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How do I use GCF (Gas Conversion Factors) with my mass flow meter or mass flow controller?

Posted by Doug Baker on Mon, Nov 02, 2020 @ 09:54 AM

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

200 Series Sensor

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.

Gas Conversion Factors (N2)
  200 Series 300 Series
Helium 1.402 1.400
Oxygen 0.981 0.978
Carbon Dioxide 0.743 0.753
Carbon Monoxide 1.001 1.001
Methane 0.770 0.779
Ammonia 0.781 0.781
Hydrogen 1.009 1.004
Argon 1.401 1.405

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.

Argon GCF

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.

H2 He GCF

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.   

HFM200 with THCD

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).

HFC with THCD

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.

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Tags: Flow Meter, mass flow conversion, mass flow controller, mass flow meter, Gas Conversion

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

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

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 – 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

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