geosciences
Article
Laboratory Testing of a MEMS Sensor System for
In-Situ Monitoring of the Engineered Barrier in
a Geological Disposal Facility
Wenbin Yang, Rebecca J. Lunn *, Alessandro Tarantino and Gráinne El Mountassir
Department of Civil and Environmental Engineering, University of Strathclyde, Glasgow G1 1XJ, UK;
[email protected] (W.Y.); [email protected] (A.T.);
[email protected] (G.E.M.)
* Correspondence: [email protected]; Tel.: +44-141-548-2826
Academic Editor: Jesus Martinez-Frias
Received: 8 April 2017; Accepted: 10 May 2017; Published: 20 May 2017
Abstract: Geological disposal facilities for radioactive waste pose significant challenges for robust
monitoring of environmental conditions within the engineered barriers that surround the waste
canister. Temperatures are elevated, due to the presence of heat generating waste, relative humidity
varies from 20% to 100%, and swelling pressures within the bentonite barrier can typically be
2–10 MPa. Here, we test the robustness of a bespoke design MEMS sensor-based monitoring system,
which we encapsulate in polyurethane resin. We place the sensor within an oedometer cell and show
that despite a rise in swelling pressure to 2 MPa, our relative humidity (RH) measurements are
unaffected. We then test the sensing system against a traditional RH sensor, using saturated bentonite
with a range of RH values between 50% and 100%. Measurements differ, on average, by 2.87% RH,
and are particularly far apart for values of RH greater than 98%. However, bespoke calibration of the
MEMS sensing system using saturated solutions of known RH, reduces the measurement difference
to an average of 1.97% RH, greatly increasing the accuracy for RH values close to 100%.
Keywords: monitoring; geological disposal; sensor; relative humidity; bentonite; engineered barrier
system; MEMS; geological disposal
1. Introduction
Real-time monitoring of deep geological disposal facilities (GDFs) for radioactive waste disposal
is a significant challenge. The operational timescales of a GDF mean that monitoring technologies must
function reliably over timescales in excess of 100 years [1]. A regulatory requirement of any GDF is
likely to be the in-situ monitoring of the thermo-hydro-mechanical-chemical (THMC) behaviour of
the engineered barrier system (EBS) that surrounds the waste canister. Monitoring creates significant
challenges: temperatures can be highly elevated due to the presence of heat generating waste, relative
humidity (RH) varies from 20% to 100%, and swelling pressures within the bentonite barrier are
typically in excess of 2 MPa.
Most geological disposal concepts, for example the Swedish KBS-3V concept, are based on
an EBS composed of a compacted bentonite buffer, which surrounds the waste canister (e.g., Figure 1).
Post deposition, the bentonite buffer saturates via groundwater ingress from the surrounding rock,
which results in a swelling pressure of between 2 and 10 MPa to ensure hydraulic sealing between the
EBS, the surrounding rock and the central waste canister. Further, the very low hydraulic permeability
of the bentonite ensures that, should canister failure occur, radionuclide transport would be extremely
slow, since it is via diffusion only. Finally, the plastic nature of the saturated bentonite within the EBS
also protects the canister from structural damage during small earthquakes [2].
Geosciences 2017, 7, 38; doi:10.3390/geosciences7020038
www.mdpi.com/journal/geosciences
[2].
Historically, an extensive range of relative humidity sensors have been deployed in radioactive
waste disposal facilities and in underground testing laboratories over the past decades [3,4]. While
the measurement principle of the sensors varies, one common restraint of these traditional sensors
lies
in the unit size of the sensor (typically in the order of 10 cm), which limits the spatial resolution
Geosciences 2017, 7, 38
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of the sensing device.
Figure 1. Schematic cross-section through the bentonite engineered barrier system.
Figure 1. Schematic cross-section through the bentonite engineered barrier system.
Historically,
extensive
range of
of arelative
humidity
sensors have been deployed
in radioactive
This
research an
focuses
on testing
MEMS-based
(Micro-Electro-Mechanical
System)
sensing
waste
disposal
facilities
and
in
underground
testing
laboratories
over
the
past
decades
[3,4].
While
the
system, developed in [5] for monitoring relative humidity within, or adjacent to, the compacted
measurement
principle
of
the
sensors
varies,
one
common
restraint
of
these
traditional
sensors
lies
bentonite buffer in the EBS. Application of MEMS sensors in GDFs and other civil engineeringin
the unitstill
sizefaces
of the
sensor
(typically
in the
order
of 10 cm),field
which
the spatial
resolution
of the
projects
several
key
challenges
in the
engineering
[6].limits
This paper
extends
our previous
sensing [5]
device.
research
by testing the performance of bespoke encapsulated MEMS sensors within saturated
This
research
focusespressures
on testing
MEMS-based
System)
sensing
bentonite under
swelling
ofof2 aMPa.
We show (Micro-Electro-Mechanical
that our encapsulated MEMS
monitoring
system,can
developed
[5] for monitoring
relative
or adjacent
the compacted
system
withstandinswelling
pressures in
excesshumidity
of 2 MPawithin,
and that,
through to,
improved
sensor
bentonite
buffer
in
the
EBS.
