MFCs Outclass Variable Area Flow Meters |
Bad flow meters do not exist - wrong applications do. There is no best flow meter for measuring gas flows in laboratories and pilot plants. However, mass flow controllers do outperform the cheaper and simpler VA-meters on points such as accuracy, measurement uncertainty and automation possibilities.
Mass flow controllers are often the best choice in low-flow gas flow measurementLiquid and gas flows in chemical and physical processes are measured with flow meters. This applies to large industrial plants as well as to measurements on a laboratory and pilot scale. There are 9 flow measurement techniques that can cover 99% of the flow applications. Hundreds of manufacturers use these techniques. The result is a large variety of flow meters for an unlimited number of applications. For measuring liquid flows in laboratories, several solutions are available; for low flow gas applications, ultimately only two measurement techniques remain: VA and thermal. A possible third concept, differential pressure measurement over a laminar flow element, is similar in performance to thermal. |
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Variable Area (VA) Flow MetersThe standard Variable Area (VA) meter, the first measurement technique, consists of a vertically positioned conical tube. Narrower at the bottom and wider at the top, with a float inside. They are also called vado-meters, float meters and sometimes rotameters. The flow direction is from bottom to top and the float is pulled down by its mass, with the flowing medium pushing it up. The higher the float rises, the greater the flow area in the measuring tube. The float thus seeks the balance between the force of gravity and the driving force of the medium (figure 1).
In itself this simple instrument is available in a variety of versions. The VA-meters for laboratory applications are almost always equipped with a glass measuring tube and a steel or ceramic spherical float. The user visually reads the scale as no measurement signal is available. To regulate the flow a needle valve can be used. As a result this simple instrument is very affordable and easy to install. As with other flow meters, VA-meters need to be calibrated annually to ensure it continues to deliver peak performance. Variable Area flow meters rely on the dimensional stability of the glass measuring tube through a controlled production process. Tolerances apply to the size, weight and roundness of the ball. All this can lead to measurement errors in the order of 5 % FS (Full Scale), which is considerable. Moreover, in practice the measurement errors are even larger because gas is compressible and temperature also plays a role. The measurement is optimized by placing a pressure reducing valve on the gas cylinder (or other gas source). With the regulating valve at the outlet side of the VA-meter the pressure in the measuring tube is kept constant. When measuring different gases, the scales need to be adjusted accordingly. The effects of process pressure and temperature should be included in the scale adjustment as well as the flow units being measured. In reality these adjustments are based on formulas and calculations, not on calibrations. Reaching for a stock instrument can lead to unpredictable measurement uncertainties. Thus, even when using such a simple measuring instrument, an error is easily made. Anyone who purchases a Variable Area flow meter should be mindful of measurement uncertainties. Make choices in advance and use them when applying this type of flow meter. Nevertheless, despite its drawbacks, this type of flow meter is perfectly acceptable for some applications. |
Figure 1
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Thermal Mass Flow Meters
The second measurement technique for gas flow measurements in labs and test facilities is thermal. They simply work on the principle of a hot sensor being cooled by a flowing gas, with the gas molecules dissipating kinetic energy. Molecules are mass, and therefore mass flow is measured by these instruments. The readout is in units of mass or standardized units of volume.
As a result, the measurement result is unambiguous, with the influence of pressure and temperature being virtually zero on the measurement performance. The accuracy specification is 1% FS, or even better. Thermal flow meters come in several forms. For larger pipes, the Constant Temperature Anemometry (CTA) technique is usually used. This works with two sensors. One sensor measures the temperature of the gas, the other is heated to a constant temperature difference in relation to the gas temperature. The amount of energy required to maintain the temperature difference is representative of the gas mass flow.
For lower flows, from less than 1 mln/min to several 100-dreds ln/min of gas, the bypass technique is used. By the way, mln/min here stands for normal millilitres per minute, and ln/min stands for normal litres per minute. There is a fixed ratio of gas mass to standardized volume units. Normal is referring to volume at 0 °C and 1 atmosphere. Bypass flow meters are also called scientific flow meters. Equipped with a control valve to regulate the gas supply, this mass flow meter (MFM) becomes a mass flow controller: MFC. In practice, both are generally referred to as MFC’s.
