Intro4u2u

There is a need for accurate flow measurement of numerous liquids, gases and vapors in many industries. For instance, food processing plants need to have an accurate measure of types of materials that go into the products on a large, automated scale. In the semiconductor industry, small amounts of gases have to be applied to the production process. Accurate delivery of these gases is essential to ensure the quality of the finished product.

As already mentioned, the mass flow or the volume flow are the measurements that are needed in these processes. In the case of mass flow, the Coriolis flow meter (also known as the inertial flow meter) is commonly used.

The Coriolis flow meter gets it’s name from the Coriolis effect that was first described by Gaspard-Gustave Coriolis in 1835. Coriolis worked in understanding the behaviors of objects in motion due to the various forces that applied to them. To this extent, the Coriolis mass flow meter works in this manner.

If a liquid or gas is passing through a tube it applies a force to the tube. When the tube is already moving, then the substance passing through it will change the movement or vibration of that tube. The change in the amplitude of the vibration of the tube can be used to determine the mass of a flow of the substance passing through the tube.

This is possible provided various other factors are known about the tube, such as the width, the type of material that it is made from, the vibrating frequency of the tube and it’s inertia. The flow density of the material passing through the tube is also needed.

Coriolis flowmeters are popular because they need little maintenance. By comparison to other devices that perform flow measurement they are well designed to the extent that little can go wrong with them. They do not need to be re-calibrated like other types of flow meters.

With this said, the flow meter does need to be checked from time to time, especially if the substances that pass through the tubing are hot or corrosive.

If you are using these types of substances the chances are you will have noted this when initially specifying the type of flow meter you need. Many flow meters will not be able to handle hot liquids or corrosive gases. A specially made flow meter would be required for these purposes.

When it comes to selecting a coriolis meter, you want to make sure it meets all your needs. You should also look for the best you can afford as this is a fair measure of the quality of the meter. Coriolis flow meters, and gas mass flow meters, are designed to be accurate and they’re designed to save you time and money so you will make this initial expense back on increased efficiency in the workplace

Coriolis flowmeters are relatively new compared to other flowmeters. They were not seen in industrial applications until 1980’s. Coriolis meters are available in a number of different designs. A popular configuration consists of one or two U-shaped, horseshoe-shaped, or tennis-racket-shaped (generalized U-shaped) flow tube with inlet on one side and outlet on the other enclosed in a sensor housing connected to an electronics unit.

The flow is guided into the U-shaped tube. When an osillating excitation force is applied to the tube causing it to vibrate, the fluid flowing through the tube will induce a rotation or twist to the tube because of the Coriolis acceleration acting in opposite directions on either side of the applied force. For example, when the tube is moving upward during the first half of a cycle, the fluid flowing into the meter resists being forced up by pushing down on the tube. On the opposite side, the liquid flowing out of the meter resists having its vertical motion decreased by pushing up on the tube. This action causes the tube to twist. When the tube is moving downward during the second half of the vibration cycle, it twists in the opposite direction. This twist results in a phase difference (time lag) between the inlet side and the outlet side and this phase difference is directly affected by the mass passing through the tube.

A more rescent single straight tube design is available to measure some dirty and/or abrasive liquids that may clog the older U-shaped design.

An advantage of Coriolis flowmeters is that it measures the mass flow rate directly which eliminates the need to compensate for changing temperature, viscosity, and pressure conditions. Please also note that the vibration of Coriolis flowmeters has very samll amplitude, usually less than 2.5 mm (0.1 in), and the frequency is near the natural frequency of the device, usually around 80 Hz. Finally, the vibration is commonly introduced by electric coils and measured by megnetic sensors.

Top of Page
Further Information

Suppose that the fluid is flowing into the U-shaped tube at velocity V and the tube is vibrating at angular velocity . Consider a small section of the fluid that is on the inlet side away from the point of flexture at distance r.

Please note that the amplitudes of the vibration and twist are extremely small compared to the size of the U-shaped tube. The above graphics are highly exaggerated for illustration purposes.

The Coriolis force on the small fluid section m is

During the down cycle, the tube applys an upward resisting force to the fluid or the fluid pushes the tube down. On the outlet side, the Coriolis force has the opposite direction.

To simply the problem, we assume that the tube has a perfect U shape with a cross section area of A. The length and width are l, d, respectively. The opposite directions of Coriolis forces on inlet and outlet sides result in a twisting moment Tc

A K factor can be introduced to compensate for the more generalized U-shape.

where Qm = AV is the mass flow rate.

