This document reviews the application of electrical resistance tomography (ERT) for conducting vessel walls. ERT is a non-invasive tomography technique that has advantages for industrial process monitoring applications. The document discusses the evolution of ERT techniques for conducting vessels over decades of research. It presents different ERT measurement strategies and challenges for conducting boundaries. It also addresses electrode fabrication and designs for effective ERT systems on conducting vessel walls.

1. Review
Electrical resistance tomography: A review of the application of
conducting vessel walls
Suzanna Ridzuan Aw a
, Ruzairi Abdul Rahim b,
⁎, Mohd Hafiz Fazalul Rahiman c
,
Fazlul Rahman Mohd Yunus b
, Chiew Loon Goh b
a
Faculty of Electrical & Automation Engineering Technology, Terengganu Advance Technical Institute University College (TATiUC), Jalan Panchor, Telok Kalong, 24000 Kemaman,
Terengganu, Malaysia
b
Process Tomography and Instrumentation Research Group (PROTOM-i), INFOCOMM Research Alliance, Faculty of Electrical Engineering, Universiti Teknologi Malaysia, 81310 UTM
Skudai, Johor, Malaysia
c
School of Mechatronic Engineering, Universiti Malaysia Perlis, Pauh Putra Campus, 02600 Arau, Perlis, Malaysia
a b s t r a c ta r t i c l e i n f o
Article history:
Received 11 July 2013
Received in revised form 3 January 2014
Accepted 10 January 2014
Available online 21 January 2014
Keywords:
Electrical resistance tomography
Conducting vessel
Conducting boundary strategy
Grounding effect
Electrode fabrication
Despite decades of research, the study on tomography continues to be a subject of great scientific interest.
Amongst all the kinds of tomography available, electrical resistance tomography (ERT) has been chosen as the
field of study because of its advantages of being low cost, suitable for various kinds and sizes of pipes and vessels,
having no radiation hazard, and being non-intrusive. In the development of ERT systems for conducting vessel
walls, prior knowledge of the fundamental process of the ERT system whilst improving the design and operation
of the process equipment is essential. In this paper, a review of the application of ERT for the conducting vessel
wall is presented, providing information about its evolution over the years. The limitations and advantages of dif-
ferent strategies of ERT are also presented besides an overview of the system. Electrode fabrication on the
conducting vessel wall is addressed.
© 2014 Elsevier B.V. All rights reserved.
Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 256
2. Recent research and applications employing ERT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 257
3. Developments of ERT/EIT on conducting vessel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 258
4. ERT system and measurement of the conducting vessel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 259
4.1. ERT system principle and components . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 259
4.2. Sensor/electrode selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 260
4.3. ERT measurement strategy for conducting vessel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 261
4.4. Electrode fabrication for conducting vessel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 262
4.5. The proposed design for electrode fabrication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 263
5. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 263
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 263
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 263
1. Introduction
The word ‘tomography’ comes originally from the Greek ‘tomos’
which means to slice and ‘graph’ meaning image. In other words,
tomography can be defined as a process of making a cross-sectional
image of an object. A cross section of the object is called a tomogram,
whilst the equipment that generates the image is called a tomographic
system [1,2]. Tomography offers a unique opportunity to reveal the
complexities of the internal structure of an object without the need
to invade it. The concept of tomography was first published by a
Norwegian physicist, Abel, for an object with axisymmetrical geometry.
Nearly 100 years later, an Austrian mathematician, Radon, extended
Abel's idea for objects with arbitrary shapes. Advances in the use of
the tomography technique, namely computerized tomography (CT)
Powder Technology 254 (2014) 256–264
⁎ Corresponding author. Tel.: +60 7 5537801; fax: +60 7 5566177.
E-mail address: ruzairi@fke.utm.my (R.A. Rahim).
0032-5910/$ – see front matter © 2014 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.powtec.2014.01.050
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journal homepage: www.elsevier.com/locate/powtec

2. and computerized axial tomography (CAT), were presented by Godfrey
Hounsfield of Great Britain and Allen Cormack of the United States
during the 1970s. Since then, tomography has become widely used as
a medical diagnostic technique. It has been developed over the last
decade into a reliable tool for imaging in numerous industrial applica-
tions [3].
