ASTM D3796-90(2004)

NOTICE: This standard has either been superseded and replaced by a new version or withdrawn.
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Designation: D3796 – 90 (Reapproved 2004)
Standard Practice for
Calibration of Type S Pitot Tubes
This standard is issued under the fixed designation D3796; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope ducts: approximately 305 to 1524 m/min or 5.08 to 25.4 m/s
[1000 to 5000 ft/min].
1.1 This practice covers the determination of Type S pitot
3.2.3 pitobe assembly—any Type S pitot tube that is cali-
tube coefficients in the gas velocity range from 305 to 1524
brated or used while attached to a conventional isokinetic
m/min or 5.08 to 25.4 m/s [1000 to 5000 ft/min]. The method
source-sampling probe (designed in accordance with Martin
applies both to the calibration of isolated Type S pitot tubes
(1) or allowable modifications thereof; see also Fig. 7).
(see 5.1), and pitobe assemblies.
1.2 This practice outlines procedures for obtaining Type S
4. Summary of Practice
pitot tube coefficients by calibration at a single-velocity setting
4.1 The coefficients of a given Type S pitot tube are
near the midpoint of the normal working range. Type S pitot
determined from alternate differential pressure measurements,
coefficients obtained by this method will generally be valid to
made first with a standard pitot tube, and then with the Type S
within 63 % over the normal working range. If a more precise
pitot tube, at a predetermined point in a confined, flowing gas
correlation between Type S pitot tube coefficient and velocity
stream. The Type S pitot coefficient is equal to the product of
is desired, multivelocity calibration technique (Annex A1)
the standard pitot tube coefficient, C (std), and the square root
p
should be used.
of the ratio of the differential pressures indicated by the
1.3 This practice may be used for the calibration of thermal
standard and Type S pitot tubes.
anemometers for gas velocities in excess of 3 m/s [10 ft/s].
1.4 This standard does not purport to address all of the
5. Significance and Use
safety concerns, if any, associated with its use. It is the
5.1 The Type S pitot tube (Fig. 1) is often used to measure
responsibility of the user of this standard to establish appro-
the velocity of flowing gas streams in industrial smokestacks
priate safety and health practices and determine the applica-
and ducts. Before a Type S pitot tube is used for this purpose,
bility of regulatory limitations prior to use.
its coefficients must be determined by calibration against a
2. Referenced Document standard pitot tube (2).
2.1 ASTM Standards:
6. Apparatus
D1356 Terminology Relating to Sampling and Analysis of
6.1 Flow System—Calibration shall be done in a flow
Atmospheres
system designed in accordance with the criteria illustrated in
Fig. 2 and described in 6.1.1 through 6.1.5.
3. Terminology
6.1.1 The flowing gas stream shall be confined within a
3.1 For definitions of terms used in this test method, refer to
definite cross-sectional area; the cross section shall be prefer-
Terminology D1356.
ably circular or rectangular (3). For circular cross sections, the
3.2 Definitions:
minimum duct diameter shall be 305 mm [12 in.]. For
3.2.1 isolated Type S pitot tube—any Type S pitot tube that
rectangular cross sections, the width shall be at least 254 mm
is calibrated or used alone (Fig. 1).
[10 in.]. Other regular cross-section geometries (for example,
3.2.2 normal working velocity range—the range of gas
hexagonal or octagonal) are permissible, provided that they
velocities ordinarily encountered in industrial smokestacks and
2 2
have cross-sectional areas of at least 645 cm [100 in. ].
6.1.2 It is recommended that the cross-sectional area of the
flow-system duct be constant over a distance of 10 or more
This practice is under the jurisdiction ofASTM Committee D22 onAir Quality
duct diameters. For rectangular cross sections, use an equiva-
and is the direct responsibility of Subcommittee D22.01 on Quality Control.
lent diameter, calculated as follows, to determine the number
Current edition approved Oct. 1, 2004. Published December 2004. Originally
of duct diameters:
approved in 1979. Last previous edition approved in 1998 as D3796 - 90 (1998).
DOI: 10.1520/D3796-90R04.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
Standards volume information, refer to the standard’s Document Summary page on The boldface numbers in parentheses refer to the list of references at the end of
the ASTM website. this practice.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
D3796 – 90 (2004)
FIG. 1 Isolated Type-S Pitot Tube
FIG. 2 Pitot Tube Calibration System
D 5 2LW/~L 1 W! (1)
6.3 Type S Pitot Tube, (isolated pitot or pitobe assembly)
e
either a commercially available model or constructed in
where:
accordance with Martin (1) or allowable modifications thereof.
