This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI 10.1109/TIM.2021.3102681, IEEE
Transactions on Instrumentation and Measurement
1
High Precision Capacitive Sensors for
Intravenous Fluid Monitoring in Hospitals
Uzma Salmaz1, M A H Ahsan2, Tarikul Islam1*, Senior Member, IEEE
Abstract— Automatic detection of the presence of intravenous
fluids (IV) and measuring their drip rates are essential for
intelligent health care applications. Conventionally, the drip rate
is monitored manually by a gravimetric method which suffers
from several drawbacks. Some smart commercial drip systems,
which mostly work on the optical technique, have inaccuracy due
to external light interference, temperature variation, and
misalignment of a photodiode and photodetector. The main
motivation of the present work is to investigate the usefulness of
the capacitive sensors for the non-contact detection and
measurement of flow rates of IV fluids. There is no previous work
on IV drop detection by the capacitive sensors. This paper
investigated three types of the capacitive sensors such as a crosscapacitive, a semi-cylindrical and a planar parallel plate to detect
the presence of IV droplets in the fluid pipe nondestructively. The
sensors are specially designed to fulfil the application needs,
simulated and fabricated with inexpensive double side copper clad
flexible PCB substrates. Experiments are conducted with the
sensors for four IV fluids typically used in hospitals to determine
their response parameters such as precision, drift, and drop rate
when the droplet passes through the inner electric field of the
sensors. There is an instantaneous change in the value of
capacitance due to sudden change in the dielectric constant of
partially filled air medium. The distinctive capacitance peak
enables to count the droplets, and drip rate (0.4 m/s crosscapacitive) with highly precise (0.036%), and drift-free readings.
All the sensors can be used for the target application but the cross
capacitive sensor has single dimension accuracy.
Index Terms— Capacitive sensors, IV fluids, droplet detection,
response parameters comparisons.
I. INTRODUCTION
In medical health care, administering intravenous fluids (IV) is
a cost-effective method that plays an essential role in
maintaining fluids in the body of a patient in case of medical
emergency [1]. Typical uses of intravenous injection are fluid
replacement for rehydration, blood transfusion, maintaining the
balance of electrolytes in the body, quick delivery of medicines
that are either ineffective or take a long time when administered
orally [2-4]. Due to increasing number of patients, a nursing
staff has to attend to several patients to monitor their health
conditions and maintaining fluid is one of the essential
requirements. IV fluids with correct dose and correct rates
should be administered to achieve the treatment goal. An
overdose can cause fluid overload, elevation of blood pressure,
electrolyte imbalance, and too fast an administration can led to
pooling of fluid in the lungs, chances of kidney failure, diabetic
emergency, and raised intracranial tension [6]. The rate of flow,
type of fluid injected, fluid in the bag, and the number of units
emptied are all critical to monitor [7-8]. Due to excess work
This work is supported by fellowship from Council of Scientific and Industrial
Research (CSIR), Human Resource Development Group, New Delhi, India
with ACK. No.- 1431086/2K19/1 and FILE NO. -09/466(0246)/2020-EMR I
(Corresponding author: Tarikul Islam)
pressure of service providers, there is a possibility of wrong
administration of fluids which lead to complication in patient’s
conditions. Hence, there is a need to develop an online system
that can monitor and alert medical attendants to take measures
in correct time.
The most common method is the gravimetric sensor which
monitors the drop rate and the remaining drug volume. But it
has some drawbacks, such as inaccurate calculation of drip rate
and volume when the solution in the bottle is small. One of the
most common methods is the optical technique using an IR
sensor installed around a drip chamber [9]. As the flow rate
deviates from its set value, an alarm rings to indicate the nurses
[10]. The optical sensors are relatively expensive, prone to
physical damage, and cause an error due to external interference
of light, temperature variation, and misalignment of devices. In
[11], an optical sensor is used to detect the presence of droplets
and the capacitive sensor is used to detect the fluid level in the
bag. It also consists of a microcontroller and GSM module for
sending alert signals. A capacitive MEMS ultrasound sensor is
used in [12] to measure the volumetric flow rate (0.05 ml/m) of
IV fluids. A noninvasive microwave-reflectometry system is
developed for automatic control and real-time monitoring of the
flow and liquid level [13]. A flexible sensor integrated with
cloud based bidirectional hetero-associative memory (BHAM)
network warning tool is reported to detect infiltration and blood
leakage [14]. In [15], a strain gauge load cell attached to the
fluid bag, and in [16], a flexible MEMS piezoresistive sensors
attached to the fluid tube is reported. However, the vibration
generated weight measurement error is reported to be about ±10
gm for the 30-40 gm bag, which is quite large. Also, the
resistive sensors being nonlinear devices require regular
calibration and consumes significant power. Lee et al. have
reported a simple and flexible coplanar capacitive sensor
integrated with wireless communication module attached at the
bottom of the bag to measure its level [17]. A RFID tag is
attached to the IV bag to monitor status of fluid bag [18] but the
device is not accurate, relatively expensive, and its
implementation is not easy. Nowadays IoT enabled drip
monitoring system is reported in the literature.
