INSTITUTE OF PHYSICS PUBLISHING

SUPERCONDUCTOR SCIENCE AND TECHNOLOGYSupercond. Sci. Technol. 19 (2006) S149–S151

doi:10.1088/0953-2048/19/3/022

SQUID NDE for in situ inspection ofcopper heat exchanger tubes
Y Hatsukade1, A Kosugi1 , N Ishizaka1 , S Okuno1, K Mori2 andS Tanaka1
1

Department of Ecological Engineering, Toyohashi University of Technology,
1-1 Hibarigaoka, Tenpaku-cho, Toyohashi, Aichi 441-8580, Japan
2
Sumitomo Light Metal Industries Ltd, Copper Works, 100 Ougishinmichi, Ichinomiya-cho,Hoi-gun, Aichi 441-1295, JapanE-mail: hatukade@eco.tut.ac.jp

Received 29 September 2005
Published 15 February 2006
Online at stacks.iop.org/SUST/19/S149
Abstract
An eddy-current-based SQUID NDE system was constructed for in situ
inspection of copper heat exchanger tubes using an HTS SQUID
gradiometer and a Helmholtz-type-coil inducer. Thin copper tubes of
6.35 mm in diameter and 0.8 mm in thickness were selected from products
as specimens. Artificial flaws of 100 µm in width, 15–25 mm in length and
10–50 µm in depth were made on the surfaces of the tubes. The tubes weremoved at a velocity of 1.6–32 m min−1 by a motor through theHelmholtz-type coil, which generated an excitation field of 10 µT at 3 kHz.
The SQUID NDE system could detect an anomalous magnetic response due
to a flaw of 10 µm in depth on the tube moving at 32 m min−1 .
(Some figures in this article are in colour only in the electronic version)

1. Introduction
In order to extract better performance, copper heat exchange
tubes used, for example, in air conditioning have had athickness of less than 1 mm. Thus, even a shallow flaw of
20–30 µm in depth on the surface of a tube, which accidentally
happened in the manufacturing process of the tube, can bea potential cause of tube breakage when the tube is bent
or flared in post-processes. Eddy current testing (ECT) has
been employed to detect defects in metallic tubes in tube
factories [1, 2]. However, the sensitivity of commercial ECT
systems is not enough to detect such shallow flaws. Thus, amore sensitive NDE technique, which can be applied in tube
factories, is much demanded by manufacturers and users ofthese tubes.
The HTS SQUID is an uncontested sensitive sensor withmagnetic field resolution of several tens of fT/Hz1/2 [3].
Taking advantage of the great sensitivity between nearly DC
and several hundred kHz, the SQUID NDE has already shown
some significant results in the detection of small defects inmetallic materials and structures [4–6]. In particular, for aSQUID NDE application on copper heat exchanger tubes,
the NDE technique must have not only high sensitivity but0953-2048/06/030149+03$30.00 © 2006 IOP Publishing Ltd

also robustness against noise and practical usability, since thetube factories will be electromagnetically noisy and the tubes
are moved at high speed (more than 100 m min−1 ) in themanufacturing process.
To meet these requests, we constructed an eddy-currentbased SQUID NDE prototype system for in situ inspection
on copper heat exchanger tubes using an HTS SQUID
gradiometer, which is expected to cancel the environmental
magnetic noise. In this paper, the details of the system anddemonstration of detection of surface flaws of a few tens ofµm in depth on the copper tubes moving at up to 32 m min−1
are described.

2. SQUID NDE prototype system for copper tubes
2.1. Principle of the SQUID NDE on tubes
In the manufacturing process of copper heat exchanger tubes,flaws that occur on the tubes are lengthened along the axis of thetube. To detect such long and shallow flaws, a Helmholtz-type
coil composed of two field coils is a suitable inducer because
it induces eddy current circulating around the circumferenceof the tube (perpendicular to the axis of the tube) when thePrinted in the UKS149

Y Hatsukade et al
HTS SQUID gradiometer
Copper heat exchange tube

Electronics

Eddy current

Micro flaw

Movementof tube

Lock-in amp.
Cryostat
XYZ coil positionadjustorz
Excitation fieldyNotePC PC
AC currentx

Current supplier

Copper tube
Helmholtz-typecoil
RailMotor

Helmholtz-type coilFigure 1. Principle of the SQUID NDE for a copper tube.

tube passes through the inducer. A schematic image of theSQUID NDE on a copper tube with a flaw is illustrated infigure 1. The copper heat exchanger tube under test is moved
through a Helmholtz-type coil, which applies an excitation
field to the specimen. An HTS SQUID gradiometer, which islocated in the middle of the Helmholtz-type coil and above thetube, measures the magnetic anomaly generated by the eddy
current, which is disturbed by the micro-flaw on the surface ofthe tube.
2.2. Details of the system
The SQUID NDE prototype system for copper heat exchanger
tubes is composed of an HTS SQUID gradiometer withelectronics, a cryostat, a Helmholtz-type coil with position
adjuster, a current supplier, a lock-in amplifier, an electricmotor, and a PC. Figure 2 shows a schematic illustrationof the system. The HTS SQUID planar gradiometer has adifferential pick-up coil composed of two rectangular coils.
The sizes of the single coil and the baseline length of thegradiometer are 2.88 mm × 3.6 mm and 3.6 mm, respectively.
The details of the gradiometer are given elsewhere [7]. We
used an FRP cryostat that can hold liquid nitrogen for 6–7 h.
The Helmholtz-type coil was set in the polymeric position
adjuster, which is used to minimize the excitation magnetic
flux coupled to the SQUID. The turn number and dimensionsof each field coil are 1000 and 50 mm in diameter and 45 mm
in length, respectively. The field coils are separated with adistance of 36 mm between each coil end while aligning theaxis. The electric motor can move a tube on a rail at 32 m min−1(maximum). The magnetic flux noise of the system measured
was about 60 µφ0 Hz−1/2 from 10 Hz to 5 kHz. Thecorresponding field gradient noise is about 6 pT cm−1 Hz−1/2 .

