The proposed calibrating unit for hot-wire anemometers can reproduce the unit of airflow velocity to within

±0.65% in the range 0.026–4.65 m/sec. Ways of improving the metrological characteristics are considered.

Key words:hot-wire anemometer, calibration.

Calibration of hot-wire anemometers for low airflow velocity requires devices that can set the airflow velocity, tem-

perature, and humidity that correspond to the operating conditions of the instruments. The unit designed and made at the

Barnaul Experimental Design Bureau for Automation (OKBA) Khimavtomatika for calibrating hot-wire anemometers [1] is

of interest in this respect. A government commission certified that calibration unit and recommended it for use.

The design of the calibration unit for hot-wire anemometers is shown in Fig. 1. It includes a stand 11, which holds

the working chamber 7. Water is poured into that chamber through a funnel and a tap 10. Inside the chamber is a float 9 with

a plug 8. Attached to that float is a cable 12, the lower end of which holds a liquid-level stabilizer 14with discharge pipes 15

and a weight 16. The cable runs inside the tube13. The float, cable, tube, and liquid-level stabilizer maintain a constant liq-

uid level h in the working chamber. That ensures a constant discharge of air into the working chamber. The weight 16 serves

to immerse the float in the water and to close the plug 8 of the inlet of the tube 13. The anemometer sensor 1 is placed in

the thermal chamber 2 so that the measuring transducer (STZ-18 or STZ-25 solid-state thermistor) would be at the center of

the airduct 4 at a distance of half the airduct radius from its inlet. That distance has been selected experimentally on the basis

of the conditions for the formation of stable air flow in the airduct and the insignificant influence of the boundary layer of

air on the accuracy of setting the velocity. In the given case, the thermistor is immersed 1 mm. The control valve 5 in the

airduct 4 is for setting the airflow velocity at the point where the thermistor is installed and the valve 6 is for opening and

closing the airflow through the airduct 4. The airduct has a known cross-sectional area S at the inlet, where the measuring

transducer 3 is installed. If that operation is complicated, a special adapter is made and connected to the airduct. The thermal

chamber 2 protects the thermistor from uncontrolled airflows and the parameters of the ambient air (temperature, humidity,

impurity content, etc.) can be set in it.

The unit functions as follows. The weight 16makes the float 9 sink and close the inlet of the tube13with the plug8.

The working chamber 7 is filled with water through the funnel and tap 10, whereupon the tap is closed, the weight 16 is

removed, and the float 9 ascends. Water from the working chamber begins to flow out through the tube 13 into the liquid-level

stabilizer 14 and through discharge pipes 15 into the measuring cylinder 17. The volume of water entering the measuring

cycle per unit time is equal to the volume of air that passed through the inlet of the airduct 4 in the same time.

The airflow velocity was calculated from

v = 0.907Q/St,

Measurement Techniques, Vol. 47, No. 2, 2004

CALIBRATION UNIT FOR HOT-WIRE

ANEMOMETERS

MECHANICAL MEASUREMENTS

I. D. Pupyshev, A. A. Chepushtanov,and Yu. V. Beketov

UDC 551.508.5

Translated from Izmeritel’naya Tekhnika, No. 2, pp. 36–38, February, 2004. Original article submitted October 22,

2003.

0543-1972/04/4702-0168©2004 Plenum Publishing Corporation168

where 0.907 is a coefficient that takes into account the difference between the working chamber air pressure and the atmo-

spheric pressure; Q is the volume of water that flowed out of the working chamber, m3; S is the area of the airduct inlet,m2;

and t is the time when the water flows out of the working chamber, sec.

Standard instruments are used to measure the parameters that appear in the formula. The results of government

acceptance tests gave the following metrological characteristics of the unit:

cross-sectional area of the airduct inlet,mm2 . . . . . . . . . . . . . . . . . . . . . . . . . . .13.24 ± 0.013

range of airflow velocities that can be set,m/sec . . . . . . . . . . . . . . . . . . . . . . . . .0.026–4.65

sensitivity threshold, m/sec . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .0.026

time required to set a stable airflow velocity, sec . . . . . . . . . . . . . . . . . . . . . . . . . less than 9.8

error of reproducing the unit of airflow velocity, % . . . . . . . . . . . . . . . . . . . . . . . no more than ±0.65

time of maintaining a given airflow velocity in the range 0.026–4 m/sec . . . . . . . no less than 45.

This unit was used for government acceptance tests and a series of AVT-1M anemometers were made at OKBA NPO

Khimavtomatika. In recent years, proposals have been made for improving the unit,enhancing its technical specifications,

and extending its measuring ranges. Let us consider those proposals.

1. The difference of the water levels h in the working chamber and the liquid-level stabilizer is of the order of 1 m.

Increasing h makes it possible to extend the range of airflow velocities set.

