Neometrix / Flow Measurement & Calibration / Calibration Wind Tunnel / NMX-CWT-78

NMX-CWT-78 · ENGINEERED-TO-ORDER CLASS — SPECIFY · SIZE · BUILD · SURVEY · CALIBRATE · PROVE

Calibration Wind Tunnel. Same air for both instruments.

Name: Calibration Wind Tunnel
Brand: Neometrix
SKU: NMX-CWT-78

A calibration wind tunnel blows steady, even air past two instruments at once: a reference whose speed is known, and the instrument being checked. The reading of one is set against the other, speed by speed, and the difference is the calibration.

So the tunnel has to make the air the same everywhere both instruments sit, hold it steady while the readings are taken, and work out the reference speed honestly.

Discuss a calibration wind tunnel Capability note (PDF)

A calibration wind tunnel on a plain white studio background: a long horizontal tunnel of pale grey painted sheet steel on a low grey steel frame, with a wide bell-shaped inlet covered by a fine mesh, a flanged straight section, a tapering contraction, a clear test section in a bright aluminium frame with a slender pitot-static tube, a widening diffuser, and at the right end a round fan housing with a grey electric motor, and no people

Fig · 01 — A calibration wind tunnel: an inlet with a honeycomb face, a settling section, a contraction, a clear test section, a diffuser and a fan at the far end — illustrative render.

The test

a comparisonreference against instrument

The air

even and steadyhoneycomb, screens, contraction

The reference

pressure and densitypitot-static and air sensors

The record

every speed stepup the range and back

Status

engineered to orderno calibration wind tunnel claimed

ISO 9001ISO 14001IEC 61400-12-1 reference standard

Overview

Why the whole job is the air.

Because a calibration only compares two readings, and the comparison is worth nothing if the two instruments felt different air.

A test section of a calibration wind tunnel on a plain white studio background: a clear acrylic box in bright aluminium frames holding a slender pitot-static tube and a small three-cup anemometer on stainless steel stands on a round aluminium mount plate, two black tubes leading to a plain grey box beside it, and no people

Fig · 02 — A test section: a reference pitot-static tube and a cup anemometer on a rotating mount side by side in a clear section — illustrative render.

Same air for both instruments.

What the tunnel is for

It calibrates anemometers, pitot tubes and air-speed probes against a reference. It suits an open-circuit tunnel with a closed test section, an open-circuit tunnel with an open jet, and a closed-circuit tunnel. The instrument is fitted in the test section as it will be used, and its output is recorded at each speed.

Why a calibration is a comparison

The tunnel does not know the speed. The reference does. Both instruments sit in the stream, the reference reports a speed and the instrument under test reports its own, and the calibration is the difference between them at every step.

Why the air must be the same for both

If the stream is faster in one place than another, the two instruments read different air, and the difference lands in the calibration. So the tunnel makes the stream even across the place where both sit. Each instrument is put at the same position and the same orientation as the reference.

How the air is made even

The air is drawn through a honeycomb that straightens it, then through fine screens that quieten it, then through a smooth contraction that speeds it up into the test section. The honeycomb takes out swirl and sideways movement. The screens reduce turbulence. The contraction does its work without adding any.

How the reference speed is found

A pitot-static tube does not read a speed. It reads a pressure difference. The speed is worked out from that pressure and from the density of the air, and the density comes from the temperature, the pressure and the humidity of the air. So the reference chain carries sensors for all three, and not only for the pressure.

Why each speed is held still

Readings are taken only when the stream has settled at the step. A controller sets the speed, waits, records, and moves to the next step, going up the range and back down, so a drift or a lag shows in the record instead of hiding in it.

Why blockage matters

An instrument in the test section blocks part of the stream, and the air speeds up around it. The larger the instrument is compared with the section, the larger the error. So the section is sized to the instruments it will carry, and the correction is part of the method.

The method

Specify, size, build, survey, calibrate, and prove.

Six steps in a fixed order. Mapping the air before the first instrument goes in, and checking the setup with a reference instrument before every session, are the two a quick order skips.

1 · Specify

The instruments, the speed range, the section and the accuracy asked of the reference are stated by the customer or by the method they name.

2 · Size

The section is sized to the largest instrument so that blockage stays small, and the fan and the contraction are sized to the speed range.

3 · Build

The fan, the diffuser, the flow conditioner, the contraction and the test section are built as one installation.