3. Specimens
Copper heat exchanger tubes that had artificial shallow
flaws with various depths on the surfaces were prepared asspecimens. These tubes were selected from commercialproducts. The dimensions of all the tubes are 6.35 mm inouter diameter, 0.8 mm in thickness and 300 mm in length.
The dimensions of the flaws are 100 µm in width and 15 mm
in length, and the depths of the flaws are 50, and 30 µm,respectively. A shallowest flaw of 100 µm in width, 25 mm
in length and 10 µm in depth was also made on the samekind of tube. The flaws were made using an electric dischargemachine.
S150

HTS SQUID gradiometer

Figure 2. Schematic diagram of the SQUID NDE system.

4. Measurements and results
4.1. Measurements
We evaluated the detection ability of the SQUID NDE system
by inspecting the flaws on the copper heat exchanger tube
specimens. A sinusoidal current of 3 kHz was supplied tothe Helmholtz-type coil to generate an excitation field. The
magnetic flux density Bx (parallel to the axis of the tube undertest) at the middle of the two field coils was about 10 µT. Thetube specimens were moved through the inducer at a velocityof 1.6–32 m min−1 . The flaws were set to be on top of thetube specimens with a lift-off distance 1.5 mm between theHTS SQUID gradiometer and the tube top. The HTS SQUID
gradiometer measured the field gradient in the x-direction of
the vertical magnetic field component, dBz /dx (see figure 1),
with a sampling frequency of 100–1000 Hz.
4.2. Results
The experimental results on the tube specimens with 50, 30,
and 10 µm-depth flaws using an excitation field of 10 µT at
3 kHz while moving the tube at 1.6 m min−1 are shown infigures 3(a)–(c), respectively. In each figure, a schematic viewof the tube specimen is depicted together for the reference ofthe position of each flaw. Anomalous magnetic responses dueto the micro flaws appear above and around the flaws. Eventhe 10 µm-depth flaw was successfully detected by the system,
which other traditional NDE techniques may fail to detect. The
signal peak-to-peak amplitude due to a flaw decreases as thedepth of a flaw becomes shallow, which is roughly proportional
to the depth of a flaw. However, this relationship between
the flaw depth and signal amplitude differs from the previousresults, where the signal amplitude was proportional to thesquare of the flaw depth [7]. In previous experiments on thesame kind of specimen, the specimens were moved stepwise
and accurately by another stepper-motor. On the other hand,in this study, the specimens were continuously moved on therail, in which a little gap between the rail and the specimen
holder was given to enable the specimen to move quickly. The
gap may cause the flaw on the specimen not to pass just under
the centre of the SQUID so that the signal response could besmaller than the case in which the flaw passed under the centre.

Signal Intensity dBz/dx [nT/cm]

Signal Intensity dBz/dx [nT/cm]

SQUID NDE for in situ inspection of copper heat exchanger tubes
3.5WLD
3.0 0.1 mm x 15 mm x 50 µm

2.52.01.5
1.015 mm

0.50.0

Flaw ends-0.5

454.0WLD
3.5 0.1 mm x 15 mm x 30 µm

3.02.52.01.515 mm

1.00.50.0

Flaw ends-0.53Measurement Time [sec]Measurement Time [sec]50-µm-depth flaw

30-µm-depth flaw

3.0(b)

0.1 mmW x 25 mm L x 10 µm D

2.52.025 mm

1.51.0

Flaw ends

0.50.045Signal Intensity dBz/dx [nT/cm]

Signal Intensity dBz/dx [nT/cm]

(a)
3.5

3.5
3.00.1 mmW x 25 mm L x 10 µm D

2.5
2.0
1.525 mm1.00.50.0

Flaw ends
0.60 .7

Measurement Time [sec]Measurement Time [sec]10-µm-depth flaw

10-µm-depth flaw(c)(d)

Figure 3. Measurement results on (a) 50 µm-depth flaw, (b) 30 µm-depth flaw, (c) 10 µm-depth flaw and (d) 10 µm-depth flaw. In themeasurement on (d), the specimen was moved at 32 m min−1 , while in the other cases ((a)–(c)) the specimens were moved at 1.6 m min−1 .
The circles in (c) indicate the noises that may be caused by the vibration of the tube being moved slowly.

Figure 3(d) shows the results on the tube with the 10 µmdepth flaw moving a bit faster, at 32 m min−1 . Almost thesame waveform as figure 3(c) was measured. The noise infigure 3(d) is somewhat smaller than in figure 3(c) (indicatedby the circles) although the moving speed in the case of (d)was faster than in (c). That may be due to a little bit larger
vibration of the specimen caused by the friction between therail and the specimen holder when the specimen was moved
slowly. It is necessary to make an improvement on the system
to suppress the tube vibration to within at least 10 µm.

5. Conclusions
An eddy-current-based SQUID NDE prototype system wasconstructed for in situ inspection of copper heat exchanger
tubes using an HTS SQUID gradiometer and a Helmholtztype coil. With an excitation field of 10 µT at 3 kHz, theSQUID NDE system could detect a magnetic response due toa 10 µm-depth flaw on a tube of 6.35 mm in outer diameter,
0.8 mm in thickness while moving the tube at 32 m min−1 .
An improvement on the system that should suppress the tube
vibration will be made. The system will be brought into a tube

factory, and then routine inspection on quickly moving tubes
in the manufacturing process will be tested.

Acknowledgment
The author would like to thank Dr Naoko Kasai (AIST, Japan)
for helpful discussions.

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