Carbon Fibre Electrodes and their Qualities in Salt Water
Lennart Crona and Anders Brage
Defence Research Establishment
Stockholm, Sweden
Abstract: For many years zinc electrodes have been
used to measure electric fields in ocean environments.
Typical stabilisation times are of the order of one week.
However, it is necessary in certain application to have
electrode
sensors
operational
immediately after
deployment. This requirement initiated the development of
a new type of electrode based on carbon fibres. Carbon
fibres are very homogenous with much lower impurity
levels than bulk graphite. Moreover, the surface in contact
with water can easily be large and well controlled by
creating a bundle of fibres.
Measurements of impedance and sensor noise have been
made in a small water tank. The electrode impedance is
found to increase rapidly with decreasing frequency,
indicating that the electrodes behaves like a large capacitor,
i.e. are insensitive to the DC component of the electric
field.
Sea trials have shown that electrodes are operational within
15 minutes after deployment. Background and sensor noise
have been measured in the archipelago of Stockholm.
Keywords: Carbon fibre, electrodes, electric sensors,
sensor noise, ELF
1. INTRODUCTION
In some applications there is a requirement for an
electric sensor to be in full function immediately after
deployment. For standard electrode materials e.g.
Ag/AgCl or zinc plates, the main problem is the slow
electrochemical process before a steady equilibrium
potential is obtained [1-3]. A process that may take
up to one week for zinc. This problem may be
overcome by special treatment and storage of the
electrodes. The conductivity varies between 0.3 and
3.3 S/m along the coast of Sweden. Therefore, there
was a need for a dry sensor which could easily be
stored, handled and deployed and taken out of the
water repeatedly without degrading. It must have low
self noise and fulfil the requirement of being
operational in any water immediately after
deployment. The carbon fibre electrodes seem to
fulfil our requirements.
2. ELECTRODE DESIGN
Carbon fibres have some advantages over bulk
graphite plates. The carbon fibres are very
homogeneous, have a large surface to weight ratio
and are chemically inert in water.
The type of fibre used is TORAY T300 with 7
micrometer diameter and the surface to weight ratio is
about 0.5 m²/gram. One electrode can consist of
totally 1.2 million fibres with the length of 10 to 20
cm. One end of the fibres are impregnated with
epoxy. After hardening the ends are ground to lay
bare the fibre ends for metal plating for the leads to
connect to, making an electric contact with the fibres.
The fibre tufts are solvent washed to take away the
finish of insulating epoxy coating in order to enhance
the fibre surface contact with water. The contact area
between lead and fibre ends are protected from water
migration by another epoxy potting.
Signal leads insulation have to be inert to water
and pressure variations and yet it should be possible
to seal it in an ordinary epoxy operation. Etched
PTFE mantled signal leads of silver coated copper
would work, and further more give a local corrosion
element if the mantling would be damaged,
preventing erroneous signals to be collected.
In Fig. 1 there is a picture of a 20 cm carbon fibre
electrode. A plastic net is used to keep fibres on
place. The electrodes are then covered with a cap of a
fibre material in order to be protected from water
movements, intrusion of mud, small animals and
plants. Water moving near the electrode surface will
give rise to noise. We have not seen any growth of
living material on the fibres after 8 month in water.
The carbon fibre diameter, 7 micrometer, is too small
for larvae to settle. The cap have to be thick enough
to be completely dark inside to minimise the growth
of algae. The fibres in the cap also seem to be too
small for settling of small animals on the outer
surface of the cap. It is necessary to make some small
holes in the cap to get rid of the air inside the cap
during deployment.
3. ELECTRODE IMPEDANCE
At frequencies above 1 Hz the impedance is
almost resistive and for a pair of electrodes below 10
ohms. Typically the resistance water-electrode is
about 3 ohms and the resistance electrode-wire is
below 1 ohm. The measurements were made in a
small water tank with the same conductivity as in the
archipelago of Stockholm, 0.8 S/m. The impedance
was measured with a carbon electrode sensor as a
receiver by using a transmitter in the same tank.
|Z| (ohms)
1000
100
10
1
0,001
With decreasing frequency the impedance is
increasing fast. The carbon fibre electrodes acts
almost like a capacitor. If the electrodes are charged
and left unloaded the voltage remains for a long time.