Application
of
MEMS
sensors
in
GDFs
and
other
civil
engineering
calibration, accurate measurements of compacted bentonite relative humidity can be achieved
even
projects
faces
up
to RH still
values
of several
100%. key challenges in the engineering field [6]. This paper extends our previous
research [5] by testing the performance of bespoke encapsulated MEMS sensors within saturated
swelling pressures of 2 MPa. We show that our encapsulated MEMS monitoring
2.bentonite
Materialsunder
and Methods
system can withstand swelling pressures in excess of 2 MPa and that, through improved sensor
MEMS sensors provide higher measurement accuracy, improved spatial resolution in a limited
calibration, accurate measurements of compacted bentonite relative humidity can be achieved even up
space, and a longer life cycle resulting from low power consumption in the order of microwatts [7].
to RH values of 100%.
A first prototype of a multi-sensor monitoring system was presented in [5]. The system contains the
® 31725 temperature sensor (Maxim Integrated, San Jose, CA, USA) [8] and the Sensirion®
Maxim
2. Materials
and Methods
SHT25 relative humidity sensor (Sensirion, Staefa Switzerland) [9]. The Maxim® 31725 temperature
MEMS sensors provide higher measurement accuracy, improved spatial resolution in a limited
sensor (Maxim Integrated, San Jose, CA, USA) has a typical precision of ± 0.5 °C for a measurement
space, and a longer life cycle resulting from low power
consumption in the order of microwatts [7].
range between −55 °C and 150 °C, and the Sensirion® (Sensirion, Staefa, Switzerland) SHT25 RH
A first prototype of a multi-sensor monitoring system was presented in [5]. The system contains the
sensor has
a labelled precision level of ±1.8% within the 10–90% RH range, and ±3% for the full RH
Maxim® 31725 temperature sensor (Maxim Integrated, San Jose, CA, USA) [8] and the Sensirion®
range. Both sensors have a chip dimension of 3 mm × 3 mm × 1 mm, and are integrated
onto a single
SHT25 relative humidity sensor (Sensirion, Staefa Switzerland) [9]. The Maxim® 31725 temperature
printed circuit board (Figure 2). To minimise size, the sensor block, 9 mm × 11
mm,
is limited to
sensor (Maxim Integrated, San Jose, CA, USA) has a typical precision of ± 0.5 ◦ C for a measurement
hosting the sensor and
its
connector;
all
other
functional
components
are
integrated
onto the
range between −55 ◦ C and 150 ◦ C, and the Sensirion® (Sensirion, Staefa, Switzerland) SHT25 RH
motherboard that can be installed outside the bentonite barrier. The power supply and signal
sensor has a labelled precision level of ±1.8% within the 10–90% RH range, and ±3% for the full RH
transmission are maintained by heat-resistant PTFE-coated wires that are compatible with
range. Both sensors have a chip dimension of 3 mm × 3 mm × 1 mm, and are integrated onto a single
temperatures between −60 °C and 200 °C. In future, these wires are planned for replacement by a
printed circuit board (Figure 2). To minimise size, the sensor block, 9 mm × 11 mm, is limited to hosting
wireless transmission system, which eliminates wire installation concerns, although at the expense
the sensor and its connector; all other functional components are integrated onto the motherboard
of slightly increased sensor size.
that can be installed outside the bentonite barrier. The power supply and signal transmission are
maintained by heat-resistant PTFE-coated wires that are compatible with temperatures between −60 ◦ C
and 200 ◦ C. In future, these wires are planned for replacement by a wireless transmission system,
which eliminates wire installation concerns, although at the expense of slightly increased sensor size.
To avoid direct contact between the sensor and the bentonite, a PTFE filter membrane cap designed
by Sensirion® (Sensirion, 8712 Staefa ZH, Switzerland) [10] was incorporated to cover the RH sensor
on the PCB board (printed circuit board) prior to encapsulation. The filter protects the sensor from
Geosciences 2017, 7, 38
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To avoid
Geosciences
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38
contact between the sensor and the bentonite, a PTFE filter membrane3cap
of 9
designed by Sensirion® (Sensirion, 8712 Staefa ZH, Switzerland) [10] was incorporated to cover the
RH sensor on the PCB board (printed circuit board) prior to encapsulation. The filter protects the
mechanical
and contamination
and prevents
liquid
waterliquid
entering
theentering
sensorsthe
by sensors
capillarity,
sensor fromimpact
mechanical
impact and contamination
and
prevents
water
by
thus
invalidating
the
measurement.
At
the
same
time,
it
allows
the
propagation
of
water
capillarity, thus invalidating the measurement. At the same time, it allows the propagation ofvapour
water
molecules
between between
the measuring
environment
and the RHand
sensor.
encapsulated
vapour molecules
the measuring
environment
the The
RH sensor
sensor.was
Thethen
sensor
was then
via
a
‘potting’
method
[11]
that
uses
polyurethane
resin
as
an
encapsulation
material.
This
resulted
encapsulated via a ‘potting’ method [11] that uses polyurethane resin as an encapsulation material.
in
a
rectangular
polyurethane
block
that
enclosed
the
sensor,
with
a
window
for
the
measurement
This resulted in a rectangular polyurethane block that enclosed the sensor, with a window for the
of
RH (Figure of
2).RH
The(Figure
encapsulated
sensor block has
similar
the sensortoboard
prior
measurement
2). The encapsulated
sensor
blockdimensions
has similar to
dimensions
the sensor
to
encapsulation,
thus
maintaining
the
small
size.
