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Questions and Answers about Axetris Mass Flow Meters, Controllers and Manifolds

Frequently Asked Questions

Axetris mass flow products

1      Introduction

 

2      Mass Flow Technology

2.1       What is a laminar flow?

2.2       What is a turbulent flow?

2.3       What is the Reynolds number?

2.4       What is the influence of pressure and temperature on volumetric flow?

 

3      MEMS mass flow

3.1       How does MEMS based flow measurement work?

 

4      Axetris Mass Flow Devices Type Key / Product Selection Key

4.1       What does the abbreviation stand for?

4.2       What does the type key of MFM / MFC 2xxx stand for?

 

5      Gas Handling

5.1       How much is the pressure loss over an Axetris Mass Flow Meter?

5.2       How much is the pressure loss over an Axetris Mass Flow Controller?

5.3       What is the difference between relative and absolute pressure?

5.4       What is the difference between internal and external leak rate?

5.5       Do Axetris devices have built-in filters?

5.6       How does positioning of the Mass Flow Meter / Mass Flow Controller affect the measurement of the device?

 

6      Performance Questions

6.1       What gases can be measured and / or controlled with an Axetris Mass Flow Meter / Mass Flow Controller?

6.2       How is the accuracy defined?

6.3       How is the settling time defined?

6.4       What is the dynamic range of an Axetris device?

6.5       How does the signal filter work?

6.6       What is a zeroing and when should I use it?  

 

7      Communication Questions

7.1       How do I convert the digital Output into a flow value?

7.2       How fast can I communicate with an Axetris device?

7.3       How do I switch between my available calibrations?

7.4       What is the valve override and what can it be used for?

1 Introduction

The below frequently asked questions will also be available in pdf shortly.

 

2 Mass Flow Technology

2.1 What is a laminar flow?

Laminar or Smooth flow tends to occur at lower flow rates through smaller pipes. In essence, the fluid particles flow in cylinders. The outermost cylinder, touching the pipe wall, does not move due to viscosity. The next cylinder flows against the unmoving fluid cylinder, which exhibits less frictional “pull” than the pipe wall. This cylinder will move the slowest. This continues, with the centermost cylinder having the greatest velocity.

 

2.2 What is a turbulent flow?

Turbulent flow is by nature chaotic. The fluid mixes irregularly during turbulent flow. Constant changes in the flow’s behavior (wakes, vortexes, eddies) make flow rates difficult, if not impossible, to accurately measure. Turbulent flow usually occurs at high flow rates and/or in larger diameter pipes. Turbulent flow is usually desirable when solids must remain suspended in the fluid to prevent settling or blockages.

 

2.3 What is the Reynolds number?

How do we know if a flow is turbulent, transitional or laminar? In the late 1800’s, Osbourne Reynolds discovered that the type of a fluid flow is related to the fluid’s density, mean velocity, diameter and viscosity. This dimensionless (no units) number helps predict changes in flow type. In simple terms, the Reynolds Number can be written as:

Re = density x mean velocity x diameter / viscosity

It is generally accepted that flow is laminar if the Reynolds Number is less than 2000. Transitional flows have a Reynolds Number between 2000 and 4000. Flows are considered turbulent when the Reynolds Number is greater than 4000. Using the Reynolds equation, we can see that reducing the density, mean velocity and/or diameter of a turbulent fluid flow (with viscosity constant) will make the flow “more” laminar. This could also be accomplished by increasing the fluid viscosity (with density, mean velocity and diameter constant). The inverse is true to make a flow more turbulent.

 

2.4 What is the influence of pressure and temperature on volumetric flow?

The conservation of mass law states, that the mass flow into a system always has to be conserved --> m1=m2 because mass flow is independent of temperature and pressure.

Since we are expressing the Mass flow device gas flow with a volume sccm unit (standard cubic centimeters), we must reference our flow to a standard state (T=0°C and p=1 atm) to account for the density of the gas that depends on temperature and pressure.

This is because the volume flow depends on the density of the gas, which depends on pressure and temperature as well:

Axetris works with the formula of the conservation of mass law that states, that the mass flow into a system always has to be conserved

3 MEMS mass flow

3.1 How does MEMS based flow measurement work?

MEMS = micro electromechanical system

Graph explaining how MEMS based flow measurement works at Axetris

On the chip there are several different resistances placed. The resistance in between is used as a heater while the two on the side measure the temperature difference.

The temperature difference increases relative to the gas flow over the chip. The ratio of temperature difference to flow is then calibrated at the Axetris clean room. Each customer receives a real gas calibrated device customized to the customer's wishes.