To discharge and stabilise the sensor a shunt resistor
was used at the input of the amplifier. In a typical
application a 330 ohms resistor was used. At 1 mHz
the impedance is about 400 ohms. In that case the
voltage will be attenuated by the factor 330/730. For
frequencies lower than 1 mHz the impedance will be
too big. Thus, the carbon fibre sensor can be used
down to 1 mHz. Fig. 2 and 3 shows examples of the
absolute value of impedance and phase shift as a
function of frequency.
0,1
1
10
Frequency (Hz)
Fig. 2. The absolute value of the impedance as a function of the
frequency in a water tank with the conductivity 0.8 S/m
Phaseshift (degree)
Fig. 1. Carbon fibre sensor. The fibres (1.2 million) are seen inside
the net. The lead is connected to the fibre ends in an epoxy
coating.
0,01
0
-10
-20
-30
-40
0,001
0,01
0,1
1
10
Frequency (Hz)
Fig.3. Phase shift as a function of the frequency in a water tank
with the conductivity 0.8 S/m
4. SENSOR NOISE
Amplifier noise have been measured by putting a
10 ohms resistor at the input of the amplifier and
connecting a FFT-analyser to the output. The resistor
was exchanged with a pair of carbon fibre electrodes
placed in the earlier mentioned small tank. The result
from the measurement shows that in the interval 3
mHz to 1000 Hz it was not possible to see any
difference in noise between 10 ohms and the carbon
fibre sensor. At 1 mHz the sensor gave 5 dB more
noise than the 10 ohms. The conclusion is that in the
tank the sensor noise is determined by the amplifier
noise down to 3 mHz.
The sensor noise was also measured out in the
sea. The natural background field is strong and has to
be effectively suppressed. One way is to place two
pairs of electrodes in parallel near each other and
subtract the signals. What is left is noncoherent noise
consisting of amplifier noise, electrode noise, noise
from water waves and currents, misalignment of
sensors and other sources of noise.
For this test a 6 m plastic pipe was used. At each
end a pair of electrodes was attached with 1 m
electrode distance. This sensor system was deployed
at the depth of 37 m in the archipelago of Stockholm.
The result after subtraction of the two sensor signals
was a reduction of the background noise with 20 -27
dB in the range 0.1 - 10 Hz. This noise reduction can
also be used as an alternative measure of sensor
noise. More noise reduction means that the coherent
part of the signals is bigger. It also means that the
S/N ratio is good for both sensors. If one of the
sensors is getting noisier the coherency will decrease
and also the noise reduction. Therefore, it is possible
to make comparisons at different occasions to see if
something have happened to the sensors.
The remaining noise level lies 5 - 14 dB over
amplifier noise. In Fig. 4 curves are drawn for
amplifier noise, sensor noise in a tank, the raw signal
and the subtracted signal from the sea measurement.
Fig. 5 shows examples of time series and spectra for
raw signals and subtracted signals. Furthermore, the
coherency is calculated and it is very high.
Sensor systems have been in use since September
1995. During this time they have also been out of the
water for some week. In May 1997, after 1½ year
they were still working without any degradation of its
performance.
Voltage dBV/(Hz)½
-100
-110
-120
-130
-140
-150
-160
-170
-180
-190
-200
0,001
C in water (1 m)
C in water, diff (1 m)
C in a tank
amp-noise (10 ohms)
0,01
0,1
1
10
100
1000
Frequency (Hz)
Fig. 4. Amplifier noise with 10 ohms at the input and sensor noise in a water tank with the conductivity 0.8 S/m coincide with one exception
for 1 mHz where sensor noise is higher. The uppermost line is an example of background noise in the archipelago of Stockholm with the
electrode distance 1 m. The water depth was 37 m and the conductivity 0.8 m/s. The middle line shows the noise level after subtracting
signals from two parallel closely placed sensors.
Fig. 5. Background noise for two parallel, closely placed, sensors in the archipelago of Stockholm. The electrode distance was 1 m, the water
depth 37 m and the conductivity 0.8 S/m. Amplifier gain was set to 80 dB and high pass filter to 0.1 Hz. (a) and (b) show time series and
spectra. In (c) the signals from the two sensors have been subtracted. (d) shows the coherency between the sensors. Note that the scale
interval is 0.9 to 1.
