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 Ultimate Accuracy: NIST

Constant-Volume Prover (~0.05%)

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 -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

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