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10 9 8 7 6 5 4 3 2 1

h

11

12

13

14

15

16

17

Fig. 1. Hot-wire anemometer calibrating unit: 1) hot-wire anemometer; 2) thermal

chamber; 3) measuring transducer; 4) airduct; 5) control valve; 6) valve; 7) working

chamber; 8) plug; 9) float; 10) valve with funnel; 11) column; 12) cable; 13) tube;

14) liquid-level stabilizer; 15) discharge pipe; 16) weight; 17) measuring cylinder.

2. Up to 10 liters of water is poured into the working chamber. The time for which a given airflow velocity is main-

tained increases proportionally as the volume of the working chamber increases.

3. The error of reproduction of the unit of airflow velocity is ±0.65%. We point out possible ways of reducing those

errors:

• choose more accurate instruments for measuring the volume, linear dimensions,time, and air pressure. For

example, use a flask of known volume that has a timer electrode in its neck and minimize the error of volume

and time measurements;

• install the thermistor more exactly at the airduct inlet. This decreases the influence of the thickness of the bound-

ary layer of air on the accuracy with which the cross-sectional area of the airduct is determined when the thermis-

tor is installed. The thickness δ of the boundary layer of air in the airduct in a laminar flow mode is given by [2]

δ = 5.2LRe–0.5,

where L is the distance from the airduct inlet; Re = vaLρa/µa is the Reynolds number; va is the airflow velocity; ρaand µa are the density and dynamic viscosity of the air. The influence of the thickness of the boundary layer of the

air can be decreased if the tolerance for the installation of the thermistor at the airduct inlet is less than 0.5d, where

d is the diameter of the thermistor fasteners (0.05 mm for STZ-18 and STZ-25 thermistors);

• take account of the evaporation of water in the working chamber. This phenomenon has an appreciable effect on the

error of reproduction of the unit of airflow velocity at the beginning of the range up to 0.2 m/sec. The influence of

evaporation can be decreased or completely eliminated by placing a thin polymer film on the surface of the water.

Calculations show that if these proposals are taken into account,the error of reproduction of the unit of airflow

velocity is less than ±0.25% and the calibration unit under discussion can be used as a reference (standard) unit.

4. In order to improve the operating characteristics,it is desirable to incorporate an indicator (scale) into the design

of the unit. Having graduated the scale in accordance with the formula given,we can calibrate and verify the instruments with

respect to that scale. This substantially reduces the work and time required to certify hot-wire anemometers. A U-shaped glass

or transparent polymer tube, filled with distilled water, can be used to indicate the airflow velocity. One end of the tube is

open to the atmosphere and the other is connected to the tube 4 near the valve 5 (see Fig. 1). The drop in the water levels in

the elbows of the tube, calibrated in airflow velocity at the tube inlet,is the scale of the calibration unit.

Let us note the advantages of the unit:

• the simple design of the system,both as shown in Fig. 1 and with the proposed improvements taken into account.

The user can make the unit independently;

• the instruments for measuring the volume, time, linear dimensions,and the air pressure are accessible for metro-

logical certif ication;

• the unit of airflow velocity is reproduced with a high degree of accuracy; and

• hot-wire anemometers are calibrated and verif ied under close to operating conditions.

The calibration unit works with an independent power supply on the principle of drawing air from the chamber. It

must be noted that using a source of excess air pressure to produce a range of fixed airflow velocities is undesirable for the

following reasons.

First, if the thermistor is placed at the airduct inlet,which is in the thermal chamber with excess air pressure, more

heat is removed from the thermistor by compressed air than is at atmospheric pressure. When used at atmospheric pressure

the velocity readings of the calibrated hot-wire anemometer are too low since the excess pressure in the thermal chamber if

varied in proportion to the set airflow velocities. Second, if the thermistor is installed at the airduct outlet,where the air pres-

sure is near-atmospheric, the error of reproduction of the airflow velocity unit increases because of the influence of the thick-

ness of the boundary layer of air in the airduct. The thickness of that boundary layer depends on the airflow velocity [2].

The unit presented here for calibrating hot-wire anemometers reproduces the unit airflow velocity measurement to

within ±0.65% in the velocity range 0.026–4.65 m/sec and has been certif ied by a government commission. The unit can set

airflow velocity parameters at which the hot-wire anemometers function. Proposals have been made for improving the cali-

170

bration unit and extending its ranges of application, which would allow it to be used as a reference unit. The calibration unit

is simple in design and the devices incorporated into it are familiar, which is why the unit can be made by a user.

REFERENCES

1. I. D. Pupyshev, Yu. V. Tudeneva, and A. I. Lisitsa, Inventor’s Certif icate 530256 USSR,Otkryt., Izobret., No. 36

(1989).

2. N. D. Slezkin,Dynamics of a Viscous Incompressible Liquid [in Russian],Nauka,Moscow (1965).

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