series_flow_sensor_insideIn our previous blog, we showed a cutaway of a thermal mass flow meter.  Now let’s take an inside look at the 300 series flow sensor:

When gas is flowing through the bypass shunt, a small pressure drop is developed which will direct a fraction of the flow through the arced / semi-circular capillary tubing in the flow sensor. On the outside of the capillary tube, there are two resistive wire coils which are tightly wound and in excellent thermal contact with the tube. These two identical windings are referred to as:

  • Upstream Heater Coil (1)
  • Downstream Heater Coil (2)

Associated with each of the two heated coils is an ambient coil. The ambient coil is in excellent thermal contact with the aluminum ambient block.  Aluminum has a very high thermal conductivity which ensures that both ends of the sensor tube and the two ambient coils (3 and 4) will be at the same temperature.

heated_coils_upstream_downstreamTwo identical Wheatstone resistance bridges are formed from the two pair of coils (see image on right).

The circuit shown in the image on the right is designed to ensure that the heated coils (upstream and downstream) are maintained at a constant temperature (ΔT) above the corresponding ambient coils.

Next, we calculate the power (W) required to maintain ΔT by:

Power_Formula

This power will be calculated for both the upstream bridge and the downstream bridge. It can be shown that:

 Upstream_downstream_bridge_formula

So, by maintaining both heaters at the same ΔT above ambient, the mass flow rate is directly proportioned to the difference in power (W) between the two bridges. For example, when no flow is passing through the capillary sensor tube, the power needed to maintain ΔT will be the same (i.e. ṁ = 0)

As gas flow increases in the tube, heat is transferred from the upstream heater to the gas stream.  This will force the upstream circuit to use more power to maintain ΔT. In turn, the gas will transfer heat to the downstream heater which will cause the downstream circuit to use less power to maintain ΔT.

LinearityNow, here is the best part: the mass flow rate is directly proportional to the power difference. In other words, LINEARITY!

In our previous blog, we discussed how excellent linearity leads to improved accuracy. And, not only does the 300 series sensor give excellent linearity, the circuit shown on the right reacts very fast to changing flow. Thus, the 300 series has excellent responsive time.

One last note, we have designed the 300 series to use relatively large diameter tubing.  This larger tubing allows flow meters to be designed with lower pressure drop than many mass flow meters on the market.  

Visit our website for more information on Teledyne Hastings 300 series Flow Meters.

Teledyne Hastings' Thermal Mass Flow Sensors are used worldwide.  Download our application note on High Throughput Leak Detection to learn about improving lead testing precision and throughput and how to reduce testing time.  

High Throughput Leak Detection

Be sure to visit our website for additional information on Teledyne Hastings Mass Flow Controllers and Mass Flow Meters

Tags: Flow Meter

Facts You Might Not Know about Teledyne Hastings Instruments

Posted by The Teledyne Hastings Team on Thu, May 14, 2015 @ 04:45 PM

Quality Teledyne Hastings ISO 9001 CertificationLast month, we passed our ISO 9001 surveillance audit.  It has been over twenty years since we first obtained ISO and we wanted to take a step back and review some significant accomplishments.  

Teledyne Hastings Instruments rich history and customer centric vision continues to support, influence and grow with those who depend on quality process control and automation.

That's why we wanted to take a moment and celebrate a milestone with our core clients and those considering a Teledyne Hastings Instruments Flow instrument or Vacuum Gauge for the first time.


2015_Infographic_ISO_20_Years_2

Teledyne Hastings Instruments' has been providing quality thermal mass flow instruments and vacuum meters and controllers for applications ranging from academic research to space exploration for over 70 years.  Let us work with you to find the best solution for your process.

