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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

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.

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.

Figure 3: Generic overview capillary mass flow meter

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.

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).

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.

MEMS Sensor Measurement Principle
Figure 4: Sensor of MEMS mass flow meter

The 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.

4 Mass Flow Controller Modular type
Figure 5: Four Mass flow controllers in a customer
specific solution

Typical Applications for Mass Flow Controllers

  •  
  • PVD-and other processes in the semiconductor industry
  • The 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 you can think about …