One of the most extensive modalities of tomography, which has
greatly evolved since it was invented in the 1980s, is electrical resis-
tance tomography (ERT), a particular case of electrical impedance to-
mography (EIT). ERT has become a promising technique in monitoring
and analysing various industrial flows due to its diverse advantages,
such as high speed, low cost, suitability for various sizes of pipes and
vessels, having no radiation hazard, and being non-intrusive [4–10]. It
has the potential of providing both qualitative analysis by providing
the data required for measurement of some flow parameters, such as
velocity distribution, and flow regime identification [11]. As a non-
intrusive, fast visualization tool, close attention has been paid to ERT
in multiphase flow research. Compared with conventional measure-
ments, ERT can provide real-time cross-sectional images of conductivity
distribution within its sensing region. Other parameters, for example
local and global gas hold-ups and radial velocity maps, can be extracted
from the reconstructed images [12].
ERT has gained acceptance as a useful means of rapidly delineating
the resistivity distribution of materials inside a process vessel or pipe-
line [13]. Most of the primary sensor geometries for the process equip-
ment of ERT concentrated on embedding invasive but non-intrusive
electrodes into a wall vessel/pipe section composed of an electrically
non-conductive material, such as acrylic, perspex or polyvinylchloride.
Conversely, the majority of industrial pipelines and process vessels are
constructed from conducting metallic composites. Thereby, the devel-
opment and improvement of ERT measurement methods to accommo-
date this environment are proposed in this research.
In this paper, a review of the application of ERT on conducting vessel
walls for process monitoring and industry is presented. Information re-
garding the evolution of this topic over the years, besides the advan-
tages and limitations are also given. Recent research on ERT and its
developments in conducting boundaries are presented and an overview
of the system, measurement strategies and electrode fabrication is
addressed.
2. Recent research and applications employing ERT
Since its exposure in the 1980s, many attractive and inspirational
works and research have been presented by researchers around the
world. In 2012, Zhang and Chen [14,15] presented a new flow pattern
identification algorithm for common two-phase flows based on electri-
cal resistance system measurement, principal component analysis–
general regression neural network (PCA–GRNN) and principal com-
ponent analysis–support vector machine (PCA–SVM). Yang et al.
[12] introduced a dual-plane ERT technique to provide a real-time
measurement of air volume fraction distribution within its sensing
region. The system could generate cross-sectional images as well as
flow velocity maps. Sharifi and Young [11,16] presented a study on
the flow and velocity profile of various milk solutions in horizontal
and vertical pipes as well as spatial 3-dimensional (3D) monitoring
using ERT. Gas hold-up in a multi-stage bubble column has been in-
vestigated by Jin et al. [17].
In measuring the multiphase flow, Dong et al., Tan et al. and Zhang
et al. [18–20] presented a new ERT system employing a fully program-
mable and reconfigurable FPGA- (field programmable gate array)
based Compact PCI (peripheral component interconnect) bus. They
are from the same research group in Tianjin University. The experi-
ments are differing in object of interest and the results consequently.
The research by [18] visualize the oil/gas/water meanwhile [19] investi-
gate the water flow through the gas dynamics simulations and experi-
ments by [20] have been performed in tap water. FPGA is adopted in
the research since it produces significantly more computation power,
through parallel implementation, compared with the traditional
instruction-driven digital signal processors. This advance in technology
brings improvements in performance such as high bandwidth and good
precision when applied to ERT systems. Moreover, the use of digital
components (FPGA chips) makes upgrading and debugging easier.
The FPGA chips are used in the new data acquisition system for im-
plementing the functions of digital filters, digital demodulations, in-
jecting strategy change and data transportation based CompactPCI
bus, etc.
Yenjaichon et al. [21] applied ERT to evaluate the mixing quality of
an industrial pulp mixer. Xu et al. [22] described a parallel ERT system
based on Compact PCI for multiphase flow measurement. Tahvildarian
et al. [23] employed ERT in investigating the mixing of micron-sized
polymeric particles in a slurry reactor. Kourunen et al. [24] applied a
3D ERT to characterize gas hold-up distribution in a laboratory flotation
cell. Other researchers [6,12,17,25–29] have analysed gas hold-up using
ERT in a bubble column.
Meng et al. [30,31] combined the ERT sensor with a Venturi meter to
measure the mass flowrate of an air–water two-phase flow. A feasibility
study has been undertaken by Kowalski et al. [32] to explore the use of
ERT for detecting the early onset of ageing in formulated products.
Karhunen et al. and Seppanen et al. [33–37] have published a number
of research papers on the ERT imaging of concrete. In a study by Jin
et al. [38], the mean phase hold-up and radial gas hold-up distributions
are discussed using ERT together with the differential pressure method
with two axial locations in a gas–liquid–solid of a bubble column.