4 · Survey

The stream is mapped across the section with a traversing probe, for evenness and for unsteadiness, at several speeds, before any instrument is calibrated.

5 · Calibrate

The reference chain is calibrated, the pressure transducer and the temperature, pressure and humidity sensors, so that the reference speed has a record of its own.

6 · Prove

A reference instrument is run first in every session, to show that the setup still gives the answer it gave before, and the record is kept with the results.

Which tunnel for which job

Tunnel	What it suits	What to watch
Open circuit, closed test section	a laboratory that calibrates many small probes at steady speeds	the walls add blockage that has to be corrected, and the tunnel takes its air from the room
Open circuit, open jet	instruments that are large, awkward to mount or need to be reached easily	the jet spreads and mixes with the room air, so the even core is smaller than the opening
Closed circuit	a laboratory that wants a quiet, steady stream and little disturbance from the room	the air warms as it circulates, so its temperature has to be watched and the density worked out for it

The step people underrate is the survey. A tunnel that is calibrated carefully and never mapped can give a tidy curve for air that was not the same at the two instruments.

Work content

What the tunnel contains, element by element.

Read it as a checklist: a tunnel missing a row will move air and give a curve, but not a curve you can defend.

A flow conditioning assembly of a wind tunnel on a plain white studio background: a pale grey painted steel duct with bolted flanges and one side cut away to show a panel of fine honeycomb cells, two fine mesh screens in bright frames and a smooth bright sheet steel contraction ending in a rectangular opening, on a low grey base, and no people

Fig · 03 — A flow conditioning assembly: a honeycomb panel, two fine screens and a smooth contraction in a duct section with one side cut away — illustrative render.

Element	What it does	What matters
Fan and drive	moves the air	a variable-speed drive that steps the speed smoothly and holds each step, with the fan and motor kept from shaking the stream
Diffuser	slows the air down after the section	a gentle widening, so that the flow does not break away from the walls and make the stream unsteady
Flow conditioner	makes the air straight and quiet	honeycomb cells lined up with the stream, and screens fine enough to reduce turbulence without choking the flow
Contraction	speeds the air up	a smooth shape that accelerates the stream into the section without adding turbulence
Test section	holds the instruments	a size matched to the instruments, with mounts that put the reference and the instrument at the same position and orientation, and a turntable to set the angle
Reference pitot-static tube	gives the reference speed	a tube calibrated for the speed range, in the stream beside the instrument
Pressure transducer	reads the pressure difference	a differential transducer chosen for the bottom of the speed range, where the pressure is smallest
Air density sensors	give the density	temperature, barometric pressure and humidity sensors, read for every run
Traverse and survey probe	map the stream	a traverse that moves a probe across the section, to measure evenness and unsteadiness
Controller and recorder	runs the steps	sets each speed, waits for it to settle, records both instruments and the state of the air, and writes the curve
Safety guards	protect people and instruments	guards on the fan inlet and outlet, a lock on the access, and an emergency stop
Procedure	fixes the sequence	specify, size, build, survey, calibrate and prove, the same for every tunnel
Testing & documentation	prove the tunnel first	the survey record, the reference chain record, a run with a reference instrument, and a record the customer can inspect

The elements that decide whether the air is the same are the flow conditioner and the contraction, and the one that decides whether the speed is true is the reference chain. The first two make the stream, and the third says how fast it is.

