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 these devices. 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 within a tapered 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 VAFM which 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. MFC flows range from cubic centimeters per minute (sccm) up to liters per
1
http://www.epa.gov/ttn/amtic/files/ambient/qaqc/OzoneTransferStandardGuidance.pdf
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 scale2. 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 flow can be calculated. Pitot tubes are relatively
inexpensive and easy to use.
2
EPA NCore guidance documents (http://www.epa.gov/ttn/amtic/ncore/guidance.html) “Operation and
Maintenance of a Mass Flow Calibration System - Training Video”
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
http://www.dhinstruments.com/prod1/pdfs/brombloc_s.pdf
EPA NCore. (2010, March 16). Operation and Maintenance of a Mass Flow Calibration
System [Other]. Retrieved from http://www.epa.gov/ttnamti1/ncore/guidance.html
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
http://www.omega.com/literature/transactions/volume4/T9904-07-DIFF.html
The Engineering ToolBox. (2005). Types of Fluid Flow Meters. Retrieved June 20, 2010,
from http://www.engineeringtoolbox.com/flow-meters-d_493.html