Hosseini et al. [39] used ERT to investigate the solid–liquid mixing in
an agitated tank equipped with a top-entering axial-flow impeller. Cui
et al. [40] proposed a twin plane ERT system on gas/liquid two-phase
flow in a vertical pipe which helps to realize the online monitoring of
flow regime classification and gas hold-up computation. Chao et al.
[41] obtained cross correlation velocity of oil–water two-phase flow in
a horizontal pipe by a dual-plane ERT. In reconstructing the conductivity
distribution of ERT, Cao et al. [42] applied electrical capacitance tomog-
raphy (ECT).
Experiments on gas–water two-phase flows have been conducted
by Tan and Dong [43] in a horizontal pipe using the ERT system and a
V-cone meter. Xu et al. [7] applied ERT for slug flow measurement of
gas/liquid flow in horizontal pipes. Razzak et al. [44] investigated liq-
uid–solid two-phase systems in a liquid–solid circulating fluidized bed
(LSCFB) for flow characteristics. Beforehand, Razzak et al. [45,46] suc-
cessfully implemented ERT in a gas–liquid–solid circulating fluidized
bed (GLSCFB) system, where the local and average phase hold-ups
and propagation velocities were determined using cross correlation
and compared it to optical fibre probe measurements. Park et al. [47]
adopted ERT in monitoring a radioactive waste separation process.
Pakzad et al. [48] used ERT to measure the mixing time of the xanthan
gum solution with the yield stress stirred in a baffled tank. In this case,
the xanthan gum solution is a pseudo-plastic fluids possessing yield
stress.
Tan et al. [49] proposed a multi-plane ERT system based on a parallel
data-acquisition system for gas/liquid two-phase flow. By applying ERT,
Lee and Bennington [50] measured the flow velocity and uniformity also
in a model batch digester. Ruzinsky and Bennington [51] applied ERT to
measure the liquor flow through a model chip digester. Kim et al. [52]
introduced ERT for the interfacial boundary recovery in stratified
flows of two immiscible liquids. A numerical procedure for tackling
shape varying bodies in ERT based on the mesh less method was
discussed by Cutrupi et al. [53]. Chen et al. [54] identified the flow re-
gime of oil/gas two-phase flow using ERT. Wang et al. [55] presented a
study on the velocity distribution and air volume fraction of gas–liquid
in a swirling flow using ERT. Earlier, Kim et al. [56] applied ERT to visu-
alize and analyse the mixing of two miscible liquids with distinct con-
ductivities in a stirred vessel. Jin et al. [57] studied the effect of sparger
geometry on gas bubble hold-up distribution using ERT. In a study by
257S.R. Aw et al. / Powder Technology 254 (2014) 256–264

3. Henningsson et al. [58], cross correlation of a dual-plane ERT has been
applied in determining the velocity profile of yogurt and its rheological
behaviour in a pipe of industrial dimensions.
A review paper by Dyakowski presented the fundamentals of electri-
cal tomography and its applications to gas–solid and liquid–solid flows.
Flow morphology within pneumatic and hydraulic conveying systems,
solids distribution within a fluidised bed and a dipleg, and solid profiles
within a hydroclone for various operating conditions have been included
as the examples in this paper [10]. Previous work on ERT applications to
Chemical Engineering has been published in a review paper by [59].
This paper is very recommended to tomography researchers worldwide
since it provides a general understanding on the current situation of
ERT related research and proven applications in the Chemical Engineering
field. The applications are categorised based on the unit operations ERT
has been applied to, the media under investigation, the purpose of ERT
measurements and also other technologies used in conjunction with ERT.
Recent research and applications employing ERT in this section are
summarized in Table 1. All of them apply non-conducting wall as the
ERT sensory system. However, as mentioned earlier, majority of indus-
trial pipelines and process vessels are constructed from conducting ma-
terial. Thus the knowledge and reviews on ERT applying conducting
vessels are the main purposes to be highlighted in this paper.
3. Developments of ERT/EIT on conducting vessel
Extracting information from industrial pipelines is important in ob-
serving the process to ensure it meets certain standards or require-
ments. Tomography seems to be one of the great applications to
accommodate this environment. Most of the vessels and pipelines in in-
dustry are made from conducting material. However, most of the re-
search on tomography has used vessels made from non-conducting
materials. This section will present the previous research on ERT
which was conducted on metal or conducting vessel walls. The motiva-
tion behind the research of ERT/EIT on conducting vessel wall was initi-
ated by Wang et al. [13,60]. By using excitation and measurement
strategy and adapting the proposed sensitivity coefficient method, use-
ful images of resistivity distribution are obtained from the metal vessel
with insulated electrodes using existing ERT systems. Yuen et al. [61]
presented a paper on ERT imaging of a metal-walled solid–liquid filter.
