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2.11.4 Ultrasonic Temperature Transducers:
Ultrasonic's, which are sound vibrations above 20.000 Hz. can be
useful when we are concerned wish rapid temperature fluctuations,
temperature extremes, limited access, nuclear, and other severe
environmental conditions and when we must measure the temperature
distribution inside solid bodies. The need to measure simultaneously the
distribution of parameters other than temperature (e.g., flow) may also
justify an ultrasonic approach. Ultrasonic's also offers possibilities of remote
sensing and sometimes can prevent penetration of the system (nonintrusive)
as shown in fig (2-43).
Fig (2-43) (a) Schematic and Oscillogram Illustrating Ultrasonic Temperature
Profiling. (y) Ultrasonic Thermometer
2.12 Humidity and Humidity Measurement:
Humidity is the measure of water vapour content in air or some
other gas. It may express in various forms such as absolute humidity,
relative humidity or dew point temperature.
Absolute humidity is the amount of water vapour actually present
in the air and is usually expressed in grams per kilogram or parts per
million.
Relative humidity (RH) is the best known and perhaps the most
widely used way of expressing the water vapour content. of air. Formally,
relative humidity is defined as the amount of the water vapour (or
moisture) actually present in the air to the maximum amount Water that
the air could possibly hold at the same temperature.
It is usually expressed a percentage, 100% relative humidity means
that air contains all the moisture that it can hold. The higher the
temperature of the air the more water vapour it can hold, so relative
humidity measurement needs knowledge of the temperature and of the
amount of water vapour that the air will hold at various temperatures.
Dew point is defined as the saturation temperature of the mixture at
the corresponding vapour pressure.
Dew point measurements are widely used in scientific and
industrial applications where precise measurement of water vapour
pressure is required.
Knowledge of the amount of water vapour in the air is very
important in the operation and/or control of many industrial processes. In
some cases the moisture contained in the ambient air is important; in
other cases the moisture contained in the product itself is more important
to the success of the industrial process.
Most methods used for measurement of humidity are based on the
fact that certain substances change their dimensions with the change in
humidity. Materials used for this purpose are human hair, animal
membrane, and some wood fibres. All of these show a variation in length
with the change in humidity but they are not sensitive to temperature
variation.
An hair hygrometer is shown in fig (2-44) Absorption of moisture
causes increase in length of the hair and the amount of moisture that it
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can absorb depends upon the relative humidity. Thus the extension of hair
is a function of relative humidity. A bunch of hair is used for increased
mechanical strength in the construction. The element is maintained at a
slight tension by a spring. Excess of stress on the hair element may lead
to a permanent set in it. Such as hygrometer is recommended for a range
of 25% to 95% relative humidity and in a temperature range of -18°C to
70° C, has to be calibrated before use. It is slow in its response and aging
affects its calibration and produces considerable drifts.
Animal membrane has a larger elongation in comparison to hair for
the same relative humidity but the calibration drift is also large. Wood
(apple) has also a good extension property particularly if it is cut at right
angles to grain orientation. But generally such devices are employed for
controlling the humidity at specific values, as their accuracy over a large
range is substantially poor.
Fig (2-44) Hair Hygrometer
Electrical type, humidity transducers are more suitable for
continuous recording and control of humidity. Some of the commonly
used electrical transducers are described below.
1. Resistive Hygrometers. Dunmore element and the Pope cell are the
two extensively used percent relative humidity (% RH) electrical
transducers.
The Dunmore sensor makes use of a bifilar-wound inert wire grid
on an isolative substrate coated with a lithium chloride (LICI) solution of
a controlled concentration. The hygroscopic nature of this salt makes it to
take up water vapour from the surrounding atmosphere.
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With the increase in relative humidity, the salt (lithium chloride)
film absorbs more and more water vapour from the air and so its
resistance decreases markedly. Thus the ac resistance of the transducer is
an indication of the prevailing % RH.
Bridge type resistance measuring circuitry with an a c excitation is
used normally as dc excitation polarizes the transducer, eventually
causing loss of calibration. The resistance-relative humidity relation is
quite non-linear.
Dunmore elements are excellent RH transducers but, because of the
characteristics of lithium chloride, are generally designed to cover a
narrow range of interest. For example, single transducer may cover from
40 to 60% RH and the transducer output is usable Ply in that range.
Where large ranges (as large as 5 to 99% RH) are required, cluster
narrow-range transducers, each designed for a specific part of the total
range, are combined in a single package. The arrangement, however,
results in a rather bulky transducer.
