# Miniaturised Optical Fibre Sensor for  Dew Detection Inside Organ Pipes

Authors: Francesco Baldini; Riccardo Falciai; Andrea Azelio Mencaglia; Folco Senesi; Dario Camuffo; Antonio della Valle; Carl Johan Bergsten
Journal: Journal of Sensors
 (2008)

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## Abstract

A new optical sensor for the continuous monitoring of the dew formation inside organ pipes was designed. This aspect is particularly critical for the conservation of organs in unheated churches since the dew formation or the condensation on the pipe surfaces can contribute to many kinds of physical and chemical disruptive mechanisms. The working principle is based on the change in the reflectivity which is observed on the surface of the fibre tip, when a water layer is formed on its distal end. Intensity changes of the order of 35% were measured, following the formation of the water layer on the distal end of a 400/430μm optical fibre. Long-term tests carried out placing the fibre tip inside the base of an in-house-made metallic foot of an organ pipe located in an external environment revealed the consistency of the proposed system.


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## Body

## 1. Introduction

Controlling the  microclimate
within museum, art galleries, and churches is increasingly recognised as an
essential feature in the protection and preservation  of invaluable works of art
[1, 2]. Important requirements for this application are of a very low
environmental impact, so as to assure minimal interference with the
visualization of the exposed works of art by visitors, continuous monitoring
and complete safety in operation. The high degree of miniaturisation, easy
handling, and absence of electric contacts make the utilisation of optical
sensors in this field very appropriate.Optical fibre sensors originated in
the eighties and their development are still expanding since they find their
application in many fields ranging from environment to medicine and industry
[3, 4].The protection and maintenance of
works of art in the museums and churches is a task which has always been
pursued but only in the last decade recent works have emphasised the relevant
role played by the microclimate.Knowledge of the microclimate, by means of an
analysis of the environmental parameters, allows the evaluation of the level of
risk to which the art objects are exposed. Clearly, this can trigger actions
directed at minimising the causes of risk and at insuring the attainment of
more appropriate conditions of preservation. Moreover, it is obvious that,
after their definition and achievement, these conditions should be kept as
constant as possible, notwithstanding the unavoidable changes induced by the
external surroundings.The pipe organ and its music
are important parts of the cultural heritage of Europe.
The organ, with its facade architecture and sound, is a multimedia and
multidisciplinary object. The organ heritage found in all countries of Europe includes more than 10 000 historical valuable
organs.The organ contains many
different types of materials like different types of wood, leather, different
types of lead-tin alloys, from pure lead to almost pure tin, brass, and iron.
It also contains a complex system of moving parts and the bellows and the wind
channels need to be airtight. This makes the organ to an object very sensitive
to microclimate changes and harmful environments. The organ can be unplayable
if only one of the materials or functions just mentioned is responding to the
environment in a negative way.Recently,
it was shown that the escalating problem of corrosion inside organ pipes is
caused by organic acids, especially acetic acid, emitted from the wooden parts
in the organ [5–7]. The organic
acids enter into the pipe foot and create a corrosive environment inside the foot. The formation of a corrosive environment in the pipes is mainly dependent on the pipe metal alloy and the temperature and humidity in the organ. High humidity and condensation can create an environment  where development of mould on the wooden parts can take place and it can also cause corrosion on the
iron parts of the organ in addition to the pipe corrosion. Also, frost crystals
formed in oxidised pipes in unheated churches may cause mechanical damage to them.For the preservation of the organ
cultural heritage, there is a great need for monitoring the climate conditions
in the organ and in the pipes especially after an organ restoration or a change
of the heating system and the ventilation conditions in the church.In the framework of the European
project SENSORGAN (sensor system to detect harmful environments for pipe
organs) which has the objective to make available new instrumentation for
monitoring and detection of harmful environments for pipe organs through
development of sensors for real time measurement, a miniaturised optical fibre
sensor for detection of dew formation, or condensation, inside/outside organ
pipes is under progress.The working principle of the sensor,
its design and the laboratory characterisation is here described.


