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Prepared By: Edward Morrell
VP of Engineering - Bios
Gas Flow Calibration Basics - May, 2012
Gas Flow Calibration Basics
Gas Flow Calibration Basics - May, 2012
• Flow is the Quantity of material transported across a defined
boundary per unit time: Flow = Quantity/Time
•Quantity can be: (Weight or Mass, Volume, Number….)
volume per unit time is typical units for gas flow
•Per unit time: indicates an action or how long for something
(how much time for a reaction to be complete, how long can a
person be exposed to a contaminate, etc.)
Gas & Flow & Calibration
Gas Flow Calibration
Volumetric Gas Flow = (Volume of gas/unit time)
The DryCal operating principal “The bucket and stopwatch”
Precisely known volume Measure time to fill the bucket
Volumetric Flow = Known volume / time to fill the bucket
This is the operating principal of a DryCal, note this only
measures steady state flow
Gas Flow Calibration
Gas Flow can also be:
Weight (mass)/ unit time – mass flow
Quantity (moles)/ unit time
Energy / unit time – natural gas (BTUs/time)
Gas Flow Calibration
However, Gas is compressible
Changing the pressure or temperature of a gas
changes the volume of the gas
A pressure change is needed to make gas flow. So as
gas flows it changes volume.
Therefore – Volumetric flow is a poor measurement of
gas flow and rarely used (air sampling is the exception)
Gas Flow Calibration Volumetric Flow – Standardized Flow
But if we also measure the gas pressure and gas
temperature we can calculated what the volume of the
gas is at defined pressure and temperature
Gas Flow Calibration Volumetric Flow –Standardized Flow
Standardization of Volumetric Gas Flow
Volumetric Flow
=volume/time
Measured Gas Pressure =Pm
Measured Gas Temperature = Tm
Volumetric Flow Standardized (Ideal Gas)
=volume/time* Pm/Ps*Ts/Tm
Standardization Pressure =Ps
Standardization Temperature = Ts
Non-Ideal Gas Correction
=Z(Ps, Ts)/Z(Pm,Tm)
Z(Ps,Ts)=Z factor @ Ps, Ts
Z(Pm,Tm)=Z factor @ Pm, Tm
Gas Flow Calibration Definition : Ideal Gas Law
Where, in compatible units
P is the absolute pressure, Pa or psia, etc.
V is the volume of the gas having molecular weight, m
N is the number of moles of the given gas
R is the Universal Gas Constant,
T is the absolute temperature K or R.
PV=NRT
For Ideal Gases:
Gas Flow Calibration
Compressibility Factor, Z (Only on ML-One)
For real gases, there are deviations from the Ideal
Gas equation, and these deviations depend on the gas and the temperature and pressure. The Compressibility Factor, Z is used to compensate for these deviations, using:
For inert gases under typical measurement
conditions Z can be safely ignored for volumetric gas
measurement to standardization conversion
PV=ZNRT
Gas Flow Calibration Bell Prover –upside down bucket and stop watch
Mesa ML 1020 replacing many of these
Gas Flow Calibration
Mercury Piston Prover
Predates DryCal, hazardous mercury, slow
DryCal = no liquids
Gas Flow Calibration DryCal Piston Prover – Positive Displacement
•Valve closes, diverting gas into
measurement cell
•Piston accelerates to the timing
start point
•Timing begins
•Timing stops after piston
reaches location of known
volume
•Piston to Tube clearances
typical .0005 inches
•Leakage past piston calibrated
and added to flow reading
Measurement Cell
Piston
Cylinder
Timing Stop Point
Timing Start Point
Piston
Acceleration
Gas Flow Calibration
Technique Examples
Differential Pressure
Orifice, Venturi, Nozzle, Pitot Tube, Laminar
Flow Element (LFEs), ”Criticals”
Momentum Turbine, Propeller, Swirl,
Coriolis…
Variable Area Rotameters, Slotted Cylinders w/piston…
Force Target, Hydrometric Pendulum…
Thermal Thermal Mass, Hot Wire or Hot Film
Anemometry…
Other Vortex Shedding,
Ultrasonic-Travel-Time, Doppler, Long Wave
Positive
Displacement Diaphragm, Piston, Bell Jar, rate of rise, PVTt ..
DryCalML One™
Primary Gas Flow Calibrator
5 to 50,000 sccm
.15 % of reading
Corrosive gas
compatibility
Z factor correction
All-in-one design
Faster reading
ML 1020
25 % of reading
500 to 500,000 sccm
Ability to measure up 1500
slm
Upstream Calibrations
45 PSIG
Bench Top Unit
1.0 % of reading
standardized -Definer
1.0 % of reading
volumetric - Defender
Three flow ranges (5 -
500ccm, 50 – 5,000ccm,
300-30,000ccm)
Small, Portable,
Rechargeable Battery
Operation
Defender 500 & Definer 220 Series
MFC Calibration with Bios and Instruments containing MFCs
for flow control •MFC as shown with
ambient exhaust
•Short direct
connection between
MFC and DryCal
•Backpressure
applications add
backpressure
regulator
downstream of MFC
upstream of Dry
Calcalibrator
•ML always
exhausts to ambient
DryCal calibrator
connected directly to air
inlet side of particulate
pollution air ambient
monitor
Air Sampler Calibration with DryCal Definer 220
Flow set by: Regulator and flow restrictor
Flow is set with upstream pressure greater then 30 PSI
One side of A- B valve to device under test (DUT) other side to calibrator
Switch valve between DUT and Calibrator, eliminates possible interaction
Calibration of variable area and other flow
meters
Gas Flow Calibration
Uncertainty – What is the statistical certainty of a measurement
Traceability – Is the measurement tied to a national standard
Accreditation – Has the calibration laboratory processes and measurement procedures been reviewed by an independent entity
NASDAQ: MLAB
For further information on Mesa Labs
DryCal Gas Flow Calibration Instruments
Visit Our Website
www.mesalabs.com