A single narrow-range Dunmore element may have an inaccuracy
of the order of 1.5% relative humidity, resolution about 0.15% relative
humidity, time constant as small as 3 seconds. These are usually of the
size of 25 mm in diameter and 50 mm in length. The working temperature
range is -40C 4o 65°C.
Since the units are sensitive to temperature, some form of
temperature compensation ay is needed.
The Pope cell uses a similar bifilar conductive grid on an isolative
substrate. In this transducer the substrate is made from polystyrene treated
with sulphuric acid in a prescribed fashion. This type of transducer
exhibits a non-linear resistance change from a few mega ohms at 0
percent RH to about 1,000  at 100% RH and a single sensor can cover
the entire range (15 to 99% RH). Accuracy is comparable to that of the
Dunmore sensor.
2. Electrolytic Hygrometer. A typical electrolytic hygrometer utilizes a
cell coated with a thin film of phosphorous pentoxide, (P2 O5) which
absorbs water from the sample gas. The cell has a bifilar winding of
inert electrodes on a fluorinated hydrocarbon capillary. Direct voltage
applied to the electrodes dissociates the water, which is absorbed by
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the phosphorous pentoxide, into hydrogen and oxygen. Two electrons
are required for electrolyzing each water molecule, and so the current
in the cell represents the number of molecules dissociated. A further
calculation, based on flow rate temperature and current yields the
humidity in ppm.
Fig (2-45) Electrolytic Hygrometer
A typical electrolytic hygrometer as shown in Fig (2-45) can cover
a range of from 0 to 2,000 ppm with an accuracy of  5% of the reading,
which is more than adequate for most industrial applications. The
transducer is suitable for most inert elemental gases and organic and
inorganic gas compounds that do not react with phosphorous pentoxide.
These hygrometers cannot be exposed to high water vapour levels for
long periods of time as these results in a high usage rate for the P2 05 and
high cell currents.
3. Aluminium Oxide Hygrometer. Aluminium oxide sensor is formed
by depositing a layer of porous aluminium oxide on a conductive
substrate and then coating the oxide with a thin film of gold. The
conductive base and the gold layer become the capacitor electrodes
and the aluminium-oxide coating becomes the capacitor's dielectric.
Water vapour penetrates into the gold layer and is absorbed by the
porous oxidation layer. The number of water molecules absorbed
determines the impedance of the capacity, which is in turn a measure
of RH.
The advantages of such transducers are that they are (i) small in
size (ii) economical in use in multiple-sensor arrangements (iii) suitable
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for very low dew point levels without the need for sensor cooling and (iv)
they have a wide measurement range.
The aluminium oxide sensors have some limitations also. These
limitations are that (i) they are secondary measurement devices and
require periodical calibration for accommodating aging effects,
hysteresis, and contamination, and (ii) they need separate calibration
curves which are typically non-linear.
These sensors are available in various types, ranging from a lowcost single-point system, including portable battery-operated models, to
multi-point microprocessor based systems with ability of computing and
displaying humidity information in various parameters such as dew
points, ppm, and % RH.
The aluminium oxide sensor is also employed for moisture
measurements in liquids (hydrocarbons) and, because of its low power
usage; it is suitable for use in explosion-proof installations. These sensors
are frequently employed in petro-chemical applications where low dew
points are required to be monitored on line and where the reduced
accuracies and other limitations are acceptable.
2.13 Digital Displacement Transducers:
Digital transducers can be considered to range from those that
count events or provide a frequency output to digital encoders. Digital
transducers signals are in the form of pulses, sinusoidal waveform, or
pattern/time sequence of 1's or 0's. The pulse count, frequency of
sinusoid, and pattern/time sequence of 1's and 0's are not essentially
dependent on the amplitude of the signals. Thus such signals can be
transmitted over long distances without incurring much distortion owing
to amplitude variation and phase shift, and they can be manipulated
without error in electronic processing circuits. It is possible to pulse
count, determine frequency or time period, and encode position very
accurately with high resolution, and the resultant read-out is
unambiguous. These are the reasons that the transducers having digital
output are preferable.
However, there are very few such devices which can provide a
direct digital output, and, therefore we usually employ an analog
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transducer to give a voltage signal and an electronic analog-to-digital
converter for realizing the desired digital data. Digital transducers are
called encoders and do exist, however, for linear or rotary displacement
and are in wide use.
2.13.1 Brush Type Encoders:
The encoder consists of a cylindrical disc with the coding patterns
(with conducting and insulating sections) arranged in concentric rings on
one side of the disc. The black areas are made of conducting material
while white areas are made of non-conducting materials. A thin layer of
insulating material is deposited on the conducting disc to form nonconducting areas. The linear version operates in a similar manner with
equal sectors along the length. The circuits of sliding contacts coming in
contact with the conducting areas are completed and the circuits coming
in contact with no conducting (insulated) areas are not completed. In this
way the encoder gives out a digital read out which is an indication of
position. Thus the encoder determines the displacement (linear or rotary).