## 2. Experimental

The working principle of the optical fibre sensor is
based on the change in the properties of the optical transmission of an optical
fibre. On the basis of the input requirements, two different configurations
were designed and tested making use of a single fibre.In
the first configuration, the formation of the water layers associated to the
dew point occurs along the fibre which is deprived of the cladding. The change
in the refractive index associated to the formation of the water layers provokes a change in the
transmission properties of the optical fibre, with an increased amount of light
which is directed out of the fibre. In order to increase the sensitivity of the
approach (i.e., an increase in the losses associated to the change of
refractive index), the fibre is U-shaped in correspondence of the region where
the cladding is removed. Practically the fibre is fixed on metallic cylinders
by means of a polymeric glue, and the curved region is polished until the clad
of the fibre is fully removed. Two different silica fibres were tested in this
configuration, with a core diameter of 1 mm and200μm, respectively (3M, HCPM0200T). The
fibre was then connected to a laser diode (Roithner Lasertechnik, LDM635/3-OLU)
on one side and to a photodetector (TAOS,
TSL14S) on the other side.The second configuration bases its
working principle on the change in the reflectivity of a400/430μm optical
fibre (3M, HCPM0400T), following the formation of a water
layer or ice crystals on its distal end. It is clear that this configuration is
simpler and more compact. A similar configuration was successfully adopted for
the continuous monitoring of respiratory rate, measuring the change of
reflectivity at the fibre end due to the change in the condensed humidity from
the airways during respiration [8]. Two optical fibres (3M, HCPM0200T,
core/clad diameter: 200/230μm) allow the connection of the sensing fibre to
the laser diode (Roithner Lasertechnik, LDM635/3-OLU) and to the photodetector
(TAOS, TSL14S).Figure1 shows schematically the optoelectronic system used for the interrogation of
the fibre in the case of the second configuration. It has three channels that are independent from
each other but share the same laser diode (LD) as light source so that three
different locations can be monitored at the same time. The monitor photodiode (MPD)
is inserted in the feedback loop of the laser diode (LD) driver, so that
constant power emission is achieved through the voltage control (P), and hybrid
photodetectors (PD1-3) are used, which provide direct light-to-voltage
conversion (OUT1-3). Then the voltage signal is converted by an
analog-to-digital converter connected to a laptop.Figure 1
Sketch of the optoelectronic
configuration used to measure the reflectivity changes which occurs at the
distal end of the optical fibre. LD: laser diode; MPD: module to monitor the
laser diode emission; P: voltage control to keep the laser diode emission
constant; PD: hybrid photodetector; OUT1-3: signal output of the
light-to-voltage converters.Commercial sensors were used to
measure the temperature (Analog Devices, AD22100) and the relative humidity (Honeywell,
HIH4000-001).


