Air Flow Calibrations I. Introduction

Air Flow Calibrations I. Introduction
Airflow Instrument Calibration
Air Flow Calibrations
Steven E. Guffey, PhD, CIH
I. Introduction
A. Evaluation of exposures
B. Direct measurement
C. Collection and analysis of samples for TWA
Steven Guffey, PhD, CIH
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Airflow Instrument Calibration
A. Evaluation of Exposures
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Measure a concentration of contaminant in air
- Collect sample of contaminant using sampling pump
or by other means
- Measure amount of air drawn during sampling
- Measure amount of contaminant collected
- Calculate the concentration
B. Direct Measurement
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Steven Guffey, PhD, CIH
Device that directly quantifies the amount
of contaminant in the air
- Real time measurement
- Series of instantaneous concentrations, or
- Continuous indication of the
concentration
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Airflow Instrument Calibration
C. Collection and Analysis of Samples for Time
Weighted Average (TWA)
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Sampling device collects the contaminant and
separates it from a volume of air
Single, integrated measurement over a length
of time allows only a time average
concentration
Later analysis of the collected contaminant(s)
More topics
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Steven Guffey, PhD, CIH
Air flow measuring devices
Advantages and limitations of each type of
device.
Proper use of each type.
Calibration of pumps and secondary
standard devices
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Airflow Instrument Calibration
Steps in sampling air
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Sample with a collection device
Draw air with an air mover
Measure the volume of air accurately
- The accuracy of the concentration depends on the
accuracy of the measurement of the air volume.
- The volume is measured by control of flow for a
set time.
Vocabulary
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Steven Guffey, PhD, CIH
Flow--the volume of air drawn per time
Flow (Q) = Volume (Vol) / time (t),
Usually in terms of liters per minute (LPM
or L/min) or mL/min.
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Airflow Instrument Calibration
II. Air Movers
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A. Pumps
- 1. Hand operated
» Single, fixed volume (detector tube pumps)
» Continuous sampling (obsolete)
- 2. Motor driven
» diaphragm, piston, vane
» adjustable flow, constant flow, pressure
regulated.
B. Pumps
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Steven Guffey, PhD, CIH
Location/size: Portable (personal) vs stationary (area)
High volume: usually too large for person to wear.
Used for bulk sampling (e.g., for free silica analyses)
and for collection of fine particulates.
Low flow (10 ml/min to 1.0 L/min): typical for
personal sampling of gases and vapors on activated
charcoal
Moderate flow (1.0 L/min to 2.0 L/min): typical for
personal sampling of aerosols
Line operated vs. battery operated vs. rechargeable
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Airflow Instrument Calibration
Pumps (Continued)
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Manual
Adjustable flow
Flow controlled
Pressure controlled
Feedback control
Programmable
III. Measurement of Flow
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Flow is measured by the volume of air
moved per unit amount of time.
» Q = V / t,
» Where Q = flow, V = Volume, and t is time.
Steven Guffey, PhD, CIH
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Airflow Instrument Calibration
Standard Levels of Flow Measurement
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Primary
- Measurements generally involve a direct
measurement of volume on the basis of the
physical dimensions of an enclosed space.
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Secondary
- Standards are reference instruments or meters
which trace their calibration to primary
standards and are capable of maintaining their
accuracy.
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Intermediate
A. Primary Standard Flow Measuring
Devices
1. Instruments measure volume based on
measured dimensions (i.e., length)
2. Examples
» Soap Bubble Meters
» Spirometers
» Sealed piston Devices.
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Airflow Instrument Calibration
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Primary standard
Can be directly related
to measured
dimensions & time
Simple, light weight,
accurate, stable
Fragile, messy liquids
for field applications
1. Soap Bubble Meters
²h
Buret
2
A=š(D/2)
Soap bubble
To pu
Schematic drawing of a
spirometer
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Primary standard
Can be directly related
to measured
dimensions & time
Accurate, used for
medical applications
Bulky, difficult to
move, require liquid
Not a field instrument.