The capacitive sensors are excellent devices for measuring
different physical and chemical parameters. Essential features
of the capacitive sensors are the possibility of non-contact
measurement with high precision and reliability, low cost, high
sensitivity, and low power consumption. Different types such
as parallel plate, coaxial cylindrical, coaxial cross-capacitor,
and coplanar interdigital capacitors can be used for sensing
applications [19]. Parallel plate capacitive sensor is reported to
measure liquid level IV bag but it has demerit of capacitance
U. Salmaz and T.Islam are with the Department of Electrical Engineering,
Jamia Millia Islamia (A Central University), New Delhi 110025, India. (e-mail:
uzmasalmaz2015@gmail.com; tislam@jmi.ac.in).
M A H Ahsan is with the department of physics, Jamia Millia Islamia (A Central
University), New Delhi 110025, India. (e-mail: mahsan@jmi.ac.in).
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variation due to change in geometrical parameters. On the other
hand, the cross-capacitor proposed by Thompson and Lampard
consists of four concentration electrodes separated by small
insulating gaps effectively can be used to precisely monitor the
droplets in IV fluids because of its single-dimensional accuracy
[20-21].
The main motivation of the present work is to investigate the
usefulness of the capacitive sensors for the non-contact
detection and measurement of flow rates of IV fluids. There is
no previous work on IV drop detection by the capacitive
sensors. Three capacitive sensors such as a cross-capacitive, a
semi-cylindrical and a planar parallel plate are designed,
simulated and fabricated with double side copper clad flexible
PCB substrates. The design and fabrication of the sensors are
simple, rugged, and inexpensive but adequate for the said
purposes. Working principles are explained in section II,
fabrication of the sensors and experimental methods are
explained in section III and IV respectively. Experimental
results are discussed in section V and the paper is concluded by
section VI.
II. WORKING PRINCIPLE OF THE SENSORS
Three types of the capacitive sensors are used for the detection
of IV fluid drops. One capacitor is a cylindrical coaxial crosscapacitor which is based on the Thompson and Lampard
theorem. The other one is a semicircular parallel plate capacitor,
and the third one is a circular planar parallel plate capacitor. The
working of the individual capacitor is explained below.
A. Parallel plate Capacitive sensor
Fig. 1(a) shows the schematic diagram of the parallel plate
capacitor. The structure consists of two circular parallel plates
made on double side copper clad PCB. The positive electrode
(high potential, H.P) is of 8 mm diameter which is guarded by
a guard ring of inner diameter 8.6 mm and thickness 2.4 mm.
The gap between the H.P electrode and the guard ring is 0.6
mm. The circular low potential (L.P) electrode of diameter 14
mm is separated from the H.P electrode by a distance of 9 mm.
Each electrode is shielded by grounded metal shield. The
electrodes are made on a rectangular PCB of dimension 40 mm
× 18 mm by chemical etching. If the dielectric medium between
the electrodes is free space, the capacitance of the sensor is
given by
A
(1)
Cp  0
d
where 𝜀𝑜 is the permittivity of free space, A is the area of the
circular plate (H.P), and d is the distance between the two
plates. If the space of the capacitor is completely filled up by a
dielectric medium  r , the capacitance value is given by C P  r
.Now consider a small spherical liquid droplet of IV fluid of
radius r (r<<d) and of dielectric constant  r is placed at the
centre of the capacitor as shown in Fig. 1 (a).
The free space is now partially filled by a dielectric sphere of
the droplet and if the sphere is not close to either of the
electrode, then the capacitance value changes from its initial
free space value C . The change in capacitance value due to
partially filled condition is given by [23,31].
C  40
Metal
r3 r 1
d 2 r  2
(2)
Insulator
Drop
εr
d= 9 mm
(a)
(b)
Fig. 1. Circular parallel plate capacitor (a) schematic of the sensor with droplet
(b) electric field distribution when a droplet is placed symmetrically at the
center.
This equation is derived for a planar parallel plate capacitor for
a spherical drop but it is valid for the electrodes of any shape
provided the electric field E due to excitation in the space is
uniform in the region which is larger than the region occupied
by the droplet at the position of the drop [23].
If the separation between the plates is 9 mm then for a water
droplet of radius 2.3 mm, the theoretical change in capacitance
determined using (2) is 17 fF. The structure in Fig. 1(a) is
designed using ANSYS Maxwell 3D software. The electrostatic
solution is used for analyzing the design of the structure.