The governing equation of twisting is

where Iu is the inertia of the U-shaped tube, Cu is the damping coefficient, Ku is the stiffness, is the twist angle, and t is time.

Recall that the Coriolis flowmeters are vibrating the U-shaped tube to generate the rotation, the real angular velocity is function of vibrating frequency :

Assuming that the damping term Cu is negligible, the equation of twisting becomes

The particular solution (steady-state solution) of the twist angle is

Furthermore, the velocity of the turning corners of the U-shaped tube are  and the displacement difference between these two corners is d/2. Therefore, the time lag between these two corners is

By measuring the time lag , the mass flow rate can be obtained

In vibration analysis, it is custom to use the natural frequency as a basis and normalize frequency terms against it. The natural frequency of the U-shaped tube system is (note that Iu includes the mass of the fluid in the tube)

The mass flow rate then becomes

Top of Page
Common Specifications

Common specifications for commercially available Coriolis flowmeters are listed below:

Fluid Phase:

Score     Phase     Condition
Liquid      Clean
Direct Mass
Dirty
Non-Newtonian
Viscous
Slurry      Abrasive
Gas      Clean
Dirty
Liquid      Corrosive
Slurry      Fibrous
: Recommended
: Limited applicability
Line Size:     6 ~ 200 mm (0.25 ~ 8 inch)
Turndown Ratio:     100 : 1

Top of Page
Pros and Cons

•     Pros:
-     Higher accuracy than most flowmeters
-     Can be used in a wide range of liquid flow conditions
-     Capable of measuring hot (e.g., molten sulphur, liquid toffee) and cold (e.g., cryogenic helium, liquid nitrogen) fluid flow
-     Low pressure drop
-     Suitable for bi-directional flow
•     Cons:
-     High initial set up cost
-     Clogging may occur and difficult to clean
-     Larger in over-all size compared to other flowmeters
-     Limited line size availability

So you want to measure flow? The answer would seem to be to purchase a flowmeter. With fluid flow defined as the amount of fluid that travels past a given location, this would seem to be straightforward — any flowmeter would suffice. However, consider the following equation describing the flow of a fluid in a pipe.

Q = A x v

Q is flow rate, A is the crosssectional area of the pipe, and v is the average fluid velocity in the pipe. Putting this equation into action, the flow of a fluid traveling at an average velocity of a 1 meter per second through a pipe with a 1 square meter cross-sectional area is 1 cubic meter per second. Note that Q is a volume per unit time, so Q is commonly denoted as the “volumetric” flow rate. Now consider the following equation:

W = rho x Q

Where W is flow rate (again - read on), and rho is the fluid density. Putting this equation into action, the flow rate will be 1 kilogram per second when 1 cubic meter per second of a fluid with a density of 1 kilogram per cubic meter is flowing. (The same can be done for the commonly-used “pounds”. Without getting into details — a pound is assumed to be a mass unit.) Note that W is a mass per unit time, so W is commonly denoted as the “mass” flow rate. Now — which flow do you want to measure? Not sure? In some applications, measuring the volumetric flow is the thing to do.

Consider filling a tank. Volumetric flow may be of interest to avoid overflowing a tank where liquids of differing densities can be added. (Then again, a level transmitter and high level switch/shutoff may obviate the need for a flowmeter.) Consider controlling fluid flow into a process that can only accept a limited volume per unit time. Volumetric flow measurement would seem applicable.

In other processes, mass flow is important. Consider chemical reactions where it is desirable to react substances A, B and C. Of interest is the number of molecules present (its mass), not its volume. Similarly, when buying and selling products (custody transfer) the mass is important, not its volume.

Having discovered that there are two types of flow rates (volumetric and mass), it should not be a surprise that some flowmeters measure mass (W) while other flowmeters measure volume (Q). However, it is not quite that simple. Repeating the equations from Part 1 (for convenience), it can be seen that, assuming A is constant, Q can be determined by measuring the average fluid velocity v. Further, assuming that rho is constant, W can be determined from Q.