Correspondingly, a work by Grieve [62] sets up an online EIT within
pressure filtration for industrial batch processing. The wall was fabricat-
ed from an electrically-conducting alloy. Finite element modelling
(FEM) was adopted for the system and then it was integrated with a
modified version of the electrical impedance tomography and diffuses
optical tomography reconstruction software (EIDORS) 3D algorithms
Table 1
Summary of recent research and applications employing ERT.
Reference Application
Zhang and Chen [14,15] Two phase flow regime identification
Yang et al. [12] Void fraction/gas holdup measurement
Sharifi and Young [11,16] Flow monitoring, velocity distribution and flowrate measurement of various milk solutions
Jin et al. [17] Gas holdup measurement
Dong et al. [18] Visualisation of multiphase flow
Tan et al. [19] Visualisation of multiphase flow
Zhang et al. [20] Visualisation of multiphase flow
Yenjaichon et al. [21] Mixing of a pulp suspension and chlorine dioxide
Xu et al. [22] Flow monitoring
Tahvildarian et al. [23] Solid–liquid mixing in a slurry reactor
Kourunen et al. [24] Gas holdup measurement in a laboratory flotation (separation process) cell
Jin et al. [6] Gas holdup profile and flow regime identification
Jin et al. [25] Bubble rise velocity and bubble size estimation
Jin et al. [26] Gas holdup profile measurement in a cocurrent bubble column
Williams et al. [27] Gas holdup measurement in flotation process
Toye et al. [28] Gas holdup in hydro-dynamics of bubble columns
Fransolet et al. [29] Gas holdup measurement
Meng et al. [30,31] Flow measurement
Kowalski et al. [32] Early onset detection of ageing in formulated products.
Karhunen et al. and Seppanen et al. [33–37] Concrete imaging
Jin et al. [38] Gas and solid holdups distribution
Hosseini et al. [39] Solid–liquid mixing
Cui et al. [40] Flow regime monitoring and gas holdup computation
Chao Tan and Feng Dong [41] Cross correlation velocity of oil–water two-phase flow in a horizontal pipe
Cao et al. [42] Flow monitoring
Chao Tan and Feng Dong [43] Flow regime identification
Xu et al. [7] Flow measurement
Razzak et al. [44] Solids and velocity holdup distributions
Razzak et al. [45] Gas and solid holdups distribution, velocity distribution
Razzak et al. [46] Gas and solid holdups distribution, velocity distribution
Park et al. [47] Monitoring of a radioactive waste separation process
Pakzad et al. [48] Investigation of mixing process
Tan et al. [49] Flow monitoring
Lee and Bennington [50] Flow velocity
Ruzinsky and Bennington [51] Liquor flow measurement
Kim et al. [52] Flow monitoring
Cutrupi et al. [53] Biomedical application
Chen et al. [54] Flow regime identification
Wang et al. [55] Velocity distribution and gas holdup in swirling flow
Kim et al. [56] Mixing
Jin et al. [57] Gas holdups and velocity distribution
Henningsson et al. [58] Velocity profile
Dyakowski et al. [10] (review Paper) Gas–solids and liquid–solids systems monitoring
Sharifi and Young [59] (review paper) Paper review on applications to chemical engineering
258 S.R. Aw et al. / Powder Technology 254 (2014) 256–264

4. to provide a three-dimensional image within the metallic vessel using
the complete electrode model.
A novel EIT diagnostic system has been developed and used by Liter
et al. [63] to quantitatively measure material distributions in opaque
multiphase within electrically-conducting (i.e. industrially relevant or
metal) vessels. The system applied seven equally spaced ring electrodes
to a thin non-conducting rod that was inserted into the vessel. In this
work, Sandia's steel pilot-scale bubble column reactor (SBCR) was
used as the plant. Only resistive EIT is the ERT considered for the pur-
pose of this work. The invasiveness of the electrode used in the system
created a non-axisymmetric flow-field disturbance that introduced a
bias in the current flow paths. The disturbance was not modelled in
the FEM simulations used to reconstruct the electrical conductivity dis-
tributions and thus presented a source of possible significant error.