Triangular mesh is used to design the model due to its high
accuracy and less simulation time. The gap between the
electrodes is 9 mm, and diameter of the H.P and L.P electrodes
are 8 mm and 14 mm respectively. The electrodes are made of
copper, and an electrostatic shield is provided to eliminate the
effect of external fields. The guard and electrostatic shields
were grounded. Fig. 1 (b) shows the electric field distribution
of the sensor. The capacitance values for air and with water
droplet (with dielectric permittivity 81) are found to be 61.7 fF
and 70.7 fF respectively.
B. Semi-cylindrical capacitive sensor
The structure of the semi-cylindrical sensor as shown in Fig.
2(a) consists of two semi-circular plates in the form of coaxial
cylinder. The basic difference between the parallel plate
capacitor and the semi-circular capacitor is that the gap between
the electrodes in the former structure is fixed but in the semicircular structure it is varying. The electrodes are excited by
potential of V and the total charge on an electrode is Q. The
curved surface electrode can be represented by n number of
distributed parallel plate capacitors with varying gaps between
the electrodes. The electric field for the capacitor with air can
be written as
Q
(3)
E
R1 h 0
where R1 is the radius of the electrode and h is the length of the
electrode. The elemental separation between AB, is L =
2R1sinϴ + g, where ϴ is the angle between the radius and the
horizontal plane of the curved surface and varies from 0 to π, so
the change in elemental length along the surface
dL  2R1 cosd . So, the potential difference can be written
as

V
Q
Q
Q
g  2R1 sin 
g
2 R1 cosd 
R1h 0
R1h 0
R1h 0
0
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capacitance value per unit length (C) for free space (  0 ) is given
The semi-circular capacitance with air is given by
by [21].
R1 h 0
Q
for 0    
(4)

V g  2 R1 sin  
In the present application, the electrodes are placed on an
insulating hollow plastic tube of IV fluid of thickness t and
dielectric constant  p as shown in Fig. 2(b). If the thickness
C  0
C sc 
t<<R1, then the capacitance value including the thin insulating
tube will approximately be given by (4). When a tiny spherical
liquid drop of IV fluid of radius r is placed at the centre of the
capacitor shown in Fig. 2(a), it causes partial filling of air space
by a dielectric liquid of permittivity  r . This situation is similar
to the situation as observed in the parallel plate capacitor. So,
the capacitance value increases partially due to the liquid
droplet with respect to the capacitor with air.
Metal
Insulator
Axis
V/2 Electrode
Droplet
A
R
θ
θ
A
εr
Drop
B
g gap
g
R
Electrode
Electrode gap
Rsinθ
B
Rsinθ
-V/2
(a)
(b)
(c)
Fig. 2. Semi-cylindrical parallel plate capacitor (a) schematic of the sensor with
droplet (b) actual sensor (c) electric field distribution when a droplet is placed
symmetrically at the center.
The change in capacitance is given by [23]
C  4 0
r 3  r 1
2
R1  r  2

F/m
(6)
So, the capacitance value depends on the length of the electrode
only. However, in the present sensor, the flexible electrodes are
mounted on a hollow insulating tube which is an IV drip
chamber made of polymer material (black color) as shown in
Fig. 3(a). The outer radius of the chamber is R2 and its inner
radius is R1. Therefore, the above equation is modified by
including a parameter k due to the chamber placed within the
electrodes of the capacitor. The value of k is constant and
depends on the material of the insulating tube. The modified
equation is given below
ln 2 F/m
(7)
C  k 0

where, 𝑘 = (1 +
IV pipe
ln 2
2 −1
𝜀𝑝
2𝜀𝑝 𝑙𝑛2
2
𝑅
3
3
𝑅
𝑅
{1+( 2 )4 }⁡{1+( 1 )2 }2 ⁡
𝑅3
𝑅3
𝑅
{⁡1+(𝑅2 )2 } {1⁡+( 1 )4 }
𝑅
ln⁡[
])
𝜀𝑝 is the dielectric constant of the tube material. The above
equation (7) is valid if the ratio of R2  1 and the dielectric
R3
constant of the insulating tube is below 5. The detailed
derivation of the cross-capacitor value for the structure shown
in Fig. 3 with a hollow insulating tube of Teflon is reported in
[22]. In the proposed work, the sensor was fabricated using
double side copper clad Upilex 50S polyimide substrate
procured from UBE, Japan. The thickness of the polyimide is
50 µm and the thickness of copper film is 12.5 µm. So, the
values of R1, R2, and R3 are measured to be 7.2 mm, 8 mm, and
8.0125 mm respectively. Therefore, by substituting R1, R2, and
R3
(5)
In practical capacitors, the electrodes are shielded by the
external metallic shield and there is a guard electrode on each
end of the electrode. The electric field distribution of the
capacitive sensor is shown in Fig.2. (c). The structure in Fig.