Full specification — expand

System	A fan and variable-speed drive; a diffuser; a flow conditioner of honeycomb and fine screens; a smooth contraction; a test section, closed or open-jet, with mounts that put the reference and the instrument under test at the same position and orientation and a turntable to set the angle; a reference chain of a pitot-static tube, a differential pressure transducer and temperature, pressure and humidity sensors for the air density; a traverse and survey probe to map the stream; a controller and recorder that step through the speeds and write the curve; guards and an emergency stop; a written test plan; and survey, calibration and documentation before handover
The One Idea	Same air for both instruments.
Why A Calibration Is A Comparison	The tunnel does not know the speed; the reference does. Both instruments sit in the stream, the reference reports a speed and the instrument under test reports its own, and the calibration is the difference between them at every step
Why The Air Must Be The Same	If the stream is faster in one place than another, the two instruments read different air and the difference lands in the calibration, so the tunnel makes the stream even across the place where both sit and each instrument is put at the same position and orientation as the reference
How The Air Is Made Even	A honeycomb straightens the air and takes out swirl, fine screens reduce turbulence, and a smooth contraction speeds the air up into the test section without adding any
How The Reference Speed Is Found	A pitot-static tube reads a pressure difference, not a speed, and the speed is worked out from that pressure and from the density of the air, which comes from the temperature, the pressure and the humidity of the air, so the reference chain carries sensors for all three
Why Blockage Matters	An instrument in the test section blocks part of the stream and the air speeds up around it, so the section is sized to the instruments it carries and the correction is part of the method
Standards	IEC 61400-12-1 is the public standard whose annex gives a wind tunnel calibration procedure for anemometers, and ISO/IEC 17025 is the public standard for the competence of calibration laboratories. This page prints none of their speeds, uncertainties or limits, and no edition year: those come from the method the customer names and from the instrument. Approval of an instrument, or of a calibration result, rests with the customer and the authority they name; none is claimed here
Configurations	An open-circuit tunnel with a closed test section, an open-circuit tunnel with an open jet, and a closed-circuit tunnel
Scope Boundary	This page is the calibration wind tunnel. It is not a wind tunnel pressure regulation system, which holds the supply pressure of a blowdown tunnel (see wind tunnel pressure regulation system), not a flow calibration rig, which weighs a liquid and times it (see flow calibration rig) and not an air data test set, which holds pitot and static pressures at the instruments (see air data test set). A high-speed tunnel and a model testing tunnel are different equipment again. No certification is claimed here
Status	Neometrix engineers calibration wind tunnels to order, and no delivered calibration wind tunnel is claimed.

Configurations

One method, three ways to build the tunnel.

The method, the flow conditioner, the reference chain and the record are shared. What changes is the circuit and the kind of test section.

Open circuit, closed section

Walls round the stream

A tunnel that draws room air through the conditioner into a closed test section, for probes and small anemometers at steady speeds.

Open circuit, open jet

A free stream to reach into

A tunnel whose stream leaves the contraction as a free jet, for larger instruments and mounts that need to be reached easily, with a traverse to map the even core.

Closed circuit

The same air round again

A recirculating tunnel for a quiet, steady stream, with a way to watch the temperature of the air as it circulates.

And the part that is not equipment at all, yet decides all three: the test plan. It fixes the speed steps, how many runs are made, how the instrument is mounted and what is recorded. It is written from the instrument and the method the customer names, not from the tunnel.

Where it is used

Wherever a wind reading is trusted, and the reading has to be earned.

The common thread is a small sensor out in the weather, and a number from it that other people will design, forecast or pay by.

Wind energy measurement

Where anemometers on masts are calibrated before they go out and checked when they come back.

Weather services and sensor makers

Where wind sensors are made, adjusted and verified against a reference.

Calibration laboratories

Where probes and air-speed instruments are compared with a reference and issued with a record.

Research and teaching laboratories

Where a known, even stream is needed to test a sensor or a small model.

Watch

See it in motion.

A short walkthrough video of the calibration wind tunnel.

FAQ

Common questions.

Longer answers, for readers who want the reasoning.

Q · 01 What does a calibration wind tunnel actually do?

It makes a steady, even stream of air and holds a reference and an instrument in it side by side. A fan moves the air, a conditioner straightens and quietens it, a contraction speeds it up into the test section, and a controller steps through a list of speeds. At every step it records what the reference says and what the instrument says, together with the state of the air. The result is a curve for the instrument. The tunnel does not decide whether the instrument is good enough: that is the job of the method and of the people who name it.

Q · 02 Why must both instruments see the same air?

Because a calibration is a comparison, and a comparison needs the same thing on both sides. If the stream is a little faster where the reference sits than where the instrument sits, the difference between their readings is partly the instrument and partly the stream, and the record cannot tell which. So the tunnel is built to make the stream even across the place where both sit, and both are mounted at the same position and orientation. It is also why the stream is mapped before any instrument is calibrated, not after.

Q · 03 How is the reference speed found if the tunnel does not know it?

From a pressure and a density. A pitot-static tube in the stream gives a pressure difference, and the speed follows from that difference and from the density of the air. The density is not a constant: it depends on the temperature, the barometric pressure and the humidity of the air that day. So the reference chain has a differential pressure transducer and three air sensors, and it records all four for every run. That is how the speed of the reference is traced to quantities that can each be measured and checked on their own.

Q · 04 Which tunnel is right: open circuit, open jet or closed circuit?

