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Measurement and control of low gas flow rates

Mass Flow Meter and Controller Overview

Introduction

There are several fundamental measurement techniques for flow meters, such as:

Other than the ones mentioned above, there are some less known unique types of flow meter technologies that often offer solutions for individual applications. There is not one “Best Flow Meter”, everything depends on the application and what the user expects from the unit.

In this article, we look at measuring instruments for relatively low gas flows, from a few cc/min to 500 or 1000 ln/min for example. In this application area is the gas mass flow meter with the thermal bypass concept the most common instrument. There are gas mass flow meters (MFM) and mass flow controllers (MFC). The latter is a flow meter equipped with a control valve, and the user can provide a setpoint that determines a generated gas flow. MFCs is the reference to these instruments.

Measurement Concept

Mass Flow Measurement Priciple

Schematic representation of the measurement principle

In the thermal bypass concept, most of the measured gas flow through a bypass. In the bypass is an element that generates a minimal pressure drop. As a result, a small quantity of the gas runs through the sensor, which runs parallel to the bypass. This sensor measures the gas flow, which is representative of the total amount of gas monitored. Each instrument is built for specific gas and a particular range of measurement.

Mass flow meters (MFM) and mass flow controllers (MFC), the names speak for themselves: these instruments measure and regulate the gas mass flow, for instance in kg/hr. The measured value can also be expressed in units of normalized volumes, such as ln/min (normal liter per minute) or SCFH (Standard Cubic Foot per Hour). In SCFH (Standard Cubic Foor per Hour)here the “S” refers to 20°C and 1 atmosphere, while NCCM (Normal Cubic centimeters per minute) with reference conditions as 0°C and 1 atmosphere. After all, there is a fixed relationship of a gas in mass units versus normalized volumes (density).

There is a distinction between the MFCs based on a capillary sensor and instruments with a MEMS (Micro-Electro-Mechanical System) sensor. Both sensor techniques have advantages and limitations. The word “cons” is here deliberately avoided because there are no bad measuring instruments. The application should drive preference example like Porsche 911 is a fantastic car, but you shouldn’t use it in the sand of the Sahara.

What is Capillary Flow Sensor?

The capillary sensor consists of a thin steel tube (0.2 to 1 mm o.d.) around which wrap with two thin platinum wires. The platinum wires act as electrical resistance and are part of a Wheatstone bridge. The resistance value of the platinum wire depends on the temperature. There is a current going through these wires, which heats them up. At zero flow is the resistance is the same, but when the flow goes through the sensor, there will be a difference in temperature that will be detected by the electronics.

Laminar Flow Element (LFE) equip in MFC with a capillary sensor in the bypass. This LFE creates laminar flow behavior, just like inside the sensor. The LFE means that the relationship between the flow through the sensor versus the flow through the bypass is almost linear over the measured range. Due to this linearity, it is possible to calibrate this MFCs with a cheap, pure gas like air and use a known factor to make the meter suitable for another exotic, explosive, poisonous or inflammable gas. If this so-called K-factor is applied, there will be additional uncertainties.  In user manuals of renowned manufacturers are further indicative inaccuracies reported to more than 2%. Parameters such as viscosity, pressure, and temperature affect the accuracy of the K-factor. It is essential to take note of this traditional calibration method, which creates uncertainties which are not often mentioned in the documentation.

Another ability of the capillary sensor is the possibility by some manufacturers to make all wetted part from one material, for instance, stainless steel or Monel so that the units are resistant to extreme corrosive gases.

Capillary Measuring Principle

Generic overview capillary mass flow meter

What is MEMS Flow Sensor?

The MEMS-based MFM/MFC has no LFE but a regular by-pass. The function is to ensure a defined percentage of the total gas flows through the sensor. The bore of the sensor is much larger so that for example the pressure drop is relatively low and the sensor less sensitive to pollution than a capillary. A MEMS sensor is also a lot more sensitive and due to that a turndown of 1000:1 is obtainable (versus 50:1 for capillary sensors).