York et al. [64] have progressively published his work on the EIT sys-
tem within metal-walled industrial production pressure filters for a
number of years. The metal wall strategy is employed in the intrinsically
safe instrument developed. Sensor architecture has been implemented
that is compliant with the process such that it is not detrimental to
the efficiency or the integrity of the associated vessel structure.
MATLAB-based EIDORS 3D software has been employed to yield images
from simulated data.
A 3D image reconstruction using real EIT measurements obtained
from a metal-walled (stainless steel) laboratory test platform has been
investigated by Davidson [65]. It is considered to be comparable to a
large-scale industrial filtration unit. Two image reconstruction tech-
niques have been applied via relatively sophisticated FEM modelling.
A generalized Tikhonov regularization method is compared to the linear
back projection (LBP) technique. It is observed that the regularized
technique is far less sensitive to the modelled geometry compared to
LBP. In addition, the regularized technique is more successful in accu-
rately reconstructing multiple inhomogeneities within an aqueous sys-
tem. A further experiment has shown similar sensitivity in a wetted
powder-based system. It is concluded that EIT via a regularization meth-
od has significant potential for detecting 3D malformations and non-
uniformities in industrial pressure filtration systems.
Industrial tomography systems (ITSs) have developed a linear ERT
sensor integrated onto a glass lined finger baffle for use in glass lined
stainless steel vessels which are commonly used in the pharmaceutical
sector [74].
4. ERT system and measurement of the conducting vessel
In this section, an overview of ERT and the parameters for sensor se-
lection are discussed. The measurement strategy and electrode fabrica-
tion on the conducting vessel wall are also presented. This review is
important before conducting research on ERT involving conducting ves-
sels and pipelines.
4.1. ERT system principle and components
The basic idea of ERT is that the conductivity of different media is
distinct from each other. Thus, the medium distribution of the measured
area can be identified if the conductivity or resistance distribution of the
sensing field is obtained [43,66]. The operation mode of an ERT system is
to provide the sensing field with exciting current (or voltage) and mea-
sure the potential difference (or current) via electrodes mounted on the
boundary of the domain [39,67]. Usually, the operating principle of the
ERT system is current exciting and voltage measurement. The current
excitation is applied into the measurement section through a pair of
electrodes and excites the sensing field. When the conductivity distribu-
tion varies, the sensing field varies with it and results in a change in the
electric potential distribution. Likewise, the boundary voltage of the
sensing field changes accordingly. The measured voltage contains infor-
mation on the conductivity in the sensing field, and the internal flow
status can be obtained from further information processing [43]. This
is shown in Fig. 1. In the case of the conducting pipes or vessels, the elec-
trodes need to be insulated from the conducting wall [68].
The ERT system is mainly composed of three units; they are sensor/
electrode array, data-acquisition system (DAS) and image reconstruc-
tion system/host computer, as shown in Fig. 2 [12]. The electrode
array that is mounted on the vessel will generate a rotational electrical
field within the region of interest by applying excitation signals, and
the resultant signals are then acquired. The sensor/electrode is used
both for excitation and detection. An accurate and stable DAS is a basic
necessity for the ERT system. It is responsible for obtaining the quantita-
tive data revealing the state of the conductivity distribution inside the
Fig. 1. Operating principle of ERT [43].
Signal
Excitation
Data
Measurement
Control Data
Acquisition
Image
Reconstruction
Electrode
Array
Object
Fig. 2. System configuration of ERT.
259S.R. Aw et al. / Powder Technology 254 (2014) 256–264

5. tank [69]. It completes a series of tasks such as excitation signal genera-
tion, electrode status control, signal conditioning and demodulation.
This has to be done accurately and quickly in order to monitor the
small changes of conductivity in real time. The acquired data are then
sent to the host computer for image reconstruction and information
extraction. Using suitable reconstruction algorithms, two and three-
dimensional (2D and 3D) conductivity distribution images are generat-
ed, from which phase hold-up, flow velocity and other information can
be extracted.
4.2. Sensor/electrode selection
Electrodes are the heart of an ERT system. It is crucial to design the
electrodes and maximize the ability of the electrode to sense conductiv-
ity changes in the region of interest. The parameters, that is, the charac-
teristics of the electrodes, that need to be considered when adopting
ERT are the materials used to construct the sensor, the shape and size
of the electrodes, the number of electrodes, and also the position of
the electrodes.