Q = A x v  W = rho x Q

Summarizing, some flowmeters measure volumetric flow, some flowmeters measure velocity from which the volumetric flow is determined, and some flowmeters measure mass flow. In addition, when the density is known or assumed, mass flow can be determined from the volumetric flow, and the volumetric flow can be determined from the mass flow. So you just wanted to measure flow — did you now? It all seemed so logical and simple at the time. Stick around — it gets worse. Some flowmeters use other principles to infer flow. The most common of these measurements measure the velocity head (1/2 rho v x v) to infer the volumetric flow. Notice that these flowmeters do NOT measure volume, do NOT measure mass, and do NOT measure velocity — but rather measure a combination of density and the square of velocity! Would it surprise you to discover that this is a description of (commonly-applied) head flowmeters, such as orifice plates, venturis, nozzles…? Further, in many applications, the inferred volumetric flow is used to determine the mass flow. Errors can enter the measurement process during each measurement and with each assumption. Is it any surprise that plant engineers often have difficulty closing material balances in their plants?

Summarizing (again), some flowmeters measure volume, some flowmeters measure mass, some flowmeters measure velocity, and some flowmeters measure inferentially. Understand the difference, but also understand that careful attention to detail can result in an inferential measurement that is better than the others.

Volumetric flow is expressed in units that reflect a volume per unit time. The example in Part 1 determines cubic meters and cubic feet per unit time to be volumetric flow units. Gallons and liters per unit time are also volumetric flow units. Mass flow is expressed in units that reflect a mass per unit time. The other example in Part 1 determines kilograms and pounds per unit time to be mass flow units. (Without getting into details — a pound is assumed to be a mass unit.) Note that the units of time are independent of whether volumetric or mass flow is measured.

Let’s have a quiz.
Are the following volumetric or mass liquid flow units?
gallons per minute
cubic feet per second
liters per minute
kilograms per hour
pounds per hour
grams per minute

Can one have a cubic foot of feathers?
yes/no

Can one have a gallon of feathers?
yes/no

Can one have a kilogram of feathers?
yes/no

If you answered volumetric to the first three questions, mass to the next three questions, and yes to the last three questions, you are on track.

Consider purchasing fuel for your car. How does a US gallon of gasoline purchased on a hot summer day in Las Vegas, Arizona compare with a US gallon of gasoline purchased on a cold winter night in Anchorage, Alaska? It was determined that a gallon is a volumetric unit, so logic would indicate that the same volume of gasoline was purchased. Yet the temperature difference would cause their densities, and hence their masses, to be different. Using this logic, more mass would be obtained by purchasing gasoline in colder weather. Thinking locally, one might conclude that it is more economical to purchase gasoline during the wee hours of the morning when the temperature is coldest.

As you might suspect, such is not the case. Gasoline pumps compensate for density variation that occurs due to temperature, and in doing so, they measure the amount (mass) of gasoline dispensed. Yet, a gallon of cold gasoline will occupy less volume than when hot. In essence, the measurement of a gallon of gasoline actually refers to its volume at a given temperature (such as 60 degF). As such, this is really a mass measurement unit because it refers to the flow of a specific substance at a given temperature, Returning to the quiz, let’s not be so hasty with the first three questions. They could be incomplete!
Part 3 discussed the use of volumetric units (such as gallons) to infer mass when the composition and temperature is known. The example given was that of purchasing a gallon of gasoline in a hot and cold climate. The assertion was that a gallon of gasoline purchased in hot and cold climates might have different sizes due to their differing temperatures, but their masses should be the same because the retail flowmeter is temperature compensated.

A number of e-mails questioning this assertion and further investigation resulted in the interesting discovery that retail gasoline flowmeters are not temperature-compensated in the United States, but are temperaturecompensated in Canada. In other words, either the measured volume (in the US) or the measured temperature-corrected volume (in Canada) is used to infer mass.

Consider the following general analysis:

1. Air temperature differences between hot and cold climates are large. In addition, air temperature fluctuations between day and night in a given location can be large.

2. There is a significant difference between ground temperatures in hot and cold climates. However, ground temperature fluctuations between day and night in a given location is very small. Ground temperature fluctuation between summer and winter in a given location is relatively small.

3. Gasoline will be warm when it leaves the refinery, but will cool in transport to the retailer’s underground tank. Given time in the tank, the temperature of the gasoline will approach the ground temperature.