This type of MFM/MFC must be calibrated with the actual gas as used in the application. The advantage is that the instrument thereby is more accurate. For some exotic gases, a correlation is possible, although the repeatability stays very low, the accuracy will suffer.

The biggest advantage of the MEMS sensor is, however, that there is no (measurable) drift. Drift is a slow shift of the zero and the measured value at a given flow hence affects the accuracy.

Sensor of MEMS mass flow meter

Sensor of MEMS mass flow meter

Pros and Cons

Capillary sensors exhibit effects of mechanical and thermal stress on the Platinum wires and aging of insulation materials. The capillary sensors must receive a “burn-in” in heated cabinets to speed up the aging process. Aged Capillary sensors exhibit less drift than, but it is still there. The effect is sometimes hidden by the manufacturer with low-flow cut-off options. The test signal is forced to zero under a specific value so hide the small drifts.

The MEMS sensors consist of two or three temperature sensors and a heater, vapor deposited as a microscopic molecular layer on a thin membrane. Conceptually the MEMS is sensor low-energetic and due to the membrane thermally and stress isolated, this ensures that MEMS sensors are free of any mechanical and thermal stress. An additional plus point: the sensor is part of an electronic circuit all mounted on the MEMS. The measured signal is immediately digitized on the MEMS sensor itself. No bad connections, no EMC distorting the signal. These are the main reasons why MEMS sensors are without drift and maintain a long-term accuracy, reproducibility, and reliability.

Other advantages are that the sensor is very fast: 50 msec response time. When you power the unit up is ready for use within one second, while capillary concepts take 15 minutes). The instrument does not need the zero adjustments on a regular basis (for capillary MFCs a standard routine). MEMS-based MFCs offer the significantly better reliability of the measured and regulated flow.

Finally, should not mention that the MEMS do need to be appropriately temperature-compensated. The more advanced manufacturers always calibrate their MEMS unit under two different temperatures. During operation, they measure the gas temperature and compensate the MFM/MFC readings, this also contributes to the accuracy and repeatability of the gas measurement/control.

Summary

Both concepts have their strong and weaker aspects. The capillary sensor in combination with the LFE leads to a reasonable linear relationship between the flow through the sensor and bypass. Due to this, it is possible to use so-called K-factors, a multiplier factor between two gases. For corrosive gases, the capillary sensor can calibrate with a safe gas like air and apply this K-Factor. In this way, the units can measure reasonable correct with corrosive gas.

However the K-factors calibrations are not always accurate, some MFCs better than others, best is always to calibrate with the real gas, although most capillary manufacturers do not do this. Other plus points are the availability of high-pressure versions and the choice of material for corrosive applications.

For the MEMS-based MFCs, we have better specifications when it comes to accuracy, response time and turndown. MEMS sensor not possible to use of K-factors, hence the MFC need calibration with the real gas of the application. The restriction is that these MFCs not suitable for each gas. Some MEMS manufacturers go a long way to temperature compensate their units; this results in a better accuracy over a more extensive temperature range.

A crucial advantage of the MEMS-based MFCs is the zero drift in the sensor. That results in a better long-term accuracy, repeatability, and reliability.

Four Mass flow controllers in a customer specific solution

Typical applications for MFCs

  • PVD-and other processes in the semiconductor industry
  • Dosage of air, O2, N2 and CO2 in bioreactors
  • Air injection in ice-cream, for a creamy product and best taste
  • Air/O2 dosage used in the fermentation process of beer
  • Gas blending for aerating of food in food packaging applications (MAP: Modified Atmosphere Packaging)
  • Laboratory applications, for instance analytics and catalyst-research
  • Leakage measurements and permeability research
  • Gas control of glass burners (O2 and fuel) Quality and costs reduction
  • Components testing with the flow/pressure method
  • Metal treatment processes
  • Flow control in analyzers
  • Flushing/blanketing gas
  • Gas consumption measurements of multiple departments in one building/industrial complex
  • Gas dosing in liquid metal purification
  • And many more …
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New Contrec 424 Batch Controller and 425 Flow Computer

Contrec 425 Flow Computer

Contrec New 424 Batch Controller & 425 Flow Computer

Contrec 425 Flow Computer

While still maintaining all the best features of the existing 400 series, the NEW 424/425 models now come with the following features as standard:

  •          Adjustable ‘white’ backlight
  •          Tactile keypad for improved operator interaction
  •          RS232 & RS485 Communications
  •          Custom Ticket Printer Header
  •          Hi & Lo Flow Alarms (425 only)
  •          Software configurable input type
  •          Universal Mains Power Supply (AC or DC)
  •          Green, Red, and Orange LED status lamps
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How to perform Gas Sampling using Digital Mass Flow Meter?