Since
the
MEMS
temperature
sensor
is
entirely
board prior to encapsulation, thus maintaining the small size. Since the MEMS temperature sensor is
encompassed
during polyurethane
encapsulation,
it is unaffected
by the bentonite,
so is not discussed
entirely encompassed
during polyurethane
encapsulation,
it is unaffected
by the bentonite,
so is not
further.
By
contrast,
the
RH
humidity
sensor
relies
on
detection
through
the
sensing
window
(Figure
2)
discussed further. By contrast, the RH humidity sensor relies on detection through the sensing
and
its
accuracy
may
be
compromised
either
by
contact
via
liquid
phase
with
the
pore-water
of
the
window (Figure 2) and its accuracy may be compromised either by contact via liquid phase with the
saturated
bentonite,
or by the
swelling or
pressure
is exerted.
pore-water
of the saturated
bentonite,
by the that
swelling
pressure that is exerted.
Figure 2. SHT25 sensor board before and after encapsulation.
Figure 2. SHT25 sensor board before and after encapsulation.
Experiment 1 focuses on verifying the mechanical robustness of the RH sensor under the
Experiment
1 focuses
verifying
the mechanical
of the
RH sensor
swelling
swelling pressure
exertedonby
hydrated
bentonite. robustness
The test was
carried
out under
in an the
engineered
pressure
exerted
by
hydrated
bentonite.
The
test
was
carried
out
in
an
engineered
oedometer
cell,
oedometer cell, specifically designed by the Universitat Politecnica de Catalunya, as shown in Figure
specifically
designed
by
the
Universitat
Politecnica
de
Catalunya,
as
shown
in
Figure
3.
The
oedometer
3. The oedometer cell is separated into two sections by a ceramic-disc-supported thin membrane. On
cell
into two
bywater
a ceramic-disc-supported
membrane.
top ofstress
the membrane,
top is
ofseparated
the membrane,
ansections
enclosed
reservoir is used to thin
apply
a known On
vertical
to the top
an
enclosed
water
reservoir
is
used
to
apply
a
known
vertical
stress
to
the
top
of
the
sample
of the sample within the range 0–2 MPa. A compacted bentonite block was fitted in the cavitywithin
below
the
range
0–2
MPa.
A
compacted
bentonite
block
was
fitted
in
the
cavity
below
the
membrane.
the membrane. The compacted MX-80 bentonite was drilled to form a 20 mm-thick cylinder block,
The
bentonite
was
to formblock
a 20 mm-thick
cylinder
block,
with
a diameter
withcompacted
a diameterMX-80
of 50 mm.
The size
ofdrilled
the bentonite
corresponded
exactly
to the
dimension
of
of
50
mm.
The
size
of
the
bentonite
block
corresponded
exactly
to
the
dimension
of
the
cavity
inside
the cavity inside the oedometer cell, in order to ensure that the top surface of the bentonite block
was
the
oedometer
order
to ensure that the top surface of the bentonite block was in firm contact
in firm
contact cell,
within
the
membrane.
with Hydration
the membrane.
of the bentonite occurred through injection of deionised water from channel A
Hydration
thetop
bentonite
through injection
deionised
from channel
A (Figure
3)
(Figure
3) onto of
the
surfaceoccurred
of the bentonite.
The gapofbetween
thewater
bentonite
block and
the side
onto
thethe
topoedometer
surface ofwas
the bentonite.
The gap between
block and
the side ring
the
ring of
sealed by polyurethane
resin,the
inbentonite
order to inhibit
the ingression
of of
water
oedometer
was
sealed
by
polyurethane
resin,
in
order
to
inhibit
the
ingression
of
water
down
the
sides
down the sides of the bentonite so as to achieve uni-directional water flow from the top surface to the
of
the bentonite
sothe
as to
achieve block.
uni-directional water flow from the top surface to the bottom surface
bottom
surface of
bentonite
of theInbentonite
order to block.
insert two sensors (side-by-side), a rectangular groove was carved into the base of
In
order
insertastwo
sensors
(side-by-side),
a rectangular
groove
wasinto
carved
into the ensuring
base of thea
the bentoniteto
block,
shown
in Figure
4. Two RH
sensors were
fitted
the groove,
bentonite
block,
as the
shown
in Figure
4. Two
RH
sensorsthe
were
fitted into
the groove,
a firm
firm contact
with
bentonite
block.
This
allowed
relative
humidity
at theensuring
bottom of
the
contact
with
the
bentonite
block.
This
allowed
the
relative
humidity
at
the
bottom
of
the
bentonite
bentonite block to be measured during hydration. This installation minimised the volume of air
block
to be
during
hydration.
This ainstallation
minimised
the volume
air between
the
between
themeasured
sample and
the sensor
ensuring
rapid response
time. The
electricalofwires
connecting
sample
and to
thethe
sensor
ensuring
rapidmeticulously
response time.
The electrical
connecting
the sensors
control
systema were
guided
throughwires
channels
beneaththe
thesensors
cavity to
to
the
control
system
were
meticulously
guided
through
channels
beneath
the
cavity
to
the
outside
the outside of the oedometer cell, and connected to the controller board. The exterior entrances of
to
the
oedometer
connected
to the controller
board. Thein
exterior
to these
channels
these
channels cell,
wereand
sealed
via application
of polyurethane,
order entrances
to block the
air ventilation
were
sealed
application
of polyurethane,
in order toevaporation
block the airfrom
ventilation
through
channel,
through
the via
channel,
and hence
to prevent pore-water
the sample
and,the
most
of all,
and
hence
to
prevent
pore-water
evaporation
from
the
sample
and,
most
of
all,
water
vapour
water vapour flow through any gap between the sample and the sensor towards the outside offlow
the
through
any
gap
between
the
sample
and
the
sensor
towards
the
outside
of
the
cell.
cell.