 

4 Axetris MFD Type Key / Product Selection Key

4.1 What does the abbreviation stand for?

MFM: Mass Flow Meter

MFC: Mass Flow Controller

 

4.2 What does the type key of MFM/MFC 2xxx stand for?

 

 Axetris Product Selection Key

 

 

5 Gas Handling

5.1 How much is the pressure loss over an Axetris mass flow meter?

 

Pressure Drop typical values for N2:

50sccm:

35Pa =

0.35mbar

250sccm: 65Pa =
0.65mbar
3'000sccm: 1800Pa =
18mbar
15'000sccm 2000Pa =
20mbar
20'000sccm:
3000Pa = 30mbar

 

5.2 How much is the pressure loss over an Axetris mass flow controller?

The following minimal pressure drop is required for the valve to be able to function properly:


Ar/N2/O2/Air

He/H2
CO2
Up to 3'000sccm: 0.5 bar
0.5 bar 0.5 bar
15'000sccm: 1.8 bar
0.5 bar
3 bar
20'000sccm: 3 bar 1 bar N/A

 

The maximum allowed pressure difference (without damaging the valve) is 7 bar. This limit is given by the working principle of the valve, using a spring to hold the plunger for closing. The magnetic coil will still be able to open the valve, but the force of the spring will not be enough to control a certain flow, so the valve will be fully open.

 

5.3 What is the difference between relative and absolute pressure?

The relative pressure is measured as a difference to the ambient pressure which is approximately 1.013 bara (depending on user location). The abbreviation bara means the pressure measured absolutely, where 0 is perfect vacuum.

The following table explains the differences between the different units, as well as between relative pressure barg (bar gauge) and absolute pressure bara (bar absolute).

Graph by Axetris explaining the difference between relative and absolute pressure 

 

5.4 What is the difference between internal and external leak rate?

An internal leak is defined as a valve that is not perfectly tight and therefore gas is able to flow through the closed valve. This is mostly visible with a small flow signal on the mass flow sensor.

Graph by Axetris explaining the internal leak rate

Solution: In these cases the valve can be defective and has to be checked by Axetris. It might be possible that the valve has to be replaced.

Certain applications in high vacuum might also create a flow through the valve, as the standard valves are not designed to be tight for vacuum applications. In these cases an additional Shutoff valve should be used in line after the Axetris mass flow controller.

MFC 2100 Series with integrated shut-off valve

An external leak is defined as gas exiting the device over non tight O-rings or fittings. Excessive pressure shocks can also cause deforming or destruction of these seals and can lead to external leakages. Axetris carries out an external leakage test with each device before sending them out. Every mass flow controller has to be leak tight down to < 1x10-8 mbar l/s (tested with Helium particles) and the mass flow meters down to < 1x10-9 mbar l/s.

Graph explaining the external leak rate that Axetris works with

Solution: If the mass flow devices show signs of external leakage (pressure loss or sound of exiting gas) the device will need to be checked by Axetris.

 

5.5 Do Axetris devices have built-in filters?

The mass flow devices from Axetris do not have built-in filters by default. It is recommended to use an inline filter of 5-200 μm. These can be bought in different sizes and forms.

Image to show that Axetris mass flow devices can have built-in filters

 

5.6 How does positioning of the mass flow meter / mass flow controller affect the measurement of the device?

Gravity can affect the way the heat is distributed around the mass flow chip. This can create a flow offset on the device, even when there is now flow present. Therefore, Axetris is recommending the following mounting positions for the mass flow devices:

Zeroing not required:

Image explainig how to position an Axetris mass flow device MFM/MFC

Solution for crossed out positioning:

If the Axetris devices are still used in the crossed-out positions, it is recommended to carry out a zeroing in the final position before the first usage to neglect an eventual offset.

 

6 Performance Questions

6.1 What gases can be measured and / or controlled with an Axetris mass flow meter / mass flow controller?

Light gas types: Helium (He), Hydrogen (H2)

Standard gas types: Air, Argon (Ar), Oxygen (O2), Nitrogen (N2)

Heavy gas types: Carbon Dioxide (CO2)

Other gases on request.

 

6.2 How is the accuracy defined?

The following sketch explains the definition of accuracy following the Norm SEMI E56. It is a combination of %F.S. (Full Scale) defined in percentage of the full available flow and in %O.R. (of reading). The combination results in the green dotted line displayed below.

Graph explaining how the accuracy of Axetris mass flow devices is defined

6.3 How is the settling time defined?

The settling time is defined as the time between the setpoint step change and when the actual flow remains within the specified band (Error band see accuracy specifications Axetris).

Graph explaining how the settling time of MFCs from Axetris is defined

Definitions of Mass Flow Controller Transient Characteristics Terminology in the case where the final set point is higher than the initial set point according SEMI E17

 

6.4 What is the dynamic range of an Axetris device?

The dynamic range is defined by the maximum available flow range divided by the minimum accurately measurable flow. The definition is meant for one device and not for one calibration.