In ERT, the electrodes need to be in continuous contact with the fluid
inside the vessel which differs from ECT where the main medium in
contact with the ERT sensors has to be conductive to allow the injected
current to pass through the medium [59]. ECT is used when the contin-
uous material does not conduct electricity such as air or oil whereas for
ERT the continuous material is electrically conducting (e.g. water, acids,
bases and ionic solutions). The attached electrodes on the periphery of
the process vessel must have low cost, ease of installation, good conduc-
tivity and resistance to corrosion/abrasion effects and the process oper-
ation environment (i.e. temperature, pressure, electrical fire hazards,
vessel wall thickness and material). The material used for the electrodes
must be more conductive than the fluids being imaged otherwise prob-
lems will arise due to contact impedance. Generally, they are fabricated
from gold, silver, platinum, brass, stainless steel or silver palladium alloy
which is commercially accessible in bolt or screw form and can often be
threaded into the vessel wall [70].
The position of the electrodes is also important in ERT since the re-
construction algorithm is based on the electrodes being located at exact-
ly defined intervals. Ideally, they are positioned equidistantly around
the boundary of the vessel at fixed locations. This is to ensure the system
can abstract the maximum amount of information from inside the ves-
sel [71]. For the selection of the number of electrodes, it is a trade-off
between image resolution and system complexity. Usually, more elec-
trodes can increase spatial resolution of the system due to the increased
number of measurements. From previous studies, more electrodes
would reduce the distance between two adjacent electrodes, which
could cause more current flow through the near field and lower sensi-
tivity to the centre. Additionally, more electrodes would raise the re-
quirements of hardware measurements and influence the real-time
performance of the system [18].
Another important factor in measuring the electric field distribution
is the electrode size. A larger surface area is required for current
injecting electrodes to ensure that an even current and enough current
are generated within the vessel. Correspondingly, in detecting the resul-
tant voltage, a smaller surface area of the voltage measuring electrodes,
ideally a needle point, is most favourable to avoid ‘averaging’ several
voltages. Nonetheless, this approach requires twice the amount of con-
nectors, cabling and associated circuitry of one with identically sized
electrodes throughout [71,72]. For reasons of simplicity, Dickin and
Wang [72] employed same size electrodes for both injection and mea-
surement. A study conducted by Pinheiro et al. [73] recommended
that electrodes should cover 60–80% of the surface of the region of inter-
est to provide a high signal-to-noise ratio.
The final consideration when incorporating the sensors/electrodes
into the vessel is the length of signal-carrying cable between the elec-
trode and the current injection or voltage measurement circuitry. It is
to be noted that a longer signal carrying cable will result on a larger as-
sociated stray capacitance and current leakage. Consequently this will
lead to the highly undesirable phase-shifted signals [72]. Thus it is im-
portant to ensure the cable is within the acceptable length.
Fig. 3. Conducting boundary strategy [62].
a) b)
Fig. 4. Electrode fabrication. (a) into an acrylic-walled vessel, (b) into a metal-walled
vessel.
metal wall
electrode
insulating
sheath
Fig. 5. Electrode fabrication by Wang et al.
260 S.R. Aw et al. / Powder Technology 254 (2014) 256–264

6. 4.3. ERT measurement strategy for conducting vessel
Measurement strategy is necessary, especially in ERT, to define the
experiment which involves a metal or conducting vessel. In ERT, quan-
titative data which describes the state of the conductivity distribution
inside the vessel is obtained. Good data collection strategies are very im-
portant because generally misleading images can be rebuilt if a full set of
independent measurements is not collected [71,74]. For all intents and
purposes, selecting the strategy that has good distinguish ability and
high sensitivity to conductivity changes in the process is necessary in
ERT. There are four main strategies in ERT: the adjacent strategy,
conducting boundary strategy, opposite strategy and diagonal strategy.
The first application of ERT only considered electrode arrangements
operating within vessels having insulating walls and applied the adja-
cent measurement strategy which is the common one. This strategy is
illustrated in Fig. 1. In this strategy, current is injected between an adja-
cent pair of electrodes and voltage is measured from successive pairs of
neighbouring electrodes. The injection pair is switched through the next
electrode pair until all independent combinations of measurements
have been completed. However, the majority of the process vessels in
industry have conducting walls and therefore provide an additional cur-
rent sink during the measurement process. This gives rise to both re-
duced sensitivity in the bulk of the material and increased difficulty in
obtaining stable measurements referenced to the injected currents [65].