D = equivalent diameter,
e
6.4 Differential Pressure Gage—An inclined manometer, or
L = length of cross section, and
equivalent device, shall be used to measure differential pres-
W = width of cross section.
sure. The gage shall be capable of measuring DP to within
For regular polygonal ducts, use an equivalent diameter,
60.13 mm water or 1.2 Pa [60.005 in. water].
equal to the diameter of the inscribed circle, to determine the
6.5 Pitot Lines—Flexible lines, made of poly(vinyl chlo-
number of duct diameters.
ride) (or similar material) shall be used to connect the standard
6.1.3 To ensure the presence of stable, well-developed flow
and Type S pitot tubes to the differential-pressure gage.
patterns at the calibration site (test section), it is recommended
that the site be located at least 8 duct diameters (or equivalent
7. Procedure
diameters) downstream and 2 diameters upstream from the
7.1 Assign a permanent identification number to the Type S
nearest flow disturbances. If the 8 and 2-diameter criteria
pitot tube. Mark or engrave this number on the body of the
cannot be met, the existence of stable, developed flow at the
tube. Mark one leg of the tube “A,” and the other, “B.”
test site must be adequately demonstrated.
7.2 Prepare the differential-pressure gage for use. If an
6.1.4 The flow-system fan shall have the capacity to gener-
inclined manometer is to be used, be sure that it is properly
ate a test-section velocity of about 909 m/min or 15.2 m/s
filled, and that the manometer fluid is free of contamination.
[3000 ft/min]; this velocity should be constant with time. The
7.3 Level and zero the manometer (if used). Inspect all pitot
fancanbelocatedeitherupstream(Fig.2)ordownstreamfrom
lines and check for leaks; repair or replace lines if necessary.
the test-section.
7.4 Turn on the flow system fan and allow the flow to
6.1.5 Two entry ports, one each for the Type S and standard
stabilize; the test section velocity should be about 909 m/min
pitot tubes, shall be cut in the test section. The standard pitot
or 15.2 m/s [3000 ft/min]. Seal the Type S entry port.
tube entry port shall be located slightly downstream of the
7.5 Determine an appropriate calibration point. Use the
Type S port, so that the standard and Type S impact openings
will lie in the same plane during calibration. To facilitate following guidelines:
alignment of the pitot tubes during calibration, it is advisable
7.5.1 For isolated Type S pitot tubes (or pitot tube-
that the test section be constructed of acrylic or similar
thermocouple combinations), select a calibration point at or
transparent material.
near the center of the duct.
6.2 Standard Pitot Tube, used to calibrate the Type S pitot 7.5.2 For pitobe assemblies, choose a point for which probe
tube. The standard pitot tube shall have a known coefficient, blockage effects are minimal; the point should be as close to
obtained preferably directly from the National Institute of thecenteroftheductaspossible.Todeterminewhetheragiven
Standards and Technology in Gaithersburg, MD.Alternatively, point will be acceptable for use as a calibration point, make a
a modified ellipsoidal-nosed pitot static tube, designed as projected-area model of the pitobe assembly (Fig. 5), with the
showninFig.3maybeused(4).Notethatthecoefficientofthe impact openings of the Type S pitot tube centered at the point.
ellipsoidal-nosed tube is a function of the stem/static hole For assemblies without external sheaths (Fig. 5(a)), the point
distance; therefore, Fig. 4 should be used as a guide for will be acceptable if the theoretical probe blockage, calculated
determining the precise coefficient value. as shown in Fig. 5, is less than or equal to 2 %. For assemblies
D3796 – 90 (2004)
FIG. 3 Ellipsoidal Nosed Pitot-Static Tube
FIG. 4 Effect of Stem/Static Hole Distance on Basic Coefficient, C (std), of Standard Pitot-Static Tubes with Ellipsoidal Nose
p
D3796 – 90 (2004)
FIG. 5 Projected-Area Models for Typical Pitobe Assemblies
with external sheaths (Fig. 5(b)), the point will be acceptable if avoid yaw and pitch angle errors. Once the standard pitot tube
the theoretical probe blockage is 3 % or less (5).
is in position, seal the entry port surrounding the tube.
7.6 Connect the standard pitot tube to the differential-
7.7 Take a differential-pressure reading with the standard
pressure gage. Position the standard tube at the calibration
pitot tube; record this value in a data table similar to the one
point; the tip of the tube should be pointed directly into the
shown in Fig. 6. Remove the standard pitot tube from the duct
flow. Particular care should be taken in aligning the tube, to
NOTE—1 in. H O = 0.249 kPa; 1 mm H O = 0.0098 kPa.
2 2
FIG. 6 Calibration Data Table, Single-Velocity Calibration
D3796 – 90 (2004)
and disconnect it from the differential pressure gage. Seal the
where:
standard pitot entry port.