Geosciences 2017, 7, 38
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4 of 9
Figure
3. Cross-sectional
of the
oedometer cell.
Figure
Cross-sectional structure
structure
Figure 3.
3. Cross-sectional
structure of
of the
the oedometer
oedometer cell.
cell.
Figure 4. Placement of the relative humidity (RH) sensors in the oedometer and the groove at the
Figure 4.
4. Placement
of
relative humidity
humidity (RH)
the oedometer
oedometer and
and the
the groove
groove at
at the
the
Figure
Placement
of the
the relative
(RH) sensors
sensors in
in the
bottom
of the
bentonite.
bottom of
of the
the bentonite.
bentonite.
bottom
The principal objective of the experiment was to test the mechanical robustness of the RH sensors
The principal objective of the experiment was to test the mechanical robustness of the RH sensors
The principal
objective
the experiment
was toincreasing
test the mechanical
robustness
ofto
thehydration
RH sensors
embedded
at the base
of theof
bentonite
block under
swelling pressure
due
of
embedded at the base of the bentonite block under increasing swelling pressure due to hydration of
embedded
at block.
the base
of the to
bentonite
block under
increasing
swelling
to hydration
the
bentonite
In order
reach swelling
pressures
in the range
of 2pressure
MPa, thedue
bentonite
block
the bentonite block. In order to reach swelling pressures in the range of 2 MPa, the bentonite block
of thebebentonite
block. Ininorder
to reach As
swelling
pressuresthe
in position
the range
MPa, the bentonite
must
fully constrained
all directions.
a consequence,
of of
the2membrane
on top of
must be fully constrained in all directions. As a consequence, the position of the membrane on top of
block must
be kept
fullystationary
constrained
all directions.
As a consequence,
the position
the membrane
bentonite
was
by in
continuously
increasing
the water pressure
in theofupper
reservoir
bentonite was kept stationary by continuously increasing the water pressure in the upper reservoir
on top
of the
bentonite
kept stationary
increasing
the swelling
water pressure
in generated
the upper
such
that
appliedwas
pressure
on the topby
of continuously
the sample was
equal to the
pressure
such that the applied pressure on the top of the sample was equal to the swelling pressure generated
reservoir
such that
the applied
pressure on the
top was
of the
sample
equal
the swelling
by
the hydrating
bentonite.
A displacement
gauge
placed
on was
top of
the to
oedometer
cell.pressure
When a
by the hydrating bentonite. A displacement gauge was placed on top of the oedometer cell. When a
generated by the
A displacement
gauge
was placed
on top
of the oedometer
cell.
displacement
washydrating
recorded bentonite.
by the displacement
gauge,
the water
pressure
controller
was manually
displacement was recorded by the displacement gauge, the water pressure controller was manually
incremented
in a step-wise
toby
restore
zero vertical displacement.
When a displacement
was fashion
recorded
the displacement
gauge, the water pressure controller was
incremented in a step-wise fashion to restore zero vertical displacement.
Experiment
2 wasindesigned
to test
the to
accuracy
of the
sensing
system within the saturated
manually
incremented
a step-wise
fashion
restore zero
vertical
displacement.
Experiment 2 was designed to test the accuracy of the sensing system within the saturated
bentonite.
There exist
several
different
methods
to measure
the relative
of the water
vapour
Experiment
2 was
designed
to test
the accuracy
of the
sensing humidity
system within
the saturated
bentonite. There exist several different methods to measure the relative humidity of the water vapour
in
equilibrium
bentonite
blocks.
Besides
the installation
of a humidity
traditional
sensorvapour
at the
bentonite.
Therewith
existthe
several
different
methods
to measure
the relative
of RH
the water
in equilibrium with the bentonite blocks. Besides the installation of a traditional RH sensor at the
point
of interestwith
(which
disturb
the Besides
sample and
be too largeofasato
be incorporated),
it isatalso
in equilibrium
the would
bentonite
blocks.
the installation
traditional
RH sensor
the
point of interest (which would disturb the sample and be too large as to be incorporated), it is also
possible
measure
the RH
using
a Chilled-Mirror
[12].asThe
psychrometer
used
this
point of to
interest
(which
would
disturb
the samplePsychrometer
and be too large
to be
incorporated),
it in
is also
possible to measure the RH using a Chilled-Mirror Psychrometer [12]. The psychrometer used in this
experimental
programme
a product
of Decagon Psychrometer
Devices, Inc. (Pullman,
WA, USA) and
is known
possible to measure
the RHisusing
a Chilled-Mirror
[12]. The psychrometer
used
in this
experimental programme is a product of Decagon Devices, Inc. (Pullman, WA, USA) and is known
as
a WP4 Dew
Point Potentiameter.
Although
psychrometer
actually
relative
experimental
programme
is a product of
Decagon the
Devices,
Inc. (Pullman,
WA,measures
USA) and the
is known
as
as a WP4 Dew Point Potentiameter. Although the psychrometer actually measures the relative
humidity
RH,
the data
are displayed
in terms
total suction actually
Ψ according
to thethe
psychrometric
law:
a WP4 Dew
Point
Potentiameter.