Voegtlin Gas Sampling

How to perform Gas Sampling using Digital Mass Flow Meter

The Vögtlin red-y compact 2 meter is used as the heart of a reliable automatic in the analytical world, low-cost gas sample taking the device. Emission or environment measurements often demand periodic sampling of a fixed amount of gas and collect these in a container or sample bag.

For example every hour a 100 scc sample. The red-y compact 2 meter with its optional smart alarm module has the complete solution without the need of any PLC to control the process.

Figure 1

Vogtlin Gas Flow Sampling with Mass Flow Meters

Figure 1 shows a very simplified diaphragm and the system runs on 24 Vdc. Totalizer, the high and low alarm can individually programmable for different alarm functions via the Red-y compact two-alarm module that has three alarm points. The capability to reset the alarms and totalizer and the timer function also utilized with an I/O input on the alarm module.

Program the Alarm 1 as a totalizer alarm with the sample requires a volume to collect in each sample. This example is 100 SCC (Standard cubic centimeter).

Duration of the alarm between 100msec and 2.78 hours programmed in that alarm function, this time sets the time between the samples. Once the process starts, the solenoid will open and a pre-set amount of gas will flow into the sample bag or container. One the defined amount is reached, the Alarm 1 will open and close the solenoid, and the gas flow will stop. Because of the programmed Alarm, 1 duration setting the alarm will stay active for the pre-programmed time, in our example 1 hour.

Once this hour is over, Alarm 1 is programmed to reset itself automatically, and the counter goes back to zero. The solenoid is activated again, the gas will flow, and the totalizer repeat until the pre-programmed value is reached, the process starts all over again. The process will continue, and every hour one sample of 100 SCC will flow into the sample container.

Diagnostics set in Alarm 3. The alarm will sound if:

  • The sample bag/container gets full
  • Water enters the sample line
  • The flow during sampling is too low (blockage of sample line)
  • The flow is too high (Too high pressure in the process)

In this case, push the reset button to deactivate the alarm and continue the process. There are many other variations possible. Every system is a bit different, you can find additions like purging, filtering, pressure reduction, gas drying, etc. out in the field.
Many of the users of these systems obtain a battery powered high accuracy red-y compact 2 meters to verify if the flow meter in the field is working correctly or if it needs calibration or cleaning. The red-y compact 2 has a significant internal diameter flow channel, has a low-pressure drop, is temperature compensated and measures the mass flow, independent of changes in pressure and temperature.

Flexibility

It is impossible to describe all process in which these type of flow meters are used, but if you see a VA meter for gas and you want to improve a process, talk to the experienced Vögtlin engineers and they will be pleased to help you to find a solution.

Conclusion

Customer around the globe makes use of the advantages in the analytical field, in research and development, in light industrial applications, in semiconductor segments and other forms. Very often we hear from the users “The beauty of the Vögtlin Thermal Mass flow instruments is once you install, set then forget them, as they work so reliable and independently.”
Vögtlin’s worldwide network of distribution and sales partners will help you select most correct equipment that matches specific needs and best for your application.

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6 Features you should know in Vogtlin Red-y Compact Mass Flow Meter

Vogtlin Compact 2

Technology brings change in our daily life with the Internet, Cloud, Mobile phone and etc. Process Instrumentation always a feel of old technology without more changes, Vogtlin brings the Red-y Compact Mass Flow Meter a new design. The improvement helps to provide better “User Experience” even in flow measurement.