2(a) is simulated by ANSYS software with geometrical
parameters: inner diameter 16 mm, length of electrode 20 mm,
the guard length 4 mm. A conducting cylinder of diameter 16
mm and thickness 0.0125 mm is divided in two halves through
the center with an insulating gap of 1 mm. The electrodes are
excited by  1 V. The simulation procedure of the sensors is the
same as described earlier, the only difference is the sensor
structure. The simulated capacitance value with air was 894.72
fF.
C. Cylindrical cross-capacitor
Fig. 3 (a) shows the schematic diagram of the cross-capacitor
having four electrodes A, B, C, and D separated by small gaps
and enclosed in a conducting shield. R2 and R3 are the inner and
outer radius of the electrodes. CAC and CBD are the crosscapacitances across the opposite faces of the electrodes without
insulating tube. If the capacitances C AC  CBD  C , then
R2
 1 , R1  8.0125  0.8985and 𝜀𝑝 = 2.1 for polypropylene,
R3
R3
7.2000
the value of k is found to be 1.01. The necessary conditions for
the modified expression are valid. Now if a spherical IV fluid
drop of radius r is placed at the centre as shown in Fig. 3(a), the
space is partially filled with dielectric medium of permittivity
 r . This situation is again similar to the condition as mentioned
for other two capacitors. The shape of the electrode of the cross
capacitor is cylindrical with separation between two crosselectrodes d = 2R1 is much greater than the radius of the drop r.
The change in capacitance can be given by (5). From (6), the
capacitance value for a 2 cm long electrode is 39.07 fF. On
including the k value because of the presence of the drip
chamber, the value of capacitance becomes 39.46 fF. So, k will
have a small effect on the overall capacitance value, provided
its thickness is small. However, in practice two of the
diagonally opposite electrodes are connected to the ground
terminal and the capacitance between the other pair of electrode
CBD is measured. The cross-capacitor then can be equivalently
represented by a pi network shown in Fig. 3(b).
according to the Thompson and Lampard (TL) theorem, the
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Insulator
A
B
R2
R1
εr
Drop
D
CBD
R3
A
D
.
IV pipe
B
C
C
(a)
(b)
Fig.5. Photograph of the cross capacitive sensor (a) front side, (b) backside side.
Electrode gap
(a)
(b)
(c)
Fig. 3. Cylindrical cross-capacitor (a) schematic of the sensor with droplet (b)
equivalent circuit (c) electric field distribution when a droplet is placed at the
center.
Recently, the cross-capacitor is used in different sensing
applications including quantification and identification of
liquid droplets of various sizes [24], precise measurement of
humidity [25], dielectric constants of liquid samples [26], and
metal debris detection in lubricating oil [27]. The cross sensor
is designed using finite element software ANSYS Maxwell 3D
software with the geometrical parameters R1 = 7.2 mm, R2 = 8
mm, R3 = 8.0125 mm, the length of the electrode 20 mm and
the gap between the electrodes 1 mm. Fig. 3 (c) shows the
electric field distribution of the cross capacitive sensor. The
cylinder is made of copper. Voltage excitation was applied to
the B & D electrodes (±1V), and the electrodes A & C including
guard electrodes and outer metal shield are grounded. The
capacitance values between B and D electrodes for air and water
droplet (with dielectric permittivity 81) are found to be 39.4 fF
and 44.2 fF respectively. The value with air is approximately
equal to the theoretically value.
III. FABRICATION OF THE CAPACITIVE SENSORS
A. Cross and cylindrical capacitive sensor
The cross-capacitive sensor shown in Fig. 4(a) was fabricated
on flexible double-sided copper-cladded polyamide substrate
(Upilex, Japan) of size 50 mm × 24 mm. The polyamide sheet
was thoroughly cleansed with acetone and DI water and then
ultrasonicated for 15 m at 60 ℃. The mask of each sensor was
designed using AutoCAD software, and it was then screen
printed on the polyamide substrate. The size of the electrode
was 8 mm θ 20 mm and the size of the gap was 1 mm. Two
guard electrodes of width 8 mm were made on each side of the
main electrodes with a gap of 1 mm. The screen-printed
substrates were then immersed in ferric chloride solution to
form the structures shown in the Fig.4 (a). The metal film on
the opposite side of the substrate was masked to avoid chemical
etching. The structure was then cleansed thoroughly. The two
edges of the polyamide sheet were connected by wrapping it on
the IV plastic chamber of diameter 16 mm.
Fig. 5 shows the image of the sensor. The sensor's terminals,
including the guard and shield electrodes, were adequately
connected to the AD7150 board to measure capacitance values.
