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ANEMOMETERS:
CALIBRATION
1. SCOPE
This NORDTEST method describes how instruments for measuring air
velocities can be calibrated.
2. FIELD OF APPLICATION
The method can be used for the calibration of any type of instruments
intended for measurement of air velocity provided the transducer is
so small that no significant effect occurs in the reference air
stream or a correction for the blockage can be made.
The instruments include mechanical instruments such as rotating vane
and cup anemometers, as well as electronic and electromechanical
instruments like thermoanemometers and electronic rotating vane and
cup anemometers.
The instruments can have a build-in indicator or they can have an
electrical output such as current, voltage or frequency or both.
published by
NORDTEST
key words
anemometers
classification
UDC 533.6.08/551.508.5pitot static tubes
calibration
test method
ISSN 0283-7226
Tekniikantie 12Fin-02150 Espoo
FINLANDwww.nordtest.org
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3. REFERENCES
1. Measurement of fluid flow in closed conduits - Velocity area
method using Pitot static tubes. IS0 3966.
2. Equation for the Determination of the Density of Moist Air(1981), P. Giacomo, BIPM, Metrologia 18,33-40 (1982).
3. Draft Recommendation on the Statement of Uncertainties,
P. Giacomo, BIPM, Metrologia 17,73 (1981).
4. DEFINITIONS
Adjustment. The operation intended to bring the instrument into
a state of freedom from bias and suitable performance
for its use.
Calibration. The set of operations which establish, under specified
conditions, the relationship between values indicated
by a measuring instrument or measuring system, or va-
lues represented by a material measure, and the corre-
sponding known values of the measurand.
Correction. The value which, added algebraically to the uncorrected
result of a measurement, compensates for an assumed
systematic error.
Traceability. The property of a result of a measurement whereby
it can be related to appropriate standards, general-
ly international or national, through an unbroken
chain of comparisons.
Uncertainty. Uncertainty of a measurement. An estimate characterizing
the range of values within which the true value of a
measurand lies.
5. SAMPLING
Not applicable.
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6. METHOD OF TEST
6.1 Principle
The transducer is placed in an air stream of known velocity or is
moved with known velocity through still air.
6.2 Apparatus
The air stream can be generated by an open jet or in a windtunnel.
The air stream shall have a known velocity distribution, turbulence
and density within the section where the transducer is placed. Low
velocities can be generated by moving the transducer through a tank
with still air.
The reference velocity can be determined by using a reference
anemometer, by measuring the flow rate through a known area or by
measuring the distance and time for the movement of a tracer in the
air stream. At higher velocities, the reference velocity can be
determined with a pitot static tube and a micromanometer in accor-
dance with IS0 3966, (ref. 1). When calibrating in a tank with still
air, the reference velocity is determined by the velocity of the
transducer movement.
For instruments with an electrical output signal such as current,
voltage or frequency, the necessary equipment for measuring of these
quantities shall be available.
Instruments for measurement of the following conditions of the air
stream shall be available:
- air temperature
- relative humidity
- barometric pressure
All measurements which are used in generation of the results in
the final report shall be traceable to national or international
standards.
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6.3 Preparation of test samples
Before calibration, the instrument is checked visually to ensure
that it is working properly.
The calibration should normally be done with the instrument used
as prescribed in the operating manual for the normal use of the
instrument.
If the operating manual is not available, only a zero adjustment
should be attempted and it should be stated in the certificate how
this was achieved.
If further adjustments are requested by the customer, the necessaryinstructions for these must be available and be followed.
6.4 Procedure
The transducer is placed in the air stream. The axis of the transducer
shall be aligned with the mean air flow direction with an angular
deviation not exceeding the manufacturers specifications and pre-
ferably less than +/- 2 degrees.
For the calibration of a measuring range 11 measurements are performed.
Each measurement consists of a determination of the reference velocity
and a reading of the instruments indication or a measurement of the
instruments output.
The measurements are performed at the following velocities:
0, 10, 20, 30, 40, 50, 60, 70, 80, 90, and 100% of the span for the
measuring range in question or limited by the maximum velocity for
the calibration laboratory.
The sequence of measurements is from low to high velocities.
In addition to the measurements above, the air temperature, rela-
tive humidity and barometric pressure are measured. Based on these
measurements the air density is calculated using the formula
recommended by BIPM, (ref. 2).
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6.5 Expression of results
The result of the calibration is shown in a table with 3 or 4
columns.
The first column shows the reference velocity. The reference velocity
shall be corrected for all known dependencies of influencial factors
such as blockage, air density, air temperature, barometric pressure,
relative humidity, turbulence, velocity distribution, etc. The
influence of unknown dependencies must be estimated and their pos-
sible effect included in the estimation of uncertainty. The reference
velocity is expressed in m/s.
The second column shows the instrument indication or the measured
value of output signal including any correction specified by the
manufacturer for the correct use of the instrument. The manufacturer
may for instance have specified corrections to be applied if the
measurement condition deviates from specified reference conditions.
These could, for instance, be 20 °C, 50% RH, 101.325 kPa. The unit
of this column is the unit of the instrument indication or of the
output signal.
The third column is only applied if the unit of the instrument
indication is m/s. It contains the correction for the instrument
calculated as:
(reference velocity - instrument indication)
The unit of correction is m/s.
The fourth column contains the uncertainty of the reference velocity
expressed in +/- m/s.
6.6 Uncertainty
The reference velocity is typically dependent on several quantities.
V = f (a, b, ....., n)
The uncertainty of each quantity can have several components. The
method described in ref. 3 shall be used to calculate the combined
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uncertainty of the reference velocity. A multiplying factor of 3
must be used as described in paragraph 5 of the method.
A formula for the estimation of the uncertainty of a velocity deter-
mined by means of a pitot static tube can be found in ref. 1,
chapter 12.5. However, please be aware that ref. 1 uses a multiply-
ing factor of 2 instead of 3 as prescribed above.
The partial derivatives for the calculation of the uncertainty of
air density can be found in ref. 2, page 39.
6.7 Test report
The calibration certificate or calibration report shall includethe following information:
d) Description of the calibrated instrument
e) Name of manufacturer or supplier of the calibrated instrument
f) Instrument type
g) Instrument and transducer serial number
h) Date of calibration
i) Reference to this NORDTEST method
j) Any deviations from this NORDTEST method
k) Calibration results
l) Conditions during the calibration (air temperature, RH, baro-
metric pressure and air density)
m) Identification of the equipment and reference instruments used
for the calibration
n) Traceability for measured reference velocity
o) Date and signature
a) Name and address of the calibrating laboratory
b) Identification number of the calibration cartificate
c) Name and address of the organization or person who ordered the
calibration