In fig the readout lamps are shown for explanatory purpose only;
the voltage on the four lamp lines could be sent to a digital computer
directly. For having visual readout these four voltages are applied to a
binary-to-decimal conversion module, which gives readout decimally on
a display. This avoids the need to mentally sum lamp readings.
While the code pattern illustrated in fig. is most convenient for
explaining how motion is represented in the familiar natural binary
system but it has got one important drawback. This drawback is that, if the brushes and segments are not perfectly aligned, an error of at least one
unit can occur in moving the disc from one position to the next.
For example, let the present reading be 9(1001 binary) and in
moving to next reading of 10 (1010 binary) the two digit changes before
the unit does because of the misalignment. This reads 1011 (11 in decimal
number), and thus the reading jumps from 9 to 11. This draw-back is
overcome by a binary coded decimal (BCD) system, such as the Gray
code wherein only one-bit change take place in the transition between,
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any two consecutive numbers. Since the Gray-code output may not be
compatible with the readout device conversion from Gray to natural
binary (or-vice-versa) may be necessary and is accomplished by
employing standard logic gates, as illustrated in fig. (2-46).
These encoders are cheaper in cost and can be made to have any
degree of accuracy provided that the sector is made large enough to
accommodate the required number of rows for binary numbers, and are
quite adequate for slowly moving systems.
Fig (2-46) Rotary and Linear Encoders
The major problem with such encoders is because of wear of
contactors and maintenance of the contacts. The resolution of these
encoders depends upon the number of digits comprising the binary
number.
The resolution is 1/2n of full scale where n is the number of digits.
The range is up to several meters with accuracy of 1 part in 20,000 of full
scale.
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The photoelectric optical method employing interference-pattern
techniques derived from the use of optical gratings is used to have a
higher degree of resolution.
2.14 Thermal Conductivity Gas Analysis:
For certain gases and gas mixtures, thermal conductivity has been
the analytical method of choice for many years. The method is based on
the fact that various gases differ considerably in their ability to conduct
heat. The thermal conductivity of air at 0C is taken as unity. The thermal
conductivities of CO2, CO helium, hydrogen, nitrogen and oxygen are
0.585, 0.958, 6.08, .7.35, 1.015 and 1.007 respectively.
The measurement of the thermal conductivity of a gas mixture
provides Knowledge about the proportions of its constituents.
The most common method are used for gas analysis involves the
use of hot-wire thermal conductivity gas analysis cell, as illustrated in
fig.(2-47) Such a cell consists of two chamber, each a wire filament. One
chamber allows the sample gas to flow through it, while the other is
sealed and contains a -reference gas, such as air.
Fig (2-47) Basic Thermal Conductivity Gas-Analyzer Circuit
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The bridge is first calibrated by allowing the same gas to surround
the two filament resistors (reference filament and measuring filament R3
and R4 The resistor R2 is adjusted to yield a balanced The bridge is first
calibrated by allowing the same gas to surround the two filament resistors
(reference filament and measuring filament R3 and R4 The resistor R2 is
adjusted to yield a balanced condition Then the gas in the measuring
chamber is replaced by the gas under test, flowing under the same
pressure and temperature conditions. If the test gas have constituents, of
different thermal conductivities than the reference gas, the measuring
filament resistor R4 will be cooled at different rate. A different cooling
rate results in a change of the temperature of the measuring filament
resistor R4 and so change in its resistance. The change in resistance R4
causes the unbalancing of the bridge. if the purity of the gas is in
question, the condition of unbalance actuates a meter which indicates the
change in composition of the gas.
The preferred circuit is the Wheatstone bridge using two cells
(four chambers), as illustrated in fig. (2-48)
Although it is a simple matter to determine the proportion of two
gases in a mixture, but it is considerably difficult to analyze a complex
gas. Special techniques have been developed for accomplishing it, such as
passing the sample through one chamber, then through an apparatus
which removes one constituent of the sample, and finally through
another chamber as illustrated in fig (2-49). The difference in the thermal
conductivity in each of the chambers is, therefore, on account of the
constituent removed thus, the percentage of the constituent removed can
determined.
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Fig (2-48)Two Cell Thermal Conductivity Bridge
Fig. (2-49) Separation of Mixture of Gases
2.15 Biomedical Instrumentation:
During the last two decades, there has been a tremendous increase
in the use of electronic equipment in the medical field for clinical and
research purposes. With the wide spread use and needs of medical
electronic instruments, it is essential to have a basic knowledge of the
principles of operation of electronic instruments used in the medical field.
There are some parameters like temperature, blood flow, blood
pressure, and respiratory functions etc which are to be routinely
monitored. These parameters are basically non-electrical in nature and
need to be converted into corresponding electrical signals with the help of
transducers. Display and recording systems are required for processing
and presenting the picked up signals of interest from the body in a form
most convenient for Interpretation. Wireless telemetry permits
examination of the physiological data of subjects in normal conditions
and in natural surroundings without discomfort o obstruction to the
person or animal under investigation. An extensive use of computers and
microprocessors Is now being made in medical instruments designed for
performing routine clinical measurements, particularly in those situations
where data computing and processing could be considered as part of the
measurement and diagnostic procedure.
Detailed study of electronic instruments used in medical field is
beyond the scope of this book. Some of the most commonly used devices,
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such as transducers, recorders, display devices, will be described here
in brief.
2.15.1 Biomedical Transducers:
1. Transducers for Body Temperature Measurement. The most popular
method of measuring temperature is by using a mercury-in-glass
thermometer. However, reliable accuracy cannot be attained by these
thermometers, especially over the wide range which is now realized to
be necessary. In many of the circumstances of lowered body,
temperature, continuous or frequent sampling of temperature is
desirable, as in the operating theatre, post-operating recovery room
and intensive care unit (ICU), and during forced dieresis, massive
blood transfusion, and accidental hypothermia. The continuous
reading facility of electronic thermometers obviously lends itself to
such applications.
Thermo-couples are normally used for Measurement of surface
skin temperature, but rectal thermocouple probes are also available.
Resistance thermometers are usually used for rectal and liquid
temperature measurement. The resistance thermometer and thermistor
measure absolute temperature, whereas the thermocouples generally
measure relative temperature.
For medical applications, copper-constantan combination is usually
preferred. With the reference junction at 0C and the other at 375C, the
output from this thermocouple is 1.5 mV. Two types of measuring
instruments can be used with thermocouples for measuring potential
differences of this order. In one, moving coil instruments are used as mill
voltmeters for measuring the thermocouple emf. They are directly
calibrated in °C. Usually in clinical thermocouple instruments, reflecting
galvanometers, or light spot galvanometers are preferred to measure and
display temperature. If the thermocouple voltages are small (less than 1
mV), they can be readily measured with a precision do potentiometer
having a Weston cadmium-mercury cell as a reference. They call also be
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read directly on digital voltmeter or by using a chopper stabilized do
amplifier followed by a panel meter of the analog or digital type.
2. Pulse Sensors. Each time the heart muscle contracts, blood is ejected
from the ventricles and a pulse of pressure is transmitted through the
circulatory system. This pressure pulse when travelling through the
vessels causes vessel wall displacement which is measurable at
various points of the peripheral circulatory system. The pulse can be
felt by placing the finger tip over the radial artery in the wrist or some
other location where an artery seems just below the skin. The timing
and wave-shape of the pressure pulse are diagnostically important as
they provide valuable information.
a. Photo-electric Pulse Transducers. Photo resistor normally used for
detecting pulsatile blood volume variations by photoelectric
method. Most common methods of photoelectric method. Most
common methods of photoelectric plethysmography are
transmittance method and reflectance method.
The arrangement used in transmittance method of photoelectric
plethysmography is shown in fig.(2-50) (a). In this arrangement a
miniature lamp and a photoresistor are mounted in an enclosure
that fits over the tip of the patient finger. Light is transmitted
determined by the amount of light reaching lt. With each
contraction of heart, blood is forced to the extremities and amount
of blood in the finger increases. It changes the
Fig (2-50) Arrangement of Photoresistor and Lamp in a
Finger Probe for Pulse Pickup
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Optical density with the result that the light transmission through
the finger reduces and the resistor or the photo-resistance increases
accordingly, the photoresistor is connected as a part to potential
divider circuit and produces a voltage that varies as the amount
blood in the finger varies. This voltage that closely follows the
pressure pulse and its wave shape may be displayed on an
oscilloscope or recorded on a strip-chart recorder.
In the arrangement used for the reflectance method, shown in
fig.(2-50) (b), the photoresistor is place adjacent to the exciter lamp
part of the light rays emitted by the lamp is reflected and scattered
from the skin and the tissues, and falls on the photoresistor. The
quantity of light reflected is determined by the blood saturation of
the capillaries and therefore, the voltage drop across drop across
the photoresistor, connected as a potential divider, will vary in
proportion to the volume variations of the blood vessels.
For photoelectric plethysmography, the transducer is usually a
miniature tungsten lamp and hotoconductive cell. Several problems
are associated when this device is employed for studying peripheral
circulation. The transducer is. Usually bulky. The tungsten lamp
gives a broad spectral output and so the reflectance from vascular
bed will be affected by both variations in peripheral circulation and
blood oxygen.
b. Piezoelectric Arterial Pulse Receptor. Piezocrystal may be
employed for detection of the pulse waves at certain places of the
peripheral system where considerable displacemen t o f the
tissue layer above the artery is involved. The arrangement consists
of a piezoelectric crystal clamped in a hermetically sealed capsule
subject to the displacement stresses. The displacement can be
transmitted to the crystal through a soft rubber diaphragm. The
crystal can be connected to an ECG recorder for recording the
pulse waveform.
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There is another variation of the finger plethysmograph in which an
air-coupled piezoelectric transducer is used. With the variations in
the blood volume in the finger during the cardiac cycle, slight
variations occur in the finger size these variations can be
transmitted as pressure variations in the air column inside the
plastic tubing. A piezoelectric transducer at the end of the tube
converts the pressure variations to a corresponding electrical
signal. Con then be amplified and displayed.
c. Strain Gauge pulse transducer. In this method, the displacement of
the vessel is transferred to the semiconductor strain gauge by
means of a feeler pin, and a leaf spring, as shown in fig.36 the
strain gauge is firmly attached to the leaf spring on one side and to
feeler pin on the other side. A ring round the feeler; of the
transducer minimizes interference caused by unsteadiness of
application of the transducer to the skin. The transducer produces
an output of V for 0.1 mm displacement. The transducer has an
internal resistance of 1 k  . and resonant frequency undamped
mechanical system as 150 Hz. The traneducer yields reliable
results as shown in fig (2-51).
Fig (2-51) Strain Gauge Pulse Transducer
3. Respiration Sensors. The primary functions of the respiratory system
are to supply oxygen to the tissues and remove carbon dioxide from
the tissues. The breathing action is controlled by muscular action
causing the volume of the lung to increase and decrease to effect a
precise and sensitive control of the tension of CO2 in the arterial
blood. Under normal circumstances, this is rhythmic action; with the
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result that the respiration rate provides a fairly good idea about the
relative respiratory activity. Several types of transducers have been
developed for the measurement of respiration rate. The choice of
particular type of transducer depends mostly upon the case of
application and their acceptance by the subject under test. Some of the
commonly used transducers for measurement of respiration rate are
explained below.
a. Strain Gauge type Chest Transducer. The respiratory cycle is
accompanied by variations in the thoracic volume.
Such variations can be detected by means of a displacement
transducer incorporating a strain gauge or a variable resistance
element. The transducer is held by an elastic band around
the chest.
The respiratory movements result in resistance variations of the
strain gauge element connected in one arm of a Wheatstone circuit.
Bridge circus output varies with chest expansion and yields signals
corresponding to respiratory activity.
Variations in chest circumference can also be detected by a rubber
tube filled with mercury. The tube is fastened firmly around the
chest. With the expansion of the chest during an inspiratory phase,
the rubber tube increases in length and so the resistance of the
mercury from one end of this tube to the other varies. Resistance
variations may be measured by passing a constant current through
it and measuring the variations in voltage developed with the
respiratory cycle.
b. Thermistor. There is a detectable difference of temperature
between inspired and expired air because air is warmed during its
passage through the lungs and respiratory tract. The temperature
difference can be best sensed by using a thermistor placed in front
of the nostrils by means of a suitable holding device. In case the
temperature difference is small, the thermistor can even be initially
heated to an appropriate temperature and the variation of its
resistance in synchronism with the respiratory rate, as a result of
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the cooling effect of the air stream, can be detected. This can be
achieved with thermistor dissipations of about 5 to 25 mW.
Excessive thermistor heating may cause discomfort to the subject.
The thermistor is placed as a part of a potential dividing circuit or
in a bridge circuit whose unbalance signal can be amplified to have
the respiratory activity. This method is simple and works well
except in the case of some patients who object to have anything
attached to their nose or face. This method is found to satisfy the
majority of clinical needs including those operative and postoperative subjects.
Respiratory activity can also be detected by measuring variations in
the impedance across the thorax. The signals are quite satisfactory,
provided the patient does not make a great deal of movement with
his thorax.
The transducer is the key link in all instruments designed for
describing and analyzing biological systems. It is beyond the scope
of this book to describe all the transducers used in biomedical
instrumentation. The transducers described here are just those
which find applications in patient monitoring systems and
experimental work; namely temperature, pulse and respiratory
activity.
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