## 3. Results and Discussion

In order to test the two different
configurations, an apparatus was realised capable to change, control, and
measure the temperature and the relative humidity of the region in proximity of
the sensing point. The sensing part of the optical fibre is fixed on an
aluminium block, the temperature of which is regulated by means of a Peltier
cell. In this way, it is possible to reach the dew point with the formation of water droplets or of a water layer on both the block piece and the sensing part of the fibre.A software program developed under
LabView allows the acquisition of the signal coming from the optical fibre and
from the sensors for humidity and temperature as well as the driving the Peltier cell.Preliminary tests showed a better
sensitivity in the case of the reflectivity-based configuration. With this
configuration, intensity changes of the order of 35% were found, whereas in the
case of the U-shaped fibres the changes were not greater than 20%.Therefore all the further tests were
made with the reflectivity-based configuration. Figure2 shows the optical
fibre deprived of the jacket located on the aluminium block. The water drops
formed on the aluminium  block due to the condensation are clearly visible.
Figure 3 shows the typical results obtained when the temperature of the block
was changed from 25°C to 5°C and back again to 25°C. The relative humidity
changes from 40% to roughly 90%. The signal coming from the optical fibre
sensor is normalised with respect to its value in dry conditions. The optical
signal, after a slight increase, starts to decrease as a consequence of the
formation of the water layer on the fibre tip. A stable value is reached and
this value persists also after the increase of the temperature, since the
humidity on the fibre tip is still present. After roughly half an hour, the
water is completely evaporated and the signal coming from the fibre comes back
to the initial value. As it can be seen, the change of the signal from the
fibre is of the order of 35%. During all the experiment, the external
temperature and the external relative humidity remain practically constant and
equal to 24°C and 38%, respectively.Figure 2
The optical fibre tip deprived of the
jacket and fixed on the surface of an aluminium block; the photo was taken
after the condensation took place, and the water drops are clearly visible on
the aluminium surface.Figure 3
Response curve of the optical fibre
sensor following a decrease of the temperature of the aluminium block from25°C to 5°C and back again to 25°C. The graph shows also the tracings of both
the temperature and the relative humidity of the aluminium block (named
internal RH and internal T, resp.) and of the external environment (named
external RH and external T, resp.). The left y-axis is related to the optical
signal, whereas the y-axis on the right is for the temperature and the related
humidity.In order to evaluate the performance
of the sensor in a situation closer to the real one, the system was characterised
by placing the sensing fibre inside the foot of a metallic organ pipe built at
the Göteborg Organ Art Center. The organ pipe was placed in an
in-house-made thermal box. Three sensors were placed at the same time in order
to evaluate the reproducibility of the results and a suitable in-house-made
spring was used to keep the fibre ends in place (tangent to the pipe's inner
surface) (Figure4). This spring was designed so as to be used also in real
conditions in organs inside churches, where the access to the  internal surfaces
of the organ pipes is performed through a small opening located in their foot.Figure 4
The metallic foot of the organ pipe
with the three optical fibre sensors.
The inset on the bottom/left shows how the metallic spring keeps the three fibre
tips in contact with the pipe’s inner surface.Thermal cycles were performed by
placing dry ice in the thermal box until the dew occurred inside the inner
surface of the pipe and then removing it until the equilibrium with the
environment was reached. These steps were repeated several times. The sensor
outputs of the three fibres, normalised to their value in dry conditions, are
shown in the upper graph of Figure5. In the lower graph the temperature and
the relative humidity recorded inside the pipe foot are shown. Also in this
case, as observed in Figure 3, the responses of the optical sensors are well
correlated with the changes in temperature and in humidity, with the optical
signal which decreases in correspondence of a temperature increase and of an RH decrease. It is
quite important to observe that the behaviour of the three optical sensors is
very similar; the differences which are observed can be ascribed to the
different locations inside the metallic organ pipe characterised by slightly
different distances of the fibre tip from the metallic surfaces. In any case,
the reproducibility of the three sensors can be considered satisfactory in view
of the application  of the sensor in real environment and an alarm threshold for
the water condensation can be established in correspondence of a 15% decrease
of the optical signal exhibited in dry conditions.Laboratory characterization of the
optical system carried out by placing the base of the organ pipe with the
sensing fibres inside a thermal box. On the top, the response curve of three
optical sensors placed inside the metallic organ pipe are shown. On the bottom,
the temperature and relative humidity curves are shown.
(a)(b)After
the laboratory characterisation, the system was exposed for a longer period in
external environment inside a weather screen with open louvers. The optical system was exposed together with the sensors for temperature
and relative humidity. Figure6
shows the results for a six-day measurement. The sampling of the optical sensor
was made every 15 minutes and recorded in a data logger. The sensor output, the
external temperature, and the external RH have been recorded. The periodic
trend is clearly correlated to the cycles of the day and of the night. The
optical sensor responds equally well to dewing and frosting, as it was recognised
by inserting the fibres inside a freezer cell, −24°C. The output drop for ice crystals
is similar to that for droplets.Long-term characterization of the
optical system in external environment. On the top the response of the sensing
fibres inserted inside the base of the organ pipe is shown; on the bottom the
external relative humidity and the external temperature are shown.
(a)(b)This
behaviour fulfils perfectly the requirements for anin situ application
of the sensor inside the organ pipes in the church; the achievement of a
dangerous level of condensation on the internal walls of the pipes can be
identified with a threshold level corresponding to a 15% decrease of the signal
exhibited in dry condition.One of the main
problems which generally affects intensity-based sensors is given by
fluctuations/losses not dependent on the measurand-induced changes, such as
source fluctuations (avoided in the present case by the use of a diode laser
driven at constant power) or changes in the fibre transmission induced by
temperature changes or fibre bending. This problem is less effective in this
application due to the binary nature of operation (dew or not dew). Even if the
device exhibits a correlation with RH in a certain range, it must be pointed
out that it is not sufficiently accurate as analog sensor, at least in this
simple configuration, without any reference channel to compensate down-lead
fluctuations in loss and without any calibration. Nevertheless, the particular application,
for which such a device has been developed, requires to monitor the occurrence
of an excess of RH (>90%), where the
output drops certainly below 80% of the value attained in dry conditions. Then,
once installed (from then, one can reasonably expect small loss fluctuations),
it can be reliably used as ON/OFF sensor simply by fixing a suitable threshold,
proportional to the value measured at low RH. Moreover, the simultaneous use of
the three sensing points with the three different fibres can be used as safety
redundancy; the concordance of the output from the three fibres can be used as
intrinsic control of the sensor reliability.


## 4. Conclusions

The
described optical sensor is capable of monitoring continuously the water
condensation and the formation of ice crystals inside organ pipes. Although not
tested yet, it should properly work also in the presence of acetic and formic
acid released by the wood deterioration, as it occurs inside the organ pipes.
As a matter of fact, the transduction of the signal occurs at the interface
quartz/air of the optical fibre, and thanks to the well-known chemical
resistance of the quartz fibre, this surface should be unaffected by the
presence of weak acids such as acetic and formic acid. In addition, the
condensation may occur earlier for the presence of hydrophilic metal oxides. Being
in direct contact with the target surface, it is able to monitor what happens
inside the organ pipe. The thorough characterisation in the laboratory and the
long-term characterisation in external environment showed the reliability of
the optical sensor and the efficiency of the adopted working principle. The next
step will be the performance of real tests on the historical organ in the Minor
Basilica of St. Andrew the Apostle in Olkusz, Poland.



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*Source: 321065-2008-09-29.xml*