Steven Guffey, PhD, CIH
Counterweight
Scale
Bell
Air
Cavity
Fluid
Three-way
valve
Stand pipe
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Airflow Instrument Calibration
Secondary Standard Devices
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Require calibration against a primary device
May need repeated calibrations
May not be as accurate as primary
B. Secondary Flow Measuring Equipment
1. Rotameters
2. Wet test meters
3. Dry gas meters
Steven Guffey, PhD, CIH
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Airflow Instrument Calibration
Upper bushing
1. Rotameters
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O-ring seal
Handy for field work.
Need calibration.
Accurate and precise
when calibrated and
maintained.
Glass or plastic
Measurement dependent
on temperature and
pressure
Upper stop
Flowmeter
tube
Plastic shield
Flowmeter ball
Lower stop
O-ring seal
Lower bushing
Six kinds of rotameter floats
Float reading
taken here
Dual floats
(glass and
stainless-steel
spheres)
Steven Guffey, PhD, CIH
Plumb-bob
float
Viscositystable
float
Ultraviscositystable
float
T-shaped
float
Combination
float
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Airflow Instrument Calibration
Typical Rotameter Floats
(Read float at widest dimension!)
Read here
Spherical
Plumb bob
Read here
Spool
Cylindrical
(marked)
1. Rotameters (Continued)
„ Measurement
dependent on
temperature (T) and pressure (P) of
airstream passing through it
Q2 = Q1 (den1/den2)0.5
= Q1 (P1T2 / P2T1)0.5
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Airflow Instrument Calibration
Calibration of Rotameters
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Calibrate against a
primary flow
measuring device
Measure the indicated
flow against the actual
flow
Repeat across the
working range of the
meter
3.5
Rotamet er f low r at e ( lpm )
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3
2.5
2
1.5
1
0.5
0
0
0.5
1
1.5
2
2.5
3
3.5
4
Burette flow rate (lpm)
Revised calibration curve for rotameter using bubble meter
3.5
Measured flow rat e ( lpm)
3
2.5
2
1.5
1
0.5
0
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
Rotometer flow rate (lpm)
Steven Guffey, PhD, CIH
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Airflow Instrument Calibration
Effect of Loading and Restricted Flow
on Rotameter Readings
10
9
Comparison of
changes in filter
resistance for five
filters
Nucleopore
AA Millipore
Glass filter
Metricel
"Dirty"
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V ac uum (inches w at er)
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7
6
5
4
3
2
1
0
0
0.2 0.4 0.6 0.8
1
1.2 1.4 1.6 1.8
2
Rotometer flow rate (lpm)
Pressure effects on pump & rotameter functioning
Pump calibration vs. a
standard at different
pressures
r2=0.99
r2=0.99
15 inches water
Ambient
25 inches water
r2=0.99
3.2
Buret te f low rat e ( lpm)
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3.8
3.6
3.4
3.0
2.8
2.6
2.4
2.2
2.0
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
0.0
1.0
2.0
3.0
4.0
Rotameter flow rate (lpm)
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Airflow Instrument Calibration
B. Other Secondary Flow Measuring Equipment
3
2
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2. Wet test meter
1
0
- Bulky
- Liquid filled
- Atmospheric
pressure
Gas pressure
gauge
Gas
thermometer
1
2
Water funnel
for filling
Gas outlet on
back of meter
3
Gas inlet on
back of meter
Water level
sight glass
Water level
Calibrating
point
Partitioned
drum (rotor)
B. Other
Secondary Flow
Measuring
Equipment
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3. Dry Gas Meters
-
Non-portable
Large
Higher flow
Stable
Industrial
applications
Steven Guffey, PhD, CIH
Mechanical valve
and counter
mechanism
Meter
readout
index
cubic
feet
Bellows or
diaphragm
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Airflow Instrument Calibration
Dry Gas Meters
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Schematic diagram
showing principle of
operation of twinlobed positive
displacement meter
C. Other Flow measuring Devices
1.Electronic bubble meters
2.Dry cal meters
3.Mass flow meters (Perkins, Pg. 732)
Steven Guffey, PhD, CIH
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Airflow Instrument Calibration
The Gilibrator (legend)
2
1
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Nomenclature:
2A
3
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1. Bubble generator assembly
2. Damper plate
2a. Pulsation damper
2b. “O” ring
3. Spacer
4. Plate, bubble breaker
5. Cable assembly
6. Sensor block connecting jack
7. Sensor block locking screw
8. Sensor block
9. Base plate assembly
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10. Mounting base
11. Charging indicator
12. On-off
4
25
24
5
23
6
7
22
8
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The Gilibrator (legend)
2
1
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Nomenclature:
13. Charging jack
14. Control unit
15. Average and Sample #
switch
16. Delete switch
17. Reset switch
18. Printer jack
19. LCD display
20. Run indicator
21. Bubble generator ring
22. Bubble initiate button
23. Air inlet boss
24. Flow tube
25. Storage tubing
26. Air outlet boss
Steven Guffey, PhD, CIH
2A
3
26
4
25
24
5
23
6
7
22
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21
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20
10
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17
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12
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Airflow Instrument Calibration
IV. Calibration of Air Flows
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Secondary devices are calibrated against primary.