A schematic of the semicylindrical parallel plate sensor is
shown in Fig. 4 (b). The structure was having two identical
electrodes made on the flexible PCB of size 20 mm × 24 mm.
The gap between the electrodes was 1 mm. The structure was
then wrapped on the outer wall of the IV chamber of diameter
16 mm with main electrodes on the inner side to form the semicircular capacitor. Two electrodes A and B were connected to
the evaluation board to measure the capacitance and the metal
shield and guard electrodes were connected to the ground
terminal of the board.
B. Circular Parallel plate sensor
To fabricate a circular parallel plate capacitive sensor shown in
Fig. 6(a) firstly the double-sided copper-cladded PCB was cut
in the dimension of 20 × 40 mm. The PCB was cleaned with
acetone and DI water and ultrasonicated for 15 m at 60 ℃. The
circular mask was made using AUTOCAD software with
dimensions and structure mentioned in the Fig. 6(b). The mask
was printed on the PCB. The electrodes are etched out using
ferric chloride solution. The sensor was then cleaned properly.
The backside of the PCB was kept intact for the shield. The
guard ring was provided to the H.P to reduce the fringing field
effect. Two parallel plates of the capacitor were separated with
by a gap of 9 mm. Terminals of the H.P and L.P including the
guard and metal shield were connected to the evaluation board
AD7150 for the measurement of the capacitance value. The
dispensing unit was placed such that liquid drops passes
through the middle of the air gap between the electrodes.
20 mm
18 mm
18 mm
8 mm
Front side of
PCB
Front side of
PCB
40 mm
Metal
Back side of
PCB
Back side of
PCB
(b)
(a)
(c)
Fig. 6. Parallel plate sensor for drip monitoring (a) experimental setup (b)
schematic of the circular capacitor (c) photo of the sensor.
IV. EXPERIMENTAL SETUP
(a)
(b)
Fig. 4. Schematic of the fabricated sensor (a) the cross capacitive sensor. (b) the
semi-cylindrical capacitive sensor.
The experimental setup consists of a tall stand with clamps to
hold the IV-fluid bag. The drip chamber of the IV tube is
connected to the bag. Then, all four electrodes are connected to
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the AD7150 board, as shown in Fig. 7 (a, b). Two diagonally
opposite electrodes were connected to the ground terminal. The
other two electrodes were connected to the Cx pin and EXC pin
respectively. An AD7150 evaluation board (EVALAD7150EBZ), is a capacitance-to-digital converter (CDC)
having two channels to connect two sensors simultaneously
[29]. Experiment was conducted with each sensor separately, so
one port was connected to the PC for data acquisition and the
other port was disabled. With the proper settings of the port,
data was acquired using data acquisition software provided by
the manufacturer. The board has different capacitance
measurement ranges. Since the capacitance value of each sensor
was small and the change in capacitance was of few tens of fF,
the minimum range of 0-0.5 pF and highest resolution 1 fF in
continuous adaptive mode was selected. The conversion time
for each port is 10 ms. The average of about 4 to 5 data points
can be acquired using the AD7150 evaluation board in 50 ms.
The interfacing circuit and the photograph of the setup are
shown in Fig. 7(a), and Fig. 7(b) respectively.
card and stored in the datasheet. The capacitance value from the
digital data (Data) was determined using the given expression
[29]
Data  12288
(8)
C
 CR
40944
where CR is the input capacitance range (0-0.5 pF).
The capacitance values corresponding to the droplets was
acquired for 10 s for cross and semi-cylindrical capacitive
sensors and 1500 ms for parallel plate sensor. The drop rate was
kept different for each sensor to ensure rapidness of droplet
detection. Variation of the capacitance values repeated for
several identical drops at different rates are shown in Fig. 8, 9
and 10 for cross, semi-cylindrical and parallel plate capacitive
sensors respectively.
Fig. 8. Capacitive peaks with cross capacitive sensor for NS IV fluid.
(a)
IV bag
Ad7150 Board
Connected to system
Sensor
AD7150
board
IV tube
Flow
regulator
It is observed that initially, the capacitance value is at reference
base value without fluid. The base value of each sensor is
different due to different expressions and geometrical values.
The moment the droplet passes through the inner chamber, the
capacitance value rises sharply, and then returns to the base
when the droplet goes out of the chamber. Almost a similar
capacitance peak is observed for each droplet.
Beaker
(b)
Fig. 7. (a) Interfacing circuit. (b) photograph of the experimental setup.
V. RESULTS AND DISCUSSION
Experiments were conducted to determine the following
parameters
A. The variation of capacitance value with IV fluid
droplets
B. Repeatability of the sensor output
C. The speed of the droplet.
D. Drift in sensor output
E. Comparative study of the sensors
.