Although
theofpsychrometer
measures
relative humidity
humidity RH, the data are displayed in terms of total suction Ψ according to the psychrometric law:
RH, the data are displayed in terms of total
suction
Ψ
according
to
the
psychrometric
law:
𝑅𝑅𝑅𝑅
𝑢𝑢𝑣𝑣
𝑅𝑅𝑅𝑅
𝛹𝛹 = − 𝑅𝑅𝑅𝑅 𝑙𝑙𝑙𝑙� 𝑢𝑢𝑣𝑣 � = − 𝑅𝑅𝑅𝑅 𝑙𝑙𝑙𝑙(𝑅𝑅𝑅𝑅)
(1)
𝛹𝛹 = − 𝑣𝑣𝑤𝑤0 𝜔𝜔𝑣𝑣 𝑙𝑙𝑙𝑙 �𝑢𝑢𝑣𝑣0 � = − 𝑣𝑣𝑤𝑤0 𝜔𝜔𝑣𝑣 𝑙𝑙𝑙𝑙(𝑅𝑅𝑅𝑅)
(1)
u𝑢𝑢v𝑣𝑣0
𝑣𝑣RT
𝑣𝑣RT
𝑤𝑤0 𝜔𝜔𝑣𝑣
𝑤𝑤0 𝜔𝜔𝑣𝑣
Ψ =−
ln
=
−
ln
RH
(1)
(
)
ωv
uv0 −1−1K−1−1), Tvw0
where R is the universal gas constantv(8.31432
Jmol
is ω
the
v absolute temperature in Kelvin, vw0
where R is the universal gas constant w0
(8.31432
Jmol
K ), T is the
absolute temperature in Kelvin, vw0
is the specific volume of water, and ωv is the molecular mass of water vapor (18.016 g/mol). uv and uv0
is the specific volume of water, and ωv is the molecular mass of water vapor (18.016 g/mol). uv and uv0
where 𝛹𝛹 is given in kPa. This equation was used to derive the relative humidity RH measured in the
air surrounding the sample from the value of total suction Ψ displayed by the instrument.
By comparing the RH measured using a WP4 dewpoint potentiometer with the RH measured
by the sensing system, within the same hydrated bentonite block, we can validate the in vivo
Geosciences 2017, 7, 38
5 of 9
measurement accuracy of the RH sensor whilst embedded in a bentonite block.
Experiment 2 was carried out using the following steps: an MX-80 bentonite block was first cut
and
drilled
to form
a short
a diameter
a height
of temperature
approximately
cm.
·K5−1cm
where R is the
universal
gascylinder
constantwith
(8.31432
J·mol−1of
), Tand
is the
absolute
in 7.5
Kelvin,
bentonite
cylinder
wasofthen
sealed
using an impermeable
and fixed
to
vThe
the specific
volume
water,
andonω vthe
is sides
the molecular
mass of watermembrane
vapor (18.016
g/mol).
w0 is
the
bottom
of
a
polycarbonate
tube,
as
shown
in
Figure
5.
The
internal
diameter
of
the
tube
was
uv and uv0 represent the partial pressure of water vapour and the vapour pressure of water vapour at
chosen to be
the same as
diameter
cylinder,
any remaining
void space
saturation,
respectively.
Forthe
water
vapourofatthe
20 ◦bentonite
C, this equation
canwith
be simplified
to:
between the bentonite and the tube wall being filled by the membrane. Water was injected from the
Ψ with
= −135022
ln( RH
(2)
)
top of the tube and was only in contact
the upper
surface
of the bentonite block. Hence, the
hydration of the bentonite block took place gradually from top to bottom, and a gradient of water
where
is given
kPa. of
This
used to
derive
the relative
humidity
RH measured
contentΨalong
the in
length
theequation
bentonitewas
cylinder
was
formed
during this
hydration
process. in the
air surrounding
theofsample
from theprocess
value ofvaried
total suction
Ψ tests
displayed
the instrument.
The duration
the hydration
between
with abyminimum
of 7 days and a
By
comparing
the
RH
measured
using
a
WP4
dewpoint
potentiometer
with
the
RH
measured
by
maximum of 20 days. This was to achieve different water content levels in the bentonite samples
such
the
within
hydrated
bentonite
block,
we can validate
vivo measurement
thatsensing
sensor system,
accuracy
couldthe
besame
tested
at a range
of relative
humidity
values. the
Theinhydrated
bentonite
accuracy
of
the
RH
sensor
whilst
embedded
in
a
bentonite
block.
block was then removed from the tube and cut into slices approximately 2.5 cm thick. Rectangular
Experiment
2 was
carried
using
MX-80the
bentonite
blockThe
washydrated
first cut
cavities
were cut into
both
sidesout
of each
2.5the
cmfollowing
bentonitesteps:
block an
to install
RH sensors.
and
drilled
to form
short
cylinder
diameter using
of 5 cm
a height ofmembrane
approximately
7.5 cm.
bentonite
block
and athe
sensors
werewith
thena wrapped
anand
impermeable
to allow
for
The
bentonite
cylinder
was
then
sealed
on
the
sides
using
an
impermeable
membrane
and
fixed
to
the
water vapour equilibrium in the air surrounding the sample, as shown in Figure 6. The RH data was
bottom
of measured
a polycarbonate
as shown
Figure
5. The of
internal
diameter
of the
tube wasRH
chosen
regularly
by thetube,
sensing
systeminover
a period
several
days until
a constant
was
to
be the same
as the that
diameter
of the distribution
bentonite cylinder,
with
anyachieved
remaining
voidthe
space
between
the
recorded,
indicating
(i) uniform
of suction
was
within
bentonite
block
bentonite
and
the
tube
wall
being
filled
by
the
membrane.
