Section 2
Types of Transfer Standards for Flow Measurement
There are many types of transfer standards used to measure flow which can be divided into different categories based on the scientific principles they use to derive a flow measurement. Within these categories are a variety of techniques and devices that may differ in accuracy, reliability, portability, economy, and convenience. No single device is necessarily the best for all situations. An agency should select an appropriate type of transfer standard based on a complete evaluation of its situation with respect to available funds, available personnel and expertise, equipment on hand, location and distance to field sites, modes of transportation used, number and calibration frequency of analyzers to be calibrated, etc.[1]
This section contains details on different categories of flow measurement devices, with abbreviated explanations of the scientific principles they rely on. There is also some discussion of the accuracy and economy of thesedevices. Consideration should be given to changing conditions in the field such as temperature, barometric pressure, or physical shock that could affect the accuracy of a transfer standard.
Mechanical Based Flow Metering
Variable-area flow rate meter
Bubble and Rotameters are two common names often associated with variable area flow meters (VAFMs). VAFMs have a linear scale that allows for interpolation procedures. They are easy to work with and have a wide measurement range, with small pressure losses. They are simple to install, low maintenance, and relatively inexpensive. The accuracy of a VAFM depends greatly on the quality of construction, and can easily be calibrated. It operates by allowing a float to move freely withinatapered tube,the float is lifted as a result of kinetic energy of the liquid flow rate and balanced by the gravitational force exerted by the float. However there are drawbacks including the size and placement of a VAFMwhich most typically must be vertical as gravity is a key component of the mathematical relations governing its accuracy.
Mass Flow Controllers (MFCs):
A mass flow controller is a unique kind of flow meter in that it can both measure and control the flow it produces. An MFC operates by measuring heat loss through the use of a thermister. As gas passes through the MFC, a small amount of gas is diverted through a bypass. This bypass contains a thermister which changes electrical resistance based on temperature change. This change in resistance corresponds with a change in voltage. The small changes in voltage are detected by the MFC which adjusts the flow accordingly. MFCflowsrange from cubic centimeters per minute (sccm) up to liters per minute (slm). An instrument containing two or more MFCs can be used to supply diluted gas for verification or calibration purposes. Top of the line MFCs can have an accuracy and precision within one percent of full scale[2]. This combination of high accuracy, precision, and flow range have made MFCs an integral part of gas dilution and quality assurance systems.
Piston Type Flow Meter
One of the most accurate gas flow measuring devices is based on piston technology. This technology uses the theory that volumetric flow can be derived from the time it takes the piston to travel a known distance in an ideal cylinder. The distance and known area of the cylinder can be used to determine the volume of air that travels through the device in a certain time. In order to standardize the gas flow calculation it is critical to consider temperature and pressure. The piston system is designed with precisely calibrated temperature and pressure sensors.
For this method, a piston is housed in a closed cylinder, which is in-line with the flow being measured. Ideally, the piston is massless, frictionless, leakproof, impermeable, and constant shape. Most piston and cylinder systems are fitted so closely to ensure the viscosity of the gas under test results in a minuscule leakage that is considered insignificant.
Differential Pressure Flow Meters
The most common type of flow meter technology encountered in industry is a variation of a differential pressure flow meter commonly referred to as an obstruction meter. The obstruction meter is a function of the pressure drop across an obstruction such as but not limited to the following: orifice plates, Venturi, and flow nozzles. In its simplest form flow rate is proportional to the velocity, kinematic viscosity and geometric constraints of the device under test. Turndown ratio is the ratio of max flow rate to minimum flow rate and is mentioned in the following section to qualify the range in which a flow standard can be used.Turbulent flows can account for as much as 10% difference in observed flow readings. This must be kept to a minimum to avoid interference that imposes undesired trends on signals and leads to erroneous errors in readings.
Pitot tube
There are many variations of the Pitot Tube such as the simple Pitot, Static Source Pitot, and the Pitot Static tube. Pitot Static Tubes are able to take flow measurements indirectly using Bernoulli’s Principle to discern fluid flow velocity which can then be used to find either mass or volumetric flow rates. Fluid flow velocity is determined by inserting the Pitot Static Tube directly into fluid stream. At the tip facing the oncoming air flow, stagnation air pressure is measured while holes perpendicular to the air flow determine the static pressure of the fluid. Using the governing equations, the velocity of the fluid, mass, or volumetric flowcan becalculated. Pitot tubes are relatively inexpensive and easy to use.
Orifice type
In Orifice type flow meters a sharp obstruction is introduced inside the tube diameter that results in increased speed at the obstruction and a resulting pressure drop. The direct placement of the pressure sensors along the tube’s length and the depth at which they are placed will have a dramatic impact on the output of the sensor. In order to achieve more accurate results it is necessary to place the sensors at a precise location and depth to reduce fluctuations that result from the mixing of turbulent eddies that form both before and after the obstruction. Turndown ratio for orifice type flow meters is 5:1 which results in poor accuracy at low flow rates.
Figure 1: Orifice type flow meter
Flow nozzles
Flow nozzles also operate on the same principle as orifice plates but are more accurate. The increased accuracy is attributed to direct placement of the pressure tap at a point removed from areas of inconsistent flow regimes. This type of flow meter is more accurate due to low noise and almost null values of interference.
Figure 2: Flow Nozzle
Venturi type flow meter
In contrast to the orifice plate described earlier the classical Venturi is limited in its application to clean, non-corrosive liquids and gases. In addition, the Venturi is more expensive to produce than the orifice due to more precise machining.The sonic nozzle in particular is based on the Venturi type flow meters and has recently demonstrated improved accuracy and repeatability to within +/- 0.25% of full scale. The added cost is justified by the reduction of eddies and the noise and interference they could impart on the output; however the same continuity principle is used in both Orifice and Venturi type obstruction flow meters
Figure 3: Venturi type flow meter
References
Bios International Coporation.(2005). Theory of Operation.In DryCal ML-500 Manual (pp. 5-6).Bulter, New Jersey.
DHI Instruments.(2007). Molbloc-S [Brochure]. Retrieved Summer, 2010, from
EPA NCore. (2010, March 16). Operation and Maintenance of a Mass Flow Calibration System [Other]. Retrieved from
Figliola, R. S., & Beasley, D. E. (2006).Chapter 10 Flow Measurements. In Theory and design for mechanical measurements (4th ed., pp. 389-405). New Jersey: John Wiley & Sons, Inc. .
Omega.(1996). Flow and Level Measurement. Retrieved Summer, 2010, from
The Engineering ToolBox.(2005). Types of Fluid Flow Meters. Retrieved June 20, 2010, from
[1]
[2] EPA NCore guidance documents ( “Operation and Maintenance of a Mass Flow Calibration System - Training Video”