Analysis of Different Water/Powder Ratios of High Strength Dental Stone
Nélia Alberto
, Rogério Nogueira
, Lídia Carvalho
, Ilda Abe
Hypolito Kalinowski
and João L. Pinto
Physics Department, University of Aveiro, Campus Universitário de Santiago, Aveiro, Portugal
Institute of Telecommunications, Campus Universitário de Santiago, Aveiro, Portugal
Dental Prosthesis IPSN, Paredes, Portugal
Federal University of Paraná, Curitiba, Brazil
Keywords: Fibre optic sensors, Bragg grating, dental gypsum, water/powder ratio.
Abstract: In this work, we present a device to measure strain and temperature, based on fibre Bragg grating (FBG)
sensors. The performance of these sensors was assessed in the study of a type of dental gypsum, namely
high strength dental stone. It was intended to know the influence of the water/powder (W/P) ratios in the
setting time and expansion. The results show that, a change in the W/P ratio influences the setting time as
well as maximum strain values. The data was compared with the values foreseen in ANSI/ADA
specification nº 25. The information that can be obtained with these sensors is quite important to dental
prosthesis technicians, in order to achieve more accurate dental prosthesis and these sensors can be a good
substitute to the actually device that is used to evaluate the setting expansion: extensometer.
Optic sensors based on fibre Bragg grating (FBG)
have attracted much attention in the past few years
due to many advantages such as high sensitivity,
immunity to electromagnetic interference,
wavelength multiplexing, lightweight, low cost and
easiness handling, which allows to be embedded into
different materials. These characteristics have
opened wide fields of applications, from optical
communications to biomedicine. Thus, we can find
FBGs in civil structures monitoring (Lima, 2007);
sensing systems in vehicles, airplanes and ships
(Castelli, 2002), in the detection of virus/antibodies
(Petrosova, 2007) or even, in linear polymerization
shrinkage monitoring of dental materials (Arenas,
In its simplest form, an FBG consists of a
periodic modulation of the refractive index along the
fibre length, which is formed by exposure of the
core to an intense optical interference pattern of
ultraviolet light. The sensing principle of a FBG-
sensor is based on the monitoring of the wavelength
shift of the reflected Bragg wavelength when it is
subjected to strain and/or temperature changes.
The Bragg wavelength is given by the following
= 2 n
Λ (1)
where n
is the effective index of the core and Λ is
the refractive index modulation period (Kersey,
Among the dental materials, gypsum is one of
most used in dental prosthesis because of its ability
to change properties by the addition of different
chemical components. When powder particles of the
gypsum are mixed with water, a chemical reaction
occurs, according with the following equation
(Phillips, 1991):
O + 3H
O + heat (2)
All manufacturers of gypsum products have its
own recommendations, concerning proper
water/powder (W/P) ratio to be used, to obtain the
best performance. However, if the W/P ratio is not
correctly followed, accordingly to the
manufacturer’s recommendations, the gypsum might
have different properties, namely the setting
Alberto N., Nogueira R., Carvalho L., Abe I., Kalinowski H. and L. Pinto J. (2008).
STUDY OF DENTAL GYPSUM USING FIBRE SENSORS - Analysis of Different Water/Powder Ratios of High Strength Dental Stone.
In Proceedings of the First International Conference on Biomedical Electronics and Devices, pages 232-235
DOI: 10.5220/0001055302320235
In this work the influence of the W/P ratios of a
high strength dental gypsum stone (type IV) was
experimentally studied, namely the setting time and
expansion, using devices based on FBG. The setting
expansion values obtained were compared with
values foreseen in ANSI/ADA specification25,
which were measured using an extensometer.
The FBGs were written into photosensitive optical
fibre (Fibercore PS1250/1500), by illuminating it
with ultraviolet light, using an automated phase-
mask interferometer system (Nogueira, 2002).
Two sensors were used in this work. One of them
was protected mechanically, in order to be only
sensitive to temperature variations. For that, the
sensor was placed inside a double needle (figure 1).
Figure 1: Temperature sensor.
The other sensor consisted in a free FBG, which was
in close contact with gypsum, and is sensitive to
strain and temperature variations. In the first
analysis it was verified that there was sliding of the
gypsum along the free FBG. In order to overcome
this situation, two plastic spheres were glued to the
fibre, down and above the sensor (figure 2),
allowing a better response of the fibre to the gypsum
Figure 2: Schematic representation of the experimental
The free sensor was put inside a hard metallic
container (diameter = 3.5 cm and height = 7 cm)
with a hole in the bottom, through which the fibre is
pulled through. The fibre is then bonded to a support
and slightly tensioned (about 700 με) allowing
measuring both expansion and shrinkage.
Relatively to the W/P ratio, three different values
were considered. One corresponds to the value
recommended by the manufacturer, which is 0.22,
one corresponding to a 15% decrease of the water
quantity (W/P = 0.19) and another corresponding to
a 15% increase of the water quantity (W/P = 0.26).