Fig.9. The capacitance peaks with a semicylindrical capacitive sensor for NS
IV fluid droplets.
A. The variation of capacitance with droplets
i.
Capacitive response of the sensors for NS IV fluid
Initially, the experiment was conducted with the cross
capacitive sensor with NS (0.9% w/v Sodium Chloride in 100
ml of water) IV fluid sample at controlled room temperature of
25°C. The droplets of NS fluid were passed through the tube
and chamber. The digital data corresponding to the actual
capacitance values was acquired through the data acquisition
Fig.10. The capacitance peaks with circular parallel plate sensor for NS IV
fluid droplets.
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Capacitance peaks for each droplet are visible and repeatable.
It was expected that peaks for the identical droplet for each
sensor should be exactly identical but there is a minor deviation.
This may due to the facts that for cross and semi-circular
capacitors experiments were conducted by mounting the
sensors on the outer wall of the IV fluid tube. The wall causes
series capacitance effect which reduces the overall sensitivity
of the sensors. Also, leads have fluctuating contribution to the
small value capacitance of the sensor. But for parallel plate,
droplets were directly allowed to pass through the inner space
of the chamber so the series capacitance effect is neglected
except the lead capacitance effect. However, one intriguing
phenomenon is observed both in Fig.8 and 9. When the drop
enters the tube, the capacitance value goes below the base value
and then rises to the peak value. All the three capacitors have
guard electrodes which were maintained at ground potential.
The electric field lines go from the positive electrode to the
negative electrode including the guard electrode. Before falling,
the drop is attached to the bulk liquid, and elongates in the
vicinity of the guard electrode. Also, the shape of the dispensed
droplet fluctuates and passes through the sensor with certain
speed. Even long jets can occur, depending on the ejection
characteristics of the liquid. The droplet then bypasses some
electric field to the ground causing less field to the negative
electrode. So, the accumulation of charges to the negative
electrode is less causing reduction in capacitance value from the
base value. But when the droplet detaches from bulk liquid and
passes away through the guard electrode, more electric field
reaches to the negative electrode causing increase in
capacitance value and it becomes maximum at the middle of the
electrodes. By the time droplet also becomes spherical. This
effect may be more if the nozzle of the drop is close to the guard
electrode. In case of parallel plate capacitor, guard electrode is
quite away from the nozzle and there is no insulating layer
between the droplet and the electrodes, so this effect is less
pronounced as shown in Fig. 10. However, we need to
investigate this negative peak phenomenon in more detail
considering all these parameters [30,31].
ii.
Capacitances change with different Intravenous fluids
The experiments were conducted with three others commonly
used IV fluids such as DNS (Sodium Chloride 0.9% w/v and
Dextrose 5% w/v in 100 ml of water, B. Braun), 5D (5% w/v
Dextrose in 100 ml of water, B. Braun), and RL (sodium
chloride 6 g/L, sodium lactate 3.1 g/L, potassium chloride 0.3
g/L, and calcium chloride 0.2 g/L, B. Braun). The variation of
the cross-capacitance of all the four different IV fluids is shown
in Fig. 11.
The fabricated sensor can detect the presence of intravenous
fluid in the IV bag, and it is also capable of delivering the
change in capacitance value for different IV fluids. The average
peak heights of drops are 5.65 fF, 6.39 fF, 5.29 fF, and 5.68 fF
for NS, DNS, 5D, and RL intravenous fluids respectively.
However, the difference is minor because of the significantly
less concentration of salts present. The average base value of
the sensor is 55.15fF. The averaged capacitance value for six
consecutive droplets from each sample are shown in Table I.
TABLE I
PEAK CAPACITANCE VALUES OF DIFFERENT IV FLUIDS
Type of fluid
NS
DNS
5D
RL
Average peak capacitance value
60.80
61.54
60.44
60.83
B. Repeatability
Repeatability is the property of a measuring instrument that
gives the same result for consecutive measurements. But the
experiments are performed with the same input, environmental
conditions, apparatus used, and experimenter. Data of each
sensor for the NS intravenous fluid sample is analyzed to test
the repeatability of the sensor output. The repeatability is given
by [24]
  % 
 C1  Cm 
2
  C2  Cm    C3  Cm   ..................   Cn  Cm 
2
2
2  n  1 Cm
2
100
(9)
Where ℜ is the repeatability index, 𝐶𝑚 is the mean capacitance,
𝐶1 𝐶2 𝐶3 …….𝐶𝑛 are obtained capacitance values at the peak,
and n is the number of readings.