Calibration should be done under identical
conditions to that of the actual sampling.
See diagrams of calibration apparatus.
Evaluation of pump errors.
Calibrate before, during, and after sampling
Diagrams of Calibration Apparatus.
Bubble meter
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Key to diagram
- Soap bubble meter
- Rotometer
- Pump
- Pressure gauge (not
shown)
Pump
Rotameter
Steven Guffey, PhD, CIH
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Airflow Instrument Calibration
Diagrams of Calibration Apparatus.
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Key to diagram
- Electronic soap
bubble meter
- Rotometer
- Pump
- Pressure gauge (not
shown)
Built-in rotometer
E. Preview of Lab Experiment I.
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In this experiment we will use the different air flow
measuring instruments and apply them to calibrate each
other.
- Part 1: Calibration of rotameter using bubble burette.
- Part 2: Calibration of sampling pump using rotameter.
- Part 3: Evaluating the effects of filters on pump
performance.
- Part 4: Evaluating the effects of atmospheric pressure
on rotameter performance.
- Part 5: Comparison of a mass flow meter and rotameter
as a function of pressure.
- Part 6: Calibration of a rotameter with a (Gilibrator)
electronic flow meter.
Steven Guffey, PhD, CIH
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Airflow Instrument Calibration
Layout
Pressure Gauge
Trap
Valve
Electronic
bubble
or dry cell
Bubble meter
Mass flow
meter
Rotameter
Pump
Calibration fastest if put all devices in series
All devices except bubble meters must be corrected for density
Remember that mass flow devices may read out in L/min, but they
control mass per time
Mass flow meters: Vol/time = MassFlow/density
Rotameters: calibrate at density will use (too confusing otherwise)
Best to calibrate at altitude will use
Density in Sealevel Lab With Resistance
1.0 Lpm
1.05
1.00
1.10
1.00
or 1.10
Electronic
bubble
or dry cell
Mass flow
meter
Rotameter
Pump
Pressure Gauge
Trap
Bubble meter
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Valve
If high resistance at needle-valve, get high SP
Bubble meter reads value at NTP
Air expanded downstream of valve
Electronic bubble meter reads correct expanded value
Rotameter values must be corrected for effects of density
Mass flow meter correct for air entering bubble meter
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Airflow Instrument Calibration
Density in Denver With No Resistance
1.0 Lpm
Pressure Gauge
Trap
Bubble meter
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1.0
1.0 Lpm
0.88
Valve
Electronic
bubble
or dry cell
Mass flow
meter
Rotameter
Pump
If no resistance at needle-valve, get room pressure
Bubble meter reads value at Denver barometric pressure
Same mass of air expanded compared to Seattle
Electronic bubble meter reads correct value
Mass flow meter and rotameter values must be corrected for
effects of density unless flowrates from them calibrated in Denver
REFERENCES
- PERKINS, CHAPTER 18
- AIR SAMPLING INSTRUMENTS, ACGIH,
Chapter 7, 1995.
Steven Guffey, PhD, CIH
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