C (s) = Type S pitot tube coefficient,
p
7.8 ConnecttheTypeSpitottubetothedifferential-pressure
C (std) = coefficient of standard pitot tube,
p
gage and open the Type S entry port. Insert and align the Type DP = differential pressure measured by standard pitot
std
S pitot tube so that its “A” side impact opening is positioned at tube, kPa (in. HOormmH O), and
2 2
DP = differential pressure measured by Type S pitot
the calibration point, and is pointed directly into the flow. Seal
s
tube, kPa (in. HOormmH O).
the entry port surrounding the tube.
2 2
7.9 Take a differential-pressure reading with the Type S 8.2 Calculate the meanAand B side coefficients of theType
pitot tube; record this value in the data table. Remove the Type
S pitot tube, as follows:
S pitot tube from the duct; disconnect the tube from the
S C ~s!
1 p
differential-pressure gage. Seal the Type S entry port. C ~sideAorB!5 (3)
p
7.10 Repeat procedures 7.6 through 7.9, until three pairs of
differential-pressure readings have been obtained.
where:
C (side A or B) = mean A or B side coefficient, and
7.11 Repeat procedures 7.6 through 7.10 above for the “B”
p
C (s) = individual value of Type S pitot coef-
side of the Type S pitot tube.
p
7.12 For pitobe assemblies in which the free space between ficient, A or B side.
the pitot tube and nozzle (dimension x, Fig. 7) is less than 19.0
8.3 Subtract the mean A side coefficient from the mean B
3 1
mm ( ⁄4 in.) with a 12.7-mm [ ⁄2-in.] inside diameter sampling
side coefficient. Take the absolute value of this difference.
nozzle in place, the value of the Type S pitot tube coefficient
8.4 Calculate the deviation of each of the A and B side
willbeafunctionofthefreespace,whichis,inturn,dependent
coefficient values from its mean value, as follows:
upon nozzle size(6); therefore, for these assemblies, a separate
Deviation ~A or B side!5 C ~s!2 C ~sideAorB! (4)
p p
calibration should be done, in accordance with procedures 7.6
8.5 Calculate the average deviation from the mean, for both
through 7.11, with each of the commonly used nozzle sizes in
place. Single-velocity calibration at the midpoint of the normal the A and B sides of the pitot tube, as follows:
working range is suitable for this purpose (7), even though 3
S @C ~s!2 C ~sideAorB!#
1 p p
nozzles larger than 6.35-mm [ ⁄4-in.] inside diameter are not s~sideAorB!5 (5)
ordinarilyusedforisokineticsamplingatvelocitiesaround909
where s(sideAor B) = average deviation of C (s) values from
p
m/min or 15.2 m/s [3000 ft/min].
the mean, A or B side.
8. Calculation
9. Precision and Bias
8.1 Calculate the value of the Type S pitot tube coefficient
for each of the six pairs of differential-pressure readings (three
9.1 Precision—The results of the calibration should not be
from side A and three from side B), as follows:
considered suspect unless the following criteria fail to be met:
9.1.1 The absolute value of the difference between the mean
DP
std
C ~s!5 C ~std! (2)
p p Œ
DP Aand B side coefficients (see 8.3) is less than or equal to 0.01.
s
3 1
NOTE—This figure shows pitot tube-nozzle separation distance (x); the Type S pitot tube coefficient is a function of x, if x < ⁄4 in. where D = ⁄2 in.
n
FIG. 7 Typical Pitobe Assembly
mm in.
13 ⁄2
19 ⁄4
76 3
D3796 – 90 (2004)
9.1.2 The A and B side values of average deviation are less 9.2.2 AType S pitot tube shall be calibrated before its initial
than or equal to 0.01. use. Thereafter, if the tube has been significantly damaged by
9.1.3 If criterion 9.1.1,or 9.1.2, or both, are not met, the
field use (for example, if the impact openings are noticeably
Type S pitot tube may not be suitable for use. In such cases,
bent out of shape, nicked, or misaligned), it should be repaired
repeat the calibration procedure two more times; do not use the
and recalibrated. The data collected should be evaluated in the
Type S pitot tube unless both of these runs give satisfactory
light of this recalibration.
results.
9.2.3 The coefficient of a calibrated isolated Type S pitot
9.2 Bias—In general, the mean A and B side coefficient
tube may change if the isolated tube is attached to a source
values obtained by this method will be accurate to within
samplingprobeandusedasapitobeassembly.Theisolatedand
63 % over the normal working range (7).
assembly coefficient values can only be considered equal when
9.2.1 When a calibrated pitobe assembly is used to measure
the intercomponent spacing requirements illustrated in Figs.
velocity in ducts havin
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