It depends on what is calibrated and where. A closed test section suits many small probes at steady speeds, but its walls add blockage that has to be corrected. An open jet is easier to reach into and suits larger instruments, but the jet mixes with the room air, so its even core is smaller than the opening. A closed circuit gives a quieter, steadier stream and less disturbance from the room, but the air warms as it circulates. We set out these trade-offs against your instruments before we propose a circuit.

Q · 05 Is this the same as the wind tunnel pressure regulation system, the flow calibration rig or the air data test set?

No, and the differences are worth stating precisely. The wind tunnel pressure regulation system page covers the valve train that holds the supply pressure of a blowdown tunnel, and it does not claim the tunnel. The flow calibration rig page covers a rig that weighs a liquid and times it. The air data test set page covers equipment that holds pitot and static pressures at instruments. This page is the calibration wind tunnel. A high-speed tunnel and a model testing tunnel are different equipment again. It does not replace any of them, and it does not claim what they claim a second time.

Q · 06 Has Neometrix built one of these?

We would rather answer this plainly than let a page imply otherwise. Neometrix engineers calibration wind tunnels to order, and no delivered calibration wind tunnel is claimed. What stands behind the offer is adjacent: Neometrix engineers a wind tunnel pressure regulation system (see the wind tunnel pressure regulation system page), a flow calibration rig (see the flow calibration rig page) and an air data test set (see the air data test set page). The fans, motors, drives, pitot-static tubes, transducers, sensors and controllers are catalogue items, bought in, not invented. What Neometrix engineers is the tunnel around them: the circuit, the flow conditioner, the contraction, the test section and its mounts, the reference chain, the control, the guards, the procedure and the test plan. So the honest position is that the tunnel is engineered to order, the neighbouring disciplines are in the building, and the first tunnel of this exact kind will be built around the customer's own instruments rather than taken off a shelf.

Q · 07 Which standard applies, and what can the tunnel not do?

IEC 61400-12-1 is the public standard whose annex gives a wind tunnel calibration procedure for anemometers, written for cup anemometers and widely used for other kinds. ISO/IEC 17025 is the public standard for the competence of calibration laboratories. This page prints none of their speeds, uncertainties or limits, and no edition year: those come from the method the customer names and from the instrument. The tunnel produces a calibration curve and keeps a record. It does not certify an instrument: this page claims no certification or accreditation, and a laboratory's own accreditation is its own. No speed, pressure, uncertainty, size or quantity figure is printed here, and approval for use rests with the customer and the authority they name; none is claimed here.

Q · 08 What do you need from us to quote?

Six things, and most of them describe your instruments rather than the tunnel. First, the instruments: their type, their size and how they are mounted. Second, the speed range you need to cover, and the steps you want. Third, the circuit and section you prefer, or the room and the access you have. Fourth, the reference: the accuracy you ask of it and the method you name. Fifth, the room: the space, the power and the noise you can accept. Sixth, the record: what you want kept for each run. From that we come back with a tunnel concept, a written test plan you can check, and a budgetary price.

The neighbouring flow and calibration pages, and how they differ from this one.

Three neighbours: one regulates a tunnel's supply, one weighs a liquid, one tests the instruments on board.

↗ A tunnel's supply

Wind Tunnel Pressure Regulation System

The valve train that holds the supply pressure of a blowdown tunnel - the supply of a different kind of tunnel, where this page is a low-speed tunnel and its reference.

↗ A scale and a clock

Flow Calibration Rig

A rig that weighs a liquid and times it to check flow meters - a liquid against a scale, where this page checks air-speed instruments against a pitot-static reference.

↗ Pressures at the instrument

Air Data Test Set

Equipment that holds pitot and static pressures to test the instruments on board - a pressure source for the instrument, where this page is a moving stream for the sensor itself.

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Get a quotation

Tell us the instruments, the speeds,
and the reference you need.

The projects desk replies within two working days with a tunnel concept, a written test plan you can check, and a budgetary quotation. Write to [email protected] or use the form.

Enquire — calibration wind tunnel Capability sheet (PDF) +91 7777 876 876

ISO 9001 / 14001 ENGINEERED-TO-ORDER CLASS — CALIBRATION WIND TUNNEL SPECIFY · SIZE · BUILD · SURVEY · CALIBRATE · PROVE — SAME AIR FOR BOTH INSTRUMENTS ENGINEERED IN NOIDA · INDIA