Calibracion anemometros NTVVS018

8/6/2019 Calibracion anemometros NTVVS018

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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.



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

Calibracion anemometros NTVVS018

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