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Teledyne Hastings Instruments Blog

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

  

 

 

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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, vacuum instruments, pressure, vacuum pressure

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

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, pressure, range, differential pressure, mass flow controller, Gas Flow Range, mass flow meter