OEM, custom applications, lead time crunch, just curious:   

Contact Us

Tags: Teledyne Hastings Instruments, Flow Controller, Flow Meter, Vacuum gauge, vacuum controllers, ISO 9001 and Thermal Mass Flow, ISO 9001 and Vacuum Gauges

Desired Characteristics of a Thermal Mass Flow Sensor - Part 1 of 2

Posted by Will Harrison on Fri, Mar 20, 2015 @ 10:59 AM

ThermalMassFlowSensorCutawayThis is part 1 of a 2 part blog on Thermal Mass Flow Sensor.  We will describe the desired characteristics of a thermal mass sensor in Part 1 and Part 2 will discuss the operation of the 300 Series flow sensor (Patent #6,125,695) from Teledyne Hastings.

A thermal mass flow meter consists of the following:

  • Electronic Circuit Card

  • Flow Sensor

  • Bypass Shunt

  • Base

A cutaway is shown in the image on the right.

In a typical thermal mass meter, gas enters the flow meter via the upstream port which is attached to the process with a fitting (VCR, Swagelok…). Most of the gas will move through the bypass shunt; however, a certain fraction will flow through the thermal mass flow sensor. Note that the shunt is selected such that amount of gas moving through the flow sensor is approximately the same at full scale flow. The gas then exits the flow meter via the downstream port.

Ideally, the thermal mass flow sensor would exhibit the following characteristics: first, it would be linear. What we mean by linear is that the sensor’s electronic output should be directly proportional to the flow rate moving through the sensor throughout its range. Linearity of the flow sensor leads to the second desired characteristic: accuracy. An accurate flow sensor can give the users the benefit of better gas flow measurement, control, and understanding of their system parameters.

FlowSensorOutput

Before we move on with our desired characteristic list, we need to discuss a little about how linearity can factor into calibration. Typically, a thermal mass flow meter is calibrated in nitrogen (or in the case of very large flows, it may be calibrated in air). The output of the flow meter can then be scaled for use in other process gases. (In other words, the flow meter technician can calibrate a flow meter for use in a corrosive process gas like silane (SiH4) – without having to use silane). A linear flow sensor will retain its linear behavior as the gas is switched from the calibration gas (N2) to the process gas.

Our next desired characteristic is fast response. Ideally, the flow sensor would respond instantaneously to a change in the flow rate. Aside from the obvious benefit of instant real-time vision of the flow in a process, fast response becomes critical when the flow meter is coupled with a proportional control valve to create a thermal mass flow controller. Finally, we would like the thermal mass flow sensor to have a low pressure drop. A low differential pressure drop across the flow meter is ideal for leak detection and gas sampling applications.

Teledyne Hastings' Thermal Mass Flow Sensors are used worldwide.  For more information on on Best Practices for Mass Flow Controllers and Mass Flow Meters download our whitepaper.

Download Whitepaper

Be sure to visit our website for additional information on Teledyne Hastings Mass Flow Controllers and Mass Flow Meters

In our next blog, we will describe the sensor that is core to the Teledyne 300 Series of mass flow meters. We will also look at how the 300 Series thermal mass flow sensor addresses each of the desired characteristics described above.

 

 

Tags: Thermal Flow

Piezoresistive Pressure Sensors and the HPM-760S

Posted by Will Harrison on Thu, Nov 20, 2014 @ 10:09 AM

Piezoresistive Pressure Sensors - Direct Vacuum Gauges

In the vacuum world, gauges can be characterized as being either “Direct” or “Indirect”. Direct gauges are so-called because they directly measure the force imparted on some surface. And since P = F /A (pressure equals force per unit area), the gauge is directly measuring the pressure. Some examples of direct gauges would include: Bourdon gauges, capacitance manometers, piezo-resistance gauges (we’ll talk more about this one later in this blog).

Bourdon Gauge Teledyne Hastings Instruments Framed

                                                            Bourdon Gauge

 

Indirect gauges do not “directly” measure the force associated with the gas in the chamber. Rather, these gauges measure some property associated with the gas. For example, thermocouple vacuum gauge tubes measure the thermal conductivity of the gas which is a function of the pressure. As another example, ionization gauges measure the ionization rate of a gas which is proportional to the pressure over a several orders of magnitude. So, thermocouple gauges and ionization gauges can both be called Indirect Gauges.

