Basics - May, 2012 Gas Flow Calibration Basics Gas Flow Calibration Basics - May, 2012 Prepared By: Edward Morrell VP of Engineering - Bios
Gas & Flow & Calibration 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.)
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 can also be: Weight (mass)/ unit time mass flow Quantity (moles)/ unit time Energy / unit time natural gas (BTUs/time)
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)
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
Volumetric Flow Standardized Flow Standardization of Volumetric Gas Flow Volumetric Flow =volume/time Volumetric Flow Standardized (Ideal Gas) =volume/time* P m /P s *T s /T m Non-Ideal Gas Correction =Z(P s, T s )/Z(P m,t m ) Measured Gas Pressure =P m Measured Gas Temperature = T m Standardization Pressure =P s Standardization Temperature = T s Z(P s,t s )=Z factor @ P s, T s Z(P m,t m )=Z factor @ P m, T m
Definition : Ideal Gas Law For Ideal Gases: PV=NRT 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.
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: PV=ZNRT For inert gases under typical measurement conditions Z can be safely ignored for volumetric gas measurement to standardization conversion
Bell Prover upside down bucket and stop watch Mesa ML 1020 replacing many of these
Mercury Piston Prover Predates DryCal, hazardous mercury, slow DryCal = no liquids
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
Ultimate Accuracy: NIST Constant-Volume Prover (~0.05%)
Technique Positive Displacement Differential Pressure Momentum Variable Area Force Thermal Other Examples Diaphragm, Piston, Bell Jar, rate of rise, PVTt.. Orifice, Venturi, Nozzle, Pitot Tube, Laminar Flow Element (LFEs), Criticals Coriolis Turbine, Propeller, Swirl, Rotameters, Slotted Cylinders w/piston Target, Hydrometric Pendulum Thermal Mass, Hot Wire or Hot Film Anemometry Vortex Shedding, Ultrasonic-Travel-Time, Doppler, Long Wave
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 -800 & ML -500 Flow Range 0.5 to 50,000 sccm Accuracy 0.15% - 0.45%
ML 1020 25 % of reading 500 to 500,000 sccm Ability to measure up 1500 slm Upstream Calibrations 45 PSIG Bench Top Unit
Defender 500 & Definer 220 Series 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
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
Air Sampler Calibration with DryCal Definer 220 DryCal calibrator connected directly to air inlet side of particulate pollution air ambient monitor
Calibration of variable area and other flow meters 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
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
For further information on Mesa Labs DryCal Gas Flow Calibration Instruments Visit Our Website www.mesalabs.com NASDAQ: MLAB