Before applying ERT to an electrically-conducting vessel, an elec-
trical path passing through the vessel wall must be taken into consid-
eration. The adjacent strategy is unsuitable for application to the
conducting vessel since much of the electrical current from the injection
electrode would travel to ground through the wall material rather than
through the multiphase mixture, greatly reducing sensitivity. This is
called the grounding effect of the vessel. One possible method of ac-
counting for the conducting vessel wall is to use the wall itself as the
ground electrode [63]. Conducting boundary strategy, as in Fig. 3, has
been proposed and developed by [13] for the conducting vessel wall
to overcome the grounding effect. The strategy considers each electrode
acting sequentially as a current source whilst the whole of the metallic
vessel behaves as a grounded current sink. In this strategy, all voltage
measurements are referenced to the same earth potential of the
conducting boundary. The number of unique measurements, N, in the
conducting boundary or ‘metal wall’ strategy can be defined as follows:
N ¼
n n−1ð Þ
2
ð1Þ
(a) Installed electrode array (b) Cable exit point
Exit point
Fig. 6. (a) Installed electrode array (b) Cable exit point.
Fig. 7. Cloth mounted radial electrode array.
(a) (b)
Fig. 8. Laboratory test filter platform; (a) interior of test filter showing hold-down bar with one central and eight outer electrodes, (b) general view of test filter.
261S.R. Aw et al. / Powder Technology 254 (2014) 256–264

7. where n is the total number of electrodes [65]. For instance, a total of 16
electrodes used will provide 120 unique measurements.
4.4. Electrode fabrication for conducting vessel
Several methods can be employed to attach the electrode to the
conducting vessels. It is to be noted that the metal electrodes for
electrically-conducting (metallic) process vessels slightly differ from
the non-conducting (plastic) vessels in which the electrodes need to
be insulated from the conducting vessel. Fig. 4(a) and (b) shows the
electrode fabrication proposed by Dickin and Wang [72] for the non-
conducting and conducting vessels respectively.
The proposed fabrication by Wang et al. [13] for vessels having
electrically-conducting boundaries is used between the metallic vessel
and a number of electrically insulated metal electrodes mounted into
the periphery of the metallic process vessel as shown in Fig. 5.
Grieve [62] designed and fabricated an electrode array from a flexi-
ble printed circuit board (PCB) for industrial batch processing to avoid
fouling of the agitator blade. Sixteen gold plated electrodes of equivalent
area were placed 100 mm above the filter cloth. The signal lines were
embedded such that they were marshalled to a single exit flange. This
is shown in Fig. 6. A multi-core cable was then used to allow the signals
to exit through a pressure gland before separating into 16 individually
screened 20 metre cables.
In the same research by Grieve, a cloth mounted radial electrode
array was adopted in the filtration process application for an industrial
scale unit as shown in Fig. 7. The rationale of this compared to Fig. 6
for this application is that it would provide greater information on the
object of interest near its centre and would be more amenable for
retrofitting to an industrial filter.
For the same filtration application, Davidson [65] applied the tech-
niques to a laboratory test vessel using a similar but smaller-scale planar
electrode system. The electrode fabrication is shown in Fig. 8.
In the work by Liter et al. [63], initial proof of concept and calibration
was completed using a stationary solid liquid mixture in a steel bench-
top standpipe. The system was applied first in a solid–liquid bench-top
experiment to measure the height of a packed bed in a liquid filled
standpipe. The system used a metal cylinder, or standpipe, of inner di-
ameter 7.14 cm with an electrically insulating base was used as the ves-
sel (ground electrode). An electrode rod was fabricated from a PVC tube
with a 2.2 cm outer diameter and a 1.5 cm inner diameter. The electrode
consisted of seven ring electrodes with the same dimensions. The elec-
trodes were 2.54 cm in length and were wrapped around the rod with a
3.5 cm edge-to-edge separation between them. The rod was positioned
coaxially inside the metal standpipe. This is shown in Fig. 9.
The system was then deployed in Sandia's slurry bubble column re-
actor (SBCR) for two and three phase flows. The SBCR is comprised of a
stainless steel column that has an inner diameter of 0.48 m, 13 mm thick
sidewalls, and an internal height of 3.15 m. All measurements were
taken at a single vertical height through the centre of the third instru-
mentation ports on the SBCR. This is as shown in Fig. 10. The only disad-
vantage of this was that the means of positioning the electrode array
throughout the flow domain led to the flow-field disturbance which
thus introduced error.
Fig. 9. Schematic of an electrode rod inserted coaxially in an electrically-conducting stand-
pipe filled with non-conducting solid polystyrene particles and liquid.