4. Flowmeter calibration is performed using standard weights, implying a calibration to mass.

These statements imply that despite wide air temperature fluctuations, the temperature of the gasoline pumped through the flowmeter should be nearly the same as the ground temperature. Because the ground temperature does not fluctuate very much, the temperature variation of the gasoline will be small throughout the year, so the mass of a gallon of gasoline should not vary much throughout the year from a given tank. Following this logic, the mass of a gallon of gasoline sold in Alaska should be the same as one sold in Nevada.

Fluctuations in gasoline temperature cause gasoline density changes. The magnitude with which these changes affect measurement accuracy can be quantified by performing an uncertainty analysis to determine if temperature compensation is appropriate. An uncertainty analysis for this measurement would likely reveal a number of sources of measurement uncertainty, such as (but not limited to) the effects of ambient air temperature, gasoline temperature leaving the refinery, transport time from the refinery to the tank, ground temperature, tank level prior to filling, the volume of gasoline in the flowmeter piping, flowmeter piping temperature, frequency of use, and composition changes. As a minimum, such analysis would likely reveal that the consumer would not be advised to purchase gasoline from a tank that was just filled with warm gasoline. A detailed analysis may reveal other significant issues.

While this is perhaps more information than one would like to know about the subject, this discussion clearly illustrates the need to understand the process — and that the same process may be different in different locations. Sometimes … it’s just not so easy.

A brief review — Part 3 addressed mass flow measurement, volumetric flow measurement, and inferred mass flow measurement. The measurement of gasoline was given as an example of the inferred mass flow measurement (using volumetric units). Comments resulted in Part 3.1 that addressed some issues associated with retail gasoline measurements. This sparked a flurry of comments regarding how gasoline is measured at the pump. This issue attempts to tie the comments together, so reading this issue without having read previous issues may prove difficult.

Gasoline pumps in the USA measure volume and are calibrated using volumetric means. In other words, they are true volumetric devices — they measure volume and indicate gallons. Even the New York Times offered advice to the consumer on this one with “… buy gasoline during the coolest time of the day — early morning or late evening — while the gasoline is most dense…” (New York Times, September 24, 2001, Empowered II Smart Energy Management, A clean car is an efficient car, page 7).

Gasoline pumps in Canada measure volume. This volume is then compensated for the actual temperature to indicate the volume of the gasoline as if it were a certain temperature. The compensated volume is an implied mass measurement. I suspect (but do not know) that these pumps are calibrated using volumetric means that are temperature compensated. In other words, they are inferred mass measurement devices and are calibrated as such — they measure volume and indicate in (temperature-compensated) liters. In Canada, the inferred mass of the gasoline received should be the same (within the limitations of the equipment) regardless of gasoline temperature. Note however that composition differences (and additives) may cause the density at a given temperature to be different than its nominal value. As an example, a 1% increase in gasoline density from its nominal value does not affect the actual volume measured, but will cause the inferred mass measurement to be 1% lower than the actual mass flow.

My comments on some readers’ responses follow:

One reader questioned whether the “wee hours of the am” would be the time when the gasoline would be at its lowest temperature in an underground tank. Thermal lag for underground gasoline storage tanks is an issue, but may not be significant. For science class, my daughter measured the temperatures 1 meter above and 1 meter below grade in the fall/winter (in the New York area). I seem to remember the ground temperature changing by only 1-2 degC over a period of months. The above ground temperature changed by 20 degC (or more?) during the same period. This issue is likely to be significant for above ground storage tanks (as suggested by other readers). Note however that filling the tank may cause larger (transient) effects caused by such issues as the quantity and temperature of the gasoline prior to fill, and the quantity and temperature of gasoline added. Not being able to sell compressed natural gas measured with a Coriolis mass flowmeter in kg or lbm (pounds mass) because it was not considered ‘marketable’ to the the public illustrates resistance to change. By the way, when will gasoline be sold by the kg or lbm — or better yet, by the BTU or Joule (as suggested by another reader)? I suspect that it will not be soon.

The comments and observations about beating the measurement were amusing. Society allows people to (reasonably) operate in their own self-interest. Parting with less money for a product is clearly in the purchaser’s self-interest. (Engineers sometimes call this an “optimization problem”, but that is an issue for another day.) Comments on how to beat the system were inevitable.