As a result, the measurement result is unambiguous, with the influence of pressure and temperature being virtually zero on the measurement performance. The accuracy specification is 1% FS, or even better. Thermal flow meters come in several forms. For larger pipes, the Constant Temperature Anemometry (CTA) technique is usually used. This works with two sensors. One sensor measures the temperature of the gas, the other is heated to a constant temperature difference in relation to the gas temperature. The amount of energy required to maintain the temperature difference is representative of the gas mass flow.
For lower flows, from less than 1 mln/min to several 100-dreds ln/min of gas, the bypass technique is used. By the way, mln/min here stands for normal millilitres per minute, and ln/min stands for normal litres per minute. There is a fixed ratio of gas mass to standardized volume units. Normal is referring to volume at 0 °C and 1 atmosphere. Bypass flow meters are also called scientific flow meters. Equipped with a control valve to regulate the gas supply, this mass flow meter (MFM) becomes a mass flow controller: MFC. In practice, both are generally referred to as MFC’s.
The bypass measurement principle
In this functional layout, the flow is from left to right. Each MFC is built for a particular application with a particular gas, measurement range, and process pressure and temperature. Based on this, a flow conditioner is chosen to be placed in the bypass. This flow conditioner also creates a pressure drop, sending a portion of the gas through a sensor parallel to the bypass. The sensor includes a thin silica plate with two temperature-sensitive resistors on it, with a heating element in between. At 0-flow, the temperature distribution is symmetrical, but asymmetrical when there is flow. The resistance values change, which is representative of the mass flow passing the sensor.
The MFC’s are built and calibrated so that the flow in the sensor is representative of the total flow through the instrument. The readout is in mass or standardized volume units.
The MFC’s are built and calibrated so that the flow in the sensor is representative of the total flow through the instrument. The readout is in mass or standardized volume units.
MFC versus Variable Area?With a bit of imagination, MFC’s and the compact from Vögtlin Instruments could also be called "electronic VA-meters". The readout is visual. These instruments are powered by an external energy source or by a built-in battery. The optional control valve is manually operated.
Compared to mechanical instruments, MFC’s offer a solution with much lower measurement uncertainties and clear measurement units. As a result of variations in, for example, the process pressure, the 'electronic VA-meter' may well show a different measured value - because the control valve is manually operated and does not automatically adjust - but it is, nevertheless, correct and unambiguous in terms of units of measurement. After all, they are mass flow meters! This is not possible with a mechanical VA-meter. These have a scale that is only valid for a certain pressure and temperature. There are several other advantages to mention such as accuracy, linearity, greater turndown (ratio of full scale to minimum usable readings), mounting vertically or horizontally, vertical with flow down, multiple gas curves in the electronic memory, etc. All of these make the switch from a VA-meter to a thermal flowmeter very logical. The ultimate? These are MFC’s that are given a set point and then independently control the flow to the set value. For example, Sierra Instruments' 100-series or Vögtlin Instruments' smart series. Both product lines can be equipped with a control valve. The setpoint is offered via the integrated display, or analog and serial signals. Various communication protocols, e.g. Modbus, Profibus, Industrial Ethernet, are available for recording and further automation. The valve is opened so far that the measured value corresponds to the set point value. For example, if the supply pressure drops, the MFC will automatically open the valve a little further. If the upstream pressure rises again, the control valve will close a bit. MFC’s are smart instruments for accurate flow measurement and control, and offer much added value over VA-meters. Why an MFC... Reasons for gas flow applications in laboratories or test facilities to choose MFC’s or mass flow controllers over Variable Area (VA) meters are: - Significantly higher accuracies - Less measurement uncertainty - Much greater turndown - Units of measure are clear - Pressure and temperature effects are negligible. - Measurement signals can be used for registration and automation - Suitable for validation of processes VA-meters are simple and price-wise, quite interesting. However, the user must be aware of the uncertainties that can ultimately lead to measurement errors in the order of tens of percent. MFC’s outclass VA-meters in every aspect. |
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