TABLE II
REPEATABILITY OF THE SENSOR OUTPUT FOR NS IV FLUID
Drop
numbers
1
2
3
4
5
6
Repeatabi
lity (%)
Cross
capacitive (fF)
60.80
60.93
61.03
60.77
60.83
60.91
0.036
Semicylindrical (fF)
960.14
959.75
960.14
960.23
960.14
959.31
0.008
Circular
parallel (fF)
96.35
96.32
95.98
95.51
95.13
95.89
0.11
The repeatability of the sensor as calculated using (9) is
approximately 0.036%, 0.015%, 0.026%, and 0.055% for NS,
DNS, 5D, and RL IV fluids, respectively. The repeatability for
all three sensors is shown in Table II. Hence, the sensors'
readings are repeatable. The difference in the highest and
lowest peak value for the cross capacitive sensor is 0.26 fF, for
the semicylindrical capacitive sensor is 0.92 fF, and for the
parallel plate sensor 1.22 fF.
Fig.11. Sensor response with other IV fluids.
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Transactions on Instrumentation and Measurement
7
C. Drop speed
E. Comparative study of the sensors
Fig. 12. A single peak of NS fluid of the cross capacitive sensor.
Drop speed is the speed with which the drop passes through the
sensor. The length of the electrode was 20mm. Fig. 12 shows
the capacitance vs time plot of a drop for the cross capacitive
sensor. The corresponding time for the droplet to enter and
leaves the sensor is 0.98 s and 1.03 s, respectively. Hence the
drop takes 50 ms to travel between the electrodes of the sensor
after detaching from bulk solution. Therefore, droplet fall freely
at the speed of 0.4 m/s. Experiments were also conducted with
variation of the speed of the droplets keeping the size of the
droplet same. The speed was varied in the range of 0.148 to 4 ×
10-3 m/s but the capacitance value varies from its mean value
(60.63) by ±0.19 fF. So, the capacitance change remains almost
same.
D. Drift
Drift is the gradual decrease in performance of the sensor over
a period of time. The experiment was conducted with the crosssensor with NS IV droplet for a month to observe the drift in
the output. The average peak capacitance and average nominal
capacitance values for the month of February are shown in
Table III. Results show that the capacitance values are quite
stable for a month. The maximum deviation from the mean
value is 0.96%
The value of capacitance for cross capacitive sensor only
depends upon the length and does not depend on radius or any
other dimension. It is also easy to install on an IV pipe. The
semi-cylindrical electrodes are also easy to install on the pipes
but depends on geometrical parameters. It depends on the inner
radius of the chamber, length of the electrode, the gap between
the electrode, and angle between the radius and horizontal plane
of the curved surface. The parallel plate sensor is not easy to
install on curved surfaces. The proposed sensors are highly
repeatable, precise, and drift-free. The capacitive sensors are
advantageous over other types of sensors. It has a simple
structure, easy fabrication, unsophisticated in design,
installation, and maintenance. The calculated, simulated, and
experimental values with the free space with typical dimensions
of the sensors are given in Table IV. Table V consists of the
literature survey for the proposed work.
Theoretical values for cross and semi-cylindrical capacitive
sensor are more than simulated values because the k factor is
included in the formula for the presence of IV chamber between
the electrodes. The experimental values for all the sensors are
larger than the simulated and calculated values because of the
presence of fringing capacitance and lead wire capacitance.
The sensitivities of the sensors to NS IV fluid are determined
from the response curve and are shown in Table VI. The table
shows that the parallel plate sensor has the highest sensitivity.
This is because the droplets are directly allowed to pass through
the electrodes and the capacitance value does not have series
capacitance effect due to plastic wall of the chamber. The
repeatability index (ℜ) shows the extent to which the data can
exist around the mean. A value less than 1 indicate good
repeatability of data. The repeatability index of the sensor is the
measure of its precision. Hence, we can say that the proposed
sensors are highly precise.
TABLE IV
AIR CAPACITANE VALUES OF THE SENSORS
Type of sensor
Theoretical
value (fF)
Simulated
value fF)
Experimental
value (fF)
Cross capacitive
Semi-cylindrical
capacitive
Parallel plate
39.46
912.4
39.4
894.7
55.2
947.7
49.5
61.7
70.8
TABLE III
PEAK AND NOMINAL CAPACITANCE VALUE OVER A MONTH
Days
1 February
8 February
15 February
23 February
29 February
Average peak
capacitance (fF)
60.80
60.77
60.28
59.82
60.33
Average nominal
capacitance (fF)
55.40
55.42
54.61
54.28
55.13
TABLE V
LITERATURE SURVEY OF THE PROPOSED WORK
Reference
Structure
Expression
Contact
Fabrication
Proposed
work
Cross Capacitive
Non-contact
Easy
Semicylindrical
Parallel plate
simple and
exact
complex
simple
Non-contact
Contact
Easy
Easy
Piezoresistive
Optical
complex
complex
Non-contact
Non-contact
complex
complex
[7], [12],
[15]
[9], [11],
[18]
Repeatabili
ty ℜ (%)
0.036
0.008
0.11
-
Drift
Drift
free
-
Demerits
The low base capacitance value
Complicated theoretical relation
Not easy to install
Temperature error, consumes more
power and requires calibration
relatively expensive, prone to
physical damage, and error due to
external interference of light,
temperature
variation,
and
misalignment of devices
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Transactions on Instrumentation and Measurement
8
[13]
Microwave
sensing
complex
Non-contact
Simple
-
-
[14], [17]
Coplanar
complex
Contact
simple
-
-
TABLE VI
COMPARISON OF PROPOSED CAPACITIVE SENSORS
Sensor
Cross
Semicylindrical
Parallel
plate
Peak
value (fF)
C
C0
Drift
Repeat.