5. RAPID DEPLOYMENT
The most important aim for developing the carbon
fibre sensor was to get a low noise sensor that could
be used immediately after deployment. To test this
quality we used the earlier described 6 m pipe with
two sensors. Fig. 6 shows raw signal (upper) and
difference or subtracted signal (lower) 15, 30 and 120
minutes after deployment. A 80 dB ac-amplifier with
a high pass filter at 0.1 Hz was used. After 120
minutes the sensor has reached full performance. At 1
Hz the noise reduction is 27 dB. Already after 15
minutes the corresponding figure is 20 dB. The
conclusion is that the carbon fibre sensor is
satisfyingly working after 15 minutes and it is in full
function within 2 hours.
Fig. 6. Electric fields (dB re 1V/m) in water as a function of frequency (Hz). Raw signal (upper) and subtracted signal (lower) after 15
minutes (a ), after 30 minutes (b) and after 2 hours (c).
6. APPLICATIONS
In connection with measurements of the induced
electrical field across a narrow channel caused by
moving water, at the West Coast of Sweden, carbon
fibre electrodes were used [4]. One of the goals was
to compare carbon fibre electrodes with Ag/AgCl
electrodes. The electrodes were connected to a high
impedance multimeter. The result confirmed that
carbon fibre electrodes can not be used for
frequencies much lower than 1 mHz. The tide that
was easily detected by the Ag/AgCl could not be seen
at all by the carbon fibre sensor. It was possible to
use the carbon electrodes as a, low noise, reference
sensor to a Ag/AgCl sensor to reduce mutual electric
field noise for frequencies where the coherency was
good.
Triaxial carbon fibre sensors are in use at a
magnetic range station to measure electric field ship
signatures. Se Fig. 7.
In the future carbon fibre electrodes can be used
as alternatives to zinc and Ag/AgCl in applications
where their is now demand for dc-measurements. The
following list gives some applications.





near field sensor at ranges
buoy-sensor
surveillance sensor
mine sensor
rapidly deployable sensor
7. SUMMARY AND CONCLUSIONS
Carbon fibre electrodes was developed and tested.
The problem with connecting the fibres to a lead have
been solved. They can be stored dry without any
special arrangement and be deployed in any water
independent of the salinity. The sensor can be used
within 15 minutes and its performance is very good
within 2 hours. Currently, the experience is that they
are still in full function after 1½ year in water. The
operational frequency range is down to 1 mHz. The
upper frequency cut-off is not tested but is higher
than 3 kHz. Under 1 mHz the behaviour is limited by
the increasing impedance. The electrode noise is
lower than the amplifier noise. It is therefore
important to construct amplifiers with even lower
noise, especially at frequencies below 10 Hz. It is
important to continue long time tests to see if they are
degrading and if biologic activity may influence the
performance.
8. PATENTS
The international patent application PCT SE 96 /
01202 has been launched, for the carbon fibre
electrode sensor, with priority demanded from the
date of the Swedish patent application 950325-4
launched September 9, 1995. Inventor Anders Brage,
Applicant National Defence Research Establishment.
9. REFERENCES
[1] D.J.G. Ives, G.J. Janz, Reference Electrodes. New York and
London: Academic Press, 1961.
[2] J.H. Filloux, Instrumentation and experimental methods for
oceanic studies, in Geomagnetism, vol.1, edited by
J.A.Jacobs. San Diego: Academic, 1987, pp. 143-248.
[3] G. Petiau and A. Dupis, ”Noise, temperature coefficient and
long time stability of electrodes for telluric observations,”
Geophysical Prospecting, 1980, 28, pp. 792-804.
[4] L. Crona, T. Fristedt, P. Lundberg and P. Sigray, to be
published.
10. BIOGRAPHIES
Lennart Crona was born in 1944 in
Stockholm, Sweden. He received the
M.Sc. in 1971 in engineering physics
from the University of Uppsala, Sweden.
Since 1970, he has been employed by the
Defence Research Establishment in
Stockholm, where he has been engaged in
research and development of electric
sensor systems.
Anders Brage was born in 1946 in
Motala, Sweden. He received the M.Sc. in
1987 in Material Science from the Royal
Institute of Technology in Stockholm,
Sweden. Since 1979 he has been
employed by the Defence Research
Establishment in Stockholm. He works at
the Department of Materials.
Fig. 7. Triaxial carbon fibre sensor. Electrode distance 1 m.