AA Battery Powered Mass Flow Meter

Battery Powered Red-y Compact Flow Regulator

Battery powered Mass Flow Meter is not a new thing in the industry, but AA Battery makes life much convenient. Most of the Battery Powered Mass Flow Meter will equip with custom-made the battery, which could cost a lot during replacement. The new Vogtlin Red-y Compact powered with 1 AA battery only with battery life as long as six months. The AA Battery even powered the Mass Flow Regulator for flow control.

Touch Screen enabled Mass Flow Meter

Touch Screen Mass Flow Meter

Touch Screen on a Mass Flow Meter is still rare in the market, hence one of the most significant advantages of the touch screen is the direct interaction between user and device. New Vogtlin Red-y Compact are standard with Touch Screen, and parameters setting aren’t hassling anymore.

USB Enabled Mass Flow Meter

Micro-USB power supply

Vogtlin Red-y Compact Series is the First USB Powered Mass Flow Meter. Besides the common AA Battery Powered, and 24VDC Powered, the inbuilt Micro USB port allows the user to power the Mass Flow Meter using any USB power source. Portable Mass Flow Meter makes possible with the Micro-USB port with the long hour of flow monitoring, while the port can be used from time to time firmware updates.

Auto Screen Rotating Mass Flow Meter

Mass Flow Meter hardly replaces Variable Area Flow Meter, which comes from 2 reasons. Pricing & Installation.

Pricing: Mass Flow Meter are generally high cost than Variable Area Flow Meter due to the electronic part and higher accuracy. In demanding industry accuracy is almost equivalent to price, therefore Mass Flow Meter delivered something that VA Flow Meter not. The accuracy & Stability.

Installation: Mass Flow Meter often display in one direction only (Horizontal or Vertical). However the new upgraded Vogtlin Red-y Compact now capable of rotating the display with 270°. Hence it solve the problem of installation in the VA Flowmeter. The readout of the flow reading now as clear as never before.

Multi Gasses Mass Flow Meter

Multi Gases Mass Flow Meter

The Vogtlin Red-y Compact can store up to 3 different curves for different gases or gas mixtures in its memory. Gas selection possible from the display if the ordered unit with multiple gases option. The maximum flow rate is shown at the back of every gas name. To purchase Three flow meters for Three different gases, the Red-y Compact save cost with One device for Three gases.

REAL Gas Calibration Mass Flow Meter

In Vogtlin Instruments accuracy is the priority, therefore all Red-y Series Mass Flow Meter out from factory with REAL Gas calibration. The Mass Flow Meter calibrated with REAL gas to ensures high accuracy and reproducibility.

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Variable Area Flow Meter or Mass Flow Meter for Gas Flow Measurements

Variable Area vs Mass Flow
Vogtlin Q Flow - Variable Area Flowmeter
Red-y Compact Meter Mass Flow Meter

Comparison between Variable Area Flow Meter and Mass Flow Meter often compare of Volumetric Flow and Mass Flow Measurements.

What good of Mass Flow Over Volumetric Flow Measurements

  • Mass Flow Meter is having widest Turndown Ratio, Fast Response Time, High Accuracy and also High Repeatability.
  • Mass Flow Rate independence to pressure and temperature changes.
  • Pressure and temperature compensation does not require direct mass flow measurement hence improve the cost-effectiveness of gas flow measurement.

VA-Flow Meter v.s. Mass Flow Meter

Industrial flow measurements applications often require a higher accuracy together with pressure and temperature compensation which not possible to achieved with conventional Variable Area Flow Meters.

Hence Variable Area Flow Meters are extremely sensitive to pressure and temperature changes. 1 Bar pressure changes in process can generate an error of 50% in gas flow measurements. Therefore the flow compensation of Mass Flow Meters creates repeatability.

Common Variable Area Flow Meter in flow measurement possible only with Vertical Installation, however, smart Digital Mass Flow Meters provide installations flexibility both horizontally and vertically. In the past the Variable Area Flow Meters having great advantages of no power supply required, however with innovation, now Mass Flow Meter with AA Battery Powered available without compromise of measurement accuracy.