Industrial and chemical processes ranging from brewing to polymerization depend on delivery of accurate quantities of gases or liquids. It is possible to measure rates of delivery in terms of either mass or volume per unit of time, and choosing between them is based on the application. In a number of situations volumetric flow measurement is sufficient, particularly if pressure and temperature are known and stable. However, as mass flow measurement directly indicates the quantity of molecules present, it has the advantage of being immune to density variation.
Most Engineers are accustomed with some of the many ways for measuring volumetric flow. In general, mass flow measurement is less well-known and understood. This OMEGA Engineering article explains how mass flow rates can be measured directly (such as via heat loss) and indirectly (by inference from pressure drop) and highlights applicable features of some commercially-available mass flow meters.
Mass Flow Meter Types and Operating Principles
The mostly extensively used types of meters are differential pressure, thermal mass and Coriolis.
Differential Pressure Flow Meter
An obstruction such as a disk with a hole of known diameter is placed inside a region of laminar flow, and fluid pressure is measured on each side. Pressure will be higher on the upstream side, with the difference in readings being relative to the distance between the two reading points, viscosity, volumetric flow rate and pipe diameter, as set out in Poiseuille’s equation. Corrections are then carried out for pressure and temperature in order to produce a standardized mass flow rate.
Thermal Mass Flow Meter
These are available in two designs: inserted probe and heated sample tube. Both derive mass from the specific heat capacity of the fluid (thus negating density variations) hence it is essential to know this property. Thermal mass flow meters are suitable for low gas flow rates.
In a heated sample tube mass flow meter, all or some of the flow passes through a high precision tube. Heat is applied to the tube and the variation in temperature measured. The difference in temperature between the two points specifies how much energy the fluid has absorbed, which relies on the mass moving through the tube.
Inserted probe mass flow meters use the same principle, but with two RTD probes located in the flow. The upstream sensor measures the fluid temperature while the second is heated to a temperature more than that of the first sensor. Heat is transferred from the second sensor to the fluid at a rate equivalent with the mass flow rate.
The Coriolis Mass Flow Meter
Coriolis mass flow meters measure mass through inertia. A dense gas or liquid flows through a tube which is vibrated by a small actuator. This acceleration generates a measurable twisting force on the tube proportional to the mass. The mass flow rate is specified without requiring any knowledge of the fluid flowing inside. More refined Coriolis meters use dual curved tubes for lower pressure drop and higher sensitivity.
Factors Influencing Accuracy
The accuracy of the differential pressure mass flow meter is affected by three factors. First, the measurement is inferred from pressure and temperature, so thus error in these reflects in the final result. Second, there are expectations about the viscosity and the degree to which laminar flow is attained (turbulence affects flow via the obstruction and produces misleading pressure measurements). Third, and perhaps the most significant, refers to the fact that the orifice may wear, becoming bigger over time and thus reducing the pressure drop. Based on the fluid passing through, there is also a risk of partial blockage.
When using thermal mass flow meters it is essential to consider the possible influence of:
- Moisture condensing on the temperature detector: Saturated gases may generate moisture, resulting in bad readings and ultimately, corrosion.
- Particulate accumulation: Low readings may also occur if heat transfer is hindered by the accumulation of residue on the sensor.
- Error in specific heat capacity assumption: Stemming from inconsistencies or variation in gas composition.
Additionally, thermal mass flow meters require time to reach a steady-state operating temperature. As soon as the device is powered-up, readings should not be taken.
Coriolis mass flow meters, while considered to be the most accurate, are vulnerable to errors caused by bubbles in liquid. These allow “splashing” within the tube, producing noise and changing the energy required for tube vibration. Huge cavities increase the energy required for tube vibration inordinately, resulting in complete failure. Additionally, separation of fluid into liquid and gas produces a damping effect on tube vibration.
Applications for Mass Flow Meters
Differential pressure mass flow meter(s) find application anywhere it can be safely assumed the fluid has reliable viscosity, and preferably where temperature does not change. Compressibility of gases can lead to problems but liquid dispensing and handling applications generally work well. Allowance must be made for the pressure drop through the meter. They are useful when a reading must be taken as soon as the meter is turned on.
Thermal mass flow meters work with both gases and liquids. They are used extensively in:
- Chemical processing
- Filter and leak detection
- Nuclear power facility air monitoring
- Semiconductor process gas measurement
Other common applications for thermal mass flow meters include laboratory analysis, such as gas chromatography.
Coriolis mass flow meters, as the most accurate, and also the most expensive, technique, are the predominant type of meter employed in scientific applications where they measure both corrosive and clean liquids and gases. They are also found in:
- Chemical processing
- Petroleum and oil
- Wastewater handling
- Pulp and paper processing
Pulp and Paper processing
Petroleum and Oil
Meters with a straight tube design are more effortlessly cleaned so are preferred for beverage and food applications and also for pharmaceuticals. They also handle the slurries usually found in mining operations.
The Latest in Mass and Volumetric Flow Meter Technology
The OMEGA® FMA6600/6700 series meters are multiparameter mass flow devices that have the potential to provide pressure, flow and temperature measurements. Intended for use with gases, mass flow is measured with the help of the heated sample tube principle. These meters handle gas flows from 0.15 to 100 LPM and are accurate to ±1% and accuracy ±11/2 percent full scale.
The stainless steel FMA3100/3300ST family of thermal mass flow meters are also designed for dry, clean gases. Using the heated sample tube principle, these are capable of handling flow rates from 0.4 to 20 SCCM to 10 to 500 LPM with temperature sensitivity of ±0.15% and repeatability ±0.25%, full-scale.
For situations where a differential pressure mass flow meter is most suitable, the FMA-1600A has a range of 0 to 0.5 SCCM up to 0 to 3,000 SLM. It comprises of more than 30 gas calibrations and displays pressure, temperature, volumetric and mass flow rate in a simultaneous manner. Typical accuracy on the FMA-1600A series is ±(0.8% of reading + 0.2% full scale).
It is possible to measure mass flow rates indirectly by differential pressure or directly with a meter employing either the Coriolis effect or specific heat capacity.
Coriolis mass flow meters generate the most accurate for most liquids but are expensive. They have the benefit of not requiring any knowledge about the fluid being carried.
Thermal mass flow meters are a less accurate but still direct measurement method. They do require knowledge of the fluid’s specific heat capacity.
The differential pressure mass flow meter generates an indirect measurement derived via Poiseuille’s equation that must be altered for fluid pressure and temperature. When the fluid is incompressible, this works well.
This information has been sourced, reviewed and adapted from materials provided by OMEGA Engineering Ltd.
For more information on this source, please visit OMEGA Engineering Ltd.