The safety point regarding gasoline expansion causing explosions and fires (after topping off a gas tank in a cold climate and then parking in a warm garage) is important. Virtually everything is potentially dangerous — even a small puddle of water that turns to ice…

Both gas and liquid flow can be measured in volumetric or mass flow rates (such as litres per second or kg/s). These measurements can be converted between one another if the materials density is known. The density for a liquid is almost independent of the liquids conditions, however this is not the case for a gas, whose density highly depends upon pressure and temperature.

In engineering contexts, the volumetric flow rate is usually given the symbol Q and the mass flow rate the symbol \dot m.

[edit] Gas

Due to the nature of an Ideal gas or a Real gas, the volumetric gas flow rate will differ for the same mass flow rate when at differing temperatures and pressures. As such gas volumetric flow rate is sometimes measured in “standard cubic centimeters per minute” (abbreviation sccm). This unit, although not an SI unit is sometimes used due to the additional information attached to the unit symbol, which indicates the temperature and pressure of the gas. Many other similar abbreviations are also in use, for two reasons, firstly mass flow and volumetric flow can be equated at known conditions, and secondly due to the imperial system older units such as standard cubic feet per minute or per second may still be used in some countries. It is often necessary to employ standard gas relationships (such as the ideal gas law) to convert between units of mass flow and volumetric flow.

[edit] Liquid

For liquids other units used depend on the application and industry but might include gallons (U.S. liquid or imperial) per minute, liters per second, bushels per minute and, when describing river flows, cumecs (cubic metres per second) or acre-feet per day.

[edit] Mechanical flow meters

There are several types of mechanical flow meter

[edit] Piston Meter

Because they are used for domestic water measurement, piston meters, also known as rotary piston or semi-positive displacement meters, are the most common flow measurement devices in the UK and are used for almost all meter sizes up to and including 40 mm (1 1/2″). The piston meter operates on the principle of a piston rotating within a chamber of known volume. For each rotation, an amount of water passes through the piston chamber. Through a gear mechanism and, sometimes, a magnetic drive, a needle dial and odometer type display is advanced.

[edit] Woltmann Meter

Woltman meters, commonly referred to as Helix meters are popular at larger sizes. Jet meters (single or Multi-Jet) are increasing in popularity in the UK at larger sizes and are commonplace in the EU.

[edit] Multi-jet Meter

A multi-jet meter is a velocity type meter which has an impeller which rotates horizontally on a vertical shaft. The impeller element is in a housing in which multiple inlet ports direct the fluid flow at the impeller causing it to rotate in a specific direction in proportion to the flow velocity. This meter works mechanically much like a paddle wheel meter except that the ports direct the flow at the impeller equally from several points around the circumference of the element, where a paddle wheel normally only receives flow from one offset flow stream.

[edit] Venturi Meter

Another method of measurement, known as a venturi meter, is to constrict the flow in some fashion, and measure the differential pressure (using a pressure sensor) that results across the constriction. This method is widely used to measure flow rate in the transmission of gas through pipelines, and has been used since Roman Empire times.

[edit] Dall Tube

The Dall tube is a shortened version of a Venturi meter with a lower pressure drop than an orifice plate. Both flow meters the flow rate of Dall tube is determined by measuring the pressure drop caused by restriction in the conduit. The pressure differential is measured using diaphragm pressure transducers with digital read out. Since these meters have significantly lower permanent pressure losses than the orifice meters, the Dall tubes have widely been used for measuring the flow rate of large pipeworks.

[edit] Orifice Plate

Another simple method of measurement uses an orifice plate, which is basically a plate with a hole through it. It is placed in the flow and constricts the flow. It uses the same principle as the venturi meter in that the differential pressure relates to the velocity of the fluid flow (Bernoulli’s principle).

[edit] Pitot tube

A Pitot tube is a pressure measuring instrument used to measure fluid flow velocity by determining the stagnation pressure. Bernoulli’s equation is used to calculate the dynamic pressure and thence fluid velocity.

[edit] Multi-hole Pressure Probe

Multi-hole pressure probes (also called impact probes) extend the theory of pitot tube to more than one dimension. A typical impact probe consists of three or more holes (depending on the type of probe) on the measuring tip arranged in a specific pattern. More holes allow the instrument to measure the direction of the flow velocity in addition to its magnitude (after appropriate calibration). Three-holes arranged in a line allow the pressure probes to measure the velocity vector in two dimensions. Introduction of more holes e.g., five holes arranged in a ‘plus’ formation allow measurement of the three-dimensional velocity vector.