Thermocouple_Guage_Tubes_Teledyne_Hastings_Instruments_framed                 Ionization Gauge IGE3000

    Thermocouple Gauge                                                       Ionization Gauge

 

One of the key features of a Direct Vacuum Gauge is that it does not matter what gas in the vacuum is being measured. In other words, if the user has 20 Torr of Argon, Helium, Methane… or Air, a Direct Gauge will read the same pressure. To say it another way, Direct Gauges are said to be Gas Composition Independent.

Teledyne Hastings provides a Direct Vacuum Gauge called the HPM-760S. (It is called the 760 because it will always provide very accurate results at atmospheric pressure.)  The HPM-760S utilizes a piezoresistive sensor. A cutaway drawing of this sensor is shown in the figure below.

 

In this cutaway, we can see the micro-machined sense die. This die contains a resistance bridge that is made up of piezo-resistors. In a piezoresistor, the resistance changes as force is applied. The resistance bridge sensor is itself in contact with silicone oil that transmits the force from the gas in the vacuum system to the sensor. And, one of the most important things to observe about this sensor is that the only wetted material actually exposed to the gas in the vacuum chamber is 316L Stainless Steel. So to summarize, the gas molecules in the vacuum system exert a force onto the stainless steel diaphragm which in turn imparts a force on the piezo-resistive sense via the silicone oil.

 

Cross Section of HPM-760 Sensor

One last thing to mention about our cutaway drawing: the sensor of the HPM-760S is referenced to vacuum. This type of arrangement gives ABSOLUTE readings. Other types of pressure sensors can be referenced to atmospheric pressure (GAUGE readings) or can be connected to another part of the process stream (DIFFERENTIAL) readings.

                 

The HPM-760S is a DIRECT, ABSOLUTE, vacuum gauge. It is an excellent gauge for use on systems that are evacuated using a diaphragm pump. These types of pumps typically operate in the region between a few Torr and atmosphere. And, as mentioned previously, the HPM-760s has only stainless steel exposed (wetted) to the gas in the vacuum chamber. In other words, any gas (including corrosives) which is compatible with stainless steel will be compatible with the HPM-760S.

 

HPM 760S Transducer Teledyne Hastings Instruments framed                                                          HPM-760S

The HPM-760S takes the output from the piezo-resistance bridge and amplifies it for the convenience of the user. At time of order entry, the user can select from four linear outputs.

 

760_Sensor_Output_Options-1

Two more quick notes about the analog output: First, selection of the 0-10 VDC version makes the conversion from voltage to pressure trivial. As a specific example, at 760 Torr, the voltage output is 7.60 Volt – SIMPLE.  Second, the 4-20 mA output is a good selection in industrial environments that might have lots of electrical noise/interface or where the pressure signal must be transmitted long distances  (>25 feet or 10 meter).

 

In addition to the linear outputs, the user can also select from several common vacuum system connections:

760_Sensor_end_fittings

 

 

 

 

 

 

 

The HPM-760s is very easy to use. See the image below. Two wires (Pins 3 & 4) are used to provide power to the HPM-760S. The two other wires (Pins 1 & 2) provide the linear output. So the HPM-760s can be used as a stand-alone vacuum gauge.

 


HPM 760S pin out Teledyne Hastings Instruments

 

In some cases, a user might like the convenience of having a readout preconfigured for the HPM-760S. The THCD-100 (shown below) can be quickly attached using the CB-760S-THCD cable. In this scenario, the user not only gets a power/display module, but the THCD-100 will also provide dual process control relays. The THCD-100 can be easily connected to a computer or PLC via RS232. And finally, by using the DisplayX software (free) for the THCD-100, the user can also easily collect and log data to a spreadsheet.

THCD-100_Teledyne_Hastings_Instruments

                THCD-100

  

 

 

  Download Tech Note  

 

 

This blog was prepared by Will (Iron Man) Harrison and Doug Baker. Will runs at least two marathons per year – this Fall, Will is going to run his first New York City Marathon.

 

Tags: Vacuum gauge, Sensor, pressure, vacuum pressure, vacuum instruments

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