Water
was
injected
from
the
top
of
the
tube
and (ii) water vapour surrounding the bentonite block achieved equilibrium with suction in the
and
was only
contact with
surface ofand
theabentonite
block.
Hence,
thewas
hydration
of the
bentonite.
Thein
bentonite
blockthe
wasupper
then unsealed
small sample
of each
block
immediately
bentonite
block
took
place
gradually
from
top
to
bottom,
and
a
gradient
of
water
content
along
the
put into the WP4. The RH of the sample could be calculated from the displayed value of total suction
length
of
the
bentonite
cylinder
was
formed
during
this
hydration
process.
Ψ using Equation 2.
Figure
5. Hydration
block in
in polycarbonate
polycarbonate tube.
tube.
Figure 5.
Hydration of
of bentonite
bentonite block
The duration of the hydration process varied between tests with a minimum of 7 days and
a maximum of 20 days. This was to achieve different water content levels in the bentonite samples such
that sensor accuracy could be tested at a range of relative humidity values. The hydrated bentonite
block was then removed from the tube and cut into slices approximately 2.5 cm thick. Rectangular
cavities were cut into both sides of each 2.5 cm bentonite block to install the RH sensors. The hydrated
bentonite block and the sensors were then wrapped using an impermeable membrane to allow for
water vapour equilibrium in the air surrounding the sample, as shown in Figure 6. The RH data
was regularly measured by the sensing system over a period of several days until a constant RH
was recorded, indicating that (i) uniform distribution of suction was achieved within the bentonite
block and (ii) water vapour surrounding the bentonite block achieved equilibrium with suction in the
Geosciences 2017, 7, 38
6 of 9
bentonite. The bentonite block was then unsealed and a small sample of each block was immediately
put into the WP4. The RH of the sample could be calculated from the displayed value of total suction
Geosciences
66ofof99
Ψ
using 2017,
Equation
Geosciences
2017,7,7,38
38 (2).
∼50
∼50mm
mm
Figure
Figure6.6.
6.Hydrated
Hydratedbentonite
bentoniteblock
blockwrapped
wrappedwith
withRH
RHsensors.
sensors.
Figure
Hydrated
bentonite
block
wrapped
with
RH
sensors.
3.3.Results
Results
3. Results
3.1.
3.1.Mechanical
MechanicalRobustness
Robustnessofof
ofthe
theSensing
SensingSystem
System
3.1.
Mechanical
Robustness
the
Sensing
System
The
Theresults
resultsof
ofExperiment
Experiment1,1,
1,the
theoedometer
oedometercell
celltest,
test,are
areplotted
plottedin
inFigure
Figure7.7.
7.Shown
Shownisis
isthe
therelative
relative
The
results
of
Experiment
the
oedometer
cell
test,
are
plotted
in
Figure
Shown
the
relative
humidity
measured
by
both
RH
sensors
alongside
the
increase
in
total
vertical
stress
(water
pressure
humidity
measured
by
both
RH
sensors
alongside
the
increase
in
total
vertical
stress
(water
pressure
humidity measured by both RH sensors alongside the increase in total vertical stress (water pressure
controlled
controlledby
bythe
theGDS),
GDS),in
inturn
turnassociated
associatedwith
withthe
theswelling
swellingpressure
pressuregenerated
generatedby
bythe
theprogressive
progressive
controlled
by
the
GDS),
in
turn
associated
with
the
swelling
pressure
generated
by
the
progressive
hydration
of
the
sample.
A
temporary
signal
loss
occurred
between
days
4
and
5
and
days
18 and
19,
hydration of
of the
the sample.
sample. A
between days
days 44 and
hydration
A temporary
temporary signal
signal loss
loss occurred
occurred between
and 55 and
and days
days 18
18 and
and 19,
19,
caused
by
a
bad
contact
on
the
sensor-to-wire
connector
under
the
influence
of
the
increasing
caused by
bya abad
bad
contact
on the
sensor-to-wire
connector
the influence
of the increasing
caused
contact
on the
sensor-to-wire
connector
underunder
the influence
of the increasing
swelling
swelling
The
for sensor
11 did
recover.
Both
sensors,
are
swelling pressure.
pressure.
The connection
connection
sensor
did not
notBoth
recover.
Bothhowever,
sensors, however,
however,
are fully
fully
pressure.
The connection
for sensorfor
1 did
not recover.
sensors,
are
fully functional
functional
during
the
entire
experimental
period
of
26
days
and
remain
unaffected
by
a
swelling
functional
during
the
entire
experimental
period
of
26
days
and
remain
unaffected
by
a
swelling
during the entire experimental period of 26 days and remain unaffected by a swelling pressure of
pressure
of
which was
for
aa10-day
period.
It isisworth
noting
that
pressure
of>2
>2MPa,
MPa,
wasmaintained
maintained
forperiod.