Graph explaining the dymamic range of an Axetris device

 

Following calculations show the dynamic range for the different channels (N2):


Max calibrated flow Min calibrated flow Smallest accurately measurable flow (0.2%F.S.) Maximum dynamic range possible for one device
A 150 scm
20 sccm
0.04 sccm 150 / 0.04 = 3750
B 250 sccm 50 sccm 0.1 sccm 250 / 0.1 = 2500
C 3´000 sccm 250 sccm 0.5 sccm 3000 / 0.5 = 6000
D 20'000 sccm 3´000 sccm 6 sccm

20´000 / 6 = 3333, 33

 

With a range of 6000, the C – Channel has one of the best dynamic ranges on the market. This shows the amazing range from 3000sccm down to 0.5sccm with the same hardware.

It is important to mention that two calibrations are necessary to be able to cover such a wide flow range.

 

6.5 How does the signal filter work?

The K-factor is a signal filter behaving similar to a moving average. This filter exponentially weights the new measurement value and divides it with 2<k-factor>. The higher the K – Factor is,

Axetris Graph explainig the K-factor

the slower the measurement reacts to changes. This will filter away unwanted fluctuations, but also slows down the measurement. Below is a graph showing the impact of the K-Factor on the reaction time of the device.

Axetris Graph showing the impact of the K-Factor on the raction tim eof the mass flow device

 

The filter default level can be changed by the customer and when changed it will be stored on the device per calibration separately. To change the K – Factor the customer will need to use the Customer GUI provided on the Homepage of Axetris. If there are difficulties with the GUI please contact customersupport@axetris.com.

 

Remark: When changing calibrations, please be aware to change the K - Factor as well, as the K – Factor is only changed in the active calibration.

 

6.6. What is a zeroing and when should I use it?

A zeroing can correct a positive or negative offset on the mass flow device, showing a value even when there is no actual flow present. The customer is then able to start a zeroing command (See respective communication protocol), which takes a couple of seconds. This will apply a fix offset correction for the selected gas calibration (can be put back to default by customer).

The following effects can cause an initial offset of the flow value on the Axetris Mass Flow Device:

  • Using a different gas than calibrated, caused by different gas properties.
  • Using a mass flow device position that is not recommended.
  • Using gas mixtures
  • Significant change in inlet pressure (compared to calibrated one)
  • Other unforeseen material property changes.

 

7 Communication Questions

7.1 How do I convert the digital Output into a flow value?

Calculation of the sensor flow with the digital returned value (2 bytes MSB first):

Digital output range = 0…10’000 = 0 % F.S….100 % F.S.

Formula, explaining conversion of the digital output into a flow value, used at Axetris

Example with a 250sccm F.S. device and a reading value equal to 3400 Dec:

Formula used at Axetris to calculate the conversion of the digital output into a flow value. Example with 250sccm F.S. device

Due to the headroom, a value of up to 110 % F.S. can be read, corresponding to 11’000Dec.


7.2 How fast can I communicate with an Axetris device?

The response time of an Axetris mass flow device is about 4ms. The next command to be sent to the complete network should not be sooner than 4ms.

When using RS485 full duplex it can be sent quicker than that, but the receiving line will have the same restrictions.

 

7.3 How do I switch between my available calibrations?

In case of multi-range and/or multi-gas calibrated device, channel (1 to 8) is selected as following for RS232 devices (see respective communication protocol datasheet):

 

Data direction
Hex data
Flow
Remark
Rx

63 06 69

64 06 02 6C

64 06 03 6D

64 06 04 6E

...

64 06 08 72

NA

Read which channel is selected

Select the channel 2

Select the channel 3

Select the channel 4

...

Select the channel 8

Tx

63 01 64

64

NA

Channel 1 is set

Request code back

 

The uncalibrated channels have all parameters equal to zero, so the flow will go to zero when choosing an uncalibrated channel. The Customer GUI can show all available calibrations, or it can be checked on the calibration certificates delivered with the mass flow device. Our Axetris Salespersons can also help to determine which calibrations are available on the present device.

 

7.4 What is the valve override and what can it be used for?

The valve override function can be used to force open or force close the valve of the Axetris mass flow controller. It is mostly used to purge or completely close the valve by the user, as closing the valve with this command is much faster, than giving a setpoint of 0sccm.

The control of the valve can be done directly with digital values. The internal PID controller is then off. Any digital value between (0…4095)Dec takes direct control of the valve, even if the analog mode was set. Any other value out of this range sets the device back into normal mode (digital or analog) with the last set point value for the digital mode and the actual analog value at the input pin for the analog mode. The relation between valve openness and flow is nonlinear.

 

Data direction
Hex data
Flow
Remark
Rx

62 1E 00 00 80

62 1E 08 00 88

62 1E 0F FF 8E

62 1E 10 00 8F

0

Valve open to 50 % of its max.

Valve full open (purge mode)

Set point (selected mode)

Valve forced close

An opening position is controlled not a flow

Digital output is max. = 110 % F.S.

Any values between 4096 and 65535

Tx 62
NA Request back

 

Or by an analog signal on the respective pin (see respective datasheet of the Axetris mass flow device).

Valve override close: 0…0.2V

Valve override open: 4.8…5V        (purge mode > F.S.)

 

Valve normal mode: 0.2…4.8V

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