ℜ (%)
5.65
12.25
0.102
0.013
free
-
0.036
0.008
Drop
speed
(m/s)
0.4
0.4
25.09
0.354
-
0.11
0.4
VI. CONCLUSION
This paper presents the design, fabrication, and experimental
validation of a drip rate monitoring system using capacitive
sensors. Three sensors, such as a cylindrical cross capacitor,
semicylindrical capacitor, and circular parallel plates, are
fabricated on copper-cladded PCB. Experiments are conducted
with four IV fluids which are typically used in health care
applications. The sensors have shown a minimal variation for
different types of IV fluids because of the nearly identical
concentration of salt present. The sensors are used to count the
drops (number of peaks) and measure the drip-rate. All the
sensors are effective to monitor the drip rates (0.4 m/s) without
any contact to the medium and the outputs are precise and driftfree. The fabrication of the sensors is easy and inexpensive but
installation, precision and accuracy of the cylindrical cross
capacitor are better than semi-cylindrical and planar parallel
plate capacitors. Hence, cross capacitive sensor is best suited
for this application.
Acknowledgement: Experiments were conducted in the Sensors
and Instrumentation lab, Jamia Millia Islamia, New Delhi with
the facilities created from sponsored projects.
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0018-9456 (c) 2021 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.
Authorized licensed use limited to: University of Glasgow. Downloaded on August 15,2021 at 16:58:43 UTC from IEEE Xplore. Restrictions apply.
This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI 10.1109/TIM.2021.3102681, IEEE
Transactions on Instrumentation and Measurement
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Uzma Salmaz received the Bachelor and Master degree
in Electrical Engineering from Jamia Millia Islamia
(JMI) University, New Delhi, in 2014 and 2017
respectively, where she is currently working toward the
Ph.D. degree. Her research interests include
development of sensors, and electronic instrumentation.
M A H Ahsan completed his MSc (Physics) from IIT
Kanpur and received his PhD (Physics) in 1998 from
IIT Bombay, Mumbai. He subsequently carried out
post-Doctoral research work at Raman Research
Institute, Bangalore (Bengaluru) from 1998 to 2000 and
later at IISc, Bangalore (Bengaluru) from 2000 to 2002.
He was a Visiting Scientist at SISSA, Italy from
September 2004 to January 2005 and later at MartinLuther University, Germany from August 2008 to
August 2009.
He is recipient of DAAD Research and Study Fellowship. Since 2002, he has
been teaching at the Department of Physics, Jamia Millia Islamia, New Delhi
where he is currently a (Full) Professor. He has taught a variety of courses
including Electromagnetic Theory, Advanced Classical Electrodynamics and
Quantum Field Theory at the Undergraduate, Postgraduate and Pre-PhD level.
He has guided seven PhD theses. His broad area of research is Condensed
Matter Physics.
Tarikul Islam (M’16-SM’20) was born in
Murshidabad, West Bengal, India. He received the
M.Sc. Engg. Degree in instrumentation and control
system from A. M. U. Aligarh, U.P. in 1997 and the
Ph. D (Engg..) degree from Jadavpur University,
Kolkata, India, in 2007. From 1997 to 2006, he was
Assistant Professor and from 2006 to 2012, he was
Associate Professor with the Electrical Engg. Deptt.
Jamia Millia Islamia (A Central University), New
Delhi. Since, 2012, he is working as professor with same university.He has
over 20 years of teaching and research experiences. He has authored/coauthored 5 book chapters, one edited book, filed two Indian patents and
published more than 155 papers in peer reviewed journals and conferences.
He received research grants from government agencies like DST, DRDO,
MHRD, CPRI, DAE of more than 260,000 (US$). His research interests
include sensors and sensing technologies, electronic instrumentation. He is a
life member of IETE (India), ISTE (India) and senior member of IEEE. He is
a topical editor, IEEE Sensors journal and IEEE Trans. Instrum. & Meas.
He is a co-editor of a specialissue of an International Journal of Electronics.
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