Ground
Current
Injection
Vessel Wall
Electrodes
Electrodes
EIT Hardware
SBCR
Fig. 10. Schematic of EIT system applied to Sandia's slurry bubble-column reactor (SBCR).
Conducting
pipe
Conducting
electrode surface
Non-conducting
electrode surface
Fig. 11. Electrode fabrication using flexible circuit board.
262 S.R. Aw et al. / Powder Technology 254 (2014) 256–264

8. 4.5. The proposed design for electrode fabrication
From the study undertaken, a design for the electrode fabrication to
be implemented in ERT system deploying conducting vessel has been
proposed. Flexible circuit board is used as the electrode. Figs. 11 and
12 show the design of electrode fabrication using flexible circuit board
and the inner cross section view of the proposed system respectively.
5. Conclusion
ERT seems to be a powerful tool for investigating and monitoring
various kinds of applications, such as mixing, filtration, multi-phase
flow and so forth. Industrial process pipelines are mostly known to be
constructed form metal which is a conducting material. From the review
that has been made, it is proven that ERT can be applied successfully on
the conducting vessel wall and pipelines both for laboratory and indus-
trial application. Conversely, from the literature, not much work has
been undertaken on ERT deploying the conducting vessel. It is believed
that further exploration on this topic can deliver valuable information to
give new insights and benefits to relevant areas and industry. Further
potential improvements to the current design and image reconstruction
of the ERT system are possible so that it can be applied with the
conducting vessel effectively and successfully.
Acknowledgements
The authors would like to thank the Ministry of Higher Education
and Terengganu Advance Technical Institute University College for
funding the study. Special thanks to Universiti Teknologi Malaysia and
PROTOM research group for their honourable support and cooperation.
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Suzanna Ridzuan Aw received her B. Eng. degree (Honours) in Electrical Engineering
(Instrumentation and Control) and her M. Eng. degree in Electrical Engineering
(Mechatronics and Automatic Control) from Universiti Teknologi Malaysia (UTM), Skudai,
Malaysia, in 2009 and 2011, respectively. Currently, she is pursuing her PhD degree at
UTM in process tomography. Her current research interest is in electrical resistance
tomography.
Ruzairi Abdul Rahim received a B. Eng. degree with Honours in Electronic System and
Control Engineering in 1992 from Sheffield City Polytechnic, UK. He received his Ph.D in
Instrumentation & Electronics Engineering from Sheffield Hallam University, UK in 1996.
At present he is a Professor and a Director of Research Management Centre, Universiti
Teknologi Malaysia. His current research interests are process tomography and sensor
technology.
Mohd Hafiz Fazalul Rahiman received a B. Eng. (Hons) degree in Electrical Engineering
(Control and Instrumentation), M. Eng. and Ph.D. degrees in Electrical Engineering from
Universiti Teknologi Malaysia (UTM), Johor, Malaysia, in 2003, 2005, and 2013 respective-
ly. In 2006, he joined Universiti Malaysia Perlis (UniMAP), Perlis, Malaysia, as a teaching
staff member and at present he holds the position of senior lecturer. His research interests
include process tomography, sensors and instrumentation.
Fazlul Rahman Mohd Yunus received a B. Eng.degree (Honours) in Electrical Engineering
(Instrumentation and Control) and an M. Eng. degree in Electrical Engineering
(Mechatronics and Automatic Control) from Universiti Teknologi Malaysia, Skudai,
Malaysia, in 1999 and 2009, respectively. In 1999, he joined ST microelectronics as test en-
gineer for two years before being called by the government of Malaysia to serve in the
Japan–Malaysia Technical Institute (JMTI), Penang (2001–2009) and the Industrial Train-
ing Institute, Ledang (2009–2012) of the Manpower Department, Ministry of Human Re-
sources Malaysia, as a vocational training officer. Currently he is working towards a Ph.D in
Process Tomography. His current research interest is in dual-modality process tomogra-
phy.
Goh Chiew Loon received her M.Sc. Master in Electrical and Electronic Engineering from
the University Technology Malaysia, Malaysia, in 2006. After several years of working in
the field of R&D engineering and software programming, she joined the process tomogra-
phy & instrumentation research group (PROTOM-i) at the Universiti Teknologi Malaysia
(UTM) as researcher in 2012. Her research interests include the design of electronic cir-
cuits, embedded systems, wireless system and application programming of image pro-
cessing for tomography systems.
264 S.R. Aw et al. / Powder Technology 254 (2014) 256–264