[edit] Paddle wheel

The paddle wheel translates the mechanical action of paddles rotating in the liquid flow around an axle into a user-readable rate of flow (gpm, lpm, etc.). The paddle tends to be inserted into the flow.

[edit] Pelton wheel

The Pelton wheel turbine (better described as a radial turbine) translates the mechanical action of the Pelton wheel rotating in the liquid flow around an axis into a user-readable rate of flow (gpm, lpm, etc.). The Pelton wheel tends to have all the flow traveling around it with the inlet flow focussed on the blades by a jet. The original Pelton wheels were used for the generation of power and consisted of a radial flow turbine with “reaction cups” which not only move with the force of the water on the face but return the flow in opposite direction using this change of fluid direction to further increase the efficiency of the turbine.

[edit] Optical Flow Meters

Optical flow meters use light to determine flow rate. Small particles which accompany natural and industrial gases pass through two laser beams focused in a pipe by illuminating optics. Laser light is scattered when a particle crosses the first beam. The detecting optics collects scattered light on a photodetector, which then generates a pulse signal. If the same particle crosses the second beam, the detecting optics collect scattered light on a second photodetector, which converts the incoming light into a second electrical pulse. By measuring the time interval between these pulses, the gas velocity is calculated as V=D/T where D is the distance between the laser beams and T is the time interval.

Laser-based optical flow meters measure the actual speed of particles, a property which is not dependent on thermal conductivity of gases, variations in gas flow or composition of gases. The different operating principle enables optical laser technology to deliver highly accurate flow data, even in challenging environments which may include high temperature, low flow rates, high pressure, high humidity, pipe vibration and acoustic noise.

Optical flow meters are very stable with no moving parts and deliver a highly repeatable measurement over the life of the product. Because distance between the two laser sheets does not change, optical flow meters do not require periodic calibration after its initial commissioning. Optical flow meters require only one installation point, instead of the two installation points typically required by other types of meters. A single installation point is simpler, requires less maintenance and is less prone to errors.

Optical flow meters are capable of measuring flow from 0.1m/s to faster than 100m/s (1000:1 turn down ratio) and have been demonstrated to be effective for the measurement of flare gases, a major global contributor to the emissions associated with climate change.[1]

[edit] Turbine flow meter

The turbine flow meter (better described as an axial turbine) translates the mechanical action of the turbine rotating in the liquid flow around an axis into a user-readable rate of flow (gpm, lpm, etc.). The turbine tends to have all the flow traveling around it.

The turbine wheel is set in the part of a fluid stream. The flowing fluid impinges on the turbine blades, imparting a force to the blade surface and setting the rotor in motion. when a steady rotation speed has been reached, the speed is proportional to fluid velocity.

[edit] Open Channel Flow Measurement

[edit] Level to Flow

The level of the water is measured at a designated point behind a hydraulic structure (a weir or flume) using various means (bubblers, ultrasonic, float, and differential pressure are common methods). This depth is converted to a flow rate according to a theoretical formula of the form Q=KHX where Q is the flow rate, K is a constant, H is the water level and X is an exponent which varies with the device used, or it is converted according to empirically derived level/flow data points (a ‘flow curve’). The flow rate can then integrated over time into volumetric flow.

[edit] Area/Velocity

The cross-sectional area of the flow is calculated from a depth measurement and the average velocity of the flow is measured directly (doppler and propeller methods are common). Velocity times the cross-sectional area yields a flow rate which can be integrated into volumetric flow.

[edit] Dye Testing

A known amount of dye per unit time is added to a flow stream. After complete mixing, the concentration of the dye is measured. The dilution rate of the dye equals the flow rate.

[edit] Thermal mass flow meters

Thermal mass flow meters generally use combinations of heated elements and temperature sensors to measure the difference between static and flowing heat transfer to a fluid and infer its flow with a knowledge of the fluid’s specific heat and density. The fluid temperature is also measured and compensated for. If the density and specific heat characteristics of the fluid are constant, the meter can provide a direct mass flow readout, and does not need any additional pressure temperature compensation over their specified range.

Technological progress allows today to manufacture thermal mass flow meters on a microscopic scale as MEMS sensors, these flow devices can be used to measure flow rates in the range of nano litres or micro litres per minute.

Thermal mass flow meters are used for compressed air, nitrogen, helium, argon, oxygen, natural gas. In fact, most gases can be measured as long as they are fairly clean and