10-day
period.
worth
noting
thatthe
theswelling
swelling
>2
MPa, which
was which
maintained
for a 10-day
It is
worthItnoting
that
the swelling
pressure
pressure
and
the
RH
recorded
by
the
two
sensors
level
off
at
the
same
time,
highlighting
the
pressure
and
the RHbyrecorded
by the level
two sensors
at highlighting
the same time,
the
and
the RH
recorded
the two sensors
off at thelevel
sameoff
time,
the highlighting
coherence of the
coherence
of
the
RH
measurement.
coherence
of
the
RH
measurement.
RH measurement.
Figure7.7. Evolution
Evolution of relativehumidity
humidity andwater
water pressureduring
during thehydration
hydration of the bentonite
bentonite
Figure
Figure 7. Evolutionofofrelative
relative humidityand
and waterpressure
pressure duringthe
the hydrationofofthe
the bentonite
within
the
oedometer
cell.
within
withinthe
theoedometer
oedometercell.
cell.
At
Atthe
theend
endof
ofthe
theexperiment,
experiment,both
bothsensors
sensorswere
wereremoved
removedfrom
fromthe
theoedometer
oedometercell
celland
andtested
tested
again
in
the
open
air.
Test
results
revealed
that
both
sensors
were
fully
functional
after
sustaining
again in the open air. Test results revealed that both sensors were fully functional after sustainingthe
the
high
highswelling
swellingpressure,
pressure,without
withoutany
anydeterioration
deteriorationin
insensor
sensoraccuracy.
accuracy.The
Theswelling
swellingpressures
pressurestested
tested
here
hereare
areatatthe
thelower
lowerend
endof
ofthose
thosethat
thatwould
wouldbe
beexperienced
experiencedin
inaageological
geologicaldisposal
disposalfacility,
facility,sensor
sensor
performance
was
entirely
unaffected
and
the
sensors
proved
to
be
robust.
The
observed
signal
performance was entirely unaffected and the sensors proved to be robust. The observed signalloss
loss
Geosciences 2017, 7, 38
7 of 9
At the end of the experiment, both sensors were removed from the oedometer cell and tested
again in the open air. Test results revealed that both sensors were fully functional after sustaining the
high swelling pressure, without any deterioration in sensor accuracy. The swelling pressures tested
here are at the lower end of those that would be experienced in a geological disposal facility, sensor
performance was entirely unaffected and the sensors proved to be robust. The observed signal loss
would be eliminated by more robust cable connection methods (these were soldered by hand) or by
the incorporation of a wireless transmission onto the sensing system.
3.2. Sensor Accuracy within the Saturated Bentonite
Table 1 shows the results for both measurement methods (WP4 and RH sensing system) for seven
different bentonite samples, as described for Experiment 2, covering a range of RH levels from 52% to
100%. Analysis of the data in Table 1 shows that the RH measured by the two methods is generally
coherent, but with a mean discrepancy of 2.87(%RH). With the exception of sample No. 1, sample
differences are less than 4%. For sample No. 1, at very high humidity there is a discrepancy of 8.1%.
Table 1. RH for seven hydrated bentonite samples from Experiment 2, measured by the psychrometer
and by the sensor.
Sample No.
Suction (MPa)
RH Calculated from Suction
RH Measured by Sensor
Difference
1
2
3
4
5
6
7
−0.09 MPa
−5.22 MPa
−9.47 MPa
−34.76 MPa
−44.69 MPa
−83.55 MPa
−96.94 MPa
99.9%
96.2%
93.3%
77.4%
71.9%
54.1%
49.0%
108.0%
97.8%
97.1%
76.8%
73.1%
52.0%
51.7%
−8.10%
−1.60%
−3.80%
0.60%
−1.20%
2.10%
−2.70%
4. Discussion
The differences noted between the measurements of the sensing system developed here may
be due both to errors in the WP4 measurement and/or the sensor measurement. The WP4 method
tends to underestimate relative humidity due to invasion of ambient air into the sealed sample
chamber, allowing some evaporation until equilibrium is established [12]. The WP4 also tends to be
inaccurate at RH values close to 100%, when even very small fluctuations of temperature can cause
drop condensation in the measurement chamber. Table 1 suggests the latter error has not been an issue:
among the different measurement techniques, the WP4 is perhaps the one that ensures the largest
measurement range at high RH values (up to 99.0–99.5%). Other commercial RH sensors, including
thermocouple and transistor psychrometers, are characterised by a shorter measurement range (up to
98.5–99.0%) [13].
For the case of the RH MEMS sensor tested in this experimental programme, another source
of inaccuracy is the non-linearity of the relationship between the air relative humidity and the
volumetric water content of the hygroscopic dielectric material placed between the two plates of
the capacitive sensor. When relative humidity approaches 100%, the sensing element approaches
saturation. As a result, variations in RH generate variations in volumetric water content of the
dielectric material that tend to become smaller and smaller as saturation is approached. In turn,
variations in capacitance and, hence, electrical signal, tend to become negligible. Since the derivative
of the capacitance versus RH function tends to zero as saturation is approached, there is a loss of
sensitivity of the instrument close to saturation.
This loss in accuracy associated with the non-linearity of the calibration curve was quantified
in [5] by the use of a variety of saturated chemical solutions, each of which had a different, known
saturated relative humidity when placed within a sealed, temperature-controlled environment.