These values were chosen, according with the
reaction’ stoichimetry and the W/P ratio
recommended by manufacturer. Through reaction’
stoichimetry we verified that a 15% decrease of
water quantity relatively to W/P ratio recommended
by manufacturer is the minimum value requested to
occur reaction thus. With a decrease of water below
15% it is impossible to have a full reaction of
gypsum. The choice of the other W/P ratio is related
to the fact that we consider that usually there is a
tendency to add a larger amount of water than the
one recommended, facilitating the handling of
For each case, the water was added to powder
and spatulated, during 45 seconds. Then, the mixture
was flowed into the metallic container and finally,
the temperature sensor was inserted into the mixture.
The Bragg wavelength measurement was made
every 5 seconds, during 3 hours, simultaneously for
the two sensors. The resolution of the system
(sensor+interrogation system) allows measurements
of displacement and temperature with a precision of
about 1 με and 0.1 ºC respectively.
The measurements were repeated several times
for each W/P ratio, being the results presented in this
work a result of an average.
The evolution of strain and temperature with time,
for the two sensors used in the experiments are
showed in the graphs of figures 3 and 4,
STUDY OF DENTAL GYPSUM USING FIBRE SENSORS - Analysis of Different Water/Powder Ratios of High Strength
Dental Stone
0 30 60 90 120 150 180
TIME (min)
(W /P) = 0.19
(W /P) = 0.22
(W /P) = 0.26
Figure 3: Evolution of strain during setting reaction, for
gypsum type IV, for three different (W/P) ratios.
Figure 4: Evolution of temperature during setting reaction,
for gypsum type IV, for three different (W/P) ratios.
The strain curve was obtained by subtracting the
effect of temperature, obtained with the temperature
sensor, in the measurements accomplished by the
free FBG. The sensitivity coefficients of the FBG to
temperature and strain was 10.6 pm/ºC and
1.1 pm/με, respectively. These values were
previously measured.
According with the results obtained, we can
observe that when we change the W/P ratio
recommended by manufacturer, the maximum strain
value and maximum thermal amplitude, reached
during the setting reaction of dental gypsum are
different (table 1), although the behaviour is similar.
Table 1: Values of maximum strain and maximum thermal
amplitude for the W/P ratios analysed.
Maximum strain
Maximum thermal
0.19 874 9.5
0.22 1120 12.3
0.26 884 10.1
Initially, we observe a shrinkage of dental
gypsum, being more accentuated in the case of the
W/P = 0.19, where it reaches about -90 με.
Following, there is an increase of strain, along with
an increased of temperature. After 55 min, for
W/P = 0.22 and after 65 min, for W/P of 0.19 and
0.26, the setting expansion stabilized and
temperature returned slowly to its initial value (room
Although there is a modification on the setting
expansions values when the W/P ratio change, the
results obtained experimentally are within the range
established by ANSI/ADA specification nº 25, that,
in this case, is between 0% and 0.15 %. This range
was determined using an extensometer that allows
the measurement of length changes within 0.01 mm
resolution in a specimen with a length of 100 mm,
(100 με of resolution). The proposed device, based
on FBG, allows a real time measurement of the
evolution of the strain and temperature in the
material. Moreover, the resolution is around 1 με.
Thus, the use of this device can be an incentive to
the revision of actual ANSI/ADA specification
25, where we suggest the measurement of the
setting expansion, using optic fibre sensors to the
detriment of the extensometer because this technique
present more resolution, with more reliable results.
When the gypsum is dry, it is also possible to
measure the linear thermal expansion coefficient.
This thermal property is other information quite
important to dental prosthesis technicians, which is
defined as the change in length per unit of length of
gypsum, when its temperature is raised or lowered
In this work we presented a device for strain and
temperature measurements, based on FBGs.
These sensors were applied in the study of the
influence of the W/P ratios of dental gypsum, in the
setting time and expansion. Also the sensor shows
applicability in the determination of thermal
expansion coefficient.
The sensors’ response showed that, a
modification of the amount of water influences the
value of maximum strain and, this difference might
interfere in the outcome of the technicians work. So,
it is advisable following the manufacturer’s
The present devices can also be a useful tool for
gypsum’ manufacturers, allowing a real time
monitoring of strain and temperature along the
production process, what can be very important in
0 30 60 90 120 150 180
TIME (min)
(W/P) = 0.19
(W/P) = 0.22
(W/P) = 0.26
BIODEVICES 2008 - International Conference on Biomedical Electronics and Devices
the control and improvement of the gypsum’s
The authors acknowledge support received from
FCT SFRH/BD/30551 and SFRH/BPD/14513/2003
and project THRONE PTDC/EEA-TEL/66840/2006
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STUDY OF DENTAL GYPSUM USING FIBRE SENSORS - Analysis of Different Water/Powder Ratios of High Strength
Dental Stone