1

Use of Sensors to monitor temperature, moisture and pH in building stone

Presented by

Sudarshan Srinivasan

Outline of the Presentation

• Introduction

• Fibre optic Relative Humidity probe– Development & fabrication of FOS-RH probe– Laboratory simulation study - Results

• Fibre optic pH probe – Calibration– Capillary rise test

• Monitoring of test wall in Oxford

• Summary

• Future work

Electrical resistance sensors

Electrical resistivity sensors

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45mm

35mm

25mm

15mm

5mm

Elapsed time, t (h)

R R

Change in resistance due to

• Moisture movement

• Chlorides

• Carbonation

Not possible to distinguish between Cl- or other ions

Water ingress Reflectance

spectrumTransmittance spectrum

Reflectance spectrum

Transmittance spectrum

Temperature/stress/strain/humidity by coating

overlay

FBG as Fibre Optic Sensor

λB: the Bragg wavelength, is defined by:

n: the average refractive index of the grating & Λ : the grating period.

Humidity probe

PI coated FBG humidity sensor

FBG Temperature sensor

Heat shrink

Pigtailed connectors to the interrogation system

Oven drying of mortar slab (Trial-4 sample) at 50°C

280

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0 40 80 120 160 200 240

time (hrs)

Unc

alib

rate

d te

mpe

ratu

re

Oven drying of mortar slab (Trial-4 sample) at 50°C

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time (hrs)

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alib

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d H

umid

ity

Challenges faced…

Capillary suction of water in Mortar(Trial-4 sample)

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time (hrs)

Unc

alib

rate

d H

umid

ity

2

Porous tubing

RH & Temperature FOSGlue

FO connectors

Sectional View

New RH sensor setup

Humidity sensor

Temp. sensor

Fabrication

• Polycarbonate tube

• 1cm dia., 20cm long

• Least cavity vol. – 30mm3

y = 0.0082x + 1536.8R2 = 0.9998

1536.90

1536.95

1537.00

1537.05

1537.10

1537.15

1537.20

1537.25

1537.30

0 10 20 30 40 50 60

Temperature (°C)

Bra

gg W

avel

engt

h (n

m)

1557

1557.1

1557.2

1557.3

1557.4

1557.5

1557.6

0 20 40 60 80 100Relative Humidity, RH (%)

Bra

gg W

avel

engt

h (n

m)

10°C20°C30°C40°C50°C

Calibration Porous Filter

Porous ceramic capDetachable porous cap

Laboratory Simulation Experiments

2.0 cm

1.0 cm3.0 cm

4.0 cm

IRT

Heat

Water

WindAmbient temp. & RH

• Sample dimension 150X150X80 mm

• ER sensors (1cm, 2cm, 3cm & 4cm)

• Temperature sensor (1cm, 2cm, 3cm & 4cm)

• FOS-RH sensor (3cm)

• Capacitance based RH sensor (3cm)

• Ambient temp. 20°C

• 30 min ON/ 15 min OFF

• Water spray 20°C/10 min

Experimental setup

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FOS readings

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0.00 10.00 20.00 30.00 40.00 50.00Time (hrs)

Tem

pera

ture

(°C

)

FOS probeCapacitance sensor

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0.00 10.00 20.00 30.00 40.00 50.00Time (hrs)

Rel

ativ

e H

umid

ity(%

)

FOS probeCapacitance sensor

ER readings

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umid

ity(%

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FOS probeCapacitance sensor

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Elec

tric

al re

sist

ance

(M o

hm)

1 cm2 cm3 cm4 cm

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Time (hrs)

Tem

pera

ture

(C)

IR temp1 cm2 cm3 cm4 cm

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Time (hrs)

Tem

pera

ture

(C)

IR temp1 cm2 cm3 cm4 cm

Dry Wet

Temperature profile

Fibre Optic pH sensor

• Calorimetric indicator + Light source + Spectrometer

• Phenol red disc: 6.5 – 8.5

• Low pH -5, High pH – 10,

Low Buffer – 7, High Buffer - 8

Calibration

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Time (min)

pH

• Portland stone 100X100X100 mm

• Drilled hole – 3cm from surface

• Tap water

• pH of dust sample – 8.3

Capillary rise test

4

Test wall monitoring - Oxford

• Test wall: 2m X 0.4m X 1.75m

• Block B7

• 10 cm recessed

• FOS-RH probe: 5cm from surface

Day1 &2

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ativ

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umid

ity (%

)

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Tem

pera

ture

(°C

)

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35

0.00 10.00 20.00 30.00 40.00 50.00Time (hrs)

Surf

ace

tem

pera

ture

(°C

)

IR temp.

FOS-RH FOS-Temp.

• Block D7

• Surfaced block/ 0cm recess

• FOS-RH probe: 5cm from surface

Day 3

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0.00 5.00 10.00 15.00 20.00 25.00 30.00Time (hrs)

Surf

ace

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pera

ture

(°C

)

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0.00 5.00 10.00 15.00 20.00 25.00 30.00Time (hrs)

Rel

ativ

e H

umid

ity (%

)

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10

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20

25

0.0 5.0 10.0 15.0 20.0 25.0 30.0Time (hrs)

Tem

pera

ture

(°C

)

IR temp.

FOS-RH FOS-Temp.

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5.5

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6.5

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0.0 2.0 4.0 6.0 8.0 10.0Time (hrs)

pH v

alue

• Block B7

• 10 cm recessed

• FOS-pH probe: 5cm from surface

pH measurement

5

Summary

• Calibration of fibre optic RH sensor – Temperature correction applied

• Fabricated new FOS-RH probe with polypropylene filter has obtained reliable results in lab and site conditions

• Electrical resistance changes correlated with FOS-RH readings at the same depth in limestone block

• Influence of retreat of stone block on temperature and moisture changes inside stone were studied in the test wall

• Calibration of Ocean optics fibre optic pH sensor with Phenol red disc, pH range 6.5-8.5 suitable for building stone

• Successfully monitored pH of limestone in both lab and site conditions

Future work

• To monitor changes in RH closer to the surface in lab. simulation experiment using FOS-RH probe

• To study influence of acid rain on pH changes in stone sample using FOS-pH probe

• To monitor temperature and moisture profile in test wall during cold weather (Nov/Dec)

Acknowlegement

• Thanks to the Engineering and Physical Sciences Research Council in the UK for their support.