Hence, these could be used as accurate reference points without the requirement for any type of
Geosciences 2017, 7, 38
8 of 9
sensor.
The
results
Geosciences
2017,
7, 38 of these experiments are reproduced in Figure 8. Data points lie on the 1:1 line
8 of 9
(in red) in the low to medium RH range, but deviate consistently at high RH values. If the relative
humidity
values
byequation
the sensor
RH an
areadjusted
treated as
raw of
sensor
data, the sensor
be calibrated
Solving
thisreturned
quadratic
gives
value
the measured
relativecan
humidity
based
using
fittedcalibration
curve in Figure
8. RH
An Aadjusted
estimate
of thefor
relative
humidity
RHRH
can
be
from
the
on thethe
sensor
curve,
. The adjusted
values
relative
humidity
A
are
shown
in
A
calibration
equation
in
Figure
8:
Table 2. The difference between the measured value (RHA) and that derived from the WP4 data has
now reduced. The mean of the differences between them is now 1.97% and the discrepancy between
2
(3)
RH = 0.001204(RH
A ) + 0.9015RH
A + 2.182
the two values for sample number
1, at high humidity,
has dropped
from 8.1% to 1.9%.
Figure8.8. Fitting
Fitting curve
curve of
ofthe
thecalibration
calibrationmodel
modelfor
forthe
theRH
RHsensor.
sensor.
Figure
MEMS this
sensor
systemsequation
are considerably
smaller value
than traditional
monitoring
allowing
Solving
quadratic
gives an adjusted
of the measured
relativedevices,
humidity
based
accurate
point
measurements
(as
opposed
to
spatially
averaged)
and
far
less
physical
disturbance
to
on the sensor calibration curve, RHA . The adjusted values for relative humidity RHA are shown
the
engineered
barrier
system
within
a
repository.
The
results
of
both
Experiment
1
and
Experiment
in Table 2. The difference between the measured value (RHA ) and that derived from the WP4 data has
2 described
here,
ourdifferences
MEMs-based
sensing
system
is a1.97%
promising
miniaturised
now
reduced.
Theshow
meanthat
of the
between
them
is now
and the
discrepancyalternative
between
to
traditional
RH
sensors
in
geological
disposal
facilities.
It
is
sufficiently
robust
to
withstand
at least
the two values for sample number 1, at high humidity, has dropped from 8.1% to 1.9%.
2 MPa of swelling pressure and, once calibrated, is capable of accurate RH measurement over the
wideTable
range
of RH values
encountered
within
an EBS.
2. Corrected
RH of(20–100%)
hydrated bentonite
samples
measured
by the psychrometer and by the sensor.
No.RH of
RHhydrated
Calculated
from Suction
RHA Corrected
Differenceand by the
Table 2. Sample
Corrected
bentonite
samples measured
by the psychrometer
sensor.
1
99.9%
101.8%
−1.90%
2
96.2%
93.1%
3.10%
Sample No.
RH Calculated from Suction
RHA Corrected
Difference
3
93.3%
92.6%
0.70%
1
99.9%
101.8%
−1.90%
4
77.4%
74.7%
2.70%
2
96.2%
93.1%
3.10%
5
71.9%
71.3%
0.60%
3
93.3%
92.6%
0.70%
6
54.1%
51.6%
2.50%
4
77.4%
74.7%
2.70%
49.0%
−2.30%
57
71.9%
71.3%51.3%
0.60%
6
7
54.1%
49.0%
51.6%
51.3%
2.50%
−2.30%
MEMS sensor systems are considerably smaller than traditional monitoring devices, allowing
accurate point measurements (as opposed to spatially averaged) and far less physical disturbance to
Acknowledgments: This research has been supported by EPSRC consortium grant EP/I036427/1 ‘SAFE
the engineered barrier system within a repository. The results of both Experiment 1 and Experiment 2
Barriers—a Systems Approach for Engineered Barriers’ (2012–2016). This was co-funded by EPSRC and Nuclear
described
here, show
that our
MEMs-based
system
is a promising
miniaturised
to
Decommissioning
Authority
Radioactive
Wastesensing
Management
Directive,
now Radioactive
Wastealternative
Management.
traditional RH sensors in geological disposal facilities. It is sufficiently robust to withstand at least 2
Author Contributions: Alessandro Tarantino, Gráinne El Mountassir and Rebecca J Lunn conceived and
MPa of swelling pressure and, once calibrated, is capable of accurate RH measurement over the wide
designed the experiments; Wenbin Yang performed the experiments; Wenbin Yang, Rebecca J Lunn and
range of RH values (20–100%) encountered within an EBS.
Alessandro Tarantino analyzed the data; Rebecca J Lunn wrote the paper.
Acknowledgments:
This
been supported
Conflicts of Interest:
Theresearch
authorshas
declare
no conflictby
of EPSRC
interest.consortium grant EP/I036427/1 ‘SAFE Barriers—
a Systems Approach for Engineered Barriers’ (2012–2016). This was co-funded by EPSRC and Nuclear
Decommissioning Authority Radioactive Waste Management Directive, now Radioactive Waste Management.
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Author Contributions: Alessandro Tarantino, Gráinne El Mountassir and Rebecca J Lunn conceived and designed
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Lidskog, R.; Wenbin
Andersson,
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Wenbin
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