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Transcript
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 - 13 - 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. - 14 - 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 -40C 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 - 15 - 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 - 16 - 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 - 17 - 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, - 18 - 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. - 19 - 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 0C 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 - 20 - 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. - 21 - 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, - 22 - 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 0C and the other at 375C, 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 - 23 - 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 - 24 - 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. - 25 - 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 - 26 - 